[wdmaudiodev] USB Audio I/F Loop Back

  • From: "Jim Leiterman" <jim@xxxxxxxxxxxxxxxxxx>
  • To: <wdmaudiodev@xxxxxxxxxxxxx>
  • Date: Mon, 16 May 2005 07:19:37 -0700

Digital Audio Loop Back deivce/driverI have a utility that is a sine wave 
generator that generates DirectX output stereo waves @44.1kHz. This works fine 
monitoring it on various USB based Audio I/F devices. I use the DirectX output 
and a head phone monitor on the device.

This tool also works fine for capturing incoming stereo snippets at 44.1kHz and 
displaying these captured waves using the multimedia capture. I use the USB 
enumerate to detect the device is plugged in, then I enumerate the list of 
multimedia input capture devices and look for an ASCII match.

The problem comes on at least one USB Audio I/F device where it seems that it 
can not output and capture at the same time. I use a patch cable to loop back 
the signal and will get moments of clarity but most of the time what sounds 
like an extra loop back or the wave pointer for the device gets lost so irratic 
sound, etc. but will momentary correct or repeat itself. The painted capture 
data during these times is garbage. De-activating the generator and 
re-activating starts the process over. Turning off the capture seems to fix the 
problem. Using a second computer to generate the wave as an external source and 
the primary computer to use the USB Audio I/F to capture works fine.

Is there any reason why it seems to work on one device and not another? Does 
windows sometimes leak the input signal back out the output? I have noticed 
that if I go into volume control properties for this USB Audio I/F device there 
are no input properties only outuput.

Any thoughts?

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