[mythtv] Software AC3 decoding when watching TV

Dave Packham Dave.Packham at utah.edu
Thu Dec 9 06:01:29 UTC 2004


This would be an ALSA or OSS issue if im not mistaken.  I too have
played with AC3 a bit to get it to decode.  I know that ALSA sound
package has software AC3 decoders and they do downmix to stereo.  Your
prolly going to have to ask some questions in the ALSA list to get help
with this.

That's what I did.  The MythTV list users are only going to respond if
it's a MythTV issue (or someone wants to) this is a sound driver issue.

Its also VERY dependant on what sound card you have and your distro you
run.

Check out ALSA's man pages and search for ac3decode.

http://www.alsa-project.org/documentation.php

http://www.linuxdvb.tv/documentation/ac3sound.html


Dave P

-----Original Message-----
From: mythtv-dev-bounces at mythtv.org
[mailto:mythtv-dev-bounces at mythtv.org] On Behalf Of htpc
Sent: Wednesday, December 08, 2004 8:55 PM
To: mythtv-dev at mythtv.org
Subject: Re: [mythtv] Software AC3 decoding when watching TV

Sighz, this is very frustrating!! I have posted no less than 3 posts
over 40 days at both mythtv-dev and mythtv-users asking about software
AC3 decoding and got zilch replies. I searched the gossamer threads
using various search combinations but found no answers. I had patiently
waited for some kind soul to shed some light on this issue but none came
forward.

Can somebody please, please tell me whether software AC3 decoding
support is working in Myth in the first place? i.e. when the audio
stream is in surround sound, decode it inside the computer and pass out
the 5.1 channels via the sound card's 3 analog outputs. I have surround
sound in MythVideo, but not when watching live HDTV. Whenever I set a
surround sound APID in my channel info, all I hear are cracks, snaps and
pops over the 3 analog outputs (note, I am not using the DVB 2.1 patch
for this).

Is this feature lacking in MythTV itself, or is it because of some
configuration error that I've made? Is surround sound part of the DVB
patch? As mentioned I am more than willing to try and implement this
feature, still it takes time for me to understand how sound is
implemented in Myth, so I welcome any help/tips/suggestions to point me
in the right direction.

Again, say something to me - abuse, flame, curses, anything to put me on
the right track.

The community help for MythTV is amazing, but somehow lacking when it
comes to surround sound for TV, sighz.


Regards.

htpc wrote:

>Dear all,
>
>Does MythTV support software AC3 decoding when watching TV with AC3
APIDs for the DVB-T format? (damn these acronyms) Everybody seem to
output AC3 via digital out but I prefer an all-in-one solution (mainly
becoz I can't afford to buy a receiver).
>
>Are they any technical issues that makes software AC3 decoding
impossible for Myth? libavcodec appears to have ac3 decoder support.
(not sure whether that is the AC3 for DVB-T or the a52 for ATSC, or
whatever).
>
>I am a C++/QT programmer (although a lousy one) by trade but it still
takes time for me to look through the source and get the big picture,
thus I appreciate any tips/pointers/references/help to help me start
working on this feature.
>
>Some more questions:
>1) Is A52 == Dolby Digital == AC3?
>2) Is it right that the AC3 audio on DVB-T doesn't have a proper AC3
header?
>
>I'm sorry if all this have been discussed before, but I can't seem to
find a definite answer or solution for this topic on Gossamer Threads,
so I'm starting a new one.
>
>
>Regards.
>
>
>
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>mythtv-dev mailing list
>mythtv-dev at mythtv.org
>http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-dev
>





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