<div class="gmail_quote">On Tue, Feb 9, 2010 at 7:39 PM, feadog <span dir="ltr"><<a href="mailto:feadog@orcon.net.nz">feadog@orcon.net.nz</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div class="im">On Tue, 2010-02-09 at 18:51 +1300, feadog wrote:<br>
> I wonder why ac3 should be<br>
> treated differently than mp2.<br>
><br>
</div>just realized we're talking about dvb-t, not dvb-s<br></blockquote><div><br>That doesn't make any difference, but I think I understand what you were getting at now. Mp2 (or mp3 or mp4 or flac or any other 2 channel format) is decoded to PCM samples by mythtv, which then passes it to alsa which then processes it according to the rules the user has set up and then sends it to the specified soundcard output. AC3 and DTS can be handled the same way - as a number of discrete channels of PCM data. But S/PDIF can't accept 6 discrete channels: if you send the soundcard 6 channel PCM data then it will generally only be able to output that via analogue connectors (some soundcards are capable of encoding those separate channels as AC3 streams, but they are rare). So most people set up mythtv (and/or mplayer, xine, etc) to "passthrough" AC3 and DTS signals. That means that mythtv doesn't decode the signal to 6 (or whatever) channels, but instead sends the encoded data to alsa which gets the soundcard to output it, still encoded, via S/PDIF. Since the data is still encoded it has to be handled differently by alsa to achieve things like volume normalisation.<br>
<br>I hope, between this and my other explanation, that you can see what I mean.<br><br>Cheers,<br>Steve<br></div></div>