[mythtv-users] Digital audio: centre channel drops out intermittently

[Marc Sherman]

I'm running 0.24-fixes on ubuntu 11.10, and the audio is connected from 
my combined fe/be machine to a JVC mini-system amp via spdif. When I'm 
watching recorded HD programming (OTA), I frequently but intermittently 
get dropouts on the centre channel (which is really bad, because that's 
usually where the dialog is).

If I switch mythtv to 2 channel output in setup (still over the spdif 
out), there's no dropouts. Similarly, if the prefs are left at 5.1 out, 
but I switch the audio channel on a recorded program to an available 
secondary 2 channel track, no dropouts. Upmixing is disabled in my 
mythtv prefs, but the minisystem has it's own 2->4 upmixing which is 
turned on.

I've tried running the speaker test built into the mini system, and 
can't reproduce the problem there.

I've tried running "speaker-test -c 6" (which is part of the alsa tools 
distro, I think), but it only outputs sound to the front 2 channels, it 
goes silent when it's supposed to be testing the other 4 surround 
channels. I think that's because it's not encoding the output as AC3? 
I'm not sure.

Does anyone have any ideas for how to further isolate the source of this 
problem so I can troubleshoot and/or replace the broken component? I'm 
still not sure if this is a software problem in mythtv (but then why 
only the centre channel?), a soundcard or alsa driver problem (same 
question?), or a hardware failure in my minisystem amp (but then why 
does the internal speaker test not show the problem?).

The only part of the system I'm convinced is ok is the physical speaker 
and the cable running to it (which was actually my first guess, because 
we've had a problem with mice :)

Thanks very much for any debugging/troubleshooting pointers...
- Marc