[fix](trx-frontend-http): measure auto squelch from the meter

Auto took the spectrum's noise floor and added 6 dB, but the threshold
is compared against the channel level the meter reports, and the two sit
a long way apart: the gap is set by the FFT size and window, the channel
bandwidth, the decimation, and peak-versus-mean statistics.  Measured on
white noise it runs +22.1 dB at 48k/8k/3k, +18.7 dB at 240k/24k/12k and
-1.2 dB at 1.92M/24k/12k — a 23 dB swing across ordinary configurations.
Only the last of those is anywhere near right, so on a narrow span Auto
set the gate some 20 dB below the noise and it never closed.

It now reads the same number the DSP compares: the 20th percentile of
the meter over the last ten seconds, plus 5 dB.  The percentile keeps a
burst of traffic inside the window from dragging the estimate up, and
5 dB clears the meter's own jitter, which measured 0.9-1.6 dB.  Nothing
in it converts between scales, so no part of the signal chain can put it
out again.  With no history yet — a fresh connection, a rig switch — it
listens for a moment rather than refusing.

The fixture gained a streaming /meter, without which there is nothing to
measure, and the spectrum test pins auto to the meter it serves.

Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/frontend/src/app.ts

@@ -3647,6 +3647,7 @@ function render(update: AppUpdate) {
       const sUnits = dbmToSUnits(update.status.rx.sig);
       sigLastSUnits = sUnits;
       sigLastDbm = update.status.rx.sig;
+      recordSquelchMeterSample(update.status.rx.sig);
       const pct = sUnits <= 9 ? Math.max(0, Math.min(100, (sUnits / 9) * 100)) : 100;
       signalBar.style.width = `${pct}%`;
       signalValue.innerHTML = formatSignal(sUnits);
@@ -5343,13 +5344,45 @@ function setSdrSquelch(thresholdDb: number, enabled: boolean, options: { submit?
   if (options.submit !== false) submitSdrSquelch();
 }
 
-/** Just above the noise, which is the level the gate has to clear. */
+// Auto reads the meter, not the spectrum. The threshold is compared against
+// the channel level the meter reports, and that sits a long way from the
+// spectrum's per-bin noise floor — the gap is set by the FFT size and window,
+// the channel bandwidth, the decimation and peak-versus-mean statistics.
+// Measured across ordinary configurations it ranges from -1 dB to +22 dB, so
+// the old "noise floor + 6 dB" left the gate 16-22 dB below the noise on a
+// narrow span and it simply never closed. Reading the same number the DSP
+// compares needs no conversion at all.
+const SQUELCH_NOISE_WINDOW_MS = 10_000;
+const SQUELCH_NOISE_MARGIN_DB = 5;
+const SQUELCH_MEASURE_MS = 1_500;
+const squelchMeterSamples: SignalSample[] = [];
+
+function recordSquelchMeterSample(db: number) {
+  if (!isFiniteNumber(db)) return;
+  const now = Date.now();
+  squelchMeterSamples.push({ t: now, v: db });
+  while (squelchMeterSamples[0] && now - squelchMeterSamples[0].t > SQUELCH_NOISE_WINDOW_MS) {
+    squelchMeterSamples.shift();
+  }
+}
+
+/** The level the channel rests at, taken low enough down the distribution that
+ *  a burst of traffic inside the window cannot drag it up. */
+function squelchNoiseFloorDb(): number | null {
+  const now = Date.now();
+  const values = squelchMeterSamples
+    .filter((sample) => now - sample.t <= SQUELCH_NOISE_WINDOW_MS)
+    .map((sample) => sample.v)
+    .sort((a, b) => a - b);
+  if (values.length < 4) return null;
+  return values[Math.floor((values.length - 1) * 0.2)] ?? null;
+}
+
+/** Just clear of the noise the meter is actually reading. */
 function autoSquelchThresholdDb(): number | null {
-  const data = lastSpectrumData || window.lastSpectrumData;
-  if (!data || !isBinsArray(data.bins) || data.bins.length === 0) return null;
-  const noiseDb = estimateNoiseFloorDb(data.bins);
-  if (noiseDb == null || !isFiniteNumber(noiseDb)) return null;
-  return clampSdrSquelchDb(noiseDb + 6);
+  const noiseDb = squelchNoiseFloorDb();
+  if (noiseDb == null) return null;
+  return clampSdrSquelchDb(noiseDb + SQUELCH_NOISE_MARGIN_DB);
 }
 
 function updateSdrSquelchControlVisibility() {
@@ -5388,17 +5421,33 @@ if (sdrSquelchToggleBtn) {
   });
 }
 
