[feat](trx-rs): make spectrum affordable over a slow link

Spectrum dominates the server↔client connection, and all three things that
govern its cost were working against a poor link.

**It was polled, one round trip per frame.** The client asked for a frame every
50 ms on a dedicated connection and waited for the reply, so the frame rate was
capped at 1/RTT — on a 200 ms link, five frames a second no matter what was
configured.  Add SubscribeSpectrum alongside the existing SubscribeMeter: the
server pushes frames from a per-rig broadcast that rig_task fills only while
somebody is subscribed.  A server too old to know the command answers with an
error and leaves the connection usable, so the client falls back to polling on
the same connection without reconnecting.

**Bins were JSON floats.** 1024 bins spelled out as decimal text is around
10 KB a frame, ~200 KB/s at full rate — while the very next hop, client to
browser, already sends the same information as base64 i8 in about 1.4 KB.  Bins
now travel base64-encoded whole dBFS, the resolution the display draws at
anyway.  Decoding still accepts the old array form.

**Nothing was tunable.** [sdr].spectrum_fft_size and [sdr].spectrum_interval_ms
replace the compile-time FFT size and cadence; [[remotes]].spectrum_interval_ms
lets the client ask for less.  512 bins at 5 frames/s is roughly 3.5 KB/s
against roughly 200 KB/s before.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-core/Cargo.toml

@@ -17,3 +17,4 @@ uuid = { workspace = true }
 ts-rs = { version = "12.0.1", features = ["uuid-impl"] }
 sgp4 = "2"
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls"] }
+base64 = "0.22"

src/trx-core/src/rig/mod.rs

@@ -19,6 +19,7 @@ pub mod command;
 pub mod controller;
 pub mod request;
 pub mod response;
+pub mod spectrum_wire;
 pub mod state;
 
 /// How this backend communicates with the rig.

src/trx-core/src/rig/spectrum_wire.rs

@@ -0,0 +1,124 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Compact wire encoding for spectrum bins.
+//!
+//! Bins are dBFS magnitudes, and the web UI has always drawn them from `i8`
+//! values — the SSE hop to the browser quantizes and base64-encodes them. The
+//! server→client hop, which is the one that crosses the operator's network,
+//! used to send the same information as a JSON array of `f32`: around ten bytes
+//! per bin instead of one, or roughly 10 KB per 1024-bin frame.
+//!
+//! Bins therefore travel as base64-encoded `i8` dBFS, about an eighth of the
+//! size, at a resolution the display already rounds to. Decoding still accepts
+//! the old array form, so a new client can read an older server.
+
+use base64::engine::general_purpose::STANDARD as BASE64;
+use base64::Engine as _;
+use serde::de::{SeqAccess, Visitor};
+use serde::{Deserializer, Serializer};
+use std::fmt;
+
+/// Quantize to whole dBFS and encode as base64.
+pub fn serialize<S: Serializer>(bins: &[f32], serializer: S) -> Result<S::Ok, S::Error> {
+    let quantized: Vec<u8> = bins
+        .iter()
+        .map(|&db| {
+            let clamped = if db.is_finite() { db } else { -128.0 };
+            clamped.round().clamp(-128.0, 127.0) as i8 as u8
+        })
+        .collect();
+    serializer.serialize_str(&BASE64.encode(quantized))
+}
+
+/// Decode base64 bins, or a plain array of numbers from an older server.
+pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<Vec<f32>, D::Error> {
+    deserializer.deserialize_any(BinsVisitor)
+}
+
+struct BinsVisitor;
+
+impl<'de> Visitor<'de> for BinsVisitor {
+    type Value = Vec<f32>;
+
+    fn expecting(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("base64-encoded i8 dBFS bins, or an array of numbers")
+    }
+
+    fn visit_str<E: serde::de::Error>(self, value: &str) -> Result<Self::Value, E> {
+        let bytes = BASE64
+            .decode(value)
+            .map_err(|e| E::custom(format!("invalid base64 spectrum bins: {e}")))?;
+        Ok(bytes.into_iter().map(|byte| byte as i8 as f32).collect())
+    }
+
+    fn visit_seq<A: SeqAccess<'de>>(self, mut seq: A) -> Result<Self::Value, A::Error> {
+        let mut bins = Vec::with_capacity(seq.size_hint().unwrap_or(1024));
+        while let Some(value) = seq.next_element::<f32>()? {
+            bins.push(value);
+        }
+        Ok(bins)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use serde::{Deserialize, Serialize};
+
+    #[derive(Debug, Serialize, Deserialize, PartialEq)]
+    struct Frame {
+        #[serde(with = "super")]
+        bins: Vec<f32>,
+    }
+
+    #[test]
+    fn test_round_trip_quantizes_to_whole_db() {
+        let frame = Frame {
+            bins: vec![-73.4, -20.6, 0.0, -120.2],
+        };
+        let json = serde_json::to_string(&frame).unwrap();
+        let back: Frame = serde_json::from_str(&json).unwrap();
+        assert_eq!(back.bins, vec![-73.0, -21.0, 0.0, -120.0]);
+    }
+
+    #[test]
+    fn test_serializes_as_a_base64_string() {
+        let json = serde_json::to_string(&Frame {
+            bins: vec![-1.0, 0.0],
+        })
+        .unwrap();
+        assert!(json.contains('"'), "bins should be a string: {json}");
+        assert!(!json.contains('['), "bins should not be an array: {json}");
+    }
+
+    #[test]
+    fn test_clamps_out_of_range_and_non_finite() {
+        let frame = Frame {
+            bins: vec![-400.0, 400.0, f32::NAN, f32::NEG_INFINITY],
+        };
+        let json = serde_json::to_string(&frame).unwrap();
+        let back: Frame = serde_json::from_str(&json).unwrap();
+        assert_eq!(back.bins, vec![-128.0, 127.0, -128.0, -128.0]);
+    }
+
+    #[test]
+    fn test_reads_the_old_array_form() {
+        let back: Frame = serde_json::from_str(r#"{"bins":[-73.25,-20.5]}"#).unwrap();
+        assert_eq!(back.bins, vec![-73.25, -20.5]);
+    }
+
+    /// The point of the change: an ordinary frame gets much smaller.
+    #[test]
+    fn test_frame_is_far_smaller_than_the_array_form() {
+        let bins: Vec<f32> = (0..1024).map(|i| -60.0 - (i % 40) as f32 * 0.37).collect();
+        let compact = serde_json::to_string(&Frame { bins: bins.clone() }).unwrap();
+        let verbose = serde_json::to_string(&bins).unwrap();
+        assert!(
+            compact.len() * 5 < verbose.len(),
+            "compact {} bytes vs array {} bytes",
+            compact.len(),
+            verbose.len()
+        );
+    }
+}

src/trx-core/src/rig/state.rs

@@ -394,6 +394,12 @@ fn default_wfm_denoise_level() -> WfmDenoiseLevel {
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, TS)]
 pub struct SpectrumData {
     /// FFT magnitude bins in dBFS, FFT-shifted so DC (centre frequency) is at index N/2.
+    ///
+    /// On the wire these are base64-encoded `i8` whole dBFS (see
+    /// `spectrum_wire`), which is what the display draws anyway; the TypeScript
+    /// type describes the decoded array the browser receives over SSE.
+    #[serde(with = "crate::rig::spectrum_wire")]
+    #[ts(type = "Array<number>")]
     pub bins: Vec<f32>,
     /// Centre frequency of the SDR capture in Hz.
     #[ts(type = "number")]