Audio Underflow's effect on latency

[John M <jdm0x0430@xxxxxxxxx>]

Hello all, 

I am developing a Voip product with a max latency requirement of 150ms mouth to ear. Our current tests show latency moving in a saw-tooth like pattern between 180ms and 140ms. 

The latency will begin at ~180ms, and then drop over time to around 160 to 140ms. Then, it jumps back up to ~180 and the process repeats.

This is all measured using oscilloscopes and similar instruments. Running the pjsua application with a call to sip:localhost shows no sawtooth pattern, and a call between two COTS Voip phones do not show the sawtooth pattern. 

What I recognized was that the latency would jump back up after a buffer underflow on a split combo port:

 17:59:27.446     scombdb-dn !Underflow, buf_cnt=1, will generate 1 frame

17:59:27.446     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

17:59:33.246     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

17:59:33.246     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

17:59:38.365     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

17:59:38.366     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

17:59:44.486     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

17:59:44.486     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

17:59:49.626     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

17:59:49.626     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

17:59:55.426     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

17:59:55.426     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

18:00:06.686     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

18:00:06.687     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

18:00:12.486     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

18:00:12.487     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

18:00:17.926     scombdb-dn  Underflow, buf_cnt=1, will generate 1 frame

18:00:17.926     scombdb-dn  Underflow, buf_cnt=126, will generate 1 frame

 

… some time later …

 

18:03:39.166     scombdb-dn  Underflow, buf_cnt=121, will generate 1 frame

18:03:39.166     scombdb-dn  Underflow, buf_cnt=21, will generate 1 frame

18:03:44.646     scombdb-dn  Underflow, buf_cnt=121, will generate 1 frame

18:03:44.646     scombdb-dn  Underflow, buf_cnt=21, will generate 1 frame

18:03:49.766     scombdb-dn  Underflow, buf_cnt=121, will generate 1 frame

18:03:49.766     scombdb-dn  Underflow, buf_cnt=21, will generate 1 frame

This reliably happens every 5 to 6 seconds. 

What could be causing this underflow? We are looking for minimum latency, so the jitter buffer is at a max of 60ms and the sound device has a play and record latency of 40ms.

Any guidance would be appreciated. 

Thanks, 

John. 

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