[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>

Cargo.lock

@@ -3165,6 +3165,7 @@ dependencies = [
 name = "trx-core"
 version = "0.1.0"
 dependencies = [
+ "base64",
  "flate2",
  "reqwest",
  "serde",

docs/User-Manual.md

@@ -153,6 +153,13 @@ When audio is enabled, at least one of `rx_enabled` or `tx_enabled` must be true
 | `sample_rate` | u32 | `1920000` | IQ capture rate in Hz |
 | `bandwidth` | u32 | `1500000` | Hardware IF filter bandwidth in Hz |
 | `center_offset_hz` | i64 | `100000` | Offset from dial to avoid DC spur |
+| `spectrum_fft_size` | usize | `1024` | Spectrum FFT bins; power of two, 128–8192 |
+| `spectrum_interval_ms` | u64 | `50` | How often a spectrum frame is pushed to subscribed clients |
+
+Spectrum is the largest thing on the client connection. On a slow or
+high-latency link, halving `spectrum_fft_size` halves the bytes per frame (at
+half the frequency resolution) and raising `spectrum_interval_ms` sends fewer of
+them; see [Spectrum over a slow link](#spectrum-over-a-slow-link).
 
 #### `[sdr.gain]`
 
@@ -295,6 +302,7 @@ Rigs without an explicit `id` get auto-generated IDs like `ft817_0`, `soapysdr_1
 |-------|------|---------|-------------|
 | `url` | string | — | Server address (e.g. `localhost:4530`) |
 | `poll_interval_ms` | u64 | `750` | State poll interval |
+| `spectrum_interval_ms` | u64 | `50` | Spectrum frame interval; also settable per `[[remotes]]` entry |
 
 #### `[remote.auth]`
 
@@ -394,6 +402,41 @@ older single `port` key and `--rigctl-port` are ignored.
 The bridge is intended for WSJT-X integration via virtual audio devices (ALSA
 loopback on Linux, BlackHole on macOS).
 
+### Spectrum over a slow link
+
+Spectrum dominates the server↔client connection: everything else is a few
+hundred bytes, a frame is a few kilobytes. Three things govern what it costs.
+
+**Frames are pushed, not polled.** The client subscribes and the server sends
+frames at `[sdr].spectrum_interval_ms`. Polling cost a round trip per frame, so
+the rate was capped at 1/RTT — on a 200 ms link you could not exceed 5 frames a
+second however often the client asked. Clients fall back to polling
+automatically against a server too old to stream.
+
+**Bins travel as whole dBFS.** They are base64-encoded `i8` on the wire, about
+an eighth of the JSON array of floats they used to be, at the resolution the
+display draws anyway.
+
+**Both ends have a rate, and the slower one wins.** The server pushes no faster
+than `[sdr].spectrum_interval_ms`; the client asks for no more than
+`[[remotes]].spectrum_interval_ms`.
+
+For a link that struggles, start here:
+
+```toml
+[trx-server.sdr]
+spectrum_fft_size = 512      # half the bins, half the bytes
+spectrum_interval_ms = 200   # 5 frames/s instead of 20
+
+[[trx-client.remotes]]
+name = "remote-site"
+url = "radio.example.com:4530"
+spectrum_interval_ms = 200
+```
+
+That is roughly 0.7 KB per frame at 5 frames/s — about 3.5 KB/s, against
+roughly 200 KB/s for 1024 float bins at 20 frames/s.
+
 ### CLI Override Summary
 
 **trx-server:**

src/trx-client/src/main.rs

@@ -288,6 +288,7 @@ async fn async_init() -> DynResult<AppState> {
         let token = cli.token.clone().or_else(|| cfg.remote.auth.token.clone());
         let poll_interval_ms = cli.poll_interval_ms.unwrap_or(cfg.remote.poll_interval_ms);
         vec![RemoteEntry {
+            spectrum_interval_ms: cfg.remote.spectrum_interval_ms,
             name,
             url: url.clone(),
             rig_id,
@@ -483,6 +484,13 @@ async fn async_init() -> DynResult<AppState> {
             .map(|e| e.poll_interval_ms)
             .min()
             .unwrap_or(750);
+        // Entries sharing a server share its connections, so the most frequent
+        // request wins: whoever wants spectrum fastest sets the rate.
+        let spectrum_interval = entries
+            .iter()
+            .map(|e| e.spectrum_interval_ms)
+            .min()
+            .unwrap_or_else(|| remote_client::DEFAULT_SPECTRUM_INTERVAL.as_millis() as u64);
 
         let (server_tx, server_rx) = mpsc::channel::<RigRequest>(RIG_TASK_CHANNEL_BUFFER);
         for entry in entries {
@@ -496,6 +504,8 @@ async fn async_init() -> DynResult<AppState> {
             known_rigs: frontend_runtime.routing.remote_rigs.clone(),
             rig_states: frontend_runtime.routing.rig_states.clone(),
             poll_interval: Duration::from_millis(poll_interval),
+            spectrum_interval: Duration::from_millis(spectrum_interval),
+            spectrum_stream_unsupported: Arc::new(std::sync::atomic::AtomicBool::new(false)),
             spectrum: frontend_runtime.spectrum.sender.clone(),
             rig_spectrums: frontend_runtime.spectrum.per_rig.clone(),
             server_connected: frontend_runtime.routing.server_connected.clone(),

src/trx-client/src/remote_client.rs

@@ -20,7 +20,7 @@ use trx_core::{RigError, RigResult};
 use trx_frontend::{RemoteRigEntry, SharedSpectrum};
 use trx_protocol::rig_command_to_client;
 use trx_protocol::types::RigEntry;
-use trx_protocol::{ClientCommand, ClientEnvelope, ClientResponse, MeterUpdate};
+use trx_protocol::{ClientCommand, ClientEnvelope, ClientResponse, MeterUpdate, SpectrumFrame};
 
 // Endpoint parsing lives in `trx-config` so config validation and the
 // connection code agree on what a URL means.
@@ -32,9 +32,10 @@ const SPECTRUM_IO_TIMEOUT: Duration = Duration::from_secs(3);
 const MAX_JSON_LINE_BYTES: usize = 256 * 1024;
 const MAX_CONSECUTIVE_POLL_FAILURES: u32 = 3;
 
-// Keep remote spectrum reasonably responsive without returning to the old
-// timeout churn caused by a much tighter request cadence.
-const SPECTRUM_POLL_INTERVAL: Duration = Duration::from_millis(50);
+// Default spectrum cadence when a config does not specify one.  Both the push
+// stream and the poll fallback run at the configured rate; see
+// `[[remotes]].spectrum_interval_ms`.
+pub const DEFAULT_SPECTRUM_INTERVAL: Duration = Duration::from_millis(50);
 
 #[derive(Clone)]
 pub struct RemoteClientConfig {
@@ -43,6 +44,12 @@ pub struct RemoteClientConfig {
     pub selected_rig_id: Arc<Mutex<Option<String>>>,
     pub known_rigs: Arc<Mutex<Vec<RemoteRigEntry>>>,
     pub poll_interval: Duration,
+    /// How often spectrum frames are wanted.  Drives the poll fallback and is
+    /// the rate the client asks the server to push at.
+    pub spectrum_interval: Duration,
+    /// Set once a server has rejected `SubscribeSpectrum`, so later
+    /// connections to it go straight to polling instead of asking again.
+    pub spectrum_stream_unsupported: Arc<AtomicBool>,
     /// Spectrum watch sender; spectrum task publishes here, SSE clients subscribe.
     pub spectrum: Arc<watch::Sender<SharedSpectrum>>,
     /// Shared flag: `true` while a TCP connection to trx-server is active.
@@ -491,6 +498,100 @@ async fn send_get_sat_passes_on(
     ))
 }
 
