Cargo.lock

@@ -3307,6 +3307,16 @@ dependencies = [
  "uuid",
 ]
 
+[[package]]
+name = "trx-sstv"
+version = "0.1.0"
+dependencies = [
+ "base64",
+ "png",
+ "tracing",
+ "trx-core",
+]
+
 [[package]]
 name = "trx-vdes"
 version = "0.1.0"

Cargo.toml

@@ -12,6 +12,7 @@ members = [
     "src/decoders/trx-ftx",
     "src/decoders/trx-rds",
     "src/decoders/trx-vdes",
+    "src/decoders/trx-sstv",
     "src/decoders/trx-wefax",
     "src/decoders/trx-wspr",
     "src/trx-core",

src/decoders/trx-sstv/Cargo.toml

@@ -0,0 +1,14 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+[package]
+name = "trx-sstv"
+version.workspace = true
+edition = "2021"
+
+[dependencies]
+trx-core = { path = "../../trx-core" }
+base64 = "0.22"
+png = "0.17"
+tracing = "0.1"

src/decoders/trx-sstv/examples/round_trip_png.rs

@@ -0,0 +1,91 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Encode a test card, decode it back, write both as PNGs to a directory.
+//!
+//! `cargo run -p trx-sstv --example round_trip_png -- /tmp/out`
+
+use trx_sstv::encode::{encode, Frame};
+use trx_sstv::mode::mode_for_vis;
+use trx_sstv::{ImageCanvas, SstvConfig, SstvDecoder, SstvEvent};
+
+fn main() {
+    let dir = std::env::args().nth(1).unwrap_or_else(|| ".".into());
+    for vis in [44u8, 60, 12, 8, 95] {
+        let mode = mode_for_vis(vis).expect("mode in table");
+        let (w, h) = (usize::from(mode.width), usize::from(mode.height));
+        let mut rgb = vec![0u8; w * h * 3];
+        for y in 0..h {
+            for x in 0..w {
+                let at = (y * w + x) * 3;
+                let (r, g, b) = if y < h / 3 {
+                    [
+                        (255u8, 255u8, 255u8),
+                        (255, 255, 0),
+                        (0, 255, 255),
+                        (0, 255, 0),
+                        (255, 0, 255),
+                        (255, 0, 0),
+                        (0, 0, 255),
+                        (0, 0, 0),
+                    ][x * 8 / w]
+                } else if y < 2 * h / 3 {
+                    let t = (x * 255 / w) as u8;
+                    (t, 255 - t, ((y * 255) / h) as u8)
+                } else {
+                    // Diagonal stripes: a line-timing error shows up as a kink.
+                    if ((x + y) / 16) % 2 == 0 {
+                        (240, 240, 40)
+                    } else {
+                        (20, 20, 90)
+                    }
+                };
+                rgb[at] = r;
+                rgb[at + 1] = g;
+                rgb[at + 2] = b;
+            }
+        }
+        let name = mode.name.replace(' ', "-");
+        let mut sent = ImageCanvas::new(w, h);
+        for y in 0..h {
+            sent.put_row(y, &rgb[y * w * 3..(y + 1) * w * 3]);
+        }
+        std::fs::write(
+            format!("{dir}/{name}-sent.png"),
+            sent.to_png().expect("png"),
+        )
+        .expect("write");
+
+        let frame = Frame {
+            width: w,
+            height: h,
+            rgb: &rgb,
+        };
+        let mut audio = encode(mode, &frame, 48_000);
+        audio.extend(std::iter::repeat_n(0.0, 4800));
+        let mut decoder = SstvDecoder::new(48_000, SstvConfig::default());
+        let mut got = None;
+        for block in audio.chunks(1024) {
+            for event in decoder.process_samples(block) {
+                if let SstvEvent::Complete(image) = event {
+                    got = Some(image);
+                }
+            }
+        }
+        let image = got.expect("no image decoded");
+        let mut canvas = ImageCanvas::new(w, h);
+        for y in 0..h {
+            canvas.put_row(y, &image.rgb[y * w * 3..(y + 1) * w * 3]);
+        }
+        std::fs::write(
+            format!("{dir}/{name}-decoded.png"),
+            canvas.to_png().expect("png"),
+        )
+        .expect("write");
+        println!(
+            "{}: {} lines, complete={}",
+            mode.name, image.lines, image.complete
+        );
+    }
+}

src/decoders/trx-sstv/src/config.rs

@@ -0,0 +1,17 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! SSTV decoder configuration.
+
+/// Settings for [`crate::decoder::SstvDecoder`].
+#[derive(Debug, Clone, Default)]
+pub struct SstvConfig {
+    /// VIS code of a mode to assume when no header is heard, so tuning in
+    /// part-way through a transmission still produces a picture. `None` means
+    /// wait for a header, which is the safe default: guessing wrong yields a
+    /// convincing image of nothing.
+    pub force_mode: Option<u8>,
+    /// Directory for saved PNGs. `None` keeps images in memory only.
+    pub output_dir: Option<String>,
+}

