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

[Stephen Worthington]

On Tue, 11 Nov 2014 09:13:46 -0500, you wrote:

>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

If your hard drive is too busy, it is possible to get dropouts just
like that where some data is never written to the disk.  I have had it
with my DVB-T tuners when I only had two hard drives and recorded too
many channels at once.