[mythtv-users] MythMusic spdif only works with some deciders-preamps

Nick Rout nick.rout at gmail.com
Fri Feb 13 00:55:32 UTC 2009

On Fri, Feb 13, 2009 at 11:10 AM, Allen Edwards
<allen.p.edwards at gmail.com> wrote:
> If I play an mp3 file from mythmusic it will play fine through my
> Technics SH-AC500D decoder which happens to be hooked up to my TV.
> The other spdif output I have out of the myth box goes to a high end
> Outlaw 990 decoder-preamp.  There is no output from the 990.  Both the
> 500D and the 990 switch to non Dolby modes with MythMusic but, like I
> said, only the 500D decodes it.  When playing a recorded ATSC TV show,
> which is Dolby Digital, both boxes indicate Dolby D and play fine.
> Can anyone explain this and let me know if there is a fix?  I could
> not find anything using google.
> I actually discovered this with mplayer but thought I would simplify
> the thread by focusing on mythmusic, but this problem is not only with
> mythmusic.
> As a side question, can anyone verify the format that goes out over
> the spdif port when playing an mp3?  Mythfrontend log says sr 44100 so
> I assume it is standard pcm 44.1KHz data.  But why won't it decode?
> Outlaw Audio tech support has never had a call where the 990 could not
> decode a spdif input so they were not helpful.  They tossed the ball
> back in my court to find out more about the spdif stream.
> Any help is appreciated.
> Allen

Are they being fed by the same soundcard? Some soundcards will pass
44.1k, some won't.

AFAIK spdif will send stereo PCM or more channels (certainly 5.1, not
sure about 7.1) via ac3 or DTS. Your mp3 will be decoded to PCM by
your player app and then the PCM passed to the receiver (perhaps with
more processing by alsa).

The spdif specs cover a range of sample rates [1], its just that some
cards don't handle anything other than 48k. You'll find pretty well
all video like broadcast TV, DVD's etc use 48k, so they work. CD music
on the other hand is always 44.1k and this is where it falls down.

At least thats my understanding of it!

Nothing in my post refers to new HD audio techniques, none of which I
have fully grokked yet.

[1] my logitech z5500 decoder's specs say it supports "PCM
(uncompressed stereo): 44.1 kHz / 16 bit through 96 kHz / 24 bit" (as
well as DD and DTS and DTS96/24)

More information about the mythtv-users mailing list