[mythtv-users] Transcoding: Audio/Video Sync

Matt Vollmar matt at vollmars.com
Tue Dec 7 20:57:01 UTC 2004

David Smith wrote:

>Hi Matt,
>I'm not familiar with the command line parameters to mythtranscode,
>but are you using the cutlist, or removing commercials as part of the
>This is just a theory, but I'm wondering if a cut point occurs between
>keyframes in the MPEG stream if this loss of synchronization will
>occur.  Perhaps the video is processed all the way to the cutpoint,
>but the audio processing stops at the keyframe, or vice-versa.  Maybe
>others can confirm or deny this theory.
>I had a similar problem with NUVEXPORT, but only on one recording. 
>I've done several others and haven't seen the same issue and I didn't
>cut them on the keyframe intentionally.
>P.S. That's one of my favorite James Bond films!
>On Tue, 07 Dec 2004 12:36:43 -0500, Matt Vollmar <matt at vollmars.com> wrote:
>>OK, I know this has been hashed around a bunch on this list, and I
>>searched for anything that might help, but I couldn't find something to
>>address this issue (I'm sure I didn't search long enough).  I transcoded
>>(manually from the command line) a movie last night, and looked at it
>>today to find an ever so slow loss of sync between audio and video.  The
>>video ends up about .5 seconds faster by the end of the movie.  In the
>>original, it is not thus.  Is there a way for me to fix this?  Here is
>>the command line stuff I use:
>>mythtranscode --showprogress -p autodetect -c 1073 -s
>>2004-11-25-12-00-00 -f . 2>&1 > /dev/null
>>ffmpeg -f rawvideo -s 480x480 -r 29.97 -i ./vidout -y -f s16le -ar 32000
>>-ac 2 -i ./audout -b 1152 -vcodec mpeg1video -s 640x480 -hq -deinterlace
>>-qmax 3 -qmin 13 -ar 44100 -aspect 4:3 "./You_Only_Live_Twice.mpg"
>>It is possible that some of my parameters are not correct.  I got these
>>from someone who said they were good quality, which they are very
>>acceptable.  Plus, it transcodes at just a hair slower than real-time,
>>so that part rules.  Should I use fifosync on the mythtranscode command
>>line or what?  Should I not deinterlace?
>>Thanks for any help,
I did indeed use cut points, but I actually transcoded them first to cut 
out the commercials, so when I am comparing these 2 videos it is really 
apples to apples.  I am sure that I cut them outside of the keyframes, 
because I move my cut-points to the exact blank spot.  I didn't realize 
that this could be a problem.  I will be more careful next time, but I 
would still like to fix this.


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