Fixing 24p video encoded as 30p

[Luke M]

Occasionally I run across video which is really 24p, but is encoded as 30p. As a result it's very juddery because players treat it as 30p video. Every 4th frame is repeated, which is very noticeable (much more than common 24p->60p judder).

My solution is to use ffmpeg:

ffmpeg -i in.mp4 -vf decimate -acodec copy out.mp4

This works, but it's a transcode operation, which means it's slow and loses quality. It would be nice if there were a way to surgically remove the repeated frames (which aren't even "physical" frames, just a repeat flag! [edit: incorrect]). But I haven't found anything that can do that.

[hello_hello]

They sound like Handbrake encodes where someone has forced a constant 29.940fps output frame rate.

I'd be surprised if they're not actual frames. and I know of no other way to remove them aside from re-encoding if they are, but if there's repeated frames resulting in a variable frame rate (effectively) you could try opening the source with MKVToolNixGUI and specifying a 24p frame rate when remuxing. All frames in the output file should then display for the same duration at 24fps, but I suspect the repeated frames are real frames and it won't work.

[sneaker]

which aren't even "physical" frames, just a repeat flag!

How did you determine this? Sample?

Theoretically it may be possible to remove such frames without loss of quality if one of the following conditions is met:
1.) no inter-frame dependencies blocking it (e.g. intra-only or repeat flags)
2.) you encode to a lossless format (file size will multiply if the source isn't lossless itself, though)

Most of the time it is not possible. We had a similar question recently whose OP had intra-frame only encoding yet it proofed difficult.

[Luke M]

My mistake - there are no frames encoded as repeated. 2% of frames are type I. The rest are type P.