[pulseaudio-discuss] [PATCH]: High-definition audio pass-through support

[Arun Raghavan]

On Fri, 2011-11-18 at 11:17 -0700, Kelly Anderson wrote:
> On 11/18/2011 10:16 AM, Colin Guthrie wrote: 
> > 'Twas brillig, and Kelly Anderson at 18/11/11 16:37 did gyre and gimble:
> > > On 11/17/2011 11:41 PM, Maarten Bosmans wrote:
> > > > 2011/11/17 Kelly Anderson<kelly at silka.with-linux.com>:
> > > > > Hi,
> > > > > 
> > > > > So I patched PulseAudio 1.1 for hdmi-surround-71 and lo and behold it
> > > > > worked.
> > > > > Here's a patch, hopefully we can get this added in the next official
> > > > > release
> > > > > of PulseAudio.  I know that there are quite a few Xbmc users that
> > > > > have been
> > > > > anxiously
> > > > > waiting for high-definition audio pass-through to work correctly.  Of
> > > > > course
> > > > > if someone
> > > > > has a better solution, that would be fine too.
> > > > Note that there's nothing inherently high-definition about the audio
> > > > data through a hdmi connector.
> > > True.  I should have said DTS-HD.  DTS-HD requires more bandwidth then 2
> > > channels @192K
> > > can handle.  8 channels @192K provides enough bandwidth (768K) to handle
> > > the high
> > > def audio formats such as DTS-HD.
> > > 
> > > > How about also adding 4.0 and 5.1? That seems more complete.
> > > They weren't necessary for the pass-through support, although they may
> > > be beneficial
> > > for non-pass-through audio.
> > I'm confused.....
> > 
> > This isn't pass through is it? I mean it's defining an 8 channel PCM
> > device. You do not pass it encoded DTS, AC3 etc. etc. data, but an 8
> > channel PCM stream.
> > 
> > Pass through is quite different, but is also supported in PA 1.1
> > (provide the client application uses the new API to enable pass through).
> > 
> > Isn't this just enabling 7.1 channel output, such that the application
> > in question does the decoding and passes the PCM stream to your receiver
> > equipment?
> This email provides the most detailed description of the what is
> necessary to
> get the bandwidth required to do DTS-HD pass-through.  The most
> significant
> piece is the 8 channel HBR requirement.  Without adding the patch at
> the
> start of this thread I can't get it to switch into 8 channel mode to
> get the 768K
> bandwidth.
> 
> http://lists.freedesktop.org/archives/pulseaudio-discuss/2011-March/009364.html
> 
> I've perused the pulseaudio code many times looking to uncover the
> secrets
> of the "new" pass-through code.  I didn't find anything like a
> conversion of raw
> DTS(-HD) packets to 0-padded pass-through data.  So, I guess I'm
> confused
> as to the advantage of the "new" pass-through code versus opening the
> stream
> with PA_STREAM_PASSTHROUGH.  I'd be happy for someone to unlock the
> secrets
> of the "new" pass-through approach.  BTW, all of the info on the web
> covers
> the "old" pass-through techniques but I don't see any info on the new
> techniques.
> It might be useful for someone to rewrite pacat.c to use the new
> techniques
> as an example.

I've replied elsewhere on the list that should hopefully clarify how
things are to be done. Part of the problem is that the current server
where the wiki is hosted can't handle being indexed, so searches
completely fail to get you relevant information. For now, this is where
the documentation on what needs to be done exists:

http://pulseaudio.org/wiki/Passthrough

-- Arun