But then I got a firestick and Kodi handles whatever.
Was still nice to learn. Gave the task to a Pi4 and ffmpeg.

Think it was a case of doing some sort of change/conversion from one version AC3 to another.

Just checked the ffmpeg command I used. Looks like I initially did an AAC reencode (slower) then found out about mapping the AC3 or something so ended up with this which if I remember rightly wasn't a reencode, more a remux or something...

ffmpeg -i input.mkv -map 0 -c:v copy -c:a ac3 -c:s copy output.mkv

Looks to me as though you're doing a frame by frame copy of the video, a copy of the subtitles (if any) and a re-encode of the audio to AC3. Which, I imagine, is what you wanted.

For my little puzzle, I first extracted the video stream.
ffmpeg -i "E:\MakeMKV\source.mkv" -c copy -f h264 "E:\MakeMKV\video.mp4"

then converted the video from 24FPS to 25FPS
ffmpeg -r 25 -i "E:\MakeMKV\video.mp4" -c copy "E:\MakeMKV\destination.mp4"

then extracted the audio from the original source, stretched it with a factor of 25/24 (near enough) and remuxed it with the "destination" video from earlier

ffmpeg -i "E:\MakeMKV\source.mkv" -r 24 -i "E:\MakeMKV\destination.mp4" -filter_complex "[0:a]atempo=1.0416666[out]" -map 1:v -map "[out]" -c:a aac -c:v copy "E:\MakeMKV\out.mp4"

 

ffmpeg is awesome!

How does it handle adding the additional video frames -- are there little 'blips' in the footage, or is it perfectly smooth?

Some people say it's not great as a muxer/demuxer (and it's certainly slower that, say, mkvtoolnix) but I've never had any problems.

For a stand alone program, it has an astonishing number of keys, flags, commands and variations