[wdmaudiodev] Understanding USB Feature Unit Descriptors
- From: RYAN ALLAN <ryanallan@xxxxxxx>
- To: wdmaudiodev@xxxxxxxxxxxxx
- Date: Fri, 14 Jan 2011 12:31:55 -0700
I am developing a USB-to-analog audio device, and am seeing strange
volume scaling behavior from Windows 7/Vista machines. The problem is
not apparent with XP machines.
I am using the
following Feature Unit Descriptors.
Get_Min: 0x01, 0x80
Get_Max: 0xFF, 0x7F
Get_Res: 0x00, 0x10This tells me that the volume will be in the
range from Min:0x8001 to Max:0x7FFFF, and each step (each time the
volume is incremented via volume+ button on a multimedia keyboard) will
be of size Res:0x1000. Is this correct?
When I move the volume slider in a Windows 7/Vista machine, the
volume (I am looking at Set_Cur packets) moves in a range from 0x7FFFF
to 0x3900 with step sizes of 0x0100. The same experiment on a XP machine
sees the volume move in a range from 0x7FFF to
0x8001 with step sizes of 0x1000. Why do the two OS's have different
behavior?
I would like volume to be in the range from -127db - +127db (as per
USB spec), and I would like step sizes of 0x1000. Basically, I would
like the Windows 7 machine to behave like the XP machine.
Thanks!
--
Ryan
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