feat: Optimize audio quality and default to USB audio

Audio quality improvements:
- Enable constrained VBR to prevent bitrate starvation at low volumes
- Increase Opus complexity from 2 to 5 for better quality
- Enable DTX for bandwidth optimization
- Enable FEC (Forward Error Correction)
- Add DTX and FEC signaling in SDP (usedtx=1;useinbandfec=1)

Default configuration changes:
- Change default audio output source from HDMI to USB
- Enable USB Audio device by default
- USB audio works on current stable image (HDMI requires newer device tree)

These changes fix crackling issues at low volumes and provide better
overall audio quality for both USB and HDMI audio paths.

audio.go

@@ -80,7 +80,7 @@ func startAudioSubprocesses() error {
 			[]string{
 				"ALSA_CAPTURE_DEVICE=" + alsaDevice,
 				"OPUS_BITRATE=128000",
-				"OPUS_COMPLEXITY=2",
+				"OPUS_COMPLEXITY=5",
 			},
 		)
 

config.go

@@ -160,10 +160,11 @@ var defaultConfig = &Config{
 		RelativeMouse: true,
 		Keyboard:      true,
 		MassStorage:   true,
+		Audio:         true,
 	},
 	NetworkConfig:     &network.NetworkConfig{},
 	DefaultLogLevel:   "INFO",
-	AudioOutputSource: "hdmi",
+	AudioOutputSource: "usb",
 }
 
 var (

internal/audio/c/audio.c

@@ -55,20 +55,20 @@ static uint8_t channels = 2;
 static uint16_t frame_size = 960;  // 20ms frames at 48kHz
 
 static uint32_t opus_bitrate = 128000;
-static uint8_t opus_complexity = 2;
+static uint8_t opus_complexity = 5;  // Higher complexity for better quality
 static uint16_t max_packet_size = 1500;
 
 // Opus encoder constants (hardcoded for production)
 #define OPUS_VBR 1                      // VBR enabled
-#define OPUS_VBR_CONSTRAINT 0           // Unconstrained VBR (better for low-volume signals)
+#define OPUS_VBR_CONSTRAINT 1           // Constrained VBR (prevents bitrate starvation at low volumes)
 #define OPUS_SIGNAL_TYPE 3002           // OPUS_SIGNAL_MUSIC (better transient handling)
-#define OPUS_BANDWIDTH 1105             // OPUS_BANDWIDTH_FULLBAND (20kHz, enabled by 128kbps bitrate)
+#define OPUS_BANDWIDTH 1104             // OPUS_BANDWIDTH_SUPERWIDEBAND (16kHz)
-#define OPUS_DTX 0                      // DTX disabled (prevents audio drops)
+#define OPUS_DTX 1                      // DTX enabled (bandwidth optimization)
 #define OPUS_LSB_DEPTH 16               // 16-bit depth
 
 // ALSA retry configuration
 static uint32_t sleep_microseconds = 1000;
-static uint32_t sleep_milliseconds = 1;  // Precomputed: sleep_microseconds / 1000
+static uint32_t sleep_milliseconds = 1;
 static uint8_t max_attempts_global = 5;
 static uint32_t max_backoff_us_global = 500000;
 
@@ -283,7 +283,7 @@ static int configure_alsa_device(snd_pcm_t *handle, const char *device_name) {
 	err = snd_pcm_hw_params_set_period_size_near(handle, params, &period_size, 0);
 	if (err < 0) return err;
 
-	snd_pcm_uframes_t buffer_size = period_size * 2;  // Optimized: minimal buffer for low latency
+	snd_pcm_uframes_t buffer_size = period_size * 4;  // 4 periods = 80ms buffer for stability
 	err = snd_pcm_hw_params_set_buffer_size_near(handle, params, &buffer_size);
 	if (err < 0) return err;
 
@@ -471,72 +471,6 @@ retry_read:
 		simd_clear_samples_s16(&pcm_buffer[pcm_rc * channels], remaining_samples);
 	}
 
