Re: Compressed stream driver
Re: Compressed stream driver
- Subject: Re: Compressed stream driver
- From: Jeff Moore <email@hidden>
- Date: Fri, 9 Jun 2006 15:47:27 -0700
In theory, it should work at the HAL/IOAudio level provided the
driver fills the format stuff out correctly. I haven't yet seen a
device that can do this, so I haven't actually ever tried it.
On the practical side of things, my guess is that applications might
not know what to make of the AC-3 data though (or any compressed
format for that matter). We do have support for writing AC-3 files
and putting AC-3 into CAF files with the AudioFile API, but outside
of our test tools, I'm not too sure of any apps that would know what
to do with AC-3 if you gave it to them.
BTW, kIOAudioStreamSampleFormatAC3 is there specifically for the
native, VBR form of AC-3. It looks vaguely like an MPEG stream and
can only be put into it's native container or a container designed
for it like CAF. kIOAudioStreamSampleFormat1937AC3 is the format that
flows across a digital interface such as SPDIF. This form of AC-3 can
live happily anywhere you can put a stereo 16 bit integer stream. It
is important to differentiate between the two because they have very
different data handling characteristics.
On Jun 9, 2006, at 3:21 PM, Phil Montoya wrote:
I'll try again. We have hardware capable of passing a Dolby
Digital AC3 stream. Now is it possible to pass the AC3 compressed
audio directly up the core audio device chain from the
IOAudioEngine. I see in the IOAudioStreamFormat struct the
fSampleFormat can be set to kIOAudioStreamSampleFormatAC3. Is this
all that is necessary then the time stamping remains the same and
happens each time our audio buffer fills up with compressed data?
Maybe it is trivial and I'm thinking this is harder than it is.
--
Jeff Moore
Core Audio
Apple
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