[mythtv-users] mythmusic iec958 help

Alen Edwards allen.edwards at oldpaloalto.com
Tue Jul 1 15:07:12 UTC 2008



Joe Henley wrote:
> Gregg,
>
> I assume (from the alsa file names) you're running Fedora Core 8.  If 
> that's correct, then the first couple of "Info" boxes on the wiki you 
> reference are saying to NOT use a modified .asoundrc file.  So go back 
> to the original from the FC8 distribution.
>
> Also, are you using PukeAudio?  If so, gert rid of it.  It's still too 
> "bleeding edge" to use.  (There are _lots_ of notes around about how to 
> do that.)
>
>  From your note, it sounds like you want to take the CDRom info, output 
> it thru the SPDIF connector to a separate amp/receiver/home theater 
> receiver/etc.  If that's correct, then you want alsa plus the sound card 
>   to do nothing to the signal; only stuff it into the SPDIF connector. 
> You want your receiver to decode and then push it to the speakers.
>
> Most receivers these days, if they can handle (digital) SPDIF at all, 
> can figure out the underlying sample frequency (44.1KHz or 48 KHz), and 
> decode it correctly.  The problem often comes when the sound card gets 
> in the middle and insists on re-coding the CDRom (native) signal as 
> being 48 KHz.  The result is that the sound is then shifted upward in 
> frequency about 10% when the receiver decodes it.
>
> Now the bad news.  IIRC, most soundblaster cards cannot output a 44.1KHz 
> signal.  So they code everything as 48KHz.  I don't like Creative stuff 
> so I'm not familiar with the specifics of their cards these days.  You 
> should be able to google "Soundblaster Live 5.1" and "bit perfect audio" 
> and get some useful info on your card's output capabilities.  I fear you 
> may be in the market soon for a replacement card.
>
> I had a similar problem awhile back and ended up getting a Turtle Beach 
> Riviera card.  It was 20 or 30 bucks and "just works" in Linux.
>
> Once you go back to the original .asoundrc file, get rid of PukeAudio, 
> get a soundcard which can output a proper 44.1KHz signal on the SPDIF 
> connector (or hopefully find a setting on SB Live card if possible), 
> then your MythTV settings above should be OK.
>
> Good luck and come back here if you have questions.
>
> Joe Henley
Excellent post Joe.  I ran across this wiki that goes into some detail 
explaining how to find out if your sound card has a spdif  passthrough 
output.  My card took until method 3 to find it.

http://alsa.opensrc.org/DigitalOut

I just want to second what Joe says.  If ALSA is getting into the act, 
change something until it isn't.  If that takes a new sound card, get 
it.  The wiki should help you figure out what you need to do.

Allen



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