[wdmaudiodev] Re: DS Audio Filters on WDM driver

  • From: Steve Thorpe <steve@xxxxxxxxxxxxx>
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Sat, 9 Jul 2005 18:34:50 -0700 (PDT)

Thanks,

Steve T.



--- Jerry Trantow <jtrantow@xxxxxxxx> wrote:

> If kmixer can not get an exact match to a USB audio
> interface, it will
> choose the most capable (highest FS) audio
> interface.  
> 
> The exact match requirement is unfortunate. Consider
> a USB device that
> reports support for 16 or 20 bits at 44.1kHz and 16
> or 20 bits at 48Khz.  If
> the audio app asks for 16 bits and 44.1Khz or 48Khz
> it will get the desired
> USB interface with no SRC.
> However, if the audio app asks for 24 bits at 44.1
> there is not an exact
> match and kmixer will use a SRC and the 20 bit 48Khz
> USB interface.
> (although the 20 bit 44.1Khz interface would have
> been a better choice)
> 
> There are several work arounds to this problem.  
> 1) Have the audio apps ask for 20 bits (yeah right,
> asking all the app
> writers to change is like herding cats) 
> 2) Change the USB descriptor to report the device
> supports 20 bit and 24
> bit. This can be accomplished in the USB device
> hardware or in a filter
> driver on the PC.
> 3) Have MicroSoft improve the kmixer audio interface
> selection logic.
> 
> Jerry J. Trantow
> Applied Signal Processing, Inc.
> jtrantow@xxxxxxxx
> 
> 
> -----Original Message-----
> From: wdmaudiodev-bounce@xxxxxxxxxxxxx
> [mailto:wdmaudiodev-bounce@xxxxxxxxxxxxx] On Behalf
> Of Tim Roberts
> Sent: Friday, July 08, 2005 11:52 AM
> To: wdmaudiodev@xxxxxxxxxxxxx
> Subject: [wdmaudiodev] Re: DS Audio Filters on WDM
> driver
> 
> 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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