[mythtv-users] AC3 passthrough still doesn't work.

Dave K. Packham Dave.Packham at utah.edu
Sun Oct 24 19:13:19 UTC 2004

Is it possible to have ALSA output the AC3 stream to the spdif AND
decode it to a stereo analog stream at the same time?    So if you have
DVD's ripped with AC3 you can watch them with your TV OR AC3 Stereo?
Just so you have a choice to turn on the Big Dolby Digital AMP or just
listen thru the TV speakers?

Dave P 

-----Original Message-----
From: mythtv-users-bounces at mythtv.org
[mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Bo Najdrovsky
Sent: Sunday, October 24, 2004 11:55 AM
To: Discussion about mythtv
Subject: Re: [mythtv-users] AC3 passthrough still doesn't work.

Kevin Kuphal wrote:

> AC3 passthrough does not mean that Myth will output on the SPDIF 
> connector.  It means that if your content has AC3 audio then it will 
> be passed untouched to the audio device.  If your recorded or live 
> content has anything but AC3 source material you will get only 
> garbage.  To get output on the SPDIF connector, you need only 
> configure ALSA OSS emulation to output everything to it or, as you 
> noted, use the ALSA:spdif feature in the audio field.

Yay!  Someone responded to me.  :-)
Anyway, I fully understand what the AC3 passthrough means, I was just 
trying to explain that when I play the recorded .nuv file via mplayer 
and tell it to send the AC3 bitstream into the SPDIF, it works perfectly

fine and my surround receiver decodes the 5.1 audio, but when I try to 
do the same thing with mythfrontend by checking the AC3 passthrough 
option, I get only about a quarter second of audio, and then silence.  
It's as if the first AC3 packet sent to the receiver is good, but then 
the following ones are somehow invalid.

> Secondly, the ALSA support in Myth is somewhat in flux and also works 
> best with the 1.0.5a version of ALSA when compiled against it.  The 
> CVS version has some fixes and also some patches are being worked on 
> that revamp the audio portion of Myth with better ALSA support.  I 
> don't know if either of these will help but I wanted to make sure you 
> were on the right track and understood what the current status is. 

Yes, I gathered that from reading of several messages on the list, so I 
went and specifically installed ALSA 1.0.5a on my myth box, and 
recompiled mythtv and mplayer against it.  As I mentioned above,  
mplayer works, but mythtv doesn't.   Perhaps throwing more hardware at 
it may help (though I doubt it)...  The current setup has Athlon XP2500+

333FSB and onboard nForce2 video, but I will be upgrading the machine to

Athlon XP3000+ 400FSB and GeForce 5200FX on Tuesday.

Since you mention that there have been some fixes in the CVS, I will go 
ahead and try to build a current CVS version to see if I get any 
different results.

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