Re: Audio Underflow's effect on latency

Bill Gardner <billg@xxxxxxxxxxxx> · Thu, 15 Jun 2017 15:55:49 -0400



    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. 
      
      
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