[wdmaudiodev] newbie questions about filter drivers

  • From: dprado@xxxxxxxxxxxxxx
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Thu, 14 Oct 2004 12:02:59 -0300

Hello All,

I have been trying for the last two weeks (without much success) to make some
sense out of the DDK documentation. I real Water Oney´s book understood some of
it :). The problem is, I want to write a driver that I believed was very
simple, but still haven´t figured out the best approach. Maybe someone can
enlighten me. Basically I have a USB audio adapter that is fulley compliant
with usbadudio.sys BUT, I would like to create two virtual devices ( called for
exemple LeftChannelUSB and RightChannelUSB ), and would like allow this devices
to behave as a "standard audio device". What I mean by this is that I would
like for example to be able to select LeftChannelUSB as a "Default Device" for
sound playback on CP Sound and Audio Devices Properties. When a user app writes
to one of these virtual audio devices ( for example LeftAudioUSB) , it should
mix Left and Right channels and write both of them on the Left Channel of
Would it be possible to write a Virtual Device driver (like a filter driver) and
stack it on top of usbaudio using the LowerFilter reg key? I successfully
intalled  MSVAD example , and a modified filter example ( from W. Onney´s book
) and was able to use MSVAD as a lowerfilter to filter.sys. When I wrote to
MSVAD it worked without the filter driver. When I selected the filter driver ,
it correctly logged debugging info and transfered the IRPs to MSVAD. 
Would I be better off trying a rewrite of usbaudio.sys?
Is there an easier way of accomplishing this? Is it easy or even possible to do
some minimal audio filtering using a filter ( in the sense of IRP filtering )
driver ( changing data before and then forwarding an IRP )?
As you can see I am totally lost, any info in this subject will be greatly

Dimitri Prado


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