[mythtvnz] A few MythTV questions

[Steve Hodge]

On Mon, Oct 13, 2008 at 2:16 PM, Steven Ellis <steven at openmedia.co.nz>wrote:

> On 14/10/2008, at 9:51 AM, Nick Rout wrote:
> On Mon, Oct 13, 2008 at 9:13 PM, Aaron Whitehouse
>
> 2. Has anyone got 5.1 sound working? Is the way that people are doing
>
> this using S/PDIF over coax/optical? How well does the S/PDIF
>
> pass-through work with recent distributions?
>
> Mine has always worked fine.
>
> Same here
>

And me. I'm using coax; PCM, AC3, and DTS all work fine.


> Most guides that I have
>
> found are quite old.  I assume that the S/PDIF carries a digital signal
>
> encoded in AC3 or PCM.
>
> by definition I think spdif must be ac3 or pcm
>
> Or DTS
>

Yes.

Some Amps won't accept anything other than 44.1 and 48KHz PCM which can
> cause issues with some internet streams, but otherwise no issues.
>

Also, some soundcards won't output one or the other via SPDIF. Hopefully
this is fairly uncommon these days but it may pay to read the small print.


Is there some negotiation between the card and

the computer that lets the computer figure out which formats it can

pass-through and which have to be re-encoded into formats that the

stereo etc. can support? Do you set all of that manually?

I think myth does it all in software, it passes ac3 and pcm streams
through. other codecs it presumably converts to pcm (as it would have
to do for analogue output anyway). Most decent digital input
amplifiers will mix sound so it comes out all speakers.

There's no negotiation, and for PCM MythTV doesn't know anything about the
process. If your soundcard is configured to output to SPDIF then it'll send
all PCM data (that's in the right format - generally 16bit 44.1 or 48kHz
stereo) to that output. Usually with ALSA these days no configuration is
needed, you just need to set your output device to the right one.

For AC3 and DTS you need to tell MythTV to pass these through otherwise Myth
will decode them to stereo.

Cheers,
Steve
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