#11249: `ffprobe` reported MPEG-2 GOP header timecodes 1 frame early
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Reporter: MaxEliaserAWS | Owner: (none)
Type: defect | Status: new
Priority: normal | Component: avcodec
Version: git-master | Resolution:
Keywords: mpeg2video | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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Comment (by MaxEliaserAWS):
> Why should the timestamps of GOP be additionally stored, when directly
derivable?
I don't really disagree, for containers/codecs that can signal a nonzero
starting PTS. I would just note that there are a lot of workflows/software
out there which still rely on timecode and which do not do this derivation
even though it is possible to do.
An example of where it might not be as easily derivable is in the
QuickTime format, where the STTS/CTTS box structure cannot easily achieve
a nonzero starting PTS, but where timecode can be signaled as a separate
track of type `tmcd` to start the timeline at 10:00:00:00.
It is still standard in television production to start timecodes at 10
hours (1 hour for film.) This practice was established because mechanical
tape/film equipment needs a little bit of time to get its motors spinning
up to full speed, so the tape can't start at zero; if the pre-roll starts
at 9:59:52:00, then the actual content can start at 10:00:00:00, a nice
round number. Although we don't use this type of equipment anymore, this
convention is still absolutely standard in television/broadcast, and there
are a lot of files out there which signal this in timecode but not in PTS.
Some information on this is here:
https://youtu.be/C-aJEhZtnVo
(Apologies for video, I would have provided a written resource if I could
find one.)
If we were to redesign all video software and file formats today, I agree
it would be better to just focus on PTS and eliminate SMPTE timecode. But
old technology still has a lot of influence from beyond the grave, and
people still want to watch the old content. (Otherwise we'd have stopped
using 29.976 FPS in north America a long time ago.)
For `ffmpeg`'s use case as a self-contained transcoder, it's probably easy
to avoid using timecode, but for `ffprobe`'s use as a video analyzer tool
or `avcodec`'s use in other video software, I would argue for its
continued usefulness.
Anyway, thanks for the opportunity to nerd out about video stuff, this was
fun!
-Max
--
Ticket URL: <https://trac.ffmpeg.org/ticket/11249#comment:14>
FFmpeg <https://ffmpeg.org>
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