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

[Mike Carron]

On 09/13/2014 02:58 PM, Justin Alcorn wrote:
>
> 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 
> <mailto:perkins1724 at hotmail.com>> wrote:
>
>
>
>     > On 14 Sep 2014, at 5:29 am, "Justin Alcorn" <justin at jalcorn.net
>     <mailto:justin at jalcorn.net>> wrote:
>     >
>     >> On Sat, Sep 13, 2014 at 3:05 PM, John Finlay
>     <finlay at moeraki.com <mailto: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
>
>
***
No, but Keyframe is one below Frame. The sequence goes ... 5 sec -> 1 
sec -> Frame -> Keyframe -> Cutpoint.

mike