+/// What ended a spectrum stream attempt.
+enum SpectrumStreamOutcome {
+    /// The stream ran and is over; the connection is spent.
+    Finished,
+    /// The server rejected the subscription.  The connection is still usable,
+    /// so the caller can poll on it.
+    Unsupported,
+}
+
+/// Subscribe to the server's spectrum push for one rig and publish frames as
+/// they arrive.
+///
+/// The server answers either with frames or, when it is too old to know the
+/// command, with an error response — which leaves the connection usable for
+/// polling, so falling back costs no reconnect.
+async fn run_spectrum_stream(
+    config: &RemoteClientConfig,
+    writer: &mut (impl AsyncWriteExt + Unpin),
+    reader: &mut (impl AsyncBufRead + Unpin),
+    short_name: &str,
+    shutdown_rx: &mut watch::Receiver<bool>,
+) -> RigResult<SpectrumStreamOutcome> {
+    let envelope = build_envelope(
+        config,
+        ClientCommand::SubscribeSpectrum,
+        Some(short_name.to_string()),
+    );
+    let mut payload = serde_json::to_string(&envelope)
+        .map_err(|e| RigError::communication(format!("JSON serialize failed: {e}")))?;
+    payload.push('\n');
+    time::timeout(SPECTRUM_IO_TIMEOUT, writer.write_all(payload.as_bytes()))
+        .await
+        .map_err(|_| RigError::communication("spectrum subscribe write timed out".to_string()))?
+        .map_err(|e| RigError::communication(format!("spectrum subscribe write failed: {e}")))?;
+    time::timeout(SPECTRUM_IO_TIMEOUT, writer.flush())
+        .await
+        .map_err(|_| RigError::communication("spectrum subscribe flush timed out".to_string()))?
+        .map_err(|e| RigError::communication(format!("spectrum subscribe flush failed: {e}")))?;
+
+    // Re-check what the UI wants: switching rigs has to end this stream so the
+    // connection can be rebuilt for the new one.
+    let mut supervisor = time::interval(Duration::from_millis(500));
+    supervisor.tick().await;
+
+    loop {
+        tokio::select! {
+            changed = shutdown_rx.changed() => {
+                match changed {
+                    Ok(()) if *shutdown_rx.borrow() => return Ok(SpectrumStreamOutcome::Finished),
+                    Ok(()) => {}
+                    Err(_) => return Ok(SpectrumStreamOutcome::Finished),
+                }
+            }
+            _ = supervisor.tick() => {
+                let wanted = active_spectrum_rig_ids(config);
+                if wanted.len() != 1 || wanted[0] != short_name {
+                    return Ok(SpectrumStreamOutcome::Finished);
+                }
+            }
+            line = read_limited_line(reader, MAX_JSON_LINE_BYTES) => {
+                let line = line
+                    .map_err(|e| RigError::communication(format!("spectrum read failed: {e}")))?
+                    .ok_or_else(|| {
+                        RigError::communication("spectrum connection closed".to_string())
+                    })?;
+                let trimmed = line.trim_end();
+                if trimmed.is_empty() {
+                    continue;
+                }
+                match serde_json::from_str::<SpectrumFrame>(trimmed) {
+                    Ok(frame) => publish_spectrum_frame(config, short_name, frame),
+                    // Anything that is not a frame means the server would rather
+                    // answer than stream: an older build rejecting the command.
+                    Err(_) => return Ok(SpectrumStreamOutcome::Unsupported),
+                }
+            }
+        }
+    }
+}
+
+/// Publish one pushed frame to the per-rig and selected-rig watch channels.
+fn publish_spectrum_frame(config: &RemoteClientConfig, short_name: &str, frame: SpectrumFrame) {
+    if let Ok(map) = config.rig_spectrums.read() {
+        if let Some(tx) = map.get(short_name) {
+            tx.send_modify(|s| s.set(Some(frame.spectrum.clone()), frame.vchan_rds.clone()));
+        }
+    }
+    if selected_rig_id(config).as_deref() == Some(short_name) {
+        config
+            .spectrum
+            .send_modify(|s| s.set(Some(frame.spectrum), frame.vchan_rds));
+    }
+}
+
 async fn handle_spectrum_connection(
     config: &RemoteClientConfig,
     stream: TcpStream,
@@ -498,7 +599,33 @@ async fn handle_spectrum_connection(
 ) -> RigResult<()> {
     let (reader, mut writer) = stream.into_split();
     let mut reader = BufReader::new(reader);
-    let mut interval = time::interval(SPECTRUM_POLL_INTERVAL);
+
+    // Prefer the push stream: polling costs a round trip per frame, so on a
+    // high-latency link the frame rate is 1/RTT no matter what interval is
+    // configured.  It only works for one rig per connection, and only against
+    // a server new enough to understand the command.
+    let streamable = active_spectrum_rig_ids(config);
+    if streamable.len() == 1 && !config.spectrum_stream_unsupported.load(Ordering::Relaxed) {
+        match run_spectrum_stream(
+            config,
+            &mut writer,
+            &mut reader,
+            &streamable[0],
+            shutdown_rx,
+        )
+        .await?
+        {
+            SpectrumStreamOutcome::Finished => return Ok(()),
+            SpectrumStreamOutcome::Unsupported => {
+                info!("Server does not support spectrum streaming; falling back to polling");
+                config
+                    .spectrum_stream_unsupported
+                    .store(true, Ordering::Relaxed);
+            }
+        }
+    }
+
+    let mut interval = time::interval(config.spectrum_interval);
     // Cache the token outside the poll loop to avoid cloning it every 50ms.
     let cached_token = config.token.clone();
 
@@ -1187,6 +1314,7 @@ mod tests {
     use super::{has_short_names, resolve_server_rig_id, resolve_short_name};
     use super::{
         parse_audio_url, parse_remote_url, RemoteClientConfig, RemoteEndpoint, SharedSpectrum,
+        DEFAULT_SPECTRUM_INTERVAL,
     };
     use std::collections::HashMap;
     use std::sync::atomic::AtomicBool;
@@ -1381,6 +1509,8 @@ mod tests {
                 selected_rig_id: Arc::new(Mutex::new(None)),
                 known_rigs: Arc::new(Mutex::new(Vec::new())),
                 poll_interval: Duration::from_millis(100),
+                spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+                spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
                 spectrum: Arc::new(spectrum_tx),
                 server_connected: Arc::new(AtomicBool::new(false)),
                 rig_server_connected: Arc::new(RwLock::new(HashMap::new())),
@@ -1426,6 +1556,8 @@ mod tests {
             selected_rig_id: Arc::new(Mutex::new(Some("sdr".to_string()))),
             known_rigs: Arc::new(Mutex::new(Vec::new())),
             poll_interval: Duration::from_millis(500),
+            spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+            spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
             spectrum: Arc::new(spectrum_tx),
             server_connected: Arc::new(AtomicBool::new(false)),
             rig_server_connected: Arc::new(RwLock::new(HashMap::new())),
@@ -1441,6 +1573,130 @@ mod tests {
         assert_eq!(envelope.rig_id.as_deref(), Some("sdr"));
     }
 
