[mythtv] AC3 passthrough

[Jason Hoos]

> I have not had any audio dropout issues, though I haven't watched any
> 720p recordings.

It actually happens on live TV as well.  Is the ABC affiliate by you a 720p 
station?  If so, does it seem to work well during live TV?  (If there's no 
720p station by you, would you be interested in testing a ~30-second clip if 
I could manage to put one together?)

I've actually done some checking by putting a call into audiooutputalsa.cpp to 
check how much of the sound card's buffer is full (something like 
snd_pcm_avail_update() I think?), and the dropouts seem to correspond to 
times when the buffer is mostly empty (which makes sense).  It seems that the 
decoder isn't keeping the buffer sufficiently full during playback, maybe 
because it's a small buffer.  I don't know what soundcards typically have for 
buffer sizes; this one is apparently 64k (16384 frames) from what I can 
tell...

> Does your receiver drop out of AC3 (back into PCM) when this happens?
> If it does, I can see even a miniscule blip turning into something
> annoying... it definitely takes mine half a second or so to re-start
> AC3 after ff/rew

It doesn't seem to.  In any case I was having the problem before the AC3 
patch; actually part of my motivation to get AC3 passthrough working was to 
see if it would fix the problem.  Oh well.  

I'm in the process of getting the other machine set up, hopefully it'll work 
better.  Either that or I dive further into the playback stuff in Myth to see 
if I can figure out what's happening...

Jason