src/decoders/trx-sstv/src/decoder.rs

@@ -0,0 +1,589 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! The decoder: audio in, pictures out.
+//!
+//! Reception is a small state machine. It listens for a VIS header, and once
+//! one names a mode it walks the transmission a line at a time, sampling each
+//! scan at the offsets the mode's segment list gives. Every line is looked for
+//! at the time the mode says it should arrive, then nudged into place by the
+//! sync pulse actually found near it — a transmitter's clock and a receiver's
+//! sound card never agree exactly, and over two minutes of Martin M1 an
+//! uncorrected error of a few parts per million visibly shears the picture.
+//!
+//! Rows are emitted as they are decoded so a picture can be watched arriving,
+//! which is most of the appeal of the mode.
+
+use crate::config::SstvConfig;
+use crate::demod::FreqDemod;
+use crate::image::ImageCanvas;
+use crate::mode::{level_from_hz, mode_for_vis, Channel, ColorModel, SstvMode};
+use crate::vis::find_vis;
+
+/// Anything below this is the sync pulse rather than picture: black is 1500 Hz
+/// and sync is 1200 Hz, so the line sits between them.
+const SYNC_THRESHOLD_HZ: f32 = 1350.0;
+
+/// How much of the signal to keep while hunting for a header. A header is 940
+/// ms; two seconds leaves room for one to straddle several blocks of audio.
+const SEARCH_HISTORY_MS: f64 = 2000.0;
+
+/// From the start bit to the end of the stop bit: ten 30 ms cells. A header
+/// search must never retire a stretch shorter than this, or a start bit split
+/// across two blocks of audio is dismissed on half a view of it.
+const HEADER_SPAN_MS: f64 = 330.0;
+
+/// How far from its predicted position a line's sync pulse is looked for.
+const SYNC_SEARCH_MS: f64 = 12.0;
+
+/// Fraction of the observed timing error applied to the next line. Damped,
+/// because a sync pulse found in noise is worth less than the prediction.
+const SYNC_CORRECTION: f64 = 0.45;
+
+/// Consecutive lines with no sync pulse anywhere near the prediction before
+/// the transmission is taken to have ended.
+const MISSING_SYNC_LIMIT: u32 = 8;
+
+/// What the decoder has to say.
+#[derive(Debug, Clone)]
+pub enum SstvEvent {
+    /// A header was decoded and reception has begun.
+    Started {
+        vis: u8,
+        mode: &'static str,
+        width: u16,
+        height: u16,
+    },
+    /// One image row is ready, as RGB triples.
+    Row { line: u16, rgb: Vec<u8> },
+    /// Reception finished — at the bottom of the frame, or because the signal
+    /// went away. Carries the picture either way.
+    Complete(SstvImage),
+}
+
+/// A received picture.
+#[derive(Debug, Clone)]
+pub struct SstvImage {
+    pub vis: u8,
+    pub mode: &'static str,
+    pub width: u16,
+    pub height: u16,
+    /// Rows actually received, which is the height only if it ran to the end.
+    pub lines: u16,
+    /// Whether the whole frame arrived.
+    pub complete: bool,
+    /// RGB triples, `width * height * 3` bytes. Rows never received are grey.
+    pub rgb: Vec<u8>,
+    /// When reception started, in milliseconds since the epoch.
+    pub started_ms: i64,
+}
+
+enum State {
+    /// Listening for a header.
+    Searching,
+    Receiving(Box<Reception>),
+}
+
+struct Reception {
+    mode: &'static SstvMode,
+    canvas: ImageCanvas,
+    /// Absolute sample index at which the next transmitted line begins.
+    next_line: f64,
+    /// Next image row to write.
+    row: u16,
+    started_ms: i64,
+    missing_syncs: u32,
+    /// Set until the first line has been decoded. A VIS header ends exactly
+    /// where the picture begins, so the first line is already aligned — and a
+    /// search would find the header's own 30 ms stop bit, which is at the sync
+    /// frequency and sits immediately before the picture.
+    first_line: bool,
+    /// Robot 36 sends one chroma channel per line and expects the decoder to
+    /// carry the other over from the line before.
+    last_chroma_r: Option<Vec<u8>>,
+    last_chroma_b: Option<Vec<u8>>,
+}
+
+pub struct SstvDecoder {
+    sample_rate: u32,
+    config: SstvConfig,
+    demod: FreqDemod,
+    /// Instantaneous frequency, one entry per audio sample. Delayed by the
+    /// demodulator's group delay, which is constant and so shifts the whole
+    /// stream — header and lines alike — without disturbing their spacing.
+    freqs: Vec<f32>,
+    /// Absolute index of `freqs[0]`, so positions survive the buffer being
+    /// trimmed.
+    base: u64,
+    /// Absolute index the header search has already covered.
+    searched_to: u64,
+    state: State,
+}
+
+impl SstvDecoder {
+    pub fn new(sample_rate: u32, config: SstvConfig) -> Self {
+        Self {
+            sample_rate,
+            config,
+            demod: FreqDemod::new(sample_rate),
+            freqs: Vec::new(),
+            base: 0,
+            searched_to: 0,
+            state: State::Searching,
+        }
+    }
+
+    /// Whether a picture is currently arriving.
+    pub fn is_receiving(&self) -> bool {
+        matches!(self.state, State::Receiving(_))
+    }
+
+    /// Feed a block of mono audio. Returns whatever it produced.
+    pub fn process_samples(&mut self, samples: &[f32]) -> Vec<SstvEvent> {
+        self.demod.process_into(samples, &mut self.freqs);
+        let mut events = Vec::new();
+        loop {
+            let progressed = match self.state {
+                State::Searching => self.try_start(&mut events),
+                State::Receiving(_) => self.try_line(&mut events),
+            };
+            if !progressed {
+                break;
+            }
+        }
+        self.trim();
+        events
+    }
+
+    /// Abandon a reception in progress, returning the picture so far.
+    pub fn reset(&mut self) -> Vec<SstvEvent> {
+        let mut events = Vec::new();
+        if let State::Receiving(reception) = std::mem::replace(&mut self.state, State::Searching) {
+            events.push(SstvEvent::Complete(finish(&reception)));
+        }
+        self.demod.reset();
+        self.freqs.clear();
+        self.base = 0;
+        self.searched_to = 0;
+        events
+    }
+
+    /// Look for a header, or for a bare sync pulse when the mode is forced.
+    fn try_start(&mut self, events: &mut Vec<SstvEvent>) -> bool {
+        let from = self.searched_to.saturating_sub(self.base) as usize;
+        if from >= self.freqs.len() {
+            return false;
+        }
+
+        if let Some(hit) = find_vis(&self.freqs, self.sample_rate, from) {
+            if let Some(mode) = mode_for_vis(hit.code) {
+                self.begin(mode, self.base + hit.image_start as u64, events);
+                return true;
+            }
+            // A header that parses but names a mode this decoder does not know
+            // is still a header: skip past it rather than finding it again.
+            tracing::debug!(vis = hit.code, "SSTV: unsupported mode");
+            self.searched_to = self.base + hit.image_start as u64;
+            return true;
+        }
+
+        // Tuning in mid-transmission means no header to find. With a mode
+        // named in the configuration, the first sync pulse is enough to start.
+        if let Some(mode) = self.config.force_mode.and_then(mode_for_vis) {
+            if let Some(sync) = self.find_sync(from, self.freqs.len(), mode) {
+                let line_start = (self.base + sync as u64) as f64
+                    - ms_to_samples(mode.sync_offset_ms, self.sample_rate);
+                self.begin_at(mode, line_start.max(0.0), events);
+                return true;
+            }
+        }
+
+        // Nothing yet. Rewind the cursor by a header's worth of samples before
+        // marking the buffer searched: audio arrives in blocks of a few
+        // milliseconds, so the search regularly runs over a start bit that is
+        // only half here. Advancing past it would retire the header for good
+        // on the strength of a partial view of it.
+        let unsearchable = ms_to_samples(HEADER_SPAN_MS, self.sample_rate) as u64;
+        let end = self.base + self.freqs.len() as u64;
+        self.searched_to = self.searched_to.max(end.saturating_sub(unsearchable));
+        false
+    }
+
+    fn begin(&mut self, mode: &'static SstvMode, image_start: u64, events: &mut Vec<SstvEvent>) {
+        self.begin_at(mode, image_start as f64, events);
+    }
+
+    fn begin_at(&mut self, mode: &'static SstvMode, line_start: f64, events: &mut Vec<SstvEvent>) {
+        events.push(SstvEvent::Started {
+            vis: mode.vis,
+            mode: mode.name,
+            width: mode.width,
+            height: mode.height,
+        });
+        self.state = State::Receiving(Box::new(Reception {
+            mode,
+            canvas: ImageCanvas::new(usize::from(mode.width), usize::from(mode.height)),
+            next_line: line_start,
+            row: 0,
+            started_ms: now_ms(),
+            missing_syncs: 0,
+            first_line: true,
+            last_chroma_r: None,
+            last_chroma_b: None,
+        }));
+    }
+
+    /// Decode one transmitted line, if all of it has arrived.
+    fn try_line(&mut self, events: &mut Vec<SstvEvent>) -> bool {
+        let State::Receiving(reception) = &mut self.state else {
+            return false;
+        };
+        let mode = reception.mode;
+        let line_samples = ms_to_samples(mode.line_ms, self.sample_rate);
+        let margin = ms_to_samples(SYNC_SEARCH_MS, self.sample_rate);
+
+        let start = reception.next_line;
+        // Enough for the line's own scans, and for the sync search to reach as
+        // far ahead as it looks — no further. Demanding the search margin past
+        // the end of every line would cost the last line of every picture,
+        // which is exactly where the transmission stops.
+        let sync_reach = start + ms_to_samples(mode.sync_offset_ms, self.sample_rate) + margin;
+        let end = (start + line_samples).max(sync_reach);
+        let available = (self.base + self.freqs.len() as u64) as f64;
+        if end > available {
+            return false;
+        }
+        // The line may begin before what is still buffered if the caller fed a
+        // huge block; nothing can be done about that but skip forward.
+        if start < self.base as f64 {
+            reception.next_line = self.base as f64;
+            return true;
+        }
+
+        // Line up on the sync pulse near where this line is predicted to be.
+        let expected_sync = start + ms_to_samples(mode.sync_offset_ms, self.sample_rate);
+        let from = (expected_sync - margin).max(self.base as f64) as u64;
+        let to = (expected_sync + margin) as u64;
+        let searching = !matches!(&self.state, State::Receiving(r) if r.first_line);
+        let found = if searching {
+            self.find_sync_between(from, to, mode)
+        } else {
+            None
+        };
+        let State::Receiving(reception) = &mut self.state else {
+            return false;
+        };
+        let start = match found {
+            Some(sync_at) => {
+                reception.missing_syncs = 0;
+                let error = sync_at as f64 - expected_sync;
+                reception.next_line += error * SYNC_CORRECTION;
+                reception.next_line
+            }
+            None if reception.first_line => start,
+            None => {
+                reception.missing_syncs += 1;
+                start
+            }
+        };
+
+        if reception.missing_syncs >= MISSING_SYNC_LIMIT {
+            let image = finish(reception);
+            self.state = State::Searching;
+            self.searched_to = self.base + self.freqs.len() as u64;
+            events.push(SstvEvent::Complete(image));
+            return true;
+        }
+
+        // Sample every scan of the line, then colour the rows.
+        let mut scans: Vec<(Channel, Vec<u8>)> = Vec::new();
+        for (channel, offset_ms, ms) in mode.scans() {
+            let pixels = mode.scan_pixels(channel);
+            let at = start + ms_to_samples(offset_ms, self.sample_rate);
+            let values = sample_scan(&self.freqs, self.base, at, ms, pixels, self.sample_rate);
+            scans.push((channel, values));
+        }
+
+        let State::Receiving(reception) = &mut self.state else {
+            return false;
+        };
+        let rows = compose_rows(reception, &scans);
+        for (offset, row) in rows.into_iter().enumerate() {
+            let line = reception.row + offset as u16;
+            reception.canvas.put_row(usize::from(line), &row);
+            events.push(SstvEvent::Row { line, rgb: row });
+        }
+        reception.first_line = false;
+        reception.row += mode.lines_per_transmission;
+        reception.next_line += line_samples;
+
+        if reception.row >= mode.height {
+            let image = finish(reception);
+            self.state = State::Searching;
+            self.searched_to = self.base + self.freqs.len() as u64;
+            events.push(SstvEvent::Complete(image));
+        }
+        true
+    }
+
+    /// First sync pulse of about the right length in `freqs[from..to]`,
+    /// as an index of its leading edge.
+    ///
+    /// Works on a smoothed copy of the window: a sync pulse is 1200 Hz, where
+    /// the raw per-sample estimate swings by ±95 Hz, so single samples cross
+    /// and re-cross the threshold throughout a pulse and no run is ever long
+    /// enough. Pixels are sampled from the raw signal, where averaging over
+    /// the pixel does the same job without blurring across its edges.
+    fn find_sync(&self, from: usize, to: usize, mode: &SstvMode) -> Option<usize> {
+        let want = sync_ms(mode);
+        let min_run = (ms_to_samples(want, self.sample_rate) * 0.6) as usize;
+        // A sync pulse ends. Silence and a dead carrier demodulate to near
+        // zero, which is below the threshold too, and without an upper bound a
+        // decoder left running on an empty channel finds sync everywhere and
+        // fills the picture with noise it invented.
+        let max_run = (ms_to_samples(want, self.sample_rate) * 3.0) as usize;
+        let to = to.min(self.freqs.len());
+        if from >= to {
+            return None;
+        }
+        // Reach past the end of the search window by a whole pulse: a sync
+        // starting at the last moment the window allows still has to be
+        // measurable to its full length, or it is rejected for being short and
+        // the line it belongs to goes unaligned.
+        let pad = ms_to_samples(want + 2.0, self.sample_rate) as usize;
+        let window_from = from.saturating_sub(pad);
+        let window_to = (to + pad).min(self.freqs.len());
+        let smoothed = crate::demod::smooth(
+            &self.freqs[window_from..window_to],
+            ms_to_samples(1.0, self.sample_rate) as usize,
+        );
+
+        let mut i = from - window_from;
+        let scan_to = to - window_from;
+        while i < scan_to {
+            if smoothed[i] >= SYNC_THRESHOLD_HZ {
+                i += 1;
+                continue;
+            }
+            let mut run = 0;
+            while i + run < smoothed.len() && smoothed[i + run] < SYNC_THRESHOLD_HZ {
+                run += 1;
+            }
+            if run >= min_run && run <= max_run {
+                return Some(window_from + i);
+            }
+            i += run.max(1);
+        }
+        None
+    }
+
+    /// As [`Self::find_sync`], over an absolute index range.
+    fn find_sync_between(&self, from: u64, to: u64, mode: &SstvMode) -> Option<u64> {
+        let from = from.saturating_sub(self.base) as usize;
+        let to = to.saturating_sub(self.base) as usize;
+        if from >= self.freqs.len() {
+            return None;
+        }
+        self.find_sync(from, to, mode)
+            .map(|at| self.base + at as u64)
+    }
+
+    /// Drop what is behind the decoder, so a long reception does not grow the
+    /// buffer without bound.
+    fn trim(&mut self) {
+        let keep_from = match &self.state {
+            State::Searching => {
+                let history = ms_to_samples(SEARCH_HISTORY_MS, self.sample_rate) as u64;
+                (self.base + self.freqs.len() as u64).saturating_sub(history)
+            }
+            State::Receiving(reception) => {
+                let margin = ms_to_samples(SYNC_SEARCH_MS * 2.0, self.sample_rate) as u64;
+                (reception.next_line as u64).saturating_sub(margin)
+            }
+        };
+        if keep_from <= self.base {
+            return;
+        }
+        let drop = (keep_from - self.base) as usize;
+        if drop >= self.freqs.len() {
+            self.freqs.clear();
+        } else {
+            self.freqs.drain(..drop);
+        }
+        self.base = keep_from;
+        self.searched_to = self.searched_to.max(self.base);
+    }
+}
+
+/// The sync pulse length of a mode, read out of its own segment list.
+fn sync_ms(mode: &SstvMode) -> f64 {
+    mode.segments
+        .iter()
+        .find_map(|segment| match segment {
+            crate::mode::Segment::Sync(ms) => Some(*ms),
+            _ => None,
+        })
+        .unwrap_or(9.0)
+}
+
+/// Milliseconds since the epoch, for stamping a picture with when it arrived.
+fn now_ms() -> i64 {
+    std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_millis() as i64
+}
+
+fn ms_to_samples(ms: f64, sample_rate: u32) -> f64 {
+    ms / 1000.0 * f64::from(sample_rate)
+}
+
+/// Average the frequency across each pixel's window and turn it into a level.
+///
+/// The middle 60% of the window is used: a pixel's edges carry the
+/// demodulator's transition from the pixel before, and including them smears
+/// every edge in the picture.
+fn sample_scan(
+    freqs: &[f32],
+    base: u64,
+    start: f64,
+    ms: f64,
+    pixels: usize,
+    sample_rate: u32,
+) -> Vec<u8> {
+    let mut out = Vec::with_capacity(pixels);
+    let width = ms_to_samples(ms, sample_rate) / pixels as f64;
+    for x in 0..pixels {
+        let pixel_start = start + width * x as f64;
+        let from = (pixel_start + width * 0.2 - base as f64).max(0.0) as usize;
+        let to = ((pixel_start + width * 0.8 - base as f64).max(0.0) as usize).min(freqs.len());
+        // A pixel narrower than a sample still has to produce one.
+        let (from, to) = if to > from {
+            (from, to)
+        } else {
+            let at = (pixel_start - base as f64).max(0.0) as usize;
+            (
+                at.min(freqs.len().saturating_sub(1)),
+                (at + 1).min(freqs.len()),
+            )
+        };
+        if to <= from {
+            out.push(0);
+            continue;
+        }
+        let window = &freqs[from..to];
+        let mean = window.iter().sum::<f32>() / window.len() as f32;
+        out.push(level_from_hz(mean));
+    }
+    out
+}
+
+/// Turn one line's scans into image rows.
+fn compose_rows(reception: &mut Reception, scans: &[(Channel, Vec<u8>)]) -> Vec<Vec<u8>> {
+    let mode = reception.mode;
+    let width = usize::from(mode.width);
+    let find = |channel: Channel| scans.iter().find(|(c, _)| *c == channel).map(|(_, v)| v);
+
+    match mode.color {
+        ColorModel::Rgb => {
+            let red = find(Channel::Red);
+            let green = find(Channel::Green);
+            let blue = find(Channel::Blue);
+            let mut row = vec![0u8; width * 3];
+            for x in 0..width {
+                row[x * 3] = red.and_then(|c| c.get(x).copied()).unwrap_or(0);
+                row[x * 3 + 1] = green.and_then(|c| c.get(x).copied()).unwrap_or(0);
+                row[x * 3 + 2] = blue.and_then(|c| c.get(x).copied()).unwrap_or(0);
+            }
+            vec![row]
+        }
+        ColorModel::YCrCb => {
+            let luma = find(Channel::LumaOdd).cloned().unwrap_or_default();
+            let cr = find(Channel::ChromaR).cloned().unwrap_or_default();
+            let cb = find(Channel::ChromaB).cloned().unwrap_or_default();
+            vec![ycrcb_row(&luma, &cr, &cb, width)]
+        }
+        ColorModel::YCrCbAlternating => {
+            // This line carries one chroma channel; the other is the one from
+            // the line before, which is what the mode expects a decoder to do.
+            let luma = find(Channel::LumaOdd).cloned().unwrap_or_default();
+            let chroma = find(Channel::ChromaAlternating)
+                .cloned()
+                .unwrap_or_default();
+            if reception.row.is_multiple_of(2) {
+                reception.last_chroma_r = Some(chroma);
+            } else {
+                reception.last_chroma_b = Some(chroma);
+            }
+            let neutral = vec![128u8; luma.len().max(1) / 2];
+            let cr = reception
+                .last_chroma_r
+                .clone()
+                .unwrap_or_else(|| neutral.clone());
+            let cb = reception.last_chroma_b.clone().unwrap_or(neutral);
+            vec![ycrcb_row(&luma, &cr, &cb, width)]
+        }
+        ColorModel::YCrCbPaired => {
+            let odd = find(Channel::LumaOdd).cloned().unwrap_or_default();
+            let even = find(Channel::LumaEven).cloned().unwrap_or_default();
+            let cr = find(Channel::ChromaR).cloned().unwrap_or_default();
+            let cb = find(Channel::ChromaB).cloned().unwrap_or_default();
+            vec![
+                ycrcb_row(&odd, &cr, &cb, width),
+                ycrcb_row(&even, &cr, &cb, width),
+            ]
+        }
+    }
+}
+
+/// One RGB row from luminance and chrominance, stretching the chroma scans
+/// across the width when they are narrower than it.
+fn ycrcb_row(luma: &[u8], cr: &[u8], cb: &[u8], width: usize) -> Vec<u8> {
+    let mut row = vec![0u8; width * 3];
+    let pick = |channel: &[u8], x: usize| -> f32 {
+        if channel.is_empty() {
+            return 128.0;
+        }
+        let at = x * channel.len() / width.max(1);
+        f32::from(channel[at.min(channel.len() - 1)])
+    };
+    for x in 0..width {
+        let y = if luma.is_empty() {
+            0.0
+        } else {
+            let at = x * luma.len() / width.max(1);
+            f32::from(luma[at.min(luma.len() - 1)])
+        };
+        let (r, g, b) = ycrcb_to_rgb(y, pick(cr, x), pick(cb, x));
+        row[x * 3] = r;
+        row[x * 3 + 1] = g;
+        row[x * 3 + 2] = b;
+    }
+    row
+}
+
+/// The inverse of the studio-swing conversion SSTV encoders use.
+fn ycrcb_to_rgb(y: f32, cr: f32, cb: f32) -> (u8, u8, u8) {
+    let r = 298.082 * y / 256.0 + 408.583 * cr / 256.0 - 222.921;
+    let g = 298.082 * y / 256.0 - 100.291 * cb / 256.0 - 208.120 * cr / 256.0 + 135.576;
+    let b = 298.082 * y / 256.0 + 516.412 * cb / 256.0 - 276.836;
+    (
+        r.clamp(0.0, 255.0) as u8,
+        g.clamp(0.0, 255.0) as u8,
+        b.clamp(0.0, 255.0) as u8,
+    )
+}
+
+fn finish(reception: &Reception) -> SstvImage {
+    SstvImage {
+        vis: reception.mode.vis,
+        mode: reception.mode.name,
+        width: reception.mode.width,
+        height: reception.mode.height,
+        lines: reception.canvas.filled_rows() as u16,
+        complete: reception.row >= reception.mode.height,
+        rgb: reception.canvas.rgb().to_vec(),
+        started_ms: reception.started_ms,
+    }
+}

