mov timebase issue with large subtitle track

[dave rice]

Summary of the bug:

I was trying to mux a vtt file into an mov but getting this error:

FATAL error, file duration too long for timebase, this file will not be
playable with QuickTime. Choose a different timebase with -video_track_timescale or a different container format

After trying the timescale suggestion, I realized that the issue is not about the video track but about the subtitle track. There's a command to generate the error below. AFAICT there's no subtitle_track_timescale and I'd rather stick with mov in this case.

How to reproduce:

ffmpeg -i test.vtt -map 0:s -c:s mov_text -tag:s:s:0 tx3g test.mov
ffmpeg version git-2021-01-28-ed51890 Copyright (c) 2000-2021 the FFmpeg developers
  built with Apple clang version 12.0.0 (clang-1200.0.32.29)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/HEAD-ed51890_9 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack
  libavutil      56. 64.100 / 56. 64.100
  libavcodec     58.119.100 / 58.119.100
  libavformat    58. 65.101 / 58. 65.101
  libavdevice    58. 11.103 / 58. 11.103
  libavfilter     7.100.100 /  7.100.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Input #0, webvtt, from 'test.vtt':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Subtitle: webvtt
Output #0, mov, to 'test.mov':
  Metadata:
    encoder         : Lavf58.65.101
    Stream #0:0: Subtitle: mov_text (tx3g / 0x67337874)
    Metadata:
      encoder         : Lavc58.119.100 mov_text
Stream mapping:
  Stream #0:0 -> #0:0 (webvtt (native) -> mov_text (native))
Press [q] to stop, [?] for help
FATAL error, file duration too long for timebase, this file will not be
playable with QuickTime. Choose a different timebase with -video_track_timescale or a different container format
size=    1669kB time=00:35:59.99 bitrate=   6.3kbits/s speed= 301x    
video:0kB audio:0kB subtitle:1047kB other streams:0kB global headers:0kB muxing overhead: 59.359367%

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.