[fix](trx-rs): GQRX-style S-meter ballistics across DSP and frontend

DSP: 400 ms attack / 1.0 s decay IIR on IQ power (block-rate corrected).
JS:  asymmetric EMA (α=0.08 attack, α=0.03 decay) with rAF coalescing.
CSS: bar transition 150 ms → 300 ms ease-out.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -7006,9 +7006,18 @@ function stopSpectrumStreaming() {
 
 // ── /meter (fast signal-strength) streaming ─────────────────────────────────
 // Dedicated SSE channel pushed at ~30 Hz by trx-server; bypasses /events so
-// meter frames are never gated by full-RigState diffing. Synchronous DOM
+// meter frames are never gated by full-RigState diffing.
-// write per frame — no rAF coalescing, per user requirement that it "feel
+//
-// instant" on the frontend.
+// Client-side asymmetric EMA smoothing (GQRX-style ballistics):
+//   attack τ ≈ 400 ms  — rises in ~12 frames at 30 Hz
+//   decay  τ ≈ 1.0 s   — falls in ~30 frames, readable
+// DOM updates are coalesced via requestAnimationFrame so the bar
+// animates at display refresh rate, not SSE rate.
+const METER_ATTACK_ALPHA = 0.08; // per-frame at ~30 Hz ≈ 400 ms τ
+const METER_DECAY_ALPHA  = 0.03; // per-frame at ~30 Hz ≈ 1.0 s τ
+let meterSmoothedDbm = null;
+let meterRafPending = false;
+
 function scheduleMeterReconnect() {
   if (meterReconnectTimer !== null) return;
   meterReconnectTimer = setTimeout(() => {
@@ -7019,6 +7028,24 @@ function scheduleMeterReconnect() {
 
 function applyMeterSample(dbm) {
   if (typeof dbm !== "number" || !Number.isFinite(dbm)) return;
+  // Asymmetric EMA: fast attack, slow decay.
+  if (meterSmoothedDbm === null) {
+    meterSmoothedDbm = dbm;
+  } else {
+    const alpha = dbm > meterSmoothedDbm ? METER_ATTACK_ALPHA : METER_DECAY_ALPHA;
+    meterSmoothedDbm += alpha * (dbm - meterSmoothedDbm);
+  }
+  // Coalesce DOM writes to display refresh rate.
+  if (!meterRafPending) {
+    meterRafPending = true;
+    requestAnimationFrame(flushMeterDom);
+  }
+}
+
+function flushMeterDom() {
+  meterRafPending = false;
+  const dbm = meterSmoothedDbm;
+  if (dbm === null) return;
   prevRenderData.sigDbm = dbm;
   const sUnits = dbmToSUnits(dbm);
   sigLastSUnits = sUnits;
@@ -7059,6 +7086,7 @@ function stopMeterStreaming() {
     clearTimeout(meterReconnectTimer);
     meterReconnectTimer = null;
   }
+  meterSmoothedDbm = null; // reset so next rig starts fresh
 }
 
 // ── Rendering ────────────────────────────────────────────────────────────────

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -1356,7 +1356,7 @@ small { color: var(--text-muted); }
 .band-tag { display: inline-block; padding: 2px 6px; border-radius: 6px; background: var(--btn-bg); color: var(--text); font-size: 0.82rem; border: 1px solid var(--border-light); margin-left: 6px; }
 .signal { display: flex; gap: 0.6rem; align-items: center; }
 .signal-bar { flex: 1 1 auto; height: 12px; border-radius: 999px; background: var(--btn-bg); border: 1px solid var(--border-light); overflow: hidden; }
-.signal-bar-fill { height: 100%; width: 0%; background: linear-gradient(90deg, var(--accent-green), var(--accent-yellow), var(--accent-red)); transition: width 150ms ease; }
+.signal-bar-fill { height: 100%; width: 0%; background: linear-gradient(90deg, var(--accent-green), var(--accent-yellow), var(--accent-red)); transition: width 300ms ease-out; }
 .signal-value { font-size: 0.95rem; color: var(--text-heading); min-width: 48px; text-align: right; }
 .meter { display: flex; gap: 0.6rem; align-items: center; }
 .meter-bar { flex: 1 1 auto; height: 12px; border-radius: 999px; background: var(--btn-bg); border: 1px solid var(--border-light); overflow: hidden; }

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

@@ -308,18 +308,22 @@ pub struct ChannelDsp {
 impl ChannelDsp {
     /// Compute asymmetric IIR coefficients for S-meter envelope tracking.
     ///
-    /// Attack: ~200 ms time constant (~6 frames at 30 Hz refresh).
+    /// Attack: ~400 ms — rises over ~12 frames at 30 Hz.
-    /// Decay:  ~600 ms time constant (~18 frames — smooth fallback).
+    /// Decay:  ~1.0 s  — falls over ~30 frames, readable.
     ///
-    /// Note: these alphas are applied once per decimated *block*, not per
+    /// Modelled after GQRX meter ballistics.  Deliberately slower than
+    /// the IARU analog-meter spec because a digital bar at 30 fps is
+    /// visually noisier than a physical needle with mechanical inertia.
+    ///
+    /// Note: alphas are applied once per decimated *block*, not per
     /// sample, with block-rate correction (`1 − (1−α)^N`).
     fn smeter_alphas(channel_sample_rate: u32) -> (f32, f32) {
         if channel_sample_rate == 0 {
             return (0.3, 0.01);
         }
         let sr = channel_sample_rate as f32;
-        let attack = (1.0 - (-1.0 / (sr * 0.200)).exp()).min(1.0); // τ = 200 ms
+        let attack = (1.0 - (-1.0 / (sr * 0.400)).exp()).min(1.0); // τ = 400 ms
-        let decay = (1.0 - (-1.0 / (sr * 0.600)).exp()).min(1.0); // τ = 600 ms
+        let decay = (1.0 - (-1.0 / (sr * 1.000)).exp()).min(1.0); // τ = 1.0 s
         (attack, decay)
     }