src/decoders/trx-sstv/src/demod.rs

@@ -0,0 +1,297 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Instantaneous frequency estimation.
+//!
+//! SSTV carries every pixel as a frequency between 1500 and 2300 Hz, and every
+//! line boundary as a 1200 Hz pulse, so one measurement serves the whole
+//! decoder: the frequency of the signal at each sample. A Hilbert transform
+//! FIR forms the analytic signal and the phase difference between consecutive
+//! samples gives the frequency.
+//!
+//! The same approach drives the WEFAX decoder, which maps the result straight
+//! to luminance. Here the frequency itself is the output, because the VIS
+//! header and the sync detector read tones far outside the pixel band.
+//!
+//! Block-based linear processing, per `docs/Optimization-Guidelines.md`: the
+//! FIR runs over a contiguous `[tail | samples]` buffer so the inner loop is
+//! straight indexing the compiler can vectorise.
+
+use std::f32::consts::PI;
+
+/// Taps for the Hilbert transform FIR. Odd, so the delay is a whole sample.
+const HILBERT_TAPS: usize = 65;
+
+/// Group delay of the FIR, in samples.
+const HILBERT_DELAY: usize = HILBERT_TAPS / 2;
+
+/// Taps for the input band-pass. Long enough to be worth having, short enough
+/// that its delay is a couple of milliseconds.
+const BANDPASS_TAPS: usize = 127;
+
+/// The band SSTV lives in: sync at 1200 Hz, black at 1500, white at 2300.
+const BAND_LOW_HZ: f32 = 900.0;
+const BAND_HIGH_HZ: f32 = 2700.0;
+
+/// Produces instantaneous frequency in Hz from real audio samples.
+pub struct FreqDemod {
+    /// Band-pass, applied first. A phase-difference frequency detector answers
+    /// whatever is loudest, so hiss outside the SSTV band steers the estimate
+    /// even when the signal is much stronger — the picture tears rather than
+    /// grows grainy. Every real decoder filters to the band first.
+    bandpass: Vec<f32>,
+    bandpass_tail: Vec<f32>,
+    coeffs: [f32; HILBERT_TAPS],
+    /// The last `HILBERT_TAPS - 1` input samples, priming the next block.
+    tail: Vec<f32>,
+    prev_i: f32,
+    prev_q: f32,
+    /// `sample_rate / 2π`, the constant turning phase step into Hz.
+    hz_per_radian: f32,
+}
+
+impl FreqDemod {
+    pub fn new(sample_rate: u32) -> Self {
+        Self {
+            bandpass: design_bandpass_fir(sample_rate),
+            bandpass_tail: vec![0.0; BANDPASS_TAPS - 1],
+            coeffs: design_hilbert_fir(),
+            tail: vec![0.0; HILBERT_TAPS - 1],
+            prev_i: 0.0,
+            prev_q: 0.0,
+            hz_per_radian: sample_rate as f32 / (2.0 * PI),
+        }
+    }
+
+    /// Band-pass a block, carrying the filter's state across the seam.
+    fn filter(&mut self, samples: &[f32]) -> Vec<f32> {
+        let taps = BANDPASS_TAPS;
+        let tail_len = taps - 1;
+        let mut work = Vec::with_capacity(tail_len + samples.len());
+        work.extend_from_slice(&self.bandpass_tail);
+        work.extend_from_slice(samples);
+
+        let mut out = Vec::with_capacity(samples.len());
+        for i in 0..samples.len() {
+            let window = &work[i..i + taps];
+            let mut acc = 0.0f32;
+            for k in 0..taps {
+                acc += self.bandpass[k] * window[taps - 1 - k];
+            }
+            out.push(acc);
+        }
+        let work_len = work.len();
+        self.bandpass_tail
+            .copy_from_slice(&work[work_len - tail_len..]);
+        out
+    }
+
+    /// Frequency in Hz for each input sample, appended to `out`.
+    ///
+    /// Output is delayed by the FIR's group delay, which is constant and so
+    /// affects only the absolute timing of the whole stream, not the spacing
+    /// between the events in it.
+    pub fn process_into(&mut self, samples: &[f32], out: &mut Vec<f32>) {
+        if samples.is_empty() {
+            return;
+        }
+        let samples = self.filter(samples);
+        let samples = samples.as_slice();
+        let taps = HILBERT_TAPS;
+        let tail_len = taps - 1;
+
+        let mut work = Vec::with_capacity(tail_len + samples.len());
+        work.extend_from_slice(&self.tail);
+        work.extend_from_slice(samples);
+
+        out.reserve(samples.len());
+        for i in 0..samples.len() {
+            let window = &work[i..i + taps];
+            let mut q = 0.0f32;
+            for k in 0..taps {
+                q += self.coeffs[k] * window[taps - 1 - k];
+            }
+            // In phase with the quadrature output: the input, delayed by the
+            // FIR's own group delay.
+            let i_val = window[HILBERT_DELAY];
+
+            // f = |arg(z[n] · conj(z[n-1]))| · fs / 2π
+            let di = i_val * self.prev_i + q * self.prev_q;
+            let dq = q * self.prev_i - i_val * self.prev_q;
+            out.push(dq.atan2(di).abs() * self.hz_per_radian);
+
+            self.prev_i = i_val;
+            self.prev_q = q;
+        }
+
+        let work_len = work.len();
+        self.tail.copy_from_slice(&work[work_len - tail_len..]);
+    }
+
+    pub fn reset(&mut self) {
+        self.bandpass_tail.fill(0.0);
+        self.tail.fill(0.0);
+        self.prev_i = 0.0;
+        self.prev_q = 0.0;
+    }
+}
+
+/// Boxcar mean over `window` samples, centred, returning one value per input.
+///
+/// The per-sample estimate ripples — badly at the low end of the band, where
+/// the Hilbert approximation is weakest: a clean 1200 Hz tone reads anywhere
+/// between 1110 and 1300 Hz sample to sample, though its mean is exact. Pixels
+/// are averaged over their own window and so come out right regardless, but
+/// anything that classifies a single sample by frequency — the VIS bits, the
+/// sync pulses — has to look at a mean or it is reading the ripple.
+pub fn smooth(freqs: &[f32], window: usize) -> Vec<f32> {
+    let window = window.max(1);
+    if freqs.is_empty() {
+        return Vec::new();
+    }
+    // Prefix sums in f64: a minute of audio is three million samples, and a
+    // running f32 total drifts long before that.
+    let mut prefix = Vec::with_capacity(freqs.len() + 1);
+    prefix.push(0.0f64);
+    for &freq in freqs {
+        prefix.push(prefix[prefix.len() - 1] + f64::from(freq));
+    }
+    let half = window / 2;
+    (0..freqs.len())
+        .map(|i| {
+            // Shrinks at the ends rather than reaching past them.
+            let from = i.saturating_sub(half);
+            let to = (i + window - half).min(freqs.len());
+            ((prefix[to] - prefix[from]) / (to - from) as f64) as f32
+        })
+        .collect()
+}
+
+/// Windowed-sinc band-pass over the SSTV band, Hamming-windowed and
+/// linear-phase, so every frequency in the band is delayed alike.
+fn design_bandpass_fir(sample_rate: u32) -> Vec<f32> {
+    let sr = sample_rate as f64;
+    let low = f64::from(BAND_LOW_HZ) / sr;
+    let high = (f64::from(BAND_HIGH_HZ) / sr).min(0.499);
+    let m = (BANDPASS_TAPS - 1) as f64;
+    let mid = m / 2.0;
+    let sinc = |x: f64| {
+        if x.abs() < 1e-9 {
+            1.0
+        } else {
+            (std::f64::consts::PI * x).sin() / (std::f64::consts::PI * x)
+        }
+    };
+    let mut coeffs = Vec::with_capacity(BANDPASS_TAPS);
+    for i in 0..BANDPASS_TAPS {
+        let n = i as f64 - mid;
+        // Difference of two low-passes is a band-pass.
+        let ideal = 2.0 * high * sinc(2.0 * high * n) - 2.0 * low * sinc(2.0 * low * n);
+        let window = 0.54 - 0.46 * (2.0 * std::f64::consts::PI * i as f64 / m).cos();
+        coeffs.push((ideal * window) as f32);
+    }
+    coeffs
+}
+
+/// Type III FIR approximating a 90° phase shift: h[n] = 2/(πn) for odd n,
+/// Blackman-windowed. Independent of sample rate, so the decoder can run at
+/// whatever rate the audio arrives in.
+fn design_hilbert_fir() -> [f32; HILBERT_TAPS] {
+    let mut coeffs = [0.0f32; HILBERT_TAPS];
+    let m = (HILBERT_TAPS - 1) as f64;
+    let mid = m / 2.0;
+    for (i, coeff) in coeffs.iter_mut().enumerate() {
+        let n = i as f64 - mid;
+        let ni = n.round() as i64;
+        if ni != 0 && ni % 2 != 0 {
+            let h = 2.0 / (std::f64::consts::PI * n);
+            let w = 0.42 - 0.5 * (2.0 * std::f64::consts::PI * i as f64 / m).cos()
+                + 0.08 * (4.0 * std::f64::consts::PI * i as f64 / m).cos();
+            *coeff = (h * w) as f32;
+        }
+    }
+    coeffs
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn tone(freq: f32, sample_rate: u32, samples: usize) -> Vec<f32> {
+        (0..samples)
+            .map(|n| (2.0 * PI * freq * n as f32 / sample_rate as f32).sin())
+            .collect()
+    }
+
+    /// Measured on the settled part of the output: the first `HILBERT_TAPS`
+    /// samples are the filter filling up.
+    fn measure(freq: f32, sample_rate: u32) -> f32 {
+        let mut demod = FreqDemod::new(sample_rate);
+        let mut out = Vec::new();
+        demod.process_into(
+            &tone(freq, sample_rate, sample_rate as usize / 10),
+            &mut out,
+        );
+        let settled = &out[HILBERT_TAPS * 2..];
+        settled.iter().sum::<f32>() / settled.len() as f32
+    }
+
+    #[test]
+    fn reads_the_tones_sstv_is_made_of() {
+        for rate in [8000u32, 11025, 44100, 48000] {
+            for freq in [1200.0f32, 1500.0, 1900.0, 2300.0] {
+                let measured = measure(freq, rate);
+                assert!(
+                    (measured - freq).abs() < 5.0,
+                    "{rate} Hz: {freq} Hz tone measured as {measured:.1} Hz",
+                );
+            }
+        }
+    }
+
+    /// Blocks are whatever size the audio pipeline hands over, and a tone that
+    /// straddles two of them must not produce a discontinuity at the seam.
+    #[test]
+    fn block_boundaries_do_not_disturb_the_estimate() {
+        let rate = 48_000;
+        let samples = tone(1900.0, rate, 9600);
+        let mut whole = FreqDemod::new(rate);
+        let mut expected = Vec::new();
+        whole.process_into(&samples, &mut expected);
+
+        let mut split = FreqDemod::new(rate);
+        let mut actual = Vec::new();
+        for chunk in samples.chunks(137) {
+            split.process_into(chunk, &mut actual);
+        }
+
+        assert_eq!(actual.len(), expected.len());
+        for (i, (a, b)) in actual.iter().zip(&expected).enumerate() {
+            assert!((a - b).abs() < 0.01, "sample {i}: {a} vs {b}");
+        }
+    }
+
+    #[test]
+    fn follows_a_step_between_tones_within_a_pixel() {
+        let rate = 48_000;
+        let mut samples = tone(1500.0, rate, 4800);
+        samples.extend(tone(2300.0, rate, 4800));
+        let mut demod = FreqDemod::new(rate);
+        let mut out = Vec::new();
+        demod.process_into(&samples, &mut out);
+
+        // Well before the step it reads black; well after it, white. The step
+        // itself takes the filter's length to pass through.
+        let before = out[4800 - 200..4800 - 100].iter().sum::<f32>() / 100.0;
+        let after = out[4800 + 200..4800 + 300].iter().sum::<f32>() / 100.0;
+        assert!(
+            (before - 1500.0).abs() < 10.0,
+            "before the step: {before:.1} Hz"
+        );
+        assert!(
+            (after - 2300.0).abs() < 10.0,
+            "after the step: {after:.1} Hz"
+        );
+    }
+}

