[wdmaudiodev] AC3 using USB Format Type I vs. II (or even III)

  • From: "Mark Rowan" <mark@xxxxxxxxxxxxxxxxxxxxx>
  • To: <wdmaudiodev@xxxxxxxxxxxxx>
  • Date: Wed, 10 Mar 2004 10:30:31 -0600

Hello list,
Can anyone help clarify some issues I have regarding sending AC3 over
USB?  I'm primarily an embedded hardware designer and am a newbie when
it comes to OS/driver interfaces.
I have a USB development kit with which I am interfacing a TI S/PDIF out
chip.  I have had no problem getting stereo audio streams passing
through the device using a Type I interface format. 
I am having trouble understanding the nuance of getting AC-3 (Dolby)
passed over the S/PDIF interface.
Using the Windows Design Note on USB Audio Devices (Ver 0.3, 4/1/03) as
a reference I wrote a descriptor for a Type II interface, but it failed
to even enumerate.  I know that document includes some pre-release
information for the 'Longhorn' OS.  But it was my understanding that the
Type II format supported currently for AC-3 data while Type III support
was slated to be added to <Usbaudio.sys>in Longhorn.  I also could not
find a corresponding document specifically for XP/2000 USB development,
so I'm proceeding on best guess here.
What further confused me was that, using a DVD app (WinDVD in this
case), if I use a digital sound card and configured the output for
"Digital (S/PDIF) out to External Processor" and then switched the
system audio output to my USB device (with the Type I descriptor).it
passes valid AC-3 that shows up as Dolby on an external receiver.  I was
further surprised when I remembered I had not yet implemented the
"non-audio" bit in the Channel Status set for the S/PDIF chip.
If I plug in my Type I device without 'priming' the interface as
digital, I only have the 'Analog' output option available.
To sum up:
Is it possible to fully support AC-3 over a Type I interface?  If so, at
what level I missing the description of stereo PCM vs. encoded AC-3?
Mark Rowan

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  • » [wdmaudiodev] AC3 using USB Format Type I vs. II (or even III)