How to decimate irregular duplicate frames with ffmpeg?

[ForSerious]

I have a whole season of a show that, for whatever reason, was interlaced. That's usually not a big deal, but for this season, I cannot un-DVD the frame rate without messing up the motion. On the seasons before it, the simple -r 23.976 command works perfectly.

Looking through sections of the videos frame by frame, it looks like it has the typical duplicate frame every fifth frame. I don't know what's going on with it.

If I were to pretend like the duplicate frames were irregular, would I be able to get the results I want? My plan of attack is this:
decimate=cycle=15:dupthresh=0.8,decimate=cycle=15: dupthresh=0.8,decimate=cycle=15:dupthresh=0.8
I might be wrong, but I think this should remove 1 duplicate in 15 frames, then another, then another, no matter how unevenly spaced they might be. I suppose I'm relying on the decimate filters running one after another.

After trying that out, it didn't work. What am I doing wrong or missing?

[ForSerious]

Some more context, and I might have figured it out. Typical.

I'm using bwdif to deinterlace and I have bob set to true. So I'm doubling the frame rate:

-filter_complex setsar=1,bwdif=mode=1:parity=-1:deint=0,decimate=cycle=15:dupthresh=0.8,decimate=cycle=15: dupthresh=0.8,decimate=cycle=15:dupthresh=0.8.

That one didn't work.
I just tried this next one with better results, but the sound became off:

-filter_complex setsar=1,bwdif=mode=1:parity=-1:deint=0,decimate=cycle=2,decimate=cycle=5

[Selur]

Haven't used ffmpegs decimate for ages, but here are my 2cents about it:

dupthresh
Set the threshold for duplicate detection. If the difference metric for a frame is less than or equal to this value, then it is declared as duplicate. Default is 1.1

(source: https://ffmpeg.org/ffmpeg-all.html#decimate-1)
But do make values below 1 even sense? (seeing that the default is 1.1, I'm not sure whether values <= 1 do make any sense)
Couldn't find how 'the difference metric' of 'decimate' works.

If the difference metric for a frame is less than or equal to this value, then it is declared as duplicate.

So unless there are still frames that differ less than 'dupthresh=0.8' (whatever that actually means), applying 'decimate=cycle=15:dupthresh=0.8' wouldn't do a thing,...

Maybe try mpdecimate instead, at least there seems to be some more details on how it works.
For mpdecimate:

hi
lo
frac

Set the dropping threshold values.

Values for hi and lo are for 8x8 pixel blocks and represent actual pixel value differences, so a threshold of 64 corresponds to 1 unit of difference for each pixel, or the same spread out differently over the block.

A frame is a candidate for dropping if no 8x8 blocks differ by more than a threshold of hi, and if no more than frac blocks (1 meaning the whole image) differ by more than a threshold of lo.

Default value for hi is 64*12, default value for lo is 64*5, and default value for frac is 0.33.

I just tried this next one with better results, but the sound became off:

especially inconsistently dropping frames without adjusting the audio might cause a/v sync issues,...


Cu Selur

[ForSerious]

Thanks for the suggestion. I cannot figure out how to make mpdecimate work in a cyclical pattern though.
I think the sound issues are from something else I'm doing with the video.

decimate=cycle=2,decimate=cycle=5

Seems to be the solution.
I had tried

shuffleframes=0|-1,decimate=cycle=5

and it didn't work. I thought decimate=cycle=2 was the same.