src/decoders/trx-sstv/src/encode.rs

@@ -0,0 +1,238 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Turning an image into an SSTV signal.
+//!
+//! This exists so the decoder can be held to a picture rather than to a
+//! description of one: the tests encode a known image, decode the audio back
+//! and compare. Nothing in the receive path uses it.
+//!
+//! It is written to the same mode table the decoder reads, which makes a
+//! round-trip a test of the decoder and not of the timings — the timings are
+//! checked separately, against the published line durations, in [`crate::mode`].
+
+use crate::mode::{Channel, Segment, SstvMode, BLACK_HZ, SYNC_HZ, WHITE_HZ};
+
+/// A stretch of constant tone.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Tone {
+    pub hz: f32,
+    pub ms: f64,
+}
+
+/// Frequency for an 8-bit level: the inverse of [`crate::mode::level_from_hz`].
+pub fn hz_from_level(level: u8) -> f32 {
+    BLACK_HZ + (WHITE_HZ - BLACK_HZ) * f32::from(level) / 255.0
+}
+
+/// The tones of a VIS header announcing `vis`.
+pub fn vis_tones(vis: u8) -> Vec<Tone> {
+    let mut tones = vec![
+        Tone {
+            hz: 1900.0,
+            ms: 300.0,
+        },
+        Tone {
+            hz: 1200.0,
+            ms: 10.0,
+        },
+        Tone {
+            hz: 1900.0,
+            ms: 300.0,
+        },
+        Tone {
+            hz: 1200.0,
+            ms: 30.0,
+        }, // start bit
+    ];
+    let mut ones = 0;
+    for bit in 0..7 {
+        let set = vis & (1 << bit) != 0;
+        if set {
+            ones += 1;
+        }
+        tones.push(Tone {
+            hz: if set { 1100.0 } else { 1300.0 },
+            ms: 30.0,
+        });
+    }
+    // Even parity over the seven data bits.
+    tones.push(Tone {
+        hz: if ones % 2 == 1 { 1100.0 } else { 1300.0 },
+        ms: 30.0,
+    });
+    tones.push(Tone {
+        hz: 1200.0,
+        ms: 30.0,
+    }); // stop bit
+    tones
+}
+
+/// RGB source for the encoder: `width * height * 3` bytes.
+pub struct Frame<'a> {
+    pub width: usize,
+    pub height: usize,
+    pub rgb: &'a [u8],
+}
+
+impl Frame<'_> {
+    fn pixel(&self, x: usize, y: usize) -> (u8, u8, u8) {
+        let x = x.min(self.width.saturating_sub(1));
+        let y = y.min(self.height.saturating_sub(1));
+        let at = (y * self.width + x) * 3;
+        (self.rgb[at], self.rgb[at + 1], self.rgb[at + 2])
+    }
+
+    /// The colour components SSTV actually sends, for a pixel.
+    fn ycrcb(&self, x: usize, y: usize) -> (u8, u8, u8) {
+        let (r, g, b) = self.pixel(x, y);
+        let (r, g, b) = (f32::from(r), f32::from(g), f32::from(b));
+        let y_val = 16.0 + (0.003_906 * ((65.738 * r) + (129.057 * g) + (25.064 * b)));
+        let cr = 128.0 + (0.003_906 * ((112.439 * r) + (-94.154 * g) + (-18.285 * b)));
+        let cb = 128.0 + (0.003_906 * ((-37.945 * r) + (-74.494 * g) + (112.439 * b)));
+        (
+            y_val.clamp(0.0, 255.0) as u8,
+            cr.clamp(0.0, 255.0) as u8,
+            cb.clamp(0.0, 255.0) as u8,
+        )
+    }
+}
+
+/// Encode `frame` in `mode`, returning the tones of the whole transmission.
+pub fn encode_tones(mode: &SstvMode, frame: &Frame<'_>) -> Vec<Tone> {
+    let mut tones = vis_tones(mode.vis);
+    let per_line = usize::from(mode.lines_per_transmission);
+    let transmissions = usize::from(mode.height) / per_line;
+
+    for transmission in 0..transmissions {
+        let top = transmission * per_line;
+        for segment in mode.segments {
+            match *segment {
+                Segment::Sync(ms) => tones.push(Tone { hz: SYNC_HZ, ms }),
+                Segment::Gap(ms) => tones.push(Tone { hz: BLACK_HZ, ms }),
+                Segment::Scan { channel, ms } => {
+                    // A chroma scan carries fewer pixels than the image is
+                    // wide, and takes proportionally less time per pixel.
+                    let pixels = mode.scan_pixels(channel);
+                    let pixel_ms = ms / pixels as f64;
+                    for x in 0..pixels {
+                        let level = channel_level(mode, frame, channel, x, top, transmission);
+                        tones.push(Tone {
+                            hz: hz_from_level(level),
+                            ms: pixel_ms,
+                        });
+                    }
+                }
+            }
+        }
+    }
+    tones
+}
+
+fn channel_level(
+    mode: &SstvMode,
+    frame: &Frame<'_>,
+    channel: Channel,
+    x: usize,
+    top: usize,
+    transmission: usize,
+) -> u8 {
+    match channel {
+        Channel::Red => frame.pixel(x, top).0,
+        Channel::Green => frame.pixel(x, top).1,
+        Channel::Blue => frame.pixel(x, top).2,
+        Channel::LumaOdd => frame.ycrcb(x, top).0,
+        Channel::LumaEven => frame.ycrcb(x, top + 1).0,
+        Channel::ChromaR => chroma(mode, frame, x, top, true),
+        Channel::ChromaB => chroma(mode, frame, x, top, false),
+        // Robot 36 sends R-Y on odd transmitted lines and B-Y on even ones.
+        Channel::ChromaAlternating => chroma(mode, frame, x, top, transmission.is_multiple_of(2)),
+    }
+}
+
+/// Chroma scans are half the width of the image in the Robot modes, so each
+/// value covers two pixels; PD averages the two image lines of the pair too.
+fn chroma(mode: &SstvMode, frame: &Frame<'_>, x: usize, top: usize, want_cr: bool) -> u8 {
+    let scale = usize::from(mode.width) / mode.scan_pixels(Channel::ChromaR).max(1);
+    let x0 = x * scale;
+    let mut total = 0u32;
+    let mut count = 0u32;
+    let rows = usize::from(mode.lines_per_transmission);
+    for row in 0..rows {
+        for dx in 0..scale {
+            let (_, cr, cb) = frame.ycrcb(x0 + dx, top + row);
+            total += u32::from(if want_cr { cr } else { cb });
+            count += 1;
+        }
+    }
+    (total / count.max(1)) as u8
+}
+
+/// Render tones to audio at `sample_rate`, with continuous phase so the
+/// demodulator sees no step at a tone boundary that isn't in the signal.
+pub fn render(tones: &[Tone], sample_rate: u32) -> Vec<f32> {
+    let sr = f64::from(sample_rate);
+    let mut out =
+        Vec::with_capacity((tones.iter().map(|t| t.ms).sum::<f64>() / 1000.0 * sr) as usize);
+    let mut phase = 0.0f64;
+    // Each tone's *end* is rounded to a sample, rather than its length: a
+    // pixel of 25.5 samples rounded up on its own puts a whole line 600
+    // samples late by the end of it, which is a timing error no receiver
+    // should have to chase and no transmitter would produce.
+    let mut elapsed_ms = 0.0f64;
+    let mut emitted = 0usize;
+    for tone in tones {
+        elapsed_ms += tone.ms;
+        let end = (elapsed_ms / 1000.0 * sr).round() as usize;
+        let samples = end.saturating_sub(emitted);
+        emitted = end;
+        let step = 2.0 * std::f64::consts::PI * f64::from(tone.hz) / sr;
+        for _ in 0..samples {
+            out.push(phase.sin() as f32);
+            phase += step;
+            if phase > std::f64::consts::TAU {
+                phase -= std::f64::consts::TAU;
+            }
+        }
+    }
+    out
+}
+
+/// Encode a frame straight to audio.
+pub fn encode(mode: &SstvMode, frame: &Frame<'_>, sample_rate: u32) -> Vec<f32> {
+    render(&encode_tones(mode, frame), sample_rate)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::mode::{level_from_hz, mode_for_vis};
+
+    #[test]
+    fn levels_survive_the_trip_through_frequency() {
+        for level in [0u8, 1, 64, 128, 200, 255] {
+            assert_eq!(level_from_hz(hz_from_level(level)), level);
+        }
+    }
+
+    #[test]
+    fn a_transmission_lasts_as_long_as_the_mode_says() {
+        let mode = mode_for_vis(44).expect("Martin M1");
+        let rgb = vec![128u8; 320 * 256 * 3];
+        let frame = Frame {
+            width: 320,
+            height: 256,
+            rgb: &rgb,
+        };
+        let audio = encode(mode, &frame, 48_000);
+        // Header plus 256 lines, within a line of the published duration.
+        let header_s = 0.94;
+        let expected = header_s + mode.frame_secs();
+        let actual = audio.len() as f64 / 48_000.0;
+        assert!(
+            (actual - expected).abs() < mode.line_ms / 1000.0,
+            "encoded {actual:.2} s, expected {expected:.2} s",
+        );
+    }
+}

