Audio gets out of sync when decimating with FFmpeg

[seifenchef]

Hello,

When restoring video files, I often encounter files whose footage was 24 or 25 fps, but which were carelessly placed in a 30 (or 29.97) timeline. This results in every 5th or 6th frame being a duplicate.
This causes unpleasant jitter.
It's especially bad when trying to create slow motion, etc.

If you remove these fake frames with decimate—which works perfectly, using the cycle=6 option even with 25 fps footage, you always end up with out-of-sync audio.

So far, I've been working around this by decoupled the audio track in DaVinci Resolve, then sped it up to 100.10%, and then recoupled it.

This worked perfectly, but it's annoying in the long run.

The error stems from the analog video era, when NTSC, the color format, was introduced and slowed down from 30 fps to 29.97 fps.

(This is due to coupling the color subcarrier with the line frequency...)
Over 45 minutes, it's about 2.5 seconds (not measured precisely).

Isn't there an option in FFmpeg decimate to prevent this error from occurring in the first place?

kind regards seifenchef

[Sharc]

You can try to adjust the audio speed in ffmpeg, for example for speeding it up by 0.1% add something like:

-filter:a "atempo=1.001"

Edit:
Alternatively you can try to slow down the video by 0.1% like

-filter:v "setpts=0.999*PTS"

[pandy]

Sharc is right - your audio duration shall be the same as video duration - you may convert audio length by factor expressed like this: 'new audio speed'= 'audio duration'/'video duration'.

-filter:a "atempo='audio duration'/'video duration'"