[mythtv-users] problem with audio underruns on 720p only

Jeffrey J. Kosowsky mythtv-users at kosowsky.org
Mon Dec 8 03:56:11 UTC 2008

Greg Zornetzer wrote at about 18:59:19 -0800 on Sunday, December 7, 2008:
 > Hi all,
 > I am running mythfrontend on a Core 2  6550 dual core machine with a
 > motherboard Nvidia 7100 display.  Transport is xv-blit.  CPU usage doesn't
 > seem to be a problem with HD content (~25-50% usage from top in 720p or
 > 1080i mode)
 > I am using the RPM version of mythtv-0.21 from rpmfusion.org.  This happens
 > with both the released -13 RPM and the newer -14 RPM in rpmfusion's testing
 > directory for fc9.  I'm running this on fedora core 9.
 > When I play 720p, either in live tv mode, or freshly recorded, I get the
 > following (recorded with -v playback):
 > NVP: Video is X.xxxx frames ahead of audio, doubling video frame interval to
 > slow down
 > (repeated several times)
 > WriteAudio: buffer underrun
 > In this case, X.xxxx is somewhere between about 7 and 12.
 > This is accompanied by stuttering of the audio every second or so.
 > I noticed that there was a bug that got fixed a few months ago regarding
 > audio buffering with 720p data (see http://svn.mythtv.org/trac/ticket/5749),
 > but the fix for this (included with the newest testing version of the rpm
 > from rpmfusion), doesn't fix the problem.
 > I have noticed that if I transcode the 720p recording using the default
 > settings, the transcode will replay flawlessly.  So, it's some kind of
 > problem with mythtv interpreting the data coming right off the TV line.
 > That's sort of good news - this way we can at least use the recordings that
 > are made.
 > Note that there is no problem with playback of 1080i material.
 > I believe that some others may be seeing the same problem.  I think that the
 > problem described here may be the same:
 > http://mythtv.org/pipermail/mythtv-users/2008-December/240895.html
That sounds like exactly the same problem I am reporting in the link
you quote. Though I seem to be seeing it with ALL digital OTA
recordings (whether SD or HD) . Interestingly, the problem occurs in
both LiveTV and playback (without transcoding) but is worse in LiveTV.

What I don't understand is the apparent contradiction between:
1. The symptom of slow video and normal speed audio "pausing" for the
   video to catch up, which is consistent with the error message:
   		 "WriteAudio: buffer underruns"

2. The error messge:
    Video is N.nnnn frames ahead of audio, doubling video frainterval
	to slow down.
	where N ranges from about 8 to 16 frames -- which would seem to be
    saying the opposite -- that audio is running slow!!!

Any resolution to this apparent contradiction or am I misunderstanding
the error messages?

 > I do have 'extra audio buffering' selected in the configuration for mythtv -
 > I saw that asked before.
 > Any suggestions?

Extra audio buffering didn't help for me and shouldn't really if the
problem is that the video is perpetually running slow.

For me the problem is not CPU, since my total CPU for mythfrontend
plus backend plus X plus audio is only about 60% (even though I just
have a lowly first gen p4 2.8GHz). 

My concern is that my base problem may be limited video bandwidth
somewhere which is causing the video to run slow (I have a new GeForce
6200 256MB AGP card running at 4x AGP).

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