Hi,
One possible avenue you might investigate is to write a WDM Filter Driver
that would sit over (upper filter) the instance of USB Audio device driver
that is installed to support your device. In this way you can modify the
requests that go to the USB driver and the information that is returned from
the USB driver. I think you could lock or effectively disable the gain this
way...as for the DSP processing, I am not sure. There might be a way for a
filter driver to send a command to the USB audio driver to disable such
processing...but I am not sure.
This solution would be compatible across 2000/xp/vista.
There are possibly other solutions that depend on some sort of USB Audio
driver vendor specific support but I am not sure. You would have to look at
the USB Audio class driver specification for such information.
I am sure others will have ideas, but perhaps this is an avenue you might
investigate.
-----Original Message-----
From: wdmaudiodev-bounce@xxxxxxxxxxxxx
[mailto:wdmaudiodev-bounce@xxxxxxxxxxxxx] On Behalf Of Keith Larson
Sent: Thursday, August 21, 2008 9:36 AM
To: wdmaudiodev@xxxxxxxxxxxxx
Subject: [wdmaudiodev] Vista: Input Volume x4 gain on microphone
Hi All
This is the first time visiting this wdmaudiodev, and I have three
questions. Being a newbie, let me begin with a kind 'Hello'.
I am here because I am working with a measurement device and the new
Vista audio interface is, well, variable. After a lot of mucking
around, I am closing on a solution that will lock the input and output
levels. This is a kludge, but at least it works. There are however
remaining snags.
USB Audio Codec -> Classified as microphone -> gain of x4 (+12db) applied
--------------------------------------------------------------------------
The device I am using is a fairly simple 2 channel in, 2 channel out
ADC/DAC. I can deal with the OS classifying the input as a
'microphone', but more importantly it seems that the default action is
to multiply the raw input stream by 4 (+12db gain). I can clearly see
this when I set the application and master volume to 100%. The input is
exactly 4x the expected value. Low signal level values are wrong, and
if the signal is large, the data is clipped.
If I set the master volume to 25% and application to 100%, things are ok
(but maybe not if reversed), but this also means I need to lock the gain
values. The issue now whether or not the x4 microphone gain ever
changes in the future. It would be *far* better if I did not have to
deal with the variable gain.
DSP processing routines
-----------------------
This time it is the output side. The DSP routines modify the outputs.
Again, this is NOT desirable for a measurement system.
Enumerating End Point and Wave Devices
--------------------------------------
My code uses the 'Wavexxx' interface and as such I can get a handle, or
an enumerated value. I can also get an enumerated listing of the 'end
point' audio devices. As expected, these enumerations do not match (an
endpoint can have multiple WAVE connections), so a direct matching is
out. Luckily(?) the identifying strings, eg "USB Audio Codec' *SEEM* to
match. That is, I am looking for something a bit more definitive.
Suggestion
----------
Would there be a way to effectively achieve this for (both input and
output) in some future release of Vista?
[X] Lock master volume at 100% *and* bypass DSP processing
(This endpoint is for a measurement device)
Best regards,
Keith
PS: My code is and needs to be compatible with all Windows platforms.
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