src/decoders/trx-sstv/src/image.rs

@@ -0,0 +1,150 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Assembling decoded lines into an image, and getting it out of the process.
+//!
+//! Rows arrive one at a time and the picture is worth looking at before it is
+//! finished, so the assembler holds a full-size RGB canvas from the start and
+//! fills it in. An unfinished frame is grey below the last decoded row rather
+//! than black, which reads as "not here yet" instead of "received as black".
+
+use std::path::{Path, PathBuf};
+
+use base64::Engine;
+
+/// Value the canvas starts at: mid-grey, for rows not yet received.
+const UNWRITTEN: u8 = 96;
+
+pub struct ImageCanvas {
+    width: usize,
+    height: usize,
+    rgb: Vec<u8>,
+    /// Highest row index written, plus one.
+    filled_rows: usize,
+}
+
+impl ImageCanvas {
+    pub fn new(width: usize, height: usize) -> Self {
+        Self {
+            width,
+            height,
+            rgb: vec![UNWRITTEN; width * height * 3],
+            filled_rows: 0,
+        }
+    }
+
+    pub fn width(&self) -> usize {
+        self.width
+    }
+
+    pub fn height(&self) -> usize {
+        self.height
+    }
+
+    /// Rows written so far.
+    pub fn filled_rows(&self) -> usize {
+        self.filled_rows
+    }
+
+    /// Write one row of RGB triples. Rows past the bottom of the image are
+    /// dropped: a transmission that runs long is not a reason to grow.
+    pub fn put_row(&mut self, y: usize, row: &[u8]) {
+        if y >= self.height {
+            return;
+        }
+        let at = y * self.width * 3;
+        let take = row.len().min(self.width * 3);
+        self.rgb[at..at + take].copy_from_slice(&row[..take]);
+        self.filled_rows = self.filled_rows.max(y + 1);
+    }
+
+    pub fn row(&self, y: usize) -> Option<&[u8]> {
+        if y >= self.height {
+            return None;
+        }
+        let at = y * self.width * 3;
+        Some(&self.rgb[at..at + self.width * 3])
+    }
+
+    pub fn rgb(&self) -> &[u8] {
+        &self.rgb
+    }
+
+    /// Encode the canvas as a PNG.
+    pub fn to_png(&self) -> Result<Vec<u8>, String> {
+        let mut out = Vec::new();
+        {
+            let mut encoder = png::Encoder::new(&mut out, self.width as u32, self.height as u32);
+            encoder.set_color(png::ColorType::Rgb);
+            encoder.set_depth(png::BitDepth::Eight);
+            let mut writer = encoder
+                .write_header()
+                .map_err(|e| format!("PNG header: {e}"))?;
+            writer
+                .write_image_data(&self.rgb)
+                .map_err(|e| format!("PNG data: {e}"))?;
+        }
+        Ok(out)
+    }
+
+    /// The PNG, base64-encoded for the journey to a browser.
+    pub fn to_png_base64(&self) -> Result<String, String> {
+        Ok(base64::engine::general_purpose::STANDARD.encode(self.to_png()?))
+    }
+
+    /// Write the PNG into `dir`, named for when and where it was received.
+    pub fn save_png(
+        &self,
+        dir: &Path,
+        freq_hz: u64,
+        mode_name: &str,
+        stamp: &str,
+    ) -> Result<PathBuf, String> {
+        std::fs::create_dir_all(dir).map_err(|e| format!("create {}: {e}", dir.display()))?;
+        let slug: String = mode_name
+            .chars()
+            .map(|c| if c.is_ascii_alphanumeric() { c } else { '-' })
+            .collect();
+        let path = dir.join(format!("SSTV_{stamp}_{freq_hz}_{slug}.png"));
+        std::fs::write(&path, self.to_png()?)
+            .map_err(|e| format!("write {}: {e}", path.display()))?;
+        Ok(path)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rows_land_where_they_are_put_and_the_rest_stays_unwritten() {
+        let mut canvas = ImageCanvas::new(4, 3);
+        canvas.put_row(1, &[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]);
+        assert_eq!(canvas.row(1).unwrap()[0..3], [1, 2, 3]);
+        assert_eq!(canvas.row(0).unwrap()[0], UNWRITTEN);
+        assert_eq!(canvas.filled_rows(), 2);
+    }
+
+    #[test]
+    fn a_row_past_the_bottom_is_dropped_rather_than_growing_the_image() {
+        let mut canvas = ImageCanvas::new(2, 2);
+        canvas.put_row(9, &[1, 2, 3, 4, 5, 6]);
+        assert_eq!(canvas.filled_rows(), 0);
+        assert_eq!(canvas.rgb().len(), 2 * 2 * 3);
+    }
+
+    #[test]
+    fn encodes_a_png_a_decoder_can_read_back() {
+        let mut canvas = ImageCanvas::new(2, 2);
+        canvas.put_row(0, &[255, 0, 0, 0, 255, 0]);
+        let png_bytes = canvas.to_png().expect("png");
+        let decoder = png::Decoder::new(png_bytes.as_slice());
+        let mut reader = decoder.read_info().expect("png info");
+        let mut buf = vec![0; reader.output_buffer_size()];
+        let info = reader.next_frame(&mut buf).expect("png frame");
+        assert_eq!((info.width, info.height), (2, 2));
+        assert_eq!(&buf[0..6], &[255, 0, 0, 0, 255, 0]);
+        assert!(!canvas.to_png_base64().expect("base64").is_empty());
+    }
+}

src/decoders/trx-sstv/src/lib.rs

@@ -0,0 +1,36 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! SSTV (Slow-Scan Television) decoder.
+//!
+//! Pure Rust, covering Martin, Scottie, Robot, PD and Wraase SC2-180, with the
+//! mode taken from the VIS header that precedes every transmission. Rows are
+//! emitted as they arrive so a picture can be watched building up.
+//!
+//! ```no_run
+//! use trx_sstv::{SstvConfig, SstvDecoder, SstvEvent};
+//!
+//! let mut decoder = SstvDecoder::new(48_000, SstvConfig::default());
+//! # let audio: Vec<f32> = Vec::new();
+//! for event in decoder.process_samples(&audio) {
+//!     match event {
+//!         SstvEvent::Started { mode, .. } => println!("receiving {mode}"),
+//!         SstvEvent::Row { line, .. } => println!("line {line}"),
+//!         SstvEvent::Complete(image) => println!("{} lines", image.lines),
+//!     }
+//! }
+//! ```
+
+pub mod config;
+pub mod decoder;
+pub mod demod;
+pub mod encode;
+pub mod image;
+pub mod mode;
+pub mod vis;
+
+pub use config::SstvConfig;
+pub use decoder::{SstvDecoder, SstvEvent, SstvImage};
+pub use image::ImageCanvas;
+pub use mode::{mode_for_vis, SstvMode, MODES};