+    fn stream_test_config(spectrum_tx: watch::Sender<SharedSpectrum>) -> super::RemoteClientConfig {
+        super::RemoteClientConfig {
+            addr: "127.0.0.1:4530".to_string(),
+            token: None,
+            selected_rig_id: Arc::new(Mutex::new(Some("sdr".to_string()))),
+            known_rigs: Arc::new(Mutex::new(Vec::new())),
+            poll_interval: Duration::from_millis(500),
+            spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+            spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
+            spectrum: Arc::new(spectrum_tx),
+            server_connected: Arc::new(AtomicBool::new(false)),
+            rig_server_connected: Arc::new(RwLock::new(HashMap::new())),
+            rig_states: Arc::new(RwLock::new(HashMap::new())),
+            rig_spectrums: Arc::new(RwLock::new(HashMap::new())),
+            rig_id_to_short_name: HashMap::new(),
+            short_name_to_rig_id: Arc::new(RwLock::new(HashMap::new())),
+            sat_passes: Arc::new(RwLock::new(None)),
+            rig_meters: Arc::new(RwLock::new(HashMap::new())),
+        }
+    }
+
+    /// A pushed frame reaches the watch channel the UI reads, with no request
+    /// from the client beyond the initial subscribe.
+    #[tokio::test]
+    async fn spectrum_stream_publishes_pushed_frames() {
+        use tokio::io::{AsyncBufReadExt, BufReader};
+
+        let (spectrum_tx, mut spectrum_rx) = watch::channel(SharedSpectrum::default());
+        let config = stream_test_config(spectrum_tx);
+        let (shutdown_tx, mut shutdown_rx) = watch::channel(false);
+
+        let (client_io, mut server_io) = tokio::io::duplex(64 * 1024);
+        let (client_read, mut client_write) = tokio::io::split(client_io);
+        let mut client_read = BufReader::new(client_read);
+
+        let task = tokio::spawn(async move {
+            super::run_spectrum_stream(
+                &config,
+                &mut client_write,
+                &mut client_read,
+                "sdr",
+                &mut shutdown_rx,
+            )
+            .await
+        });
+
+        // The server sees the subscribe, then pushes without being asked.
+        let mut server = BufReader::new(&mut server_io);
+        let mut subscribe = String::new();
+        server.read_line(&mut subscribe).await.expect("subscribe");
+        assert!(
+            subscribe.contains("subscribe_spectrum"),
+            "unexpected command: {subscribe}"
+        );
+
+        let frame = trx_protocol::SpectrumFrame {
+            rig_id: "sdr".to_string(),
+            spectrum: trx_core::rig::state::SpectrumData {
+                bins: vec![-70.0, -30.0],
+                center_hz: 14_200_000,
+                sample_rate: 1_920_000,
+                rds: None,
+            },
+            vchan_rds: None,
+        };
+        let mut line = serde_json::to_string(&frame).unwrap();
+        line.push('\n');
+        server_io.write_all(line.as_bytes()).await.expect("push");
+        server_io.flush().await.expect("flush");
+
+        spectrum_rx.changed().await.expect("spectrum published");
+        let published = spectrum_rx.borrow().clone();
+        let spectrum = published.frame.expect("spectrum present");
+        assert_eq!(spectrum.center_hz, 14_200_000);
+        assert_eq!(spectrum.bins, vec![-70.0, -30.0]);
+
+        let _ = shutdown_tx.send(true);
+        let _ = task.await;
+    }
+
+    /// An older server answers the unknown command with an error instead of
+    /// frames.  That has to read as "poll instead", not as a dead connection.
+    #[tokio::test]
+    async fn spectrum_stream_falls_back_when_unsupported() {
+        use tokio::io::{AsyncBufReadExt, BufReader};
+
+        let (spectrum_tx, _spectrum_rx) = watch::channel(SharedSpectrum::default());
+        let config = stream_test_config(spectrum_tx);
+        let (_shutdown_tx, mut shutdown_rx) = watch::channel(false);
+
+        let (client_io, mut server_io) = tokio::io::duplex(64 * 1024);
+        let (client_read, mut client_write) = tokio::io::split(client_io);
+        let mut client_read = BufReader::new(client_read);
+
+        let task = tokio::spawn(async move {
+            super::run_spectrum_stream(
+                &config,
+                &mut client_write,
+                &mut client_read,
+                "sdr",
+                &mut shutdown_rx,
+            )
+            .await
+        });
+
+        let mut server = BufReader::new(&mut server_io);
+        let mut subscribe = String::new();
+        server.read_line(&mut subscribe).await.expect("subscribe");
+
+        server_io
+            .write_all(
+                b"{\"success\":false,\"state\":null,\"error\":\"Invalid JSON: unknown variant\"}\n",
+            )
+            .await
+            .expect("error response");
+        server_io.flush().await.expect("flush");
+
+        let outcome = task.await.expect("join").expect("stream result");
+        assert!(
+            matches!(outcome, super::SpectrumStreamOutcome::Unsupported),
+            "an error response should fall back to polling"
+        );
+    }
+
     #[test]
     fn build_envelope_translates_short_name_to_server_rig_id() {
         let (spectrum_tx, _spectrum_rx) = watch::channel(SharedSpectrum::default());
@@ -1454,6 +1710,8 @@ mod tests {
             selected_rig_id: Arc::new(Mutex::new(Some("home-hf".to_string()))),
             known_rigs: Arc::new(Mutex::new(Vec::new())),
             poll_interval: Duration::from_millis(500),
+            spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+            spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
             spectrum: Arc::new(spectrum_tx),
             server_connected: Arc::new(AtomicBool::new(false)),
             rig_server_connected: Arc::new(RwLock::new(HashMap::new())),
@@ -1486,6 +1744,8 @@ mod tests {
             selected_rig_id: Arc::new(Mutex::new(None)),
             known_rigs: Arc::new(Mutex::new(Vec::new())),
             poll_interval: Duration::from_millis(500),
+            spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+            spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
             spectrum: Arc::new(spectrum_tx),
             server_connected: Arc::new(AtomicBool::new(false)),
             rig_server_connected: Arc::new(RwLock::new(HashMap::new())),
@@ -1510,6 +1770,8 @@ mod tests {
             selected_rig_id: Arc::new(Mutex::new(None)),
             known_rigs: Arc::new(Mutex::new(Vec::new())),
             poll_interval: Duration::from_millis(500),
+            spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+            spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
             spectrum: Arc::new(spectrum_tx),
             server_connected: Arc::new(AtomicBool::new(false)),
             rig_server_connected: Arc::new(RwLock::new(HashMap::new())),
@@ -1550,6 +1812,8 @@ mod tests {
             selected_rig_id: Arc::new(Mutex::new(None)),
             known_rigs: known_rigs.clone(),
             poll_interval: Duration::from_millis(500),
+            spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+            spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
             spectrum: Arc::new(spectrum_tx),
             server_connected: Arc::new(AtomicBool::new(false)),
             rig_server_connected: Arc::new(RwLock::new(HashMap::new())),
@@ -1623,6 +1887,8 @@ mod tests {
             selected_rig_id,
             known_rigs,
             poll_interval: Duration::from_millis(500),
+            spectrum_interval: DEFAULT_SPECTRUM_INTERVAL,
+            spectrum_stream_unsupported: Arc::new(AtomicBool::new(false)),
             spectrum: Arc::new(spectrum_tx),
             server_connected: Arc::new(AtomicBool::new(false)),
             rig_server_connected: Arc::new(RwLock::new(HashMap::new())),

src/trx-client/trx-frontend/trx-frontend-http/frontend/src/api/generated.ts

@@ -78,6 +78,10 @@ export type RdsData = { pi?: number | null, program_service?: string | null, rad
 export type 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.
  */
 bins: Array<number>, 
 /**

src/trx-config/src/client.rs

@@ -76,6 +76,8 @@ pub struct RemoteConfig {
     pub auth: RemoteAuthConfig,
     /// Poll interval in milliseconds.
     pub poll_interval_ms: u64,
+    /// Spectrum frame interval in milliseconds.
+    pub spectrum_interval_ms: u64,
 }
 
