[pulseaudio-discuss] Nvidia hdmi passthrough patch

[Colin Guthrie]

'Twas brillig, and David Henningsson at 19/01/12 10:33 did gyre and gimble:
> On 01/16/2012 10:09 AM, Peter Humphreys wrote:
>> Hi there,
>>
>> I just found this post:
>>
>> https://tango.0pointer.de/pipermail/pulseaudio-discuss/2011-March/009291.html
>>
>>
>> Is this relevant to HDMI passthrough to receivers IE: so the NVIDIA
>> isn't decoding the audio stream, allowing the receiver to get DTS-HD MA,
>> True HD, FLAC etc encoded streams and do the decoding on the receiver??
>>
>> Because at the moment I only get PCM on my receiver from the NVIDIA,
>> where as in Windows 7 I can use powerdvd which passes through the signal
>> for the receiver to decode, in Ubuntu I have no options to do that.
> 
> The a52 sink is not in Ubuntu for patent reasons [1], but that's only
> about encoding.
> 
> Passthrough is, unless Arun corrects me, about having already compressed
> data being decoded in the receiver. Ubuntu 11.10's pulseaudio has
> support for that, but the rest of the pieces aren't there - in
> particular, no client application supports PulseAudio's passthrough API
> yet.

Absolutely.

Just to clarify, the a52 module (and another module which is similar but
does dts encoding rather than ac3: https://gitorious.org/dtsenc/dtsenc)
do not technically do anything by way of "passthrough", they simply take
a multi-channel stream from a client and encode it.

That means if you source material is AC3 data, the client application
would have to decode it, supply it to PA as a 6ch PCM stream, PA would
then use the a52 (or dtsenc) module to re-encode the data and pass and
then "passthrough" the encoded stream.

So this really isn't true "passthrough". While this approach may seem
wasteful, it has the advantage of being able to support mixing of the
stream with other sounds. For movie watching (the typical case where
audio data is pre-encoded) this is typically NOT very desirable anyway
(you don't want beeps and bongs interrupting you - although caveats for
integrated VoIP systems etc. may somewhat counteract this argument!) so
it's not really an advantage in that context anyway.

Col

-- 

Colin Guthrie
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http://colin.guthr.ie/

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