Re: Timing problems with USB sound cards

[Harold Mills]

We have seen the same sort of problem that Doug reports for USB devices 
with PCI sound cards when the recording CPU is heavily utilized. As he 
suggested, I suspect the problem is that the CPU is so busy doing other 
things that it doesn't have time to process some of the incoming audio 
data before they are no longer available.

As far as I know, there is often no way for an application to detect 
dropped samples other than to try to compare the number of samples it 
has received during a recording to the number expected given the 
purported sample rate. Unfortunately this is usually not very accurate, 
especially over shorter time periods, since it's not possible to obtain 
either the precise duration of a recording or the precise number of 
samples that a sound card has converted. For example, it may not be 
possible for an application to account for samples buffered on the sound 
card, in its driver, in the host operating system, and/or within an 
audio API.

We have also seen a recording duration computed from the number of 
samples received and the purported sample rate differ from real time at 
a small but roughly constant rate, even when the recording CPU is 
lightly utilized. I suspect that this is due to small inaccuracies in 
the A/D clock.

    Harold Mills
    Bioacoustics Research Program
    Cornell Lab of Ornithology