 impl Default for RemoteConfig {
@@ -85,6 +87,7 @@ impl Default for RemoteConfig {
             rig_id: None,
             auth: RemoteAuthConfig::default(),
             poll_interval_ms: 750,
+            spectrum_interval_ms: default_spectrum_interval_ms(),
         }
     }
 }
@@ -121,12 +124,23 @@ pub struct RemoteEntry {
     /// Poll interval in milliseconds. Defaults to 750.
     #[serde(default = "default_poll_interval_ms")]
     pub poll_interval_ms: u64,
+    /// How often spectrum frames are wanted from this server, in milliseconds.
+    /// Defaults to 50 (20 frames/s).
+    ///
+    /// Raise it on a slow or high-latency link: spectrum is by far the largest
+    /// thing on the connection, and the server pushes no faster than this.
+    #[serde(default = "default_spectrum_interval_ms")]
+    pub spectrum_interval_ms: u64,
 }
 
 fn default_poll_interval_ms() -> u64 {
     750
 }
 
+fn default_spectrum_interval_ms() -> u64 {
+    50
+}
+
 /// Frontend configurations.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(default)]
@@ -421,6 +435,7 @@ impl ClientConfig {
                 rig_id: self.remote.rig_id.clone(),
                 auth: self.remote.auth.clone(),
                 poll_interval_ms: self.remote.poll_interval_ms,
+                spectrum_interval_ms: self.remote.spectrum_interval_ms,
             }]
         } else {
             Vec::new()
@@ -603,12 +618,21 @@ impl ClientConfig {
                     i, entry.name
                 ));
             }
+            if entry.spectrum_interval_ms == 0 {
+                return Err(format!(
+                    "[[remotes]][{}].spectrum_interval_ms must be > 0 (name \"{}\")",
+                    i, entry.name
+                ));
+            }
         }
 
         // Legacy [remote], kept for backward compatibility.
         if self.remote.poll_interval_ms == 0 {
             return Err("[remote].poll_interval_ms must be > 0".to_string());
         }
+        if self.remote.spectrum_interval_ms == 0 {
+            return Err("[remote].spectrum_interval_ms must be > 0".to_string());
+        }
         if let Some(url) = &self.remote.url {
             if url.trim().is_empty() {
                 return Err("[remote].url must not be empty when set".to_string());
@@ -834,6 +858,7 @@ impl ClientConfig {
                         token_file: None,
                     },
                     poll_interval_ms: 750,
+                    spectrum_interval_ms: 50,
                 },
                 RemoteEntry {
                     name: "home-vhf".to_string(),
@@ -844,6 +869,7 @@ impl ClientConfig {
                         token_file: None,
                     },
                     poll_interval_ms: 750,
+                    spectrum_interval_ms: 50,
                 },
             ],
             frontends: FrontendsConfig {
@@ -994,6 +1020,7 @@ impl ConfigFile for ClientConfig {
                 rig_id: Some("hf".to_string()),
                 auth: RemoteAuthConfig::default(),
                 poll_interval_ms: default_poll_interval_ms(),
+                spectrum_interval_ms: 50,
             }],
             ..Default::default()
         };
@@ -1311,6 +1338,7 @@ url = "remote.example.com:4530"
                 rig_id: None,
                 auth: RemoteAuthConfig::default(),
                 poll_interval_ms: 750,
+                spectrum_interval_ms: 50,
             }],
             ..Default::default()
         };
@@ -1330,6 +1358,7 @@ url = "remote.example.com:4530"
                     token_file: None,
                 },
                 poll_interval_ms: 750,
+                spectrum_interval_ms: 50,
             },
             ..Default::default()
         };
@@ -1368,6 +1397,7 @@ url = "remote.example.com:4530"
                 rig_id: None,
                 auth: RemoteAuthConfig::default(),
                 poll_interval_ms: 750,
+                spectrum_interval_ms: 50,
             }],
             ..Default::default()
         };
@@ -1386,6 +1416,7 @@ url = "remote.example.com:4530"
                     rig_id: None,
                     auth: RemoteAuthConfig::default(),
                     poll_interval_ms: 750,
+                    spectrum_interval_ms: 50,
                 },
                 RemoteEntry {
                     name: "dup".to_string(),
@@ -1393,6 +1424,7 @@ url = "remote.example.com:4530"
                     rig_id: None,
                     auth: RemoteAuthConfig::default(),
                     poll_interval_ms: 750,
+                    spectrum_interval_ms: 50,
                 },
             ],
             ..Default::default()
@@ -1409,6 +1441,7 @@ url = "remote.example.com:4530"
                 rig_id: None,
                 auth: RemoteAuthConfig::default(),
                 poll_interval_ms: 750,
+                spectrum_interval_ms: 50,
             }],
             ..Default::default()
         };
@@ -1427,6 +1460,7 @@ url = "remote.example.com:4530"
                 rig_id: None,
                 auth: RemoteAuthConfig::default(),
                 poll_interval_ms: 750,
+                spectrum_interval_ms: 50,
             }],
             ..Default::default()
         };
@@ -1445,6 +1479,7 @@ url = "remote.example.com:4530"
                 rig_id: None,
                 auth: RemoteAuthConfig::default(),
                 poll_interval_ms: 0,
+                spectrum_interval_ms: 50,
             }],
             ..Default::default()
         };
@@ -1499,6 +1534,45 @@ url = "remote.example.com:4530"
         assert!(warnings.is_empty(), "unexpected warnings: {warnings:?}");
     }
 
+    // --- Spectrum cadence ---
+
+    #[test]
+    fn test_spectrum_interval_defaults_and_parses() {
+        let config: ClientConfig = toml::from_str(
+            r#"
+[[remotes]]
+name = "slow-link"
+url = "host:4530"
+spectrum_interval_ms = 500
+
+[[remotes]]
+name = "lan"
+url = "host2:4530"
+"#,
+        )
+        .unwrap();
+        assert_eq!(config.remotes[0].spectrum_interval_ms, 500);
+        assert_eq!(config.remotes[1].spectrum_interval_ms, 50);
+        assert!(config.validate().is_ok());
+    }
+
+    #[test]
+    fn test_validate_rejects_zero_spectrum_interval() {
+        let config: ClientConfig = toml::from_str(
+            r#"
+[[remotes]]
+name = "hf"
+url = "host:4530"
+spectrum_interval_ms = 0
+"#,
+        )
+        .unwrap();
+        assert!(config
+            .validate()
+            .unwrap_err()
+            .contains("spectrum_interval_ms"));
+    }
+
     // --- Secret indirection ---
 
     fn secret_file(content: &str) -> tempfile::NamedTempFile {
@@ -1537,6 +1611,7 @@ url = "remote.example.com:4530"
                     token_file: Some(f.path().to_str().unwrap().to_string()),
                 },
                 poll_interval_ms: 750,
+                spectrum_interval_ms: 50,
             }],
             ..Default::default()
         };
@@ -1570,6 +1645,7 @@ url = "remote.example.com:4530"
                 rig_id: None,
                 auth: RemoteAuthConfig::default(),
                 poll_interval_ms: 750,
+                spectrum_interval_ms: 50,
             })
             .collect()
     }

src/trx-config/src/server.rs

@@ -399,12 +399,30 @@ pub struct SdrConfig {
     /// Default: 4.
     #[serde(default = "default_max_virtual_channels")]
     pub max_virtual_channels: usize,
+    /// FFT bin count for the spectrum display.  Must be a power of two.
+    ///
+    /// Halving it halves both the DSP cost and the bytes each frame puts on
+    /// the network, at half the frequency resolution — worth it on a slow link.
+    #[serde(default = "default_spectrum_fft_size")]
+    pub spectrum_fft_size: usize,
+    /// How often the server pushes a spectrum frame to a subscribed client, in
+    /// milliseconds.  Raise it on a slow or metered link.
+    #[serde(default = "default_spectrum_interval_ms")]
+    pub spectrum_interval_ms: u64,
 }
 
