[FFmpeg-trac] #7506(undetermined:new): EIA-608 Closed Captions distortion on the output.

FFmpeg trac at avcodec.org
Sun Nov 18 08:45:07 EET 2018

#7506: EIA-608 Closed Captions distortion on the output.
             Reporter:  w_boba       |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
              Version:  git-master   |  undetermined
             Keywords:               |               Resolution:
  EIA-608,a53cc, closed captions     |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |

Comment (by The_Bart_The):

 I've done some research and I think I've cracked it.

 Each frame of video can hold up to two characters of closed caption data.
 In line 5921 of your log, you'll see 367 duplicated frames and zero
 dropped frames.  That's why you see duplicated letters.

 If you use -vsync 0 like so:
 % ffmpeg.exe -y -v 99 -loglevel 99 -i sample-1min.ts -c:a aac -c:v libx264
 sample-1min.mp4 -vsync 0
 FFmpeg won't try to adjust the framerate of your video by duplicating &
 dropping frames and your subtitles should remain intact.

 I figured I was on to something when I remembered my local NBC affiliate
 was causing me the same problem.  I remembered that all those years I was
 using HandBrakeCLI, NBC was the only network doing this to me:


 Once I realized NBC was the only network changing framerates back and
 forth (Video and Film are code for 30 fps and 24 fps, respectively), and
 saw duplicated frames in my own FFmpeg logs I went looking for how to
 preserve the original framerate. I came across "-vsync 0" and here we are.

 Of course, the bug's still valid despite this workaround.

Ticket URL: <https://trac.ffmpeg.org/ticket/7506#comment:3>
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