Fix DMR audio stream failing with "no supported source found"

Digital voice is intermittent — dsd-fme only outputs PCM during active
voice transmissions. Without input, ffmpeg never wrote the WAV header
and the browser got an empty response. Add an audio bridge thread that
feeds 100ms silence chunks during voice gaps so ffmpeg always has input
and the browser receives a continuous WAV stream. Add auto-reconnect
on the frontend if the audio stream drops while the decoder is running.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/dmr.py

@@ -205,6 +205,45 @@ def parse_dsd_output(line: str) -> dict | None:
 
 _HEARTBEAT_INTERVAL = 3.0  # seconds between heartbeats when decoder is idle
 
+# 100ms of silence at 8kHz 16-bit mono = 1600 bytes
+_SILENCE_CHUNK = b'\x00' * 1600
+
+
+def _audio_bridge(dsd_stdout, ffmpeg_stdin):
+    """Bridge DSD audio output to ffmpeg, inserting silence during gaps.
+
+    Digital voice is intermittent — the decoder only outputs PCM during
+    active voice transmissions.  Without this bridge, ffmpeg would block
+    waiting for its first input bytes and never write the WAV header,
+    causing the browser ``<audio>`` element to fail with
+    "no supported source found".
+
+    The bridge feeds 100ms silence chunks during gaps so ffmpeg always
+    has input, the WAV header is written immediately, and the browser
+    receives a continuous audio stream (silence when idle, decoded voice
+    when active).
+    """
+    try:
+        while dmr_audio_running:
+            ready, _, _ = select.select([dsd_stdout], [], [], 0.1)
+            if ready:
+                data = os.read(dsd_stdout.fileno(), 4096)
+                if not data:
+                    break
+                ffmpeg_stdin.write(data)
+                ffmpeg_stdin.flush()
+            else:
+                # No audio from decoder — feed silence to keep stream alive
+                ffmpeg_stdin.write(_SILENCE_CHUNK)
+                ffmpeg_stdin.flush()
+    except (BrokenPipeError, OSError, ValueError):
+        pass
+    finally:
+        try:
+            ffmpeg_stdin.close()
+        except Exception:
+            pass
+
 
 def _queue_put(event: dict):
     """Put an event on the DMR queue, dropping oldest if full."""
@@ -443,7 +482,10 @@ def start_dmr() -> Response:
         # Allow rtl_fm to send directly to dsd
         dmr_rtl_process.stdout.close()
 
-        # Start ffmpeg to transcode DSD 8kHz s16le PCM → 44.1kHz WAV
+        # Start ffmpeg to transcode DSD 8kHz s16le PCM → 44.1kHz WAV.
+        # A bridge thread reads DSD stdout and writes to ffmpeg stdin,
+        # inserting silence during voice gaps so ffmpeg always has input
+        # and the browser receives a continuous WAV stream.
         if ffmpeg_path and dmr_dsd_process.stdout:
             encoder_cmd = [
                 ffmpeg_path, '-hide_banner', '-loglevel', 'error',
@@ -454,14 +496,18 @@ def start_dmr() -> Response:
             ]
             dmr_audio_process = subprocess.Popen(
                 encoder_cmd,
-                stdin=dmr_dsd_process.stdout,
+                stdin=subprocess.PIPE,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.PIPE,
             )
             register_process(dmr_audio_process)
-            # Allow DSD to pipe directly to ffmpeg
-            dmr_dsd_process.stdout.close()
             dmr_audio_running = True
+            # Bridge: reads DSD audio, feeds ffmpeg with silence fill
+            threading.Thread(
+                target=_audio_bridge,
+                args=(dmr_dsd_process.stdout, dmr_audio_process.stdin),
+                daemon=True,
+            ).start()
             # Drain ffmpeg stderr to prevent blocking
             threading.Thread(
                 target=lambda p: [None for _ in p.stderr],
@@ -605,10 +651,10 @@ def stream_dmr_audio() -> Response:
         if not proc or not proc.stdout:
             return
         try:
-            # Digital voice is intermittent — allow longer first-chunk
-            # timeout since the decoder only produces audio when there
-            # is an active voice transmission on the channel.
-            first_chunk_deadline = time.time() + 5.0
+            # The audio bridge thread feeds silence during voice gaps,
+            # so ffmpeg always produces output and the first chunk
+            # arrives quickly.  We still use select() to avoid blocking
+            # forever if the process dies unexpectedly.
             while dmr_audio_running and proc.poll() is None:
                 ready, _, _ = select.select([proc.stdout], [], [], 2.0)
                 if ready:
@@ -618,9 +664,6 @@ def stream_dmr_audio() -> Response:
                     else:
                         break
                 else:
-                    if time.time() > first_chunk_deadline:
-                        logger.warning("DMR audio stream timed out waiting for first chunk")
-                        break
                     if proc.poll() is not None:
                         break
         except GeneratorExit:

static/js/modes/dmr.js

@@ -557,7 +557,18 @@ function startDmrAudio() {
     if (volSlider) audioPlayer.volume = volSlider.value / 100;
 
     audioPlayer.onplaying = () => updateDmrAudioStatus('STREAMING');
-    audioPlayer.onerror = () => updateDmrAudioStatus('ERROR');
+    audioPlayer.onerror = () => {
+        // Retry if decoder is still running (stream may have dropped)
+        if (isDmrRunning && dmrHasAudio) {
+            console.warn('[DMR] Audio stream error, retrying in 2s...');
+            updateDmrAudioStatus('RECONNECTING');
+            setTimeout(() => {
+                if (isDmrRunning && dmrHasAudio) startDmrAudio();
+            }, 2000);
+        } else {
+            updateDmrAudioStatus('OFF');
+        }
+    };
 
     audioPlayer.play().catch(e => {
         console.warn('[DMR] Audio autoplay blocked:', e);