 fn default_max_virtual_channels() -> usize {
     4
 }
 
+fn default_spectrum_fft_size() -> usize {
+    1024
+}
+
+fn default_spectrum_interval_ms() -> u64 {
+    50
+}
+
 impl Default for SdrConfig {
     fn default() -> Self {
         Self {
@@ -417,6 +435,8 @@ impl Default for SdrConfig {
             noise_blanker: SdrNoiseBlankerConfig::default(),
             channels: Vec::new(),
             max_virtual_channels: default_max_virtual_channels(),
+            spectrum_fft_size: default_spectrum_fft_size(),
+            spectrum_interval_ms: default_spectrum_interval_ms(),
         }
     }
 }
@@ -913,6 +933,9 @@ fn validate_rig_instance(
             errors.push(format!("{prefix}[sdr.gain].max_value must be >= 0"));
         }
     }
+    if let Err(e) = validate_spectrum_config(prefix, &rig.sdr) {
+        errors.push(e);
+    }
     if let Err(e) = validate_sdr_squelch_config(&format!("{prefix}[sdr.squelch]"), &rig.sdr.squelch)
     {
         errors.push(e);
@@ -1070,6 +1093,26 @@ fn validate_access(prefix: &str, access: &AccessConfig) -> Result<(), String> {
     Ok(())
 }
 
+/// A non-power-of-two FFT size still plans, but costs far more per frame; the
+/// bounds keep the display useful without letting a typo allocate a huge FFT.
+fn validate_spectrum_config(prefix: &str, sdr: &SdrConfig) -> Result<(), String> {
+    let size = sdr.spectrum_fft_size;
+    if !(128..=8192).contains(&size) {
+        return Err(format!(
+            "{prefix}[sdr].spectrum_fft_size must be in range 128..=8192"
+        ));
+    }
+    if !size.is_power_of_two() {
+        return Err(format!(
+            "{prefix}[sdr].spectrum_fft_size must be a power of two (got {size})"
+        ));
+    }
+    if sdr.spectrum_interval_ms == 0 {
+        return Err(format!("{prefix}[sdr].spectrum_interval_ms must be > 0"));
+    }
+    Ok(())
+}
+
 fn validate_sdr_squelch_config(path: &str, squelch: &SdrSquelchConfig) -> Result<(), String> {
     if !squelch.threshold_db.is_finite() {
         return Err(format!("{path}.threshold_db must be finite"));
@@ -1813,6 +1856,93 @@ port = 4531
         assert!(warnings.is_empty(), "unexpected warnings: {warnings:?}");
     }
 
+    // --- Spectrum transport knobs ---
+
+    #[test]
+    fn test_spectrum_defaults() {
+        let cfg = SdrConfig::default();
+        assert_eq!(cfg.spectrum_fft_size, 1024);
+        assert_eq!(cfg.spectrum_interval_ms, 50);
+    }
+
+    fn sdr_cfg_with_spectrum(fft_size: usize, interval_ms: u64) -> ServerConfig {
+        let mut cfg = ServerConfig::default();
+        cfg.rig.access.port = Some("/dev/ttyUSB0".to_string());
+        cfg.rig.access.baud = Some(9600);
+        cfg.sdr.spectrum_fft_size = fft_size;
+        cfg.sdr.spectrum_interval_ms = interval_ms;
+        cfg
+    }
+
+    #[test]
+    fn test_validate_accepts_smaller_power_of_two_fft() {
+        assert!(sdr_cfg_with_spectrum(256, 200).validate().is_ok());
+    }
+
+    #[test]
+    fn test_validate_rejects_non_power_of_two_fft() {
+        let err = sdr_cfg_with_spectrum(1000, 50)
+            .validate()
+            .expect_err("expected a power-of-two error");
+        assert!(err.contains("power of two"), "unexpected error: {err}");
+    }
+
+    #[test]
+    fn test_validate_rejects_out_of_range_fft() {
+        assert!(sdr_cfg_with_spectrum(64, 50).validate().is_err());
+        assert!(sdr_cfg_with_spectrum(16384, 50).validate().is_err());
+    }
+
+    #[test]
+    fn test_validate_rejects_zero_spectrum_interval() {
+        let err = sdr_cfg_with_spectrum(1024, 0)
+            .validate()
+            .expect_err("expected an interval error");
+        assert!(
+            err.contains("spectrum_interval_ms"),
+            "unexpected error: {err}"
+        );
+    }
+
+    /// Per-rig validation covers the spectrum knobs too, and names the rig.
+    #[test]
+    fn test_validate_rejects_bad_spectrum_size_in_rig_entry() {
+        let toml_str = r#"
+[[rigs]]
+id = "sdr"
+[rigs.rig]
+model = "soapysdr"
+[rigs.rig.access]
+type = "sdr"
+args = "driver=rtlsdr"
+[rigs.audio]
+port = 4532
+tx_enabled = false
+[rigs.sdr]
+spectrum_fft_size = 999
+"#;
+        let cfg: ServerConfig = toml::from_str(toml_str).unwrap();
+        let err = cfg.validate().expect_err("expected a power-of-two error");
+        assert!(
+            err.contains("power of two") && err.contains("sdr"),
+            "unexpected error: {err}"
+        );
+    }
+
+    #[test]
+    fn test_parse_spectrum_config_from_toml() {
+        let cfg: ServerConfig = toml::from_str(
+            r#"
+[sdr]
+spectrum_fft_size = 512
+spectrum_interval_ms = 250
+"#,
+        )
+        .unwrap();
+        assert_eq!(cfg.sdr.spectrum_fft_size, 512);
+        assert_eq!(cfg.sdr.spectrum_interval_ms, 250);
+    }
+
     // --- Secret indirection ---
 
     #[test]

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")]

src/trx-protocol/src/lib.rs

@@ -18,4 +18,6 @@ pub use auth::{NoAuthValidator, SimpleTokenValidator, TokenValidator};
 pub use codec::{mode_to_string, parse_envelope, parse_mode};
 pub use decoders::{DecoderActivation, DecoderDescriptor, DECODER_REGISTRY};
 pub use mapping::{client_command_to_rig, rig_command_to_client};
-pub use types::{ClientCommand, ClientEnvelope, ClientResponse, MeterUpdate, RigEntry};
+pub use types::{
+    ClientCommand, ClientEnvelope, ClientResponse, MeterUpdate, RigEntry, SpectrumFrame,
+};

src/trx-protocol/src/mapping.rs

@@ -102,7 +102,7 @@ macro_rules! define_command_mapping {
 
 define_command_mapping! {
     // ── Client-only variants (no RigCommand counterpart) ─────────────
-    client_only: GetRigs, GetSatPasses, SubscribeMeter;
+    client_only: GetRigs, GetSatPasses, SubscribeMeter, SubscribeSpectrum;
 
     // ── Unit variants (no payload) ───────────────────────────────────
     unit:

src/trx-protocol/src/types.rs

@@ -130,6 +130,28 @@ pub enum ClientCommand {
     /// newline-delimited `MeterUpdate` JSON frames and no further commands or
     /// regular responses are sent.  Intended for a dedicated TCP connection.
     SubscribeMeter,
+    /// Subscribe to a per-rig spectrum stream on this connection.  Like
+    /// `SubscribeMeter`, the connection becomes a one-way flow of
+    /// newline-delimited `SpectrumFrame` JSON and no further commands or
+    /// regular responses are sent.
+    ///
+    /// Polling `GetSpectrum` costs a round trip per frame, which caps the frame
+    /// rate at 1/RTT no matter how often the client asks; the server pushes at
+    /// its own cadence instead.  Clients fall back to polling when the server
+    /// rejects this command.
+    SubscribeSpectrum,
+}
+
+/// One spectrum frame pushed by the server on a dedicated spectrum stream.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+pub struct SpectrumFrame {
+    /// Rig identifier this frame belongs to.
+    pub rig_id: String,
+    /// The frame itself; bins travel base64-encoded (see `spectrum_wire`).
+    pub spectrum: trx_core::rig::state::SpectrumData,
+    /// Virtual-channel RDS state, mirroring what `GetSpectrum` returned.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub vchan_rds: Option<Vec<trx_core::rig::state::VchanRdsEntry>>,
 }
 