+function applyAutoSquelch(threshold: number) {
+  setSdrSquelch(threshold, true);
+  showHint(`Squelch ${threshold} dB`, 1500);
+}
+
 const sdrSquelchAutoBtn = document.getElementById("sdr-squelch-auto") as HTMLButtonElement | null;
 if (sdrSquelchAutoBtn) {
   sdrSquelchAutoBtn.addEventListener("click", () => {
     if (!sdrSquelchSupported) return;
     const threshold = autoSquelchThresholdDb();
-    if (threshold == null) {
-      showHint("No spectrum to measure the noise from", 1800);
+    if (threshold != null) {
+      applyAutoSquelch(threshold);
       return;
     }
-    setSdrSquelch(threshold, true);
-    showHint(`Squelch ${threshold} dB`, 1500);
+    // Nothing recorded yet — right after a connection or a rig switch. Listen
+    // for a moment rather than refusing, which is what a radio does.
+    sdrSquelchAutoBtn.disabled = true;
+    showHint("Measuring the noise…");
+    setTimeout(() => {
+      sdrSquelchAutoBtn.disabled = false;
+      const measured = autoSquelchThresholdDb();
+      if (measured == null) {
+        showHint("No meter to measure the noise from", 1800);
+        return;
+      }
+      applyAutoSquelch(measured);
+    }, SQUELCH_MEASURE_MS);
   });
 }
 
@@ -7147,6 +7196,7 @@ function flushMeterDom() {
   const sUnits = dbmToSUnits(dbm);
   sigLastSUnits = sUnits;
   sigLastDbm = dbm;
+  recordSquelchMeterSample(dbm);
   const pct = sUnits <= 9 ? Math.max(0, Math.min(100, (sUnits / 9) * 100)) : 100;
   if (signalBar) signalBar.style.width = `${pct}%`;
   if (signalValue) signalValue.innerHTML = formatSignal(sUnits);

src/trx-client/trx-frontend/trx-frontend-http/frontend/tests/spectrum-layout.mjs

@@ -35,11 +35,16 @@ const BANDPLAN = {
 const BAND_WITH_CONTENT = 7074000;
 const BAND_WITHOUT_CONTENT = 14074000;
 
+// The meter sits well away from the spectrum's noise floor, so a squelch that
+// took its level from the plot would land somewhere else entirely.
+const METER_DB = -70;
+
 const fixture = await startWebFixture({
   spectrum: true,
   bookmarks: BOOKMARKS,
   bandplan: BANDPLAN,
   bandplanEnabled: true,
+  meterDb: METER_DB,
 });
 const { browser, page, runtimeErrors } = await startBrowser(chromium);
 
@@ -125,10 +130,14 @@ try {
   // row, with nothing on screen to relate the number to.
   await page.locator("summary", { hasText: "Audio controls" }).click();
   await page.locator("#sdr-squelch-toggle").click();
-  // Auto parks it just above the noise, which is mid-axis and leaves room to
-  // drag in either direction.
+  // Auto parks it just above the noise the meter reports — not above the
+  // spectrum's noise floor, which sits anywhere from 1 dB below to 22 dB above
+  // the meter depending on span, bandwidth and decimation.
   await page.locator("#sdr-squelch-auto").click();
-  await page.waitForTimeout(300);
+  await page.waitForTimeout(400);
+  const auto = await page.evaluate(() => Number(document.getElementById("sdr-squelch-db").value));
+  assert.ok(Math.abs(auto - (METER_DB + 5)) <= 1,
+    `auto put the threshold at ${auto} dB with the meter at ${METER_DB} dB`);
   const squelchOn = await page.evaluate(() => {
     const line = document.getElementById("spectrum-squelch-line");
     return {

src/trx-client/trx-frontend/trx-frontend-http/frontend/tests/web-fixture.mjs

@@ -64,6 +64,7 @@ function assetPath(urlPath) {
 export async function startWebFixture({
   spectrum = false,
   tx = false,
+  meterDb = -70,
   bookmarks = [],
   bandplan = {},
   bandplanEnabled = false,
@@ -111,7 +112,7 @@ export async function startWebFixture({
         signal_meter: spectrum,
       },
     },
-    status: { freq: { hz: 100_000_000 }, mode: "FM", tx_en: false, vfo: null, tx: null, rx: { sig: -70 }, lock: null },
+    status: { freq: { hz: 100_000_000 }, mode: "FM", tx_en: false, vfo: null, tx: null, rx: { sig: meterDb }, lock: null },
     // Reported only by SDR backends, and what makes the client show the
     // squelch control at all.
     filter: spectrum
@@ -213,6 +214,20 @@ export async function startWebFixture({
       request.on("close", () => clearInterval(timer));
       return;
     }
+    // The meter streams like the server's does: the squelch reads its noise
+    // level from here, so a static snapshot would leave it nothing to measure.
+    if (url.pathname === "/meter") {
+      response.writeHead(200, {
+        "cache-control": "no-cache",
+        connection: "keep-alive",
+        "content-type": "text/event-stream",
+      });
+      const timer = setInterval(() => {
+        response.write(`data: ${JSON.stringify({ sig: meterDb })}\n\n`);
+      }, 120);
+      request.on("close", () => clearInterval(timer));
+      return;
+    }
     if (["/events", "/decode", "/spectrum", "/meter"].includes(url.pathname)) {
       response.writeHead(200, {
         "cache-control": "no-cache",