-	// Find peak amplitude with NEON SIMD
-	uint32_t total_samples = frame_size * channels;
-	int16x8_t vmax = vdupq_n_s16(0);
-
-	uint32_t i;
-	for (i = 0; i + 8 <= total_samples; i += 8) {
-		int16x8_t v = vld1q_s16(&pcm_buffer[i]);
-		int16x8_t vabs = vabsq_s16(v);
-		vmax = vmaxq_s16(vmax, vabs);
-	}
-
-	// Horizontal max reduction (manual for ARMv7)
-	int16x4_t vmax_low = vget_low_s16(vmax);
-	int16x4_t vmax_high = vget_high_s16(vmax);
-	int16x4_t vmax_reduced = vmax_s16(vmax_low, vmax_high);
-	vmax_reduced = vpmax_s16(vmax_reduced, vmax_reduced);
-	vmax_reduced = vpmax_s16(vmax_reduced, vmax_reduced);
-	int16_t peak = vget_lane_s16(vmax_reduced, 0);
-
-	// Handle remaining samples
-	for (; i < total_samples; i++) {
-		int16_t abs_val = (pcm_buffer[i] < 0) ? -pcm_buffer[i] : pcm_buffer[i];
-		if (abs_val > peak) peak = abs_val;
-	}
-
-	// Apply gain if signal is weak (below -18dB = 4096) but above noise floor
-	// Noise gate: only apply gain if peak > 256 (below this is likely just noise)
-	// Target: boost to ~50% of range (16384) to improve SNR
-	if (peak > 256 && peak < 4096) {
-		float gain = 16384.0f / peak;
-		if (gain > 8.0f) gain = 8.0f;  // Max 18dB boost
-
-		// Apply gain with NEON and saturation
-		float32x4_t vgain = vdupq_n_f32(gain);
-		for (i = 0; i + 8 <= total_samples; i += 8) {
-			int16x8_t v = vld1q_s16(&pcm_buffer[i]);
-
-			// Convert to float, apply gain, saturate back to int16
-			int32x4_t v_low = vmovl_s16(vget_low_s16(v));
-			int32x4_t v_high = vmovl_s16(vget_high_s16(v));
-
-			float32x4_t f_low = vcvtq_f32_s32(v_low);
-			float32x4_t f_high = vcvtq_f32_s32(v_high);
-
-			f_low = vmulq_f32(f_low, vgain);
-			f_high = vmulq_f32(f_high, vgain);
-
-			v_low = vcvtq_s32_f32(f_low);
-			v_high = vcvtq_s32_f32(f_high);
-
-			// Saturate to int16 range
-			int16x4_t result_low = vqmovn_s32(v_low);
-			int16x4_t result_high = vqmovn_s32(v_high);
-
-			vst1q_s16(&pcm_buffer[i], vcombine_s16(result_low, result_high));
-		}
-
-		// Handle remaining samples
-		for (; i < total_samples; i++) {
-			int32_t boosted = (int32_t)(pcm_buffer[i] * gain);
-			if (boosted > 32767) boosted = 32767;
-			if (boosted < -32768) boosted = -32768;
-			pcm_buffer[i] = (int16_t)boosted;
-		}
-	}
-
 	nb_bytes = opus_encode(encoder, pcm_buffer, frame_size, out, max_packet_size);
 	return nb_bytes;
 }

ui/src/components/popovers/AudioPopover.tsx

@@ -10,7 +10,7 @@ import notifications from "@/notifications";
 
 export default function AudioPopover() {
   const { send } = useJsonRpc();
-  const [audioOutputSource, setAudioOutputSource] = useState<string>("hdmi");
+  const [audioOutputSource, setAudioOutputSource] = useState<string>("usb");
   const [audioOutputEnabled, setAudioOutputEnabled] = useState<boolean>(true);
   const [audioInputEnabled, setAudioInputEnabled] = useState<boolean>(true);
   const [usbAudioEnabled, setUsbAudioEnabled] = useState<boolean>(false);

ui/src/routes/devices.$id.tsx

@@ -191,7 +191,7 @@ export default function KvmIdRoute() {
           console.warn("[SDP] Opus 48kHz stereo not found in answer - stereo may not work");
         } else {
           const pt = opusMatch[1];
-          const stereoParams = 'stereo=1;sprop-stereo=1;maxaveragebitrate=128000';
+          const stereoParams = 'stereo=1;sprop-stereo=1;maxaveragebitrate=128000;usedtx=1;useinbandfec=1';
           const fmtpRegex = new RegExp(`a=fmtp:${pt}\\s+(.+)`, 'i');
           const fmtpMatch = remoteDescription.sdp.match(fmtpRegex);
 
@@ -463,7 +463,7 @@ export default function KvmIdRoute() {
             console.warn("[SDP] Opus 48kHz stereo not found in offer - stereo may not work");
           } else {
             const pt = opusMatch[1];
-            const stereoParams = 'stereo=1;sprop-stereo=1;maxaveragebitrate=128000';
+            const stereoParams = 'stereo=1;sprop-stereo=1;maxaveragebitrate=128000;usedtx=1;useinbandfec=1';
             const fmtpRegex = new RegExp(`a=fmtp:${pt}\\s+(.+)`, 'i');
             const fmtpMatch = offer.sdp.match(fmtpRegex);