 /// Fast meter sample pushed by the server on a dedicated meter stream.

src/trx-server/src/listener.rs

@@ -494,6 +494,58 @@ where
             }
         };
 
+        // SubscribeSpectrum: turns this connection into a one-way spectrum
+        // stream.  Polling GetSpectrum costs a round trip per frame, so a
+        // client on a slow link could never reach the frame rate it asked for;
+        // pushing decouples the rate from the latency.
+        if matches!(envelope.cmd, ClientCommand::SubscribeSpectrum) {
+            let mut spectrum_rx = handle.spectrum_tx.subscribe();
+            let io_timeout = timeouts.io_timeout;
+            info!(
+                "Client {} subscribed to spectrum stream for rig '{}'",
+                addr, target_rig_id
+            );
+            loop {
+                tokio::select! {
+                    frame = spectrum_rx.recv() => {
+                        match frame {
+                            Ok(frame) => {
+                                let Ok(mut line) = serde_json::to_string(&frame) else { continue };
+                                line.push('\n');
+                                let write = time::timeout(
+                                    io_timeout,
+                                    writer.write_all(line.as_bytes()),
+                                ).await;
+                                match write {
+                                    Ok(Ok(())) => {}
+                                    Ok(Err(e)) => {
+                                        info!("Client {} spectrum write failed: {}", addr, e);
+                                        break;
+                                    }
+                                    Err(_) => {
+                                        info!("Client {} spectrum write timed out", addr);
+                                        break;
+                                    }
+                                }
+                            }
+                            // A client that cannot keep up skips to the newest
+                            // frame; stale spectrum is not worth drawing.
+                            Err(tokio::sync::broadcast::error::RecvError::Lagged(_)) => continue,
+                            Err(tokio::sync::broadcast::error::RecvError::Closed) => break,
+                        }
+                    }
+                    changed = shutdown_rx.changed() => {
+                        match changed {
+                            Ok(()) if *shutdown_rx.borrow() => break,
+                            Ok(()) => {}
+                            Err(_) => break,
+                        }
+                    }
+                }
+            }
+            break;
+        }
+
         // SubscribeMeter: turns this connection into a one-way meter stream.
         // No regular responses are produced; the connection lives until the
         // client disconnects or shutdown fires.
@@ -732,6 +784,7 @@ mod tests {
         let (state_tx, state_rx) = watch::channel(state);
         let _state_tx = state_tx;
         let (meter_tx, _) = tokio::sync::broadcast::channel(8);
+        let (spectrum_tx, _) = tokio::sync::broadcast::channel(4);
         let handle = RigHandle {
             rig_id: "default".to_string(),
             display_name: "Default Rig".to_string(),
@@ -739,6 +792,7 @@ mod tests {
             state_rx,
             audio_port: 4531,
             meter_tx,
+            spectrum_tx,
         };
         let mut map = HashMap::new();
         map.insert("default".to_string(), handle);
@@ -933,6 +987,7 @@ mod tests {
             state_rx: state_rx_a,
             audio_port: 4531,
             meter_tx: meter_tx_a,
+            spectrum_tx: tokio::sync::broadcast::channel(4).0,
         };
 
         let (tx_b, rx_b) = mpsc::channel::<RigRequest>(8);
@@ -945,6 +1000,7 @@ mod tests {
             state_rx: state_rx_b,
             audio_port: 4532,
             meter_tx: meter_tx_b,
+            spectrum_tx: tokio::sync::broadcast::channel(4).0,
         };
 
         let mut map = HashMap::new();
@@ -953,6 +1009,67 @@ mod tests {
         (Arc::new(map), "rig_hf".to_string(), rx_a, rx_b)
     }
 
+    /// Polling spectrum costs a round trip per frame, so the rate a client can
+    /// reach is capped by latency rather than by what it asked for.  Subscribed
+    /// clients get frames pushed instead; this is that path end to end.
+    #[tokio::test]
+    async fn subscribe_spectrum_pushes_frames() {
+        use trx_core::rig::state::SpectrumData;
+
+        let (rigs, default_id) = make_rigs(sample_state());
+        let spectrum_tx = rigs.get("default").expect("rig").spectrum_tx.clone();
+        let ctx = make_ctx(rigs, default_id, HashSet::new());
+        let (mut reader, mut writer, handle, shutdown_tx) = spawn_client_io(ctx);
+
+        writer
+            .write_all(br#"{"cmd":"subscribe_spectrum"}"#)
+            .await
+            .expect("write");
+        writer.write_all(b"\n").await.expect("newline");
+        writer.flush().await.expect("flush");
+
+        // The subscription is registered asynchronously; publish until it takes.
+        let frame = trx_protocol::SpectrumFrame {
+            rig_id: "default".to_string(),
+            spectrum: SpectrumData {
+                bins: vec![-73.4, -20.6, 0.0],
+                center_hz: 14_200_000,
+                sample_rate: 1_920_000,
+                rds: None,
+            },
+            vchan_rds: None,
+        };
+        let mut line = String::new();
+        for _ in 0..50 {
+            let _ = spectrum_tx.send(frame.clone());
+            tokio::select! {
+                read = reader.read_line(&mut line) => {
+                    if read.expect("read") > 0 && !line.trim().is_empty() {
+                        break;
+                    }
+                    line.clear();
+                }
+                _ = tokio::time::sleep(Duration::from_millis(20)) => {}
+            }
+        }
+
+        let received: trx_protocol::SpectrumFrame =
+            serde_json::from_str(line.trim()).unwrap_or_else(|e| panic!("frame {line:?}: {e}"));
+        assert_eq!(received.rig_id, "default");
+        assert_eq!(received.spectrum.center_hz, 14_200_000);
+        // Bins survive the trip quantized to whole dBFS, which is the
+        // resolution the display draws at anyway.
+        assert_eq!(received.spectrum.bins, vec![-73.0, -21.0, 0.0]);
+        // And they travel as base64 rather than a JSON array of floats.
+        assert!(
+            !line.contains("-73"),
+            "bins should not be spelled out on the wire: {line}"
+        );
+
+        let _ = shutdown_tx.send(true);
+        let _ = handle.await;
+    }
+
     #[tokio::test]
     async fn multi_rig_state_isolation() {
         let state_hf = sample_state_custom("HF-Dummy", 14_200_000, trx_core::RigMode::USB);

src/trx-server/src/main.rs

@@ -360,6 +360,7 @@ fn build_sdr_rig_from_instance(rig_cfg: &RigInstanceConfig) -> SdrRigBuildResult
         max_virtual_channels: rig_cfg.sdr.max_virtual_channels,
         nb_enabled: rig_cfg.sdr.noise_blanker.enabled,
         nb_threshold: rig_cfg.sdr.noise_blanker.threshold,
+        spectrum_fft_size: rig_cfg.sdr.spectrum_fft_size,
     })?;
 
     let pcm_rx = sdr_rig.subscribe_pcm();
@@ -460,6 +461,7 @@ fn build_rig_task_config(
         prebuilt_rig: None,
         command_exec_timeout: Duration::from_millis(timeouts.command_exec_timeout_ms),
         poll_refresh_timeout: Duration::from_millis(timeouts.poll_refresh_timeout_ms),
+        spectrum_interval_ms: rig_cfg.sdr.spectrum_interval_ms,
     }
 }
 
