[FFmpeg-user] FFMpeg problem using fieldmatch and decimate filters to IVTC

[Steve Boyer]

Hey all,

Originally, I posted this to the wrong mailing list and had a request to
execute an ffprobe command. I am reposting and including the output of the
commands.

The ffprobe.txt file is for the sample.vob and the ffprobe.4.18.txt is for
the full, ~2GB vob.

I'm trying to encode a large number of videos so I can watch them on my
phone, but not having luck doing a proper IVTC using the fieldmatch and
decimate filters. I have compiled from source yesterday afternoon as
outlined on https://trac.ffmpeg.org/wiki/CompilationGuide/Ubuntu. Using the
command:

ffmpeg -i 4.18 -map 0:1 -map 0:3 -vf crop=704:480:10:0 -vf
fieldmatch,decimate -c:v libx264 -preset veryfast -level 41 -crf 22 -c:a
libfdk_aac -t 360 4.18.fieldmatch.decimate.mp4

I result in a proper IVTC'ed video stream, but the video seems to go at an
incorrect framerate (although running "ffmpeg -i 4.18.mp4" shows it at a
23.976 fps). I did other versions (using -vf fieldmatch, -vf yadif, -vf
pullup,fps=24000/1001, or -vf pullup -r 24000/1001, etc) and the audio
track remains in sync with the video consistently. Adding the decimate
filter in to the mix seems to make the video play faster than the audio
leading to the desynchronization. I see that
https://trac.ffmpeg.org/ticket/3019 claims this as fixed, but I'm still
experiencing it.

Samples (a snippet of the .vob cut using dd, MP4s going through various
filter combinations, and then a 6 minute run of the encode using the
command above to really show the AV desync) along with text output files
are available on my dropbox at:

https://www.dropbox.com/sh/v9jp4pdxzad4a7g/djUTH4OYcA

If I can help in any way, let me know! A good IVTC solution is the ONLY
thing I miss from Windows so far. I've had hit and miss results with pullup
and -r and/or -fps

Steven Boyer