[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)
