[mythtv] sound + xvmc + livetv = problem, why?

[Daniel Kristjansson]

I've been looking at ticket #774 a little more closely since it 
seems to be effecting more users and it makes the EPIA pretty
useless as a frontend in LiveTV mode.

I'm not very familiar with how the audio decoding works in MythTV.
But it seems that the audio buffers aren't getting filled quickly
enough when you are in LiveTV and using XvMC. This causes the A/V
sync routines then kick in to slow down the video to match the audio
speed, which in the meantime plays a sample, mutes, plays a sample,
mutes, etc.

This does not happen when you playback a recording, and the CPU
utilization is lower when you play the same stream in LiveTV vs.
playing back the recording. What I did notice is that with XvMC,
when you pause a playing recording the audio '-v audio' debug
keeps going, but it stops when you pause in LiveTV. With XVideo
output, it keeps going in either case.

I've tried upping the ringbuffering and I'm using ChromaKey OSD
so I don't think the problem is with video (and the CPU is at 40%
in LiveTV on a 1080i stream, and 70% playing a 1080i recording).

AFAICT Everything points to audio, but I don't understand how that
would interact with LiveTV mode. (I even tried removing the avlock
locking, to see if there was some lock contention, it had no effect.)

Anyone have an idea what might be happening here?

-- Daniel