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Fix overflow mixing #3601

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Fix overflow mixing #3601

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6eecdfd
Fix sum overflow
whatisor Nov 6, 2025
d88e257
Cleanup comments
whatisor Nov 6, 2025
21ebec0
Rename overflow check function and constants
whatisor Nov 13, 2025
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13 changes: 12 additions & 1 deletion src/plugins/janus_audiobridge.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1275,6 +1275,17 @@ void janus_audiobridge_hangup_media(janus_plugin_session *handle);
void janus_audiobridge_destroy_session(janus_plugin_session *handle, int *error);
json_t *janus_audiobridge_query_session(janus_plugin_session *handle);

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

@lminiero lminiero Nov 12, 2025

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To avoid risks of clashes, better to add a unique prefix to those defines, e.g. JANUS_AUDIOBRIDGE_SHRT_MAX_TUNED etc.


/* Optimized inline function to clamp 16-bit audio samples */
static inline opus_int16 overflow_check(int sum) {

@lminiero lminiero Nov 12, 2025

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Same thing for this function here, so something like janus_audiobridge_overflow_check.

As a side note, better to define sum as int32_t, as that's what the original sumBuffer is. A generic int should be fine in general anyway, but considering this could be compiled on different systems, better be explicit about it and avoid surprises.

sum = sum > SHRT_MAX_TUNED ? SHRT_MAX_TUNED : sum;
sum = sum < SHRT_MIN_TUNED ? SHRT_MIN_TUNED : sum;
return (opus_int16)sum;
}

/* Plugin setup */
static janus_plugin janus_audiobridge_plugin =
JANUS_PLUGIN_INIT (
Expand Down Expand Up @@ -8900,7 +8911,7 @@ static void *janus_audiobridge_mixer_thread(void *data) {
}
for(i=0; i<samples; i++)
/* FIXME Smoothen/Normalize instead of truncating? */
outBuffer[i] = sumBuffer[i];
outBuffer[i] = overflow_check(sumBuffer[i]);
/* 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