[mythtv-users] seeking/editing in mythfrontend

John Pilkington johnpilk222 at gmail.com
Sat Mar 27 18:46:53 UTC 2021

On 27/03/2021 17:17, Leo Butler wrote:
> John Pilkington <johnpilk222 at gmail.com> writes:
>> On 27/03/2021 08:26, Stephen Worthington wrote:
>>> On Fri, 26 Mar 2021 20:50:26 -0500, you wrote:
>>>> I transcode my recordings to an x264-encoded video stream in a matroska
>>>> container. I use ffprobe to update the recordedseek table after
>>>> transcoding. I have been doing this for a couple years.
>>>> In both v0.28 and v0.29 (from the mythbuntu ppa), I was able to seek and
>>>> edit these mkv files without difficulty. In v0.31/fixes from the same
>>>> ppa, seeking and editing is broken for these transcoded files--it
>>>> appears that any movement in the video is only by a keyframe. Playback
>>>> is fine.
>>>> Seeking and playback in fine in vlc and ffplay, fwiw.
>>>> This seems like a regression to pre v0.28 behaviour.
>>>> Has anyone seen similar behaviour? Suggestions on how to workaround the
>>>> problems? Ideas about what caused the regression?
>>>> TIA,
>>>> Leo
>>> Have you tried using the mythcommflag --rebuild option to rebuild
>>> the
>>> seek table using the official method?
>> TTBOMK the MythTV seektable isn't used for playback of files in
>> matroska format.  If you somehow create one it may upset things.  ISTR
>> that 'recordings' and 'videos' get different treatment.
>> I 'transcode' to mythffmpeg  -f mpegts format, which is myth-native
>> and plays well for me in the frontend or via UPnP or Firestick 4k.
>> But all my video cutting, mostly from mpeg2 recordings, is
>> keyframe-based;  I find that acceptable.  I still get the best results
>> by using ProjectX to demux and sync-adjust before remuxing, but can't
>> do that for h264.
> Re: -f mpegts.
> I tried the following experiment. I cut a matroska container and remuxed
> it to mpegts and matroska separately. I rebuilt the seektable for the
> mpegts container. Seeking (in the editor or playback) works fine with
> that video.
> But...the file size of the mpegts is 9.5% larger than the mkv. That
> 9-10% overhead seems consistent across a few other recordings that I
> have remuxed, too.
> Leo
Yes.  I have recently been recording from 'Sky Arts' which became 
available via Freeview UK last September, so although it's mainly 
repeats they are new to me.  Format is 544x576 mpeg2 progressive, but 
seems adequate.  The recordings often show a surround-sound icon but 
it's joint stereo at 128kb/s.  When cut, concatenated and sync-adjusted 
by ProjectX, mythffmpeg  .ts output shows a muxing overhead around 5.5%, 
which is higher than I used to get with mplex and a near-DVD output 
format.  I guess the overhead is there in .ts for better potential 
recovery of transmission errors.

Peter's leanfront player used to stutter on most files in the mplex 
format, and long ago I was told that mplex would fail at high bitrates. 
I haven't tried them together recently, but the .ts files play flawlessly.

I mentioned the decoder setting because it does (or did) affect stepping 
in the editor.  If I used nvdec, in one direction only alternate key 
presses were effective.  And h264 still-frames were blurred...

John P

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