@@ -1215,6 +1217,9 @@ async fn main() -> DynResult<()> {
         let (state_tx, state_rx) = watch::channel(initial_state);
         let (meter_tx, _) =
             broadcast::channel::<trx_protocol::MeterUpdate>(rig_handle::METER_BROADCAST_CAPACITY);
+        let (spectrum_tx, _) = broadcast::channel::<trx_protocol::SpectrumFrame>(
+            rig_handle::SPECTRUM_BROADCAST_CAPACITY,
+        );
 
         let mut task_config = build_rig_task_config(
             rig_cfg,
@@ -1242,12 +1247,14 @@ async fn main() -> DynResult<()> {
         let rig_shutdown_rx = shutdown_rx.clone();
         let rig_id_supervisor = rig_cfg.id.clone();
         let meter_tx_task = meter_tx.clone();
+        let spectrum_tx_task = spectrum_tx.clone();
         task_handles.push(tokio::spawn(async move {
             let result = rig_task::run_rig_task(
                 task_config,
                 rig_rx,
                 state_tx.clone(),
                 meter_tx_task,
+                spectrum_tx_task,
                 rig_shutdown_rx,
             )
             .await;
@@ -1294,6 +1301,7 @@ async fn main() -> DynResult<()> {
         rig_handles.insert(
             rig_cfg.id.clone(),
             RigHandle {
+                spectrum_tx: spectrum_tx.clone(),
                 rig_id: rig_cfg.id.clone(),
                 display_name: rig_cfg.display_name().to_string(),
                 rig_tx,

src/trx-server/src/rig_handle.rs

@@ -8,13 +8,18 @@ use tokio::sync::{broadcast, mpsc, watch};
 
 use trx_core::rig::request::RigRequest;
 use trx_core::rig::state::RigState;
-use trx_protocol::MeterUpdate;
+use trx_protocol::{MeterUpdate, SpectrumFrame};
 
 /// Bounded broadcast capacity for the meter stream.  Keeps ~0.5 s of buffered
 /// samples at 30 Hz — more than enough slack to tolerate a scheduling blip
 /// without forcing the producer to block or drop silently.
 pub const METER_BROADCAST_CAPACITY: usize = 16;
 
+/// Bounded broadcast capacity for the spectrum stream.  Frames are large and
+/// only the newest one is worth drawing, so the buffer stays shallow: a slow
+/// client lags and skips rather than making the server hold stale frames.
+pub const SPECTRUM_BROADCAST_CAPACITY: usize = 4;
+
 /// A handle to a single running rig backend.
 ///
 /// One `RigHandle` is created per rig in `main.rs` and stored in the shared
@@ -34,4 +39,8 @@ pub struct RigHandle {
     /// ~6–7 Hz (CAT).  Consumed by `SubscribeMeter` clients; independent of
     /// the slower `state_rx` snapshot path.
     pub meter_tx: broadcast::Sender<MeterUpdate>,
+    /// Per-rig spectrum frames published by `rig_task` while at least one
+    /// client is subscribed.  Consumed by `SubscribeSpectrum` clients; the
+    /// producer skips the work entirely when nobody is listening.
+    pub spectrum_tx: broadcast::Sender<SpectrumFrame>,
 }

src/trx-server/src/rig_task.rs

@@ -23,7 +23,7 @@ use trx_core::rig::request::RigRequest;
 use trx_core::rig::state::{RigMode, RigSnapshot, RigState};
 use trx_core::rig::{RigCat, RigRxStatus, RigTxStatus};
 use trx_core::{DynResult, RigError, RigResult};
-use trx_protocol::MeterUpdate;
+use trx_protocol::{MeterUpdate, SpectrumFrame};
 
 use crate::audio::DecoderHistories;
 use crate::error::is_invalid_bcd_error;
@@ -64,6 +64,8 @@ pub struct RigTaskConfig {
     pub command_exec_timeout: Duration,
     /// Maximum time for a CAT poll refresh cycle.
     pub poll_refresh_timeout: Duration,
+    /// How often to push a spectrum frame to subscribed clients, in ms.
+    pub spectrum_interval_ms: u64,
 }
 
 impl Default for RigTaskConfig {
@@ -94,6 +96,7 @@ impl Default for RigTaskConfig {
             prebuilt_rig: None,
             command_exec_timeout: DEFAULT_COMMAND_EXEC_TIMEOUT,
             poll_refresh_timeout: DEFAULT_POLL_REFRESH_TIMEOUT,
+            spectrum_interval_ms: 50,
         }
     }
 }
@@ -115,6 +118,7 @@ pub async fn run_rig_task(
     mut rx: mpsc::Receiver<RigRequest>,
     state_tx: watch::Sender<RigState>,
     meter_tx: broadcast::Sender<MeterUpdate>,
+    spectrum_tx: broadcast::Sender<SpectrumFrame>,
     mut shutdown_rx: watch::Receiver<bool>,
 ) -> DynResult<()> {
     let histories = config.histories.clone();
@@ -273,6 +277,13 @@ pub async fn run_rig_task(
     } else {
         Duration::from_millis(150)
     };
+    // Spectrum frames get their own tick so a subscribed client never pays a
+    // round trip per frame.  The FFT is computed by the SDR thread either way;
+    // this only reads the latest result, and only while somebody is subscribed.
+    let spectrum_tick_duration = Duration::from_millis(config.spectrum_interval_ms.max(1));
+    let mut spectrum_tick: std::pin::Pin<Box<tokio::time::Sleep>> =
+        Box::pin(tokio::time::sleep(spectrum_tick_duration));
+
     let meter_task_start = Instant::now();
     let meter_state_delta_db: f64 = 0.25;
     let rig_id = config.rig_id.clone();
@@ -302,6 +313,21 @@ pub async fn run_rig_task(
                     Err(_) => break,
                 }
             }
+            // Push the latest spectrum frame to subscribed clients.
+            _ = &mut spectrum_tick => {
+                spectrum_tick = Box::pin(tokio::time::sleep(spectrum_tick_duration));
+                // `send` fails only when nobody is listening, but building the
+                // frame clones a few KB of bins, so check before doing the work.
+                if spectrum_tx.receiver_count() > 0 {
+                    if let Some(spectrum) = rig.as_sdr_ref().and_then(|s| s.get_spectrum()) {
+                        let _ = spectrum_tx.send(SpectrumFrame {
+                            rig_id: rig_id.clone(),
+                            spectrum,
+                            vchan_rds: rig.as_sdr_ref().and_then(|s| s.get_vchan_rds()),
+                        });
+                    }
+                }
+            }
             // Fast meter-only refresh between full polls.
             _ = &mut meter_tick => {
                 meter_tick = Box::pin(tokio::time::sleep(meter_tick_duration));

src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp.rs

@@ -154,6 +154,7 @@ impl SdrPipeline {
         squelch_cfg: VirtualSquelchConfig,
         nb_cfg: NoiseBlankerConfig,
         channels: &[(f64, RigMode, u32)],
+        spectrum_fft_size: usize,
     ) -> Self {
         const IQ_BROADCAST_CAPACITY: usize = 64;
         let (iq_tx, _iq_rx) = broadcast::channel::<Vec<Complex<f32>>>(IQ_BROADCAST_CAPACITY);
@@ -219,6 +220,7 @@ impl SdrPipeline {
             .name("sdr-iq-read".to_string())
             .spawn(move || {
                 iq_read_loop(
+                    spectrum_fft_size,
                     source,
                     sdr_sample_rate,
                     thread_dsps,
@@ -317,6 +319,7 @@ pub const IQ_BLOCK_SIZE: usize = 4096;
 
