[mythtv-users] Lossless Cut and keyframe cuts off

HP-mini blm-ubunet at slingshot.co.nz
Wed Nov 14 19:55:59 UTC 2012

> There could be several causes for this..I have no experience with mpeg2
> recordings but have spent months playing with cut-lists & dd based
> cutting.
> 1. The cut-list editor is misleading  with H264 video recordings
> (mpeg2-ts from dvb-t). H264 uses out-of-order frames & keyframes can be
> 2 sec apart & can even be intra-refresh B frames.
> Mpeg2 uses out of order frames as well.
> For example a recording with first keyframe at frame =24.
> The editor shows a valid picture at frames that it can not (before the
> first keyframe).
> The editor can not frame advance from frame 1 (not wrong) but there is
> no possible image.
> The editor seems to display the picture created by the first possible
> returned decoded frame. It seems to stuff as many frames in until the
> decoder returns anything. So the editor shows an image from 1 frame to
> 20+ frames in advance of the current time.
> This results in  cut-list offset error.
> A possible explanation is that ffmpeg decoder only returns decoded
> frames when "enough" frames are passed to it. With out-of-order
> encoding, this can be of the order of 1 sec (25+ frames)..
> 2. mkvmerge cut points are at the keyframe immediately preceding the cut
> time.
> 3. could be bug in lossless_cut keyframe adjust..moving the wrong way..
> Possibly setting the tuner to start on a sequence header might help
> reduce the offset error.
> I have not tried this. 

The start on sequence header is a tick box in mythtv-setup tuner setup..

Try a simple expt:
- invoke cut-list editor on a obvious transition.
- note the frame number
- navigate around transition with cut-list editor
- does the transition still occur at same frame number ?
- some recordings could be worse than others due to location of

More information about the mythtv-users mailing list