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

[Daryl McDonald]

On Sat, Sep 13, 2014 at 5:58 PM, Justin Alcorn <justin at jalcorn.net> 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> 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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>
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No, keep going down until it says"keyframe" instead of a time increment
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