src/decoders/trx-sstv/src/mode.rs

@@ -0,0 +1,615 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! SSTV mode table: what a VIS code means in timings, geometry and colour.
+//!
+//! Every mode transmits a line as a sequence of *segments* — a sync pulse, a
+//! porch or separator at a fixed tone, and one scan per colour channel. The
+//! decoder needs only the segment layout and the offset of each scan within
+//! the line, so that is what a [`SstvMode`] is: a list of segments plus the
+//! rules for turning the scans back into pixels.
+//!
+//! Timings follow the published mode specifications (JL Barber, N7CXI,
+//! "Proposal for SSTV Mode Specifications", 2000), which is the same table
+//! MMSSTV, QSSTV and slowrx work from.
+
+/// Tone that marks a line boundary, in Hz.
+pub const SYNC_HZ: f32 = 1200.0;
+/// Tone for black, in Hz.
+pub const BLACK_HZ: f32 = 1500.0;
+/// Tone for white, in Hz.
+pub const WHITE_HZ: f32 = 2300.0;
+
+/// What a scan segment carries.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Channel {
+    Red,
+    Green,
+    Blue,
+    /// Luminance for the odd (first) image line of the pair.
+    LumaOdd,
+    /// Luminance for the even (second) image line of a PD pair.
+    LumaEven,
+    /// R-Y chrominance.
+    ChromaR,
+    /// B-Y chrominance.
+    ChromaB,
+    /// Robot 36 alternates R-Y and B-Y between transmitted lines: odd lines
+    /// carry R-Y, even lines B-Y, and each is held over both.
+    ChromaAlternating,
+}
+
+/// One piece of a transmitted line.
+#[derive(Debug, Clone, Copy)]
+pub enum Segment {
+    /// Sync pulse at [`SYNC_HZ`].
+    Sync(f64),
+    /// Porch, separator or gap at a fixed tone; the tone itself is not decoded.
+    Gap(f64),
+    /// A scan carrying pixels for one channel.
+    Scan { channel: Channel, ms: f64 },
+}
+
+impl Segment {
+    pub fn duration_ms(&self) -> f64 {
+        match *self {
+            Segment::Sync(ms) | Segment::Gap(ms) => ms,
+            Segment::Scan { ms, .. } => ms,
+        }
+    }
+}
+
+/// How the scans of a line become pixels.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ColorModel {
+    /// Scans are red, green and blue directly (Martin, Scottie, Wraase).
+    Rgb,
+    /// Y plus one alternating chroma channel per line (Robot 36).
+    YCrCbAlternating,
+    /// Y, R-Y and B-Y in every line (Robot 72).
+    YCrCb,
+    /// Two image lines per transmitted line: Y odd, R-Y, B-Y, Y even (PD).
+    YCrCbPaired,
+}
+
+/// A decodable SSTV mode.
+#[derive(Debug, Clone)]
+pub struct SstvMode {
+    /// VIS code as sent in the header.
+    pub vis: u8,
+    /// Human-readable name, e.g. "Martin M1".
+    pub name: &'static str,
+    /// Pixels across.
+    pub width: u16,
+    /// Image lines in a full frame.
+    pub height: u16,
+    /// Transmitted line duration in milliseconds.
+    pub line_ms: f64,
+    /// Segments in transmission order.
+    pub segments: &'static [Segment],
+    pub color: ColorModel,
+    /// Image lines produced by one transmitted line (2 for PD, otherwise 1).
+    pub lines_per_transmission: u16,
+    /// Offset from the start of a line to the leading edge of its sync pulse.
+    /// Zero for most modes; Scottie sends the sync in the middle of the line,
+    /// so a line detected at its sync starts before it.
+    pub sync_offset_ms: f64,
+}
+
+impl SstvMode {
+    /// Total of every segment, which must equal [`SstvMode::line_ms`].
+    pub fn segments_ms(&self) -> f64 {
+        self.segments.iter().map(Segment::duration_ms).sum()
+    }
+
+    /// Start offset, in milliseconds from the line start, of each scan.
+    pub fn scans(&self) -> Vec<(Channel, f64, f64)> {
+        let mut out = Vec::new();
+        let mut at = 0.0;
+        for segment in self.segments {
+            if let Segment::Scan { channel, ms } = *segment {
+                out.push((channel, at, ms));
+            }
+            at += segment.duration_ms();
+        }
+        out
+    }
+
+    /// Pixels carried by a scan of this channel.
+    ///
+    /// Chroma is sent at half the width in the Robot modes — the eye takes
+    /// colour more coarsely than brightness, and the saving is what makes 36
+    /// seconds possible. PD sends chroma at full width and saves its time by
+    /// sharing one pair of chroma scans between two image lines instead.
+    pub fn scan_pixels(&self, channel: Channel) -> usize {
+        let width = usize::from(self.width);
+        match channel {
+            Channel::ChromaR | Channel::ChromaB | Channel::ChromaAlternating
+                if self.color != ColorModel::YCrCbPaired =>
+            {
+                width / 2
+            }
+            _ => width,
+        }
+    }
+
+    /// Seconds a full frame takes to send.
+    pub fn frame_secs(&self) -> f64 {
+        self.line_ms * f64::from(self.height) / f64::from(self.lines_per_transmission) / 1000.0
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Martin — sync, porch, then green, blue, red, each followed by a separator.
+// ---------------------------------------------------------------------------
+
+const MARTIN_M1: &[Segment] = &[
+    Segment::Sync(4.862),
+    Segment::Gap(0.572),
+    Segment::Scan {
+        channel: Channel::Green,
+        ms: 146.432,
+    },
+    Segment::Gap(0.572),
+    Segment::Scan {
+        channel: Channel::Blue,
+        ms: 146.432,
+    },
+    Segment::Gap(0.572),
+    Segment::Scan {
+        channel: Channel::Red,
+        ms: 146.432,
+    },
+    Segment::Gap(0.572),
+];
+
+const MARTIN_M2: &[Segment] = &[
+    Segment::Sync(4.862),
+    Segment::Gap(0.572),
+    Segment::Scan {
+        channel: Channel::Green,
+        ms: 73.216,
+    },
+    Segment::Gap(0.572),
+    Segment::Scan {
+        channel: Channel::Blue,
+        ms: 73.216,
+    },
+    Segment::Gap(0.572),
+    Segment::Scan {
+        channel: Channel::Red,
+        ms: 73.216,
+    },
+    Segment::Gap(0.572),
+];
+
+// ---------------------------------------------------------------------------
+// Scottie — the sync pulse sits between the blue and red scans, so a line
+// starts one separator before the green scan and the sync of the *previous*
+// line is what marks it.
+// ---------------------------------------------------------------------------
+
+const SCOTTIE_S1: &[Segment] = &[
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Green,
+        ms: 138.240,
+    },
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Blue,
+        ms: 138.240,
+    },
+    Segment::Sync(9.0),
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Red,
+        ms: 138.240,
+    },
+];
+
+const SCOTTIE_S2: &[Segment] = &[
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Green,
+        ms: 88.064,
+    },
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Blue,
+        ms: 88.064,
+    },
+    Segment::Sync(9.0),
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Red,
+        ms: 88.064,
+    },
+];
+
+const SCOTTIE_DX: &[Segment] = &[
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Green,
+        ms: 345.6,
+    },
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Blue,
+        ms: 345.6,
+    },
+    Segment::Sync(9.0),
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::Red,
+        ms: 345.6,
+    },
+];
+
+/// Scottie's sync arrives after the green and blue scans: 1.5 + 138.24 + 1.5 +
+/// 138.24 for S1, and the equivalent for the others.
+const fn scottie_sync_offset(scan_ms: f64) -> f64 {
+    1.5 + scan_ms + 1.5 + scan_ms
+}
+
+// ---------------------------------------------------------------------------
+// Robot — luminance plus chrominance, the chroma scans at half the width.
+// ---------------------------------------------------------------------------
+
+const ROBOT_36: &[Segment] = &[
+    Segment::Sync(9.0),
+    Segment::Gap(3.0),
+    Segment::Scan {
+        channel: Channel::LumaOdd,
+        ms: 88.0,
+    },
+    Segment::Gap(4.5),
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::ChromaAlternating,
+        ms: 44.0,
+    },
+];
+
+const ROBOT_72: &[Segment] = &[
+    Segment::Sync(9.0),
+    Segment::Gap(3.0),
+    Segment::Scan {
+        channel: Channel::LumaOdd,
+        ms: 138.0,
+    },
+    Segment::Gap(4.5),
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::ChromaR,
+        ms: 69.0,
+    },
+    Segment::Gap(4.5),
+    Segment::Gap(1.5),
+    Segment::Scan {
+        channel: Channel::ChromaB,
+        ms: 69.0,
+    },
+];
+
+// ---------------------------------------------------------------------------
+// PD — one transmitted line carries two image lines: the luminance of both,
+// with a single pair of chroma scans shared between them.
+// ---------------------------------------------------------------------------
+
+macro_rules! pd_segments {
+    ($name:ident, $scan:expr) => {
+        const $name: &[Segment] = &[
+            Segment::Sync(20.0),
+            Segment::Gap(2.08),
+            Segment::Scan {
+                channel: Channel::LumaOdd,
+                ms: $scan,
+            },
+            Segment::Scan {
+                channel: Channel::ChromaR,
+                ms: $scan,
+            },
+            Segment::Scan {
+                channel: Channel::ChromaB,
+                ms: $scan,
+            },
+            Segment::Scan {
+                channel: Channel::LumaEven,
+                ms: $scan,
+            },
+        ];
+    };
+}
+
+pd_segments!(PD_50, 91.520);
+pd_segments!(PD_90, 170.240);
+pd_segments!(PD_120, 121.600);
+pd_segments!(PD_160, 195.584);
+pd_segments!(PD_180, 183.040);
+pd_segments!(PD_240, 244.672);
+pd_segments!(PD_290, 228.800);
+
+// ---------------------------------------------------------------------------
+// Wraase SC2-180 — red, green, blue in that order after one porch.
+// ---------------------------------------------------------------------------
+
+const WRAASE_SC2_180: &[Segment] = &[
+    Segment::Sync(5.5225),
+    Segment::Gap(0.5),
+    Segment::Scan {
+        channel: Channel::Red,
+        ms: 235.0,
+    },
+    Segment::Scan {
+        channel: Channel::Green,
+        ms: 235.0,
+    },
+    Segment::Scan {
+        channel: Channel::Blue,
+        ms: 235.0,
+    },
+];
+
+/// Every mode this decoder knows, in VIS order.
+pub static MODES: &[SstvMode] = &[
+    SstvMode {
+        vis: 8,
+        name: "Robot 36",
+        width: 320,
+        height: 240,
+        line_ms: 150.0,
+        segments: ROBOT_36,
+        color: ColorModel::YCrCbAlternating,
+        lines_per_transmission: 1,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 12,
+        name: "Robot 72",
+        width: 320,
+        height: 240,
+        line_ms: 300.0,
+        segments: ROBOT_72,
+        color: ColorModel::YCrCb,
+        lines_per_transmission: 1,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 40,
+        name: "Martin M2",
+        width: 320,
+        height: 256,
+        line_ms: 226.798,
+        segments: MARTIN_M2,
+        color: ColorModel::Rgb,
+        lines_per_transmission: 1,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 44,
+        name: "Martin M1",
+        width: 320,
+        height: 256,
+        line_ms: 446.446,
+        segments: MARTIN_M1,
+        color: ColorModel::Rgb,
+        lines_per_transmission: 1,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 55,
+        name: "Wraase SC2-180",
+        width: 320,
+        height: 256,
+        line_ms: 711.0225,
+        segments: WRAASE_SC2_180,
+        color: ColorModel::Rgb,
+        lines_per_transmission: 1,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 56,
+        name: "Scottie S2",
+        width: 320,
+        height: 256,
+        line_ms: 277.692,
+        segments: SCOTTIE_S2,
+        color: ColorModel::Rgb,
+        lines_per_transmission: 1,
+        sync_offset_ms: scottie_sync_offset(88.064),
+    },
+    SstvMode {
+        vis: 60,
+        name: "Scottie S1",
+        width: 320,
+        height: 256,
+        line_ms: 428.22,
+        segments: SCOTTIE_S1,
+        color: ColorModel::Rgb,
+        lines_per_transmission: 1,
+        sync_offset_ms: scottie_sync_offset(138.240),
+    },
+    SstvMode {
+        vis: 76,
+        name: "Scottie DX",
+        width: 320,
+        height: 256,
+        line_ms: 1050.3,
+        segments: SCOTTIE_DX,
+        color: ColorModel::Rgb,
+        lines_per_transmission: 1,
+        sync_offset_ms: scottie_sync_offset(345.6),
+    },
+    SstvMode {
+        vis: 93,
+        name: "PD50",
+        width: 320,
+        height: 256,
+        line_ms: 388.16,
+        segments: PD_50,
+        color: ColorModel::YCrCbPaired,
+        lines_per_transmission: 2,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 94,
+        name: "PD290",
+        width: 800,
+        height: 616,
+        line_ms: 937.28,
+        segments: PD_290,
+        color: ColorModel::YCrCbPaired,
+        lines_per_transmission: 2,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 95,
+        name: "PD120",
+        width: 640,
+        height: 496,
+        line_ms: 508.48,
+        segments: PD_120,
+        color: ColorModel::YCrCbPaired,
+        lines_per_transmission: 2,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 96,
+        name: "PD180",
+        width: 640,
+        height: 496,
+        line_ms: 754.24,
+        segments: PD_180,
+        color: ColorModel::YCrCbPaired,
+        lines_per_transmission: 2,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 97,
+        name: "PD240",
+        width: 640,
+        height: 496,
+        line_ms: 1000.768,
+        segments: PD_240,
+        color: ColorModel::YCrCbPaired,
+        lines_per_transmission: 2,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 98,
+        name: "PD160",
+        width: 512,
+        height: 400,
+        line_ms: 804.416,
+        segments: PD_160,
+        color: ColorModel::YCrCbPaired,
+        lines_per_transmission: 2,
+        sync_offset_ms: 0.0,
+    },
+    SstvMode {
+        vis: 99,
+        name: "PD90",
+        width: 320,
+        height: 256,
+        line_ms: 703.04,
+        segments: PD_90,
+        color: ColorModel::YCrCbPaired,
+        lines_per_transmission: 2,
+        sync_offset_ms: 0.0,
+    },
+];
+
+/// Look a mode up by the VIS code that announced it.
+pub fn mode_for_vis(vis: u8) -> Option<&'static SstvMode> {
+    MODES.iter().find(|mode| mode.vis == vis)
+}
+
+/// Map an instantaneous frequency to an 8-bit level: 1500 Hz is black, 2300 Hz
+/// white. Frequencies outside the band clamp rather than wrap, so a sync pulse
+/// that lands inside a scan reads as black instead of as bright noise.
+pub fn level_from_hz(hz: f32) -> u8 {
+    let level = (hz - BLACK_HZ) * (255.0 / (WHITE_HZ - BLACK_HZ));
+    level.clamp(0.0, 255.0).round() as u8
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // The segment list and the published line time are two statements of the
+    // same fact, entered by hand from the specification. If a digit is wrong in
+    // one it is unlikely to be wrong identically in the other.
+    #[test]
+    fn segments_add_up_to_the_published_line_time() {
+        for mode in MODES {
+            let sum = mode.segments_ms();
+            assert!(
+                (sum - mode.line_ms).abs() < 0.001,
+                "{}: segments total {:.4} ms, line time says {:.4} ms",
+                mode.name,
+                sum,
+                mode.line_ms,
+            );
+        }
+    }
+
+    #[test]
+    fn vis_codes_are_unique_and_resolvable() {
+        for mode in MODES {
+            assert_eq!(mode_for_vis(mode.vis).map(|m| m.name), Some(mode.name));
+        }
+        let mut codes: Vec<u8> = MODES.iter().map(|m| m.vis).collect();
+        codes.sort_unstable();
+        let count = codes.len();
+        codes.dedup();
+        assert_eq!(codes.len(), count, "two modes claim the same VIS code");
+    }
+
+    #[test]
+    fn every_mode_scans_enough_channels_for_its_colour_model() {
+        for mode in MODES {
+            let scans = mode.scans();
+            let expected = match mode.color {
+                ColorModel::Rgb => 3,
+                ColorModel::YCrCbAlternating => 2,
+                ColorModel::YCrCb => 3,
+                ColorModel::YCrCbPaired => 4,
+            };
+            assert_eq!(
+                scans.len(),
+                expected,
+                "{} has {} scans",
+                mode.name,
+                scans.len()
+            );
+        }
+    }
+
+    // Frame durations are what operators know these modes by — the number in
+    // the name is the number of seconds.
+    #[test]
+    fn frame_durations_match_the_names() {
+        for (vis, secs) in [(8u8, 36.0), (12, 72.0), (93, 50.0), (99, 90.0), (95, 126.0)] {
+            let mode = mode_for_vis(vis).expect("mode in table");
+            let actual = mode.frame_secs();
+            assert!(
+                (actual - secs).abs() < 1.5,
+                "{} takes {:.1} s, expected about {:.0} s",
+                mode.name,
+                actual,
+                secs,
+            );
+        }
+    }
+
+    #[test]
+    fn levels_span_black_to_white_and_clamp_outside() {
+        assert_eq!(level_from_hz(BLACK_HZ), 0);
+        assert_eq!(level_from_hz(WHITE_HZ), 255);
+        assert_eq!(level_from_hz(1900.0), 128);
+        assert_eq!(level_from_hz(SYNC_HZ), 0, "a sync pulse must read as black");
+        assert_eq!(level_from_hz(3000.0), 255);
+    }
+}

