[FFmpeg-trac] #9440(undetermined:new): timeline-free dash muxing not possible with all framerates

FFmpeg trac at avcodec.org
Sun Sep 26 05:41:18 EEST 2021

#9440: timeline-free dash muxing not possible with all framerates
             Reporter:  Gregory      |                     Type:  defect
  Beauregard                         |
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:  dash         |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 Summary of the bug: ffmpeg's dash muxer is not compatible with all
 framerates in no-timeline mode

 In DASH it is required that the segment duration be specified very
 accurately when `use_template` is enabled and `use_timeline` is disabled
 in order to get the modern templated playlists. In particular,
 submicrosecond errors result in the stream failing to play after a couple
 segments with e.g. dash.js. The DASH spec provides the timebase so that in
 the mpd the segment duration is effectively specified as a rational
 number. This allows the segment duration to be an exact multiple of the

 Currently, ffmpeg accepts a "duration" field for the `-seg_duration`
 option of the dash muxer that it converts into an integer number of
 microseconds and a timebase of 1000000 meaning that the ffmpeg DASH muxer
 in modes without a timeline is only compatible with segment durations that
 can be represented exactly as an integer number of microseconds.

 Not all slightly uncommon framerates in the wild have '''any''' reasonable
 segment duration that can be represented exactly with an integer number of
 microseconds, meaning that ffmpeg's dash muxer does not support these
 framerates in no-timeline mode.

 To demonstrate this, consider the slightly uncommon framerate 23.976
 (2997/125). In order to have a truncating float representation, the
 segment duration (=GOP_SIZE/framerate) must, at minimum, have a reduced
 fraction representation with only facts of 2 and 5 in the denominator.
 Because the GOP_SIZE is an integer and 2997 has no factors of 2 or 5, the
 GOP_SIZE must at minimum have 2997 as a factor in order to be compatible
 with ffmpeg's DASH muxer. If this minimum value is chosen, the segment
 duration would be 2997/23.976=125 seconds. So the ffmpeg DASH muxer is
 unusable in no-timeline mode with framerates of 23.976 and segment lengths
 under 125 seconds (not generally usable for streaming). Note in addition
 to this, to be compatible with the DASH muxer the reduced fraction
 representation of the segment length must have a denominator <=10^6 due to
 the microseconds used, but that's not relevant to see the failure here.

 How to reproduce:
 Generate a DASH playlist with `use_template` enabled and `use_timeline`
 disabled using a file with a framerate of 23.976 (2997/125). dash.js will
 not be able to play more than a couple segments without going out of sync
 and failing to load the stream.
Ticket URL: <https://trac.ffmpeg.org/ticket/9440>
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