[wdmaudiodev] Re: Dolby 5.1 audio and USB
- From: "DJ Sisolak" <dsisolak@xxxxxxxxxxxxxxxxxxxxxx>
- To: <wdmaudiodev@xxxxxxxxxxxxx>
- Date: Wed, 11 Dec 2002 09:56:29 -0800
Hello Ajai et al,
A few things to keep in mind when using the Microsoft USBAudio driver.
If you do implement a hardware AC-3 decoder in your device, be aware
that up until now only Type II (non-SPDIF) AC-3 transport has been
implemented. Type III (SPDIF) will be in upcoming versions of the
driver. The driver does accept multi-channel PCM but, as Bob mentioned,
at the cost of bandwidth. Go to 24 bit and you may have difficulty on
USB 1.1. This is obviously alleviated by USB 2.0 but we are still in the
process of defining USB Audio for USB 2.0.
One other thing to note, once AC-3/SPDIF (Type III) data is allowed by
the driver, it may take a bit more time to get out a version which does
the DSound Looped streaming interface. Thus only a limited number of the
DVD players listed below will be able to use this capability as a
straight WaveIn interface.
As I have very little hardware (1 prototype device) to test these code
paths with I have not given them the attention they deserve. If there is
any way I can work with you on this, please let me know.
Thanks,
DJ
-----Original Message-----
From: Bob Johnston [mailto:Bob_Johnston@xxxxxxxxx]=20
Sent: Wednesday, December 11, 2002 5:19 AM
To: wdmaudiodev@xxxxxxxxxxxxx
Subject: [wdmaudiodev] Re: Dolby 5.1 audio and USB
Somehow, my text got garbled on the last post. Trying again.
=20
Ajai,
The Microsoft OS's have no "free" AC3 decoder that ship with them. It's
a pretty hefty cost for each seat of the DD license, especially for 6
channel decode, and it would be very cost prohibitive for MS to support
it in the plain OS.
=20
You're only options are to:
=20
1. Use a hardware AC3 decoder at the end point of your speaker system
and implement a driver that supports wave-format tag 0x92 to support AC3
over SP/DIF. This is very do-able for USB since it keeps the data rate
low (192-448Kbps) since the AC3 stream stays compressed over the USB.
Most every software based DVD player's audio decoder can support AC3
SP/DIF (Cineplayer, WinDVD, PowerDVD, NVDVD, etc...)
=20
2. Use a software based multi-channel AC3 decoder and blow 6 channels of
straight PCM across the USB bus. Although this method alleviates the
cost of a hardware AC3 decoder, it will tax the USB bandwidth musch more
than the SP/DIF case however: 48,000(KHz sample rate)*(16 bits per
sample) * 6 (channels) =3D 4.6 mbs or approximately 37% of the maximum =
USB
bandwidth. Multi-channel AC3 audio decoders that can support 6 or more
channels of decoded PCM are ussually included in the more expensive
versions of software based DVD players since they need to pay a higher
license fee to Dolby.
=20
Bob Johnston
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