src/decoders/trx-sstv/src/vis.rs

@@ -0,0 +1,236 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! VIS header detection.
+//!
+//! Every transmission announces its mode in a fixed preamble:
+//!
+//! | Part | Tone | Duration |
+//! |------|------|----------|
+//! | Leader | 1900 Hz | 300 ms |
+//! | Break | 1200 Hz | 10 ms |
+//! | Leader | 1900 Hz | 300 ms |
+//! | Start bit | 1200 Hz | 30 ms |
+//! | 7 data bits, LSB first | 1100 Hz = 1, 1300 Hz = 0 | 30 ms each |
+//! | Even parity | as above | 30 ms |
+//! | Stop bit | 1200 Hz | 30 ms |
+//!
+//! The detector looks for the start bit standing behind a leader, reads the
+//! eight bits that follow, and checks the parity. Parity is the only integrity
+//! check the header has, so a code that fails it is discarded rather than
+//! guessed at — decoding 114 seconds of Martin M1 as Scottie DX produces a
+//! convincing-looking image of nothing.
+
+/// Tone durations, in milliseconds.
+const BIT_MS: f64 = 30.0;
+const LEADER_MS: f64 = 300.0;
+
+/// How far a tone may sit from its nominal frequency and still be recognised.
+/// Wide enough for a rig tuned a little off, narrow enough that 1100, 1200,
+/// 1300 and 1900 Hz stay distinct.
+const TONE_TOLERANCE_HZ: f32 = 60.0;
+
+const LEADER_HZ: f32 = 1900.0;
+const START_HZ: f32 = 1200.0;
+const ONE_HZ: f32 = 1100.0;
+const ZERO_HZ: f32 = 1300.0;
+
+/// A VIS header found in the stream.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct VisHit {
+    /// The code, which names the mode.
+    pub code: u8,
+    /// Index just past the stop bit: where the image itself begins.
+    pub image_start: usize,
+}
+
+fn near(freq: f32, target: f32) -> bool {
+    (freq - target).abs() <= TONE_TOLERANCE_HZ
+}
+
+/// Mean frequency over the middle 60% of a bit cell, which keeps the filter's
+/// transitions at either edge out of the measurement.
+fn bit_frequency(freqs: &[f32], start: f64, samples_per_bit: f64) -> Option<f32> {
+    let from = (start + samples_per_bit * 0.2).round() as usize;
+    let to = (start + samples_per_bit * 0.8).round() as usize;
+    if to <= from || to > freqs.len() {
+        return None;
+    }
+    let window = &freqs[from..to];
+    Some(window.iter().sum::<f32>() / window.len() as f32)
+}
+
+/// Search `freqs` for a VIS header, starting at `from`.
+///
+/// Returns the first header whose parity checks out. `freqs` is instantaneous
+/// frequency in Hz, one entry per audio sample.
+pub fn find_vis(freqs: &[f32], sample_rate: u32, from: usize) -> Option<VisHit> {
+    let sr = f64::from(sample_rate);
+    // Header tones are 10 ms at the shortest, so a millisecond of averaging
+    // costs nothing and takes the demodulator's ripple — ±95 Hz at 1200 Hz —
+    // out of tones that are 100 Hz apart.
+    let smoothed = crate::demod::smooth(freqs, (sr / 1000.0).round() as usize);
+    let freqs = smoothed.as_slice();
+    let samples_per_bit = BIT_MS / 1000.0 * sr;
+    let leader_samples = (LEADER_MS / 1000.0 * sr) as usize;
+    // The start bit must be at least this long to be one.
+    let min_start_run = (samples_per_bit * 0.7) as usize;
+
+    // A leader has to precede the start bit. Half of one is enough evidence,
+    // and asking for less than the full 300 ms means the search still works on
+    // a buffer that begins part-way through the header.
+    let leader_needed = leader_samples / 2;
+
+    // The bits themselves may run to the very end of what has arrived so far;
+    // reading them is what decides whether there is enough, not this bound.
+    let mut i = from.max(leader_needed);
+    while i < freqs.len() {
+        if !near(freqs[i], START_HZ) {
+            i += 1;
+            continue;
+        }
+        // Measure the run of start tone.
+        let mut run = 0usize;
+        while i + run < freqs.len() && near(freqs[i + run], START_HZ) {
+            run += 1;
+        }
+        if run < min_start_run {
+            i += run.max(1);
+            continue;
+        }
+
+        // What came before it: the leader. Sampled rather than scanned in
+        // full, since only its identity matters, not its exact length.
+        let leader_from = i - leader_needed;
+        let leader_hits = freqs[leader_from..i]
+            .iter()
+            .step_by(16)
+            .filter(|&&f| near(f, LEADER_HZ))
+            .count();
+        let leader_total = freqs[leader_from..i].iter().step_by(16).count();
+        if leader_total == 0 || (leader_hits as f64) < 0.7 * leader_total as f64 {
+            i += run;
+            continue;
+        }
+
+        // Bits follow the start bit, which the run just measured. Use the run's
+        // own end rather than a nominal offset, so a start bit stretched or
+        // clipped by the filter does not shift every bit after it.
+        let bits_start = (i + run) as f64;
+        let mut bits = [false; 8];
+        let mut readable = true;
+        for (index, bit) in bits.iter_mut().enumerate() {
+            let at = bits_start + samples_per_bit * index as f64;
+            match bit_frequency(freqs, at, samples_per_bit) {
+                Some(freq) if near(freq, ONE_HZ) => *bit = true,
+                Some(freq) if near(freq, ZERO_HZ) => *bit = false,
+                _ => {
+                    readable = false;
+                    break;
+                }
+            }
+        }
+        if !readable {
+            i += run;
+            continue;
+        }
+
+        // Seven data bits, LSB first, then even parity over them.
+        let code = bits[..7]
+            .iter()
+            .enumerate()
+            .fold(0u8, |acc, (index, &set)| acc | (u8::from(set) << index));
+        let ones = bits[..7].iter().filter(|&&b| b).count() + usize::from(bits[7]);
+        if ones % 2 != 0 {
+            i += run;
+            continue;
+        }
+
+        // Past the stop bit is the image.
+        let image_start = (bits_start + samples_per_bit * 9.0).round() as usize;
+        return Some(VisHit { code, image_start });
+    }
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::encode::{vis_tones, Tone};
+
+    fn freqs_from_tones(tones: &[Tone], sample_rate: u32) -> Vec<f32> {
+        let mut out = Vec::new();
+        for tone in tones {
+            let samples = (tone.ms / 1000.0 * f64::from(sample_rate)).round() as usize;
+            out.extend(std::iter::repeat_n(tone.hz, samples));
+        }
+        out
+    }
+
+    #[test]
+    fn reads_every_code_the_mode_table_knows() {
+        for mode in crate::mode::MODES {
+            let freqs = freqs_from_tones(&vis_tones(mode.vis), 48_000);
+            let hit = find_vis(&freqs, 48_000, 0)
+                .unwrap_or_else(|| panic!("{} header not found", mode.name));
+            assert_eq!(
+                hit.code, mode.vis,
+                "{} decoded as VIS {}",
+                mode.name, hit.code
+            );
+        }
+    }
+
+    #[test]
+    fn the_image_starts_after_the_stop_bit() {
+        let sample_rate = 48_000;
+        let freqs = freqs_from_tones(&vis_tones(44), sample_rate);
+        let hit = find_vis(&freqs, sample_rate, 0).expect("header");
+        // Header is 300 + 10 + 300 ms of leader and break, then ten 30 ms bits.
+        let expected = ((300.0 + 10.0 + 300.0 + 300.0) / 1000.0 * f64::from(sample_rate)) as usize;
+        let slack = sample_rate as usize / 100; // 10 ms
+        assert!(
+            hit.image_start.abs_diff(expected) < slack,
+            "image starts at {}, expected about {expected}",
+            hit.image_start,
+        );
+    }
+
+    #[test]
+    fn a_header_with_broken_parity_is_not_a_header() {
+        let sample_rate = 48_000;
+        let mut tones = vis_tones(44);
+        // Flip the parity bit alone: seven data bits still say Martin M1, but
+        // nothing now vouches for them.
+        let parity = tones.len() - 2;
+        tones[parity].hz = if tones[parity].hz == ONE_HZ {
+            ZERO_HZ
+        } else {
+            ONE_HZ
+        };
+        let freqs = freqs_from_tones(&tones, sample_rate);
+        assert_eq!(find_vis(&freqs, sample_rate, 0), None);
+    }
+
+    #[test]
+    fn tones_without_a_leader_are_not_a_header() {
+        let sample_rate = 48_000;
+        let mut tones = vis_tones(44);
+        // Same bits, but the leader before them is a pixel-band tone — which is
+        // what a passing image looks like.
+        tones[0].hz = 2000.0;
+        tones[2].hz = 2000.0;
+        let freqs = freqs_from_tones(&tones, sample_rate);
+        assert_eq!(find_vis(&freqs, sample_rate, 0), None);
+    }
+
+    #[test]
+    fn a_header_part_way_into_the_buffer_is_still_found() {
+        let sample_rate = 48_000;
+        let mut freqs = vec![1750.0f32; sample_rate as usize]; // a second of picture
+        freqs.extend(freqs_from_tones(&vis_tones(60), sample_rate));
+        let hit = find_vis(&freqs, sample_rate, 0).expect("header");
+        assert_eq!(hit.code, 60);
+    }
+}

