[wdmaudiodev] Re: DS Audio Filters on WDM driver

  • From: Tim Roberts <timr@xxxxxxxxx>
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Fri, 08 Jul 2005 09:52:13 -0700

Steve Thorpe wrote:

I have a WDM audio driver for our USB Video Capture
It seems to work okay on most systems but we have the following problem: When the driver is inserted it into a capture graph
using one of the DirectShow filters on some systems the sample frequency is 44100hz and on others it is

This makes no discernable difference to audio preview
but our clients report that when capturing to file (using their proprietory USB Video Recording application) if the audio filter has a sample rate of 44100hz after one or two hours of recording the audio is dropping out of sync with the video.

We need to understand :

1) Why do some systems insert the same audio filter with a different sample frequency ?

This depends on a number of factors, some of which I have never been able to discern. Generally, if the kernel mixer is involved, your filter will be asked to supply the maximum possible data rate. However, in some cases, it might be able to reduce the CPU load in mixing by matching some other source.

So, I guess the reason is, "because".

2) why, if it selcts 44100hz instead of 48000hz would it cause audio/video capture to go out of sync? Is
this likely to be a driver, DS filter or application problem?

Is your device supplying a lower data rate, or is the sample rate being reduced by the kernel mixer? Do you have confidence that you really are delivering exactly 44,100 samples per second over long periods? Audio is usually the "master" time source for the graph, so if the audio rate is not correct, things do get confused.

Tim Roberts, timr@xxxxxxxxx
Providenza & Boekelheide, Inc.


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