[mythtv-users] Bizarre troubles with AC-3 (5.1) audio, center channel intermittent mix

Brad Templeton brad+myth at templetons.com
Wed Dec 1 23:44:06 UTC 2004


As part of our Digital TV liberation front project at the EFF
( http://www.eff.org/broadcastflag/ ) I recently jumped in and decided
to build Myth boxes.   Alas, it has not been a picnic, but I have been
able to resolve most of my many myth related problems by searching the mailing
list archives.

However, here's a problem that I have not seen the answer to.  I have
a pchdtv-3000 and Fedora Core 3 (upgraded Sunday from RH9).  The sound
is the nforce2 chipset on the motherboard.

I have the standard stereo speakers on the PC right now, I am not using
SPDIF, though I plan to use it for my own setup down the road once it all
works well enough to move downstairs.

Some programs -- presumably those in just stereo and mono -- give good
audio, other than the fact it's out of sync for a short time when you
start playing.   However other programs, I am presuming ones that were
broadcast in more than 2 channels of sound, give very bizarre audio.
Notably, the voices -- I presume the center channel -- cut in and out.
Mostly they're out but sometimes with some seeking I can get them back
in for a while.  It varies with the phase of the moon.

It's actually good on Football, as you get the full crowd noise but no
announcers.  But not what I generally want.

It seems the system is deciding I do have an independent center channel
output (sometimes) and other times deciding to correctly mix the center
channel into the left and right.

mplayer, on the other hand, always mixes the audio properly.  In fact I
have ended up having to watch the 5.1 shows I have recorded with mplayer
and not in myth.

So yesterday I tried some alsa tricks.  /dev/adsp doesn't work as a device,
even though it is there and I made it writable.   ALSA:default had various
behaviours.  The most recent has been to work on the 2 channel shows
but give no sound on the >2 channel shows, and send the frontend into
a series of errors:

    2004-12-01 00:17:14 Not in the running state, state=2
    2004-12-01 00:17:14 Not in the running state, state=2
    2004-12-01 00:17:14 Not in the running state, state=2
    2004-12-01 00:17:14 Not in the running state, state=2
    2004-12-01 00:17:14 Not in the running state, state=2
    ...
    2004-12-01 00:17:14 Audio buffer overflow, audio datalost! afree=0 need=18432

Which slows down the system so much it can't play video any more.
It's an Athlon 3000.

Anybody seen this or know a way around it, or other things to try.
Should I try ALSA:nforce2 -- that's already the default.  It doesn't
understand having a default mixer as some suggested.  alsamixer doesn't
do anything, though if you mute the PCM channel you drop all audio.


I have a few other problems which I will wait until later to introduce
in other threads to keep this one about the joys of AC-3 audio.


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