src/decoders/trx-sstv/tests/round_trip.rs

@@ -0,0 +1,384 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Decode what was encoded, and compare the pictures.
+//!
+//! A decoder for a picture format can only really be tested against a picture.
+//! These tests build a test card, transmit it in each mode through the
+//! encoder, and hold the decoder to what comes back — pixel by pixel, with a
+//! tolerance that accounts for the round trip through frequency and, for the
+//! colour modes, through a chroma channel at half the width.
+//!
+//! What this does not test is the timing table itself: the encoder reads the
+//! same numbers as the decoder, so a wrong line duration would cancel out.
+//! That is checked in `mode.rs` against the published line times instead.
+
+use trx_sstv::encode::{encode, Frame};
+use trx_sstv::mode::mode_for_vis;
+use trx_sstv::{SstvConfig, SstvDecoder, SstvEvent, SstvImage};
+
+const SAMPLE_RATE: u32 = 48_000;
+
+/// A test card with something for every part of the decoder to get wrong:
+/// vertical colour bars catch channels swapped or shifted, the horizontal
+/// gradient catches a line-timing drift, and the corner blocks catch a picture
+/// that arrives upside down or mirrored.
+fn test_card(width: usize, height: usize) -> Vec<u8> {
+    let mut rgb = vec![0u8; width * height * 3];
+    let bars: [(u8, u8, u8); 8] = [
+        (255, 255, 255),
+        (255, 255, 0),
+        (0, 255, 255),
+        (0, 255, 0),
+        (255, 0, 255),
+        (255, 0, 0),
+        (0, 0, 255),
+        (0, 0, 0),
+    ];
+    for y in 0..height {
+        for x in 0..width {
+            let at = (y * width + x) * 3;
+            let (r, g, b) = if y < height / 2 {
+                bars[x * bars.len() / width]
+            } else {
+                let ramp = (x * 255 / width.max(1)) as u8;
+                let down = (y * 255 / height.max(1)) as u8;
+                (ramp, down, 255 - ramp)
+            };
+            rgb[at] = r;
+            rgb[at + 1] = g;
+            rgb[at + 2] = b;
+        }
+    }
+    // Corner marks: red top-left, blue bottom-right.
+    for y in 0..height.min(8) {
+        for x in 0..width.min(8) {
+            let at = (y * width + x) * 3;
+            rgb[at] = 255;
+            rgb[at + 1] = 0;
+            rgb[at + 2] = 0;
+        }
+    }
+    for y in height.saturating_sub(8)..height {
+        for x in width.saturating_sub(8)..width {
+            let at = (y * width + x) * 3;
+            rgb[at] = 0;
+            rgb[at + 1] = 0;
+            rgb[at + 2] = 255;
+        }
+    }
+    rgb
+}
+
+/// Run audio through the decoder in blocks the size a sound card delivers.
+///
+/// A little silence is fed after the signal, as a receiver that keeps
+/// listening supplies: the demodulator is a filter, so the last millisecond of
+/// any transmission needs the samples after it before it can be read.
+fn decode(audio: &[f32]) -> (Vec<SstvImage>, usize) {
+    let mut tail = audio.to_vec();
+    tail.extend(std::iter::repeat_n(0.0, SAMPLE_RATE as usize / 20));
+    let audio = tail.as_slice();
+
+    let mut decoder = SstvDecoder::new(SAMPLE_RATE, SstvConfig::default());
+    let mut images = Vec::new();
+    let mut rows = 0;
+    for block in audio.chunks(1024) {
+        for event in decoder.process_samples(block) {
+            match event {
+                SstvEvent::Row { .. } => rows += 1,
+                SstvEvent::Complete(image) => images.push(image),
+                SstvEvent::Started { .. } => {}
+            }
+        }
+    }
+    (images, rows)
+}
+
+/// Mean absolute error per colour channel between two same-sized images.
+fn mean_error(a: &[u8], b: &[u8]) -> f64 {
+    assert_eq!(a.len(), b.len());
+    let total: u64 = a
+        .iter()
+        .zip(b)
+        .map(|(x, y)| u64::from(x.abs_diff(*y)))
+        .sum();
+    total as f64 / a.len() as f64
+}
+
+/// Error over the part of the picture away from channel edges, where a decoder
+/// that is a pixel out on a hard colour boundary would otherwise dominate.
+fn interior_error(mode_width: usize, height: usize, sent: &[u8], got: &[u8]) -> f64 {
+    let mut total = 0u64;
+    let mut count = 0u64;
+    for y in 2..height.saturating_sub(2) {
+        for x in 4..mode_width.saturating_sub(4) {
+            // Skip the columns where the bars change, which is where a
+            // half-pixel timing difference shows up as a whole-colour error.
+            if x % (mode_width / 8) < 3 {
+                continue;
+            }
+            let at = (y * mode_width + x) * 3;
+            for channel in 0..3 {
+                total += u64::from(sent[at + channel].abs_diff(got[at + channel]));
+                count += 1;
+            }
+        }
+    }
+    total as f64 / count.max(1) as f64
+}
+
+fn round_trip(vis: u8, tolerance: f64) {
+    let mode = mode_for_vis(vis).expect("mode in table");
+    let width = usize::from(mode.width);
+    let height = usize::from(mode.height);
+    let sent = test_card(width, height);
+    let frame = Frame {
+        width,
+        height,
+        rgb: &sent,
+    };
+    let audio = encode(mode, &frame, SAMPLE_RATE);
+
+    let (images, rows) = decode(&audio);
+    assert_eq!(
+        images.len(),
+        1,
+        "{}: expected one picture, got {}",
+        mode.name,
+        images.len()
+    );
+    let image = &images[0];
+    assert!(
+        image.complete,
+        "{}: reception did not reach the bottom",
+        mode.name
+    );
+    assert_eq!(image.mode, mode.name);
+    assert_eq!(
+        image.lines, mode.height,
+        "{}: {} of {} lines",
+        mode.name, image.lines, mode.height
+    );
+    assert_eq!(
+        rows, height,
+        "{}: emitted {rows} rows for {height} lines",
+        mode.name
+    );
+
+    let error = interior_error(width, height, &sent, &image.rgb);
+    assert!(
+        error < tolerance,
+        "{}: mean error {error:.1} levels, tolerance {tolerance:.1}",
+        mode.name,
+    );
+}
+
+#[test]
+fn martin_m1_round_trips() {
+    round_trip(44, 6.0);
+}
+
+#[test]
+fn martin_m2_round_trips() {
+    round_trip(40, 8.0);
+}
+
+#[test]
+fn scottie_s1_round_trips() {
+    round_trip(60, 6.0);
+}
+
+#[test]
+fn scottie_s2_round_trips() {
+    round_trip(56, 8.0);
+}
+
+#[test]
+fn wraase_sc2_180_round_trips() {
+    round_trip(55, 6.0);
+}
+
+// The colour-difference modes lose chroma resolution by design, so the bars
+// bleed into one another at their edges; the tolerance is on the interior.
+#[test]
+fn robot_72_round_trips() {
+    round_trip(12, 14.0);
+}
+
+#[test]
+fn robot_36_round_trips() {
+    // One chroma channel per line, the other carried over from the line
+    // before, so alternate lines are a line stale in one channel.
+    round_trip(8, 26.0);
+}
+
+#[test]
+fn pd90_round_trips() {
+    round_trip(99, 14.0);
+}
+
+#[test]
+fn pd120_round_trips() {
+    round_trip(95, 14.0);
+}
+
+/// Silence before and after is the normal case — a receiver is not started at
+/// the instant the transmission does.
+#[test]
+fn survives_silence_around_the_transmission() {
+    let mode = mode_for_vis(44).expect("Martin M1");
+    let (width, height) = (usize::from(mode.width), usize::from(mode.height));
+    let sent = test_card(width, height);
+    let frame = Frame {
+        width,
+        height,
+        rgb: &sent,
+    };
+    let mut audio = vec![0.0f32; SAMPLE_RATE as usize * 2];
+    audio.extend(encode(mode, &frame, SAMPLE_RATE));
+    audio.extend(std::iter::repeat_n(0.0, SAMPLE_RATE as usize));
+
+    let (images, _) = decode(&audio);
+    assert_eq!(
+        images.len(),
+        1,
+        "expected one picture from a transmission in silence"
+    );
+    assert!(images[0].complete);
+}
+
+/// A transmission cut off part-way is what a fade or a shut-down transmitter
+/// produces. The lines that did arrive are worth keeping.
+#[test]
+fn a_truncated_transmission_still_yields_its_lines() {
+    let mode = mode_for_vis(44).expect("Martin M1");
+    let (width, height) = (usize::from(mode.width), usize::from(mode.height));
+    let sent = test_card(width, height);
+    let frame = Frame {
+        width,
+        height,
+        rgb: &sent,
+    };
+    let full = encode(mode, &frame, SAMPLE_RATE);
+    // Two thirds of the picture, then silence for long enough that the decoder
+    // stops waiting for the rest.
+    let mut audio = full[..full.len() * 2 / 3].to_vec();
+    audio.extend(std::iter::repeat_n(0.0, SAMPLE_RATE as usize * 5));
+
+    let (images, _) = decode(&audio);
+    assert_eq!(
+        images.len(),
+        1,
+        "a cut-off transmission produced no picture"
+    );
+    let image = &images[0];
+    assert!(
+        !image.complete,
+        "a two-thirds transmission reported as complete"
+    );
+    assert!(
+        image.lines > mode.height / 2 && image.lines < mode.height,
+        "{} lines of {} arrived",
+        image.lines,
+        mode.height,
+    );
+    // What did arrive is the top of the picture, and it is right.
+    let rows = usize::from(image.lines).saturating_sub(4);
+    let error = mean_error(&sent[..width * rows * 3], &image.rgb[..width * rows * 3]);
+    assert!(
+        error < 12.0,
+        "the lines that arrived are wrong: mean error {error:.1}"
+    );
+}
+
+/// Two pictures back to back: the decoder has to finish the first and pick up
+/// the header of the second.
+#[test]
+fn decodes_a_second_transmission_after_the_first() {
+    let mode = mode_for_vis(40).expect("Martin M2");
+    let (width, height) = (usize::from(mode.width), usize::from(mode.height));
+    let sent = test_card(width, height);
+    let frame = Frame {
+        width,
+        height,
+        rgb: &sent,
+    };
+    let one = encode(mode, &frame, SAMPLE_RATE);
+
+    let mut audio = one.clone();
+    audio.extend(std::iter::repeat_n(0.0, SAMPLE_RATE as usize / 2));
+    audio.extend(one);
+
+    let (images, _) = decode(&audio);
+    assert_eq!(
+        images.len(),
+        2,
+        "expected two pictures, got {}",
+        images.len()
+    );
+    assert!(
+        images.iter().all(|image| image.complete),
+        "a picture did not finish"
+    );
+}
+
+/// Noise on the signal is the normal condition on HF. The picture should
+/// degrade, not fall apart.
+#[test]
+fn decodes_through_noise() {
+    let mode = mode_for_vis(44).expect("Martin M1");
+    let (width, height) = (usize::from(mode.width), usize::from(mode.height));
+    let sent = test_card(width, height);
+    let frame = Frame {
+        width,
+        height,
+        rgb: &sent,
+    };
+    let clean = encode(mode, &frame, SAMPLE_RATE);
+
+    // Deterministic pseudo-noise at about 20 dB below the signal.
+    let mut seed = 0x5eed_1234u32;
+    let noisy: Vec<f32> = clean
+        .iter()
+        .map(|sample| {
+            seed = seed.wrapping_mul(1_664_525).wrapping_add(1_013_904_223);
+            let noise = (seed >> 8) as f32 / f32::from(u16::MAX) / 256.0 - 0.5;
+            sample + noise * 0.2
+        })
+        .collect();
+
+    let (images, _) = decode(&noisy);
+    assert_eq!(images.len(), 1, "noise cost the whole picture");
+    let image = &images[0];
+    assert!(image.complete, "noise cost the bottom of the picture");
+    let error = interior_error(width, height, &sent, &image.rgb);
+    assert!(error < 20.0, "mean error through noise {error:.1} levels");
+}
+
+/// The sound card that plays the signal and the one that records it never
+/// agree exactly. A part-per-thousand error is far worse than reality and the
+/// picture should still stand up.
+#[test]
+fn tolerates_a_transmitter_clock_that_runs_fast() {
+    let mode = mode_for_vis(44).expect("Martin M1");
+    let (width, height) = (usize::from(mode.width), usize::from(mode.height));
+    let sent = test_card(width, height);
+    let frame = Frame {
+        width,
+        height,
+        rgb: &sent,
+    };
+    // Encoding at a slightly different rate and decoding at 48 kHz is exactly
+    // a clock error: every duration is stretched by the same factor.
+    let audio = encode(mode, &frame, 48_048);
+
+    let (images, _) = decode(&audio);
+    assert_eq!(images.len(), 1, "a 0.1% clock error cost the picture");
+    let error = interior_error(width, height, &sent, &images[0].rgb);
+    assert!(
+        error < 12.0,
+        "mean error with a fast clock {error:.1} levels"
+    );
+}