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Smoothen AudioBridge mix instead of clamping/truncating (see #3593 and #3601) #3610

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Smoothen AudioBridge mix instead of clamping/truncating (see #3593 and #3601) #3610

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46 changes: 33 additions & 13 deletions src/plugins/janus_audiobridge.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1254,6 +1254,10 @@ room-<unique room ID>: {

#define JANUS_AUDIOBRIDGE_MAX_GROUPS 5

/* Audio mixing with overflow protection - tuned to 0.9 of MAX/MIN to prevent clipping */
#define JANUS_AUDIOBRIDGE_SHRT_MAX_TUNED 29491
#define JANUS_AUDIOBRIDGE_SHRT_MIN_TUNED -29491

/* Plugin methods */
janus_plugin *create(void);
int janus_audiobridge_init(janus_callbacks *callback, const char *config_path);
Expand Down Expand Up @@ -8451,6 +8455,30 @@ static void janus_audiobridge_update_wav_header(janus_audiobridge_room *audiobri
}
}

/* Helper to limit/clamp mixed audio */
static inline void janus_audiobridge_limit_mix(opus_int32 *sumBuffer, opus_int16 *outBuffer, int samples) {
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@whatisor whatisor Dec 7, 2025

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These are overkill and not reliable. 1 random false noise could make whole mixing result worse.

/* Find the loudest sample first, if there's one */
opus_int32 loudest = 0, sample = 0;
int i = 0;
for(i=0; i<samples; i++) {
sample = abs(sumBuffer[i]);
if(sample > JANUS_AUDIOBRIDGE_SHRT_MAX_TUNED && sample > loudest)
loudest = sample;
}
int gain = 100;
if(loudest > JANUS_AUDIOBRIDGE_SHRT_MAX_TUNED) {
/* We need to apply a negative gain to contain the mix */
float gainF = (float)JANUS_AUDIOBRIDGE_SHRT_MAX_TUNED / (float)loudest;
gainF *= 100;
gain = gainF;
}
/* Transform to 16-bit samples, limiting if necessary */
for(i=0; i<samples; i++) {
sample = (gain == 100) ? sumBuffer[i] : ((sumBuffer[i]*gain)/100);
outBuffer[i] = sample;
}
}

/* Thread to mix the contributions from all participants */
static void *janus_audiobridge_mixer_thread(void *data) {
JANUS_LOG(LOG_VERB, "Audio bridge thread starting...\n");
Expand Down Expand Up @@ -8834,10 +8862,7 @@ static void *janus_audiobridge_mixer_thread(void *data) {
}
/* Are we recording the mix? (only do it if there's someone in, though...) */
if(audiobridge->recording != NULL && g_list_length(participants_list) > 0) {
for(i=0; i<samples; i++) {
/* FIXME Smoothen/Normalize instead of truncating? */
outBuffer[i] = buffer[i];
}
janus_audiobridge_limit_mix(buffer, outBuffer, samples);
fwrite(outBuffer, sizeof(opus_int16), samples, audiobridge->recording);
/* Every 5 seconds we update the wav header */
gint64 now = janus_get_monotonic_time();
Expand Down Expand Up @@ -8905,9 +8930,7 @@ static void *janus_audiobridge_mixer_thread(void *data) {
}
}
}
for(i=0; i<samples; i++)
/* FIXME Smoothen/Normalize instead of truncating? */
outBuffer[i] = sumBuffer[i];
janus_audiobridge_limit_mix(sumBuffer, outBuffer, samples);
/* Enqueue this mixed frame for encoding in the participant thread */
janus_audiobridge_rtp_relay_packet *mixedpkt = g_malloc(sizeof(janus_audiobridge_rtp_relay_packet));
mixedpkt->data = g_malloc(samples*2);
Expand Down Expand Up @@ -8965,8 +8988,7 @@ static void *janus_audiobridge_mixer_thread(void *data) {
if(go_on) {
/* By default, let's send the mixed frame to everybody */
if(groups_num == 0) {
for(i=0; i<samples; i++)
outBuffer[i] = buffer[i];
janus_audiobridge_limit_mix(buffer, outBuffer, samples);
have_opus[0] = FALSE;
have_alaw[0] = FALSE;
have_ulaw[0] = FALSE;
Expand All @@ -8990,13 +9012,11 @@ static void *janus_audiobridge_mixer_thread(void *data) {
if(groups_num > 0) {
if(rfm->group == 0) {
/* We're forwarding the main mix */
for(i=0; i<samples; i++)
outBuffer[i] = buffer[i];
janus_audiobridge_limit_mix(buffer, outBuffer, samples);
} else {
/* We're forwarding a group mix */
index = rfm->group-1;
for(i=0; i<samples; i++)
outBuffer[i] = *(groupBuffers + index*samples + i);
janus_audiobridge_limit_mix(groupBuffers + index*samples, outBuffer, samples);
}
}
if(rfm->codec == JANUS_AUDIOCODEC_OPUS) {
Expand Down