[mythtv-users] HD-PVR dropped frames / MythTV AV glitches

[Will Dormann]

Hi folks,

I've recently been digging into a problem that I've concluded to be that
my HD-PVR is dropping frames.   The symptoms are that once every 10
minutes or so, the video seems to lag for a little bit.   And shortly
thereafter, my digital audio receiver audio signal (and subsequently
sound) goes off for a fraction of a second.

Initially, I figured this was due to system load, and that it is a
playback-only problem.   When encountering such a glitch, if I rewind
the video, it happens in the same place, 100% of the time.   Which makes
me think that the problem is not in playback, but actually in the
recording.   But since the HD-PVR does hardware encoding, it should be
able to record reliably all of the time, regardless of what's going on
with the computer system it's plugged into.

Looking at mythfrontend.log, I'll see lines like this at the point of
the glitch:
W Decoder avformatdecoder.cpp:4908 (GetFrame) AFD: Audio 194347 ms
behind video but already 220 video frames queued. AV-Sync might be broken.


When I look at the point in question of the recorded video on another
(non-Myth) system, it seems as if there may be a single dropped frame or
something.  However, the audio and video are pretty seamless as far as I
can tell.


Current theories:

1) The cable box is fine, and something with the HD-PVR is causing it to
produce a flawed stream.   I've heard of overheating and power supply
issues with the HD-PVR, but from what I gather, either of those issues
will result in a lock up, rather than droped frames.   I also tried
taking the cover off, and making sure that it's running in a cool
environment, but that made no difference.  I also tried updating the
firmware of the HD-PVR to the latest, and that also made no difference.

2) The cable box (Comcast Motorola 6200) is actually emitting video with
occasional dropped frames.  And that glitch is cascading through the
chain of MythTV, VDPAU, Pluseaudio, SPDIF, digital audio receiver, etc.,
to the point where it's noticeable.


If it's option 1), then I suppose that there's a chance that it's a
little bit more under my control.  (e.g., if the HD-PVR is dying, maybe
it needs to be replaced).   If it's 2), then I suppose that the best I
can do is to just minimize the noticeable impact of a dropped frame.

Given that my symptoms have been brief periods of dropped audio, my
first reaction was to play with options like Realtime priority threads,
"Extra audio buffering" or tinkering with
/proc/asound/card0/pcm1p/sub0/prealloc
However, given my latest theory that the problem is likely with a
dropped frame or two and the subsequent re-synchronization of AV, I'm
wondering if my attempts to get rid of audio glitches have actually had
the opposite effect of increasing the recovery time for dropped frames.


Has anybody else experienced these symptoms, and if so, have you found a
fix?   Or even, are there generic techniques for minimizing the impact
of dropped frames?



Thanks
-WD