Re: Audio Underflow's effect on latency

[Bill Gardner <billg@xxxxxxxxxxxx>]

USB can't alter the audio sampling rate, but if the audio buffers don't keep up then a missing record buffer would also trigger wsola reconstruction. If you increase the audio buffer sizes does the latency become more stable? Ideally the record and playback buffer callbacks should alternate, but often you will get repeats (bursts), depending on platform and audio device. Perhaps you are getting a repeat playback every 5 seconds causing the record buffer to underflow.

On 6/15/2017 4:42 PM, John M wrote:

Thanks for the quick response. 

Our input and output are both powered by the same master clock. They communicate to the system over USB Audio though. Could it be possible timing issues in USB transmission can cause a similar issue? 

On Jun 15, 2017 3:56 PM, "Bill Gardner" <billg@xxxxxxxxxxxx> wrote:

Hi John,

It's likely a mismatch in the sampling rates of the record and playback devices. Unless they are using the same hardware clock there will be drift and hence the wsola delay buf is necessary to prevent discontinuities.

Regards,

Bill

On 6/15/2017 3:19 PM, John M wrote:

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. 

_______________________________________________
Visit our blog: http://blog.pjsip.org

pjsip mailing list
pjsip@xxxxxxxxxxxxxxx
http://lists.pjsip.org/mailman/listinfo/pjsip_lists.pjsip.org

_______________________________________________
Visit our blog: http://blog.pjsip.org

pjsip mailing list
pjsip@xxxxxxxxxxxxxxx
http://lists.pjsip.org/mailman/listinfo/pjsip_lists.pjsip.org

_______________________________________________
Visit our blog: http://blog.pjsip.org

pjsip mailing list
pjsip@xxxxxxxxxxxxxxx
http://lists.pjsip.org/mailman/listinfo/pjsip_lists.pjsip.org

_______________________________________________
Visit our blog: http://blog.pjsip.org

pjsip mailing list
pjsip@xxxxxxxxxxxxxxx
http://lists.pjsip.org/mailman/listinfo/pjsip_lists.pjsip.org