[mythtv-users] FFMPeg Commercial cutting puts audio out of sync

[Justin Alcorn]

My jump goes from .5 sec to 1 frame. That means one key frame?

-- Sent from my Droid
On Sep 13, 2014 5:36 PM, "Mark Perkins" <perkins1724 at hotmail.com> wrote:

>
>
> > On 14 Sep 2014, at 5:29 am, "Justin Alcorn" <justin at jalcorn.net> wrote:
> >
> >> On Sat, Sep 13, 2014 at 3:05 PM, John Finlay <finlay at moeraki.com>
> wrote:
> >> ffmpeg will only cut at a keyframe (AFAIK) and will skip to the closest
> >> keyframe following the position you specify. Therefore to use the
> >> mythfronteend editor to specify cutpoints you'll have to specify
> keyframes
> >> as the cutpoints around commercials. Unfortunately, the keyframes may
> not
> >> line up with the start and end of commercials - this is what I
> experience
> >> with ATSC MPEG2 streams.
> >>
> >> An additional problem with the mythfrontend editor is that it uses
> 1-based
> >> counting of frames and ffmpeg uses 0-based so if you take the
> mythfrontend
> >> cutlist of keyframes and use it in ffmpeg you will find that ffmpeg
> will cut
> >> at the following keyframe the one specified. The workaround is to
> subtract 1
> >> from every cutlist frame number.
> >>
> >> I hacked together a python script as a lossless transcoding Job to use
> >> ffmpeg (mythffmpeg) to cut out commercials using the mythfrontend
> cutlist.
> >> It works for MPEG2 but I don't know about H264 but in theory it should
> work.
> >> Look at the ffmpeg segment muxer
> >> (
> https://www.ffmpeg.org/ffmpeg-formats.html#segment_002c-stream_005fsegment_002c-ssegment
> )
> >> for documentation.
> >
> > OK, that info helps.  One problem is that the script I found
> > translates the Myth cutlist to seconds and uses that to specify the
> > cuts.  I guess I better switch to using frames. I assume there's some
> > way to do that in ffmpeg?
> >
> > The biggest unanswered question - HOW do I always specify keyframes as
> > cutpoints?  I don't know how to find keyframes.
> >
>
> When using the cutlist editor you can use up/down arrows to change the
> jump by amount. When I get to where I want the cut I just change to jump by
> keyframes then move left / right to pick the keyframe I want or suits the
> best.
>
> > Sorry, but I'm still pretty new at video.
> >
> > --
> > Justin B. Alcorn
> > PGP Fingerprint A36D D691 C5B0 BE15 5A2A AF49 AA1C 372C
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