[wdmaudiodev] Re: Dolby 5.1 audio and USB
- From: "Bob Johnston" <Bob_Johnston@xxxxxxxxx>
- To: <wdmaudiodev@xxxxxxxxxxxxx>
- Date: Wed, 11 Dec 2002 08:19:21 -0500
Somehow, my text got garbled on the last post. Trying again.
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.
You're only options are to:
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...)
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) =
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.
Bob Johnston
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