[wdmaudiodev] Re: DS Audio Filters on WDM driver
- From: Steve Thorpe <steve@xxxxxxxxxxxxx>
- To: wdmaudiodev@xxxxxxxxxxxxx
- Date: Sat, 9 Jul 2005 18:36:01 -0700 (PDT)
Thanks,
That sounds like it could be the source of the
problem.
Steve T
--- Tim Roberts <timr@xxxxxxxxx> wrote:
> Steve Thorpe wrote:
>
> >I have a WDM audio driver for our USB Video Capture
> >chip.
> >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
> >48000hz.
> >
> >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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