[wdmaudiodev] Re: DS Audio Filters on WDM driver

  • From: "Jerry Trantow" <jtrantow@xxxxxxxx>
  • To: <wdmaudiodev@xxxxxxxxxxxxx>
  • Date: Fri, 8 Jul 2005 12:22:50 -0500

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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