 #[allow(clippy::too_many_arguments)]
 fn iq_read_loop(
+    spectrum_fft_size: usize,
     mut source: Box<dyn IqSource>,
     sdr_sample_rate: u32,
     channel_dsps: Arc<RwLock<Vec<Arc<Mutex<ChannelDsp>>>>>,
@@ -335,7 +338,7 @@ fn iq_read_loop(
     };
     let throttle = !source.is_blocking();
 
-    let mut spectrum = SpectrumSnapshotter::new();
+    let mut spectrum = SpectrumSnapshotter::new(spectrum_fft_size);
     let mut read_error_streak: u32 = 0;
     let mut zero_read_streak: u32 = 0;
     let mut overflow_log_window_start: Option<Instant> = None;
@@ -572,6 +575,7 @@ mod tests {
             VirtualSquelchConfig::default(),
             NoiseBlankerConfig::default(),
             &[(200_000.0, RigMode::USB, 3000)],
+            1024,
         );
         assert_eq!(pipeline.pcm_senders.len(), 1);
         assert_eq!(pipeline.channel_dsps.read().unwrap().len(), 1);
@@ -590,6 +594,7 @@ mod tests {
             VirtualSquelchConfig::default(),
             NoiseBlankerConfig::default(),
             &[],
+            1024,
         );
         assert_eq!(pipeline.pcm_senders.len(), 0);
         assert_eq!(pipeline.channel_dsps.read().unwrap().len(), 0);

src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp/spectrum.rs

@@ -9,28 +9,36 @@ use num_complex::Complex;
 use rustfft::num_complex::Complex as FftComplex;
 use rustfft::FftPlanner;
 
-/// Number of FFT bins for the spectrum display.
-pub(super) const SPECTRUM_FFT_SIZE: usize = 1024;
+/// Default number of FFT bins for the spectrum display, used when the config
+/// does not say otherwise.
+pub(super) const DEFAULT_SPECTRUM_FFT_SIZE: usize = 1024;
 
 /// Update the spectrum buffer every this many IQ blocks (~10 Hz at 1.92 MHz / 4096 block).
 pub(super) const SPECTRUM_UPDATE_BLOCKS: usize = 4;
 
 pub(super) struct SpectrumSnapshotter {
+    fft_size: usize,
     hann_window: Vec<f32>,
     fft: std::sync::Arc<dyn rustfft::Fft<f32>>,
     counter: usize,
 }
 
 impl SpectrumSnapshotter {
-    pub(super) fn new() -> Self {
-        let hann_window: Vec<f32> = (0..SPECTRUM_FFT_SIZE)
-            .map(|i| 0.5 * (1.0 - (2.0 * PI * i as f32 / (SPECTRUM_FFT_SIZE - 1) as f32).cos()))
+    pub(super) fn new(fft_size: usize) -> Self {
+        let fft_size = if fft_size >= 2 {
+            fft_size
+        } else {
+            DEFAULT_SPECTRUM_FFT_SIZE
+        };
+        let hann_window: Vec<f32> = (0..fft_size)
+            .map(|i| 0.5 * (1.0 - (2.0 * PI * i as f32 / (fft_size - 1) as f32).cos()))
             .collect();
 
         let mut planner = FftPlanner::<f32>::new();
-        let fft = planner.plan_fft_forward(SPECTRUM_FFT_SIZE);
+        let fft = planner.plan_fft_forward(fft_size);
 
         Self {
+            fft_size,
             hann_window,
             fft,
             counter: 0,
@@ -48,7 +56,7 @@ impl SpectrumSnapshotter {
         }
         self.counter = 0;
 
-        let take = samples.len().min(SPECTRUM_FFT_SIZE);
+        let take = samples.len().min(self.fft_size);
         let mut buf: Vec<FftComplex<f32>> = samples[..take]
             .iter()
             .enumerate()
@@ -59,16 +67,15 @@ impl SpectrumSnapshotter {
                 )
             })
             .collect();
-        buf.resize(SPECTRUM_FFT_SIZE, FftComplex::new(0.0, 0.0));
+        buf.resize(self.fft_size, FftComplex::new(0.0, 0.0));
         self.fft.process(&mut buf);
 
-        let half = SPECTRUM_FFT_SIZE / 2;
+        let half = self.fft_size / 2;
         let bins: Vec<f32> = buf[half..]
             .iter()
             .chain(buf[..half].iter())
             .map(|value| {
-                let mag =
-                    (value.re * value.re + value.im * value.im).sqrt() / SPECTRUM_FFT_SIZE as f32;
+                let mag = (value.re * value.re + value.im * value.im).sqrt() / self.fft_size as f32;
                 20.0 * mag.max(1e-10_f32).log10()
             })
             .collect();

src/trx-server/trx-backend/trx-backend-soapysdr/src/lib.rs

@@ -73,6 +73,11 @@ pub struct SoapySdrConfig {
     pub nb_enabled: bool,
     /// Noise blanker impulse threshold multiplier.
     pub nb_threshold: f64,
+    /// FFT bin count for the spectrum display; a power of two.
+    ///
+    /// Fewer bins cost less DSP and put fewer bytes on the network per frame,
+    /// which is what a slow server↔client link cares about.
+    pub spectrum_fft_size: usize,
 }
 
 impl Default for SoapySdrConfig {
@@ -99,6 +104,7 @@ impl Default for SoapySdrConfig {
             max_virtual_channels: 4,
             nb_enabled: false,
             nb_threshold: 10.0,
+            spectrum_fft_size: 1024,
         }
     }
 }
@@ -194,6 +200,7 @@ impl SoapySdrRig {
         let max_virtual_channels = config.max_virtual_channels;
         let nb_enabled = config.nb_enabled;
         let nb_threshold = config.nb_threshold;
+        let spectrum_fft_size = config.spectrum_fft_size;
         tracing::info!(
             "initialising SoapySDR backend (args={:?}, gain_mode={:?}, gain_db={}, max_gain_db={:?})",
             args,
@@ -293,6 +300,7 @@ impl SoapySdrRig {
                 threshold: nb_threshold as f32,
             },
             &all_channels,
+            spectrum_fft_size,
         ));
 
         let info = RigInfo {
@@ -418,6 +426,7 @@ impl SoapySdrRig {
         nb_threshold: f64,
     ) -> DynResult<Self> {
         Self::new_from_config(SoapySdrConfig {
+            spectrum_fft_size: 1024,
             args: args.to_string(),
             channels: channels.to_vec(),
             gain_mode: gain_mode.to_string(),

src/trx-server/trx-backend/trx-backend-soapysdr/src/vchan_impl.rs

@@ -439,6 +439,7 @@ mod tests {
             VirtualSquelchConfig::default(),
             NoiseBlankerConfig::default(),
             &[(0.0, RigMode::USB, 3_000)],
+            1024,
         ))
     }
 

trx-rs.toml.example

@@ -120,6 +120,8 @@ wfm_deemphasis_us = 50
 center_offset_hz = 100000
 channels = []
 max_virtual_channels = 4
+spectrum_fft_size = 1024
+spectrum_interval_ms = 50
 
 # "auto" for hardware AGC, or "manual".
 [trx-server.sdr.gain]
@@ -160,6 +162,7 @@ log_level = "info"
 # Legacy single-remote form; prefer [[remotes]] below.
 [trx-client.remote]
 poll_interval_ms = 750
+spectrum_interval_ms = 50
 
 [trx-client.remote.auth]
 
@@ -168,6 +171,7 @@ name = "home-hf"
 url = "192.168.1.100:4530"
 rig_id = "hf"
 poll_interval_ms = 750
+spectrum_interval_ms = 50
 
 [trx-client.remotes.auth]
 token = "my-token"
@@ -177,6 +181,7 @@ name = "home-vhf"
 url = "192.168.1.100:4530"
 rig_id = "vhf"
 poll_interval_ms = 750
+spectrum_interval_ms = 50
 
 [trx-client.remotes.auth]
 token = "my-token"