[mythtv] ffmpeg sync

Bruce Markey bjm at lvcm.com
Sat Apr 21 20:22:15 UTC 2007

Janne Grunau wrote:
> On Tuesday 17 April 2007 20:58:44 Bruce Markey wrote:
> Updated patch: http://www.grunau.be/ffmpeg_sync_8742_13272.diff.bz2

I've been using this in production for several days to look
for MPEG-4 problems and it seems pretty solid. I had tested
recording a 90min rebroadcast of a College Basketball game
then playing it back at 3X smooth motion and didn't find any
broken frames or artifacts. The current SVN did have occasional
encoding problems in addition to the decode problems you fixed.
The encoding problems (assumed because the same artifacts would
appear when played with and earlier frontend) were harder to
isolate so I never followed up. However, the 8742 patch does
not have these and this is an improvement.

I tested the DCT encoding and interlace motion options. These
do increase the CPU usage though I can't say that I can identify
a difference in the video. These had been broken in a earlier
version of myth's ffmpeg but do appear to work in current SVN
and with this patch. So, MPEG-4 seems to be good.

MPEG-2... not so good. If I go to Watch TV and "Y" to an ivtv
card then press the up arrow to change channels every few seconds,
about half the time there are broken frame between the old and
new channel playback. These appear to be broken frame artifacts
from the old channel before playback starts for the new channel.
Next, if I hit left arrow, or any other button that will back up
into the previous channel recorded, the current channel image
will break up before playback restarts from the other recorded

Next, I tried DVD playback but after trying 4 or 5 DVD, I never
got one to play a movie though I got a couple to show the vendor
logos. The screen would remain black at the start of the chapter
and the console would fill with messages like:

[mpeg2video @ 0xb75b3f28]ac-tex damaged at 6 8
[mpeg2video @ 0xb75b3f28]ac-tex damaged at 6 8
[mpeg2video @ 0xb75b3f28]ac-tex damaged at 6 8

Any attempt to skip or jump forward would reset the time
counter to zero and continue spewing the same messages.
I intended to test AC3 with the opening shot from the DVD
"Contact" but it would not play. It does play with current
SVN and surround sound clearly works.

>> Janne, could you please do the following? Create a patch with
>> "svn di",
> No sorry, but I can't. The patches are in two branches of my personal 
> git mirror of the mythtv.org subversion repository.

I try to work on separate things in separate trees so that
I won't screw up (as much =) but I can understand having
diffs in two trees. All the more reason to apply the ambiguous
diff to a clean checkout to see that it applies, compiles, and
preforms the advertised function (after all, that's what you are
asking others to do when they receive the patch). Once this
sanity check works, then you can "svn di" in this tree and
know that the patch is exactly what you've just verified.

--  bjm

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