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

[Marc Sherman]

On 1/14/2012 5:12 PM, Jean-Yves Avenard wrote:
>>
>> 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

Sorry, I meant 2->5 up mixing -- the centre channel is actually active 
in this mode (presumably being sent the common signal from the L and R 
input channels).

> 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.

Are you sure? I thought that in 0.24, myth actually decodes, processes, 
and re-encodes the AC3 stream? For example, mythtv's internal volume 
controls (if enabled with the software mixer) do affect AC3 audio, and 
AC3 audio is sped up or slowed down to sync with Time Stretched playback.

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

Nope, not corrupt recordings, because when it happens, if I rewind and 
replay the part that was bad, it won't play the same (but often will 
still have dropouts, just at different points). It does seem to often 
have dropouts near the same place, but not at exactly the same place. 
And if I play the same recording with 2 channel output from myth, 
everything sounds fine.

> 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.

Well, if myth is decoding/processing/reencoding the AC3 stream as I 
suggested above, then it could be dropping one of the channels in this 
code path.

- Marc