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

[Jean-Yves Avenard]

On Sunday, 15 January 2012, Marc Sherman <msherman at projectile.ca> wrote:
> 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.

How could there be a centre channel drop out, if there's no centre channel
to start with?

> 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.

Same with a 2 > 4 up mixing: no centre channel. Additionally, by mini
system I'm guessing you are referring to your audio system

If you are playing 5.1 audio over spdif and have up mixing turned off in
myth, the only way to have multi channel audio is via AC3 or DTS
passthrough. Myth does nothing but pass on the original audio stream.

That leaves only one possible culprit: your audio system (amplifier,
processor, speakers whatever)
That's where you need to look.

>
> 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.

You can't play more than stereo over spdif, so at best you can do
speaker-test -c 2

>
> 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?).

You're saying it's intermittent. The test speaker screen is mainly there to
identify issue on the mythfrontend side and things like locate if your
speakers are properly connected.

But from your description, I would look into your mini system.
Or it's the audio file itself that is corrupted.

If it was in myth or sound card, you would loose all channels , not just
one, as the card only output two channels at any given time, and the
content is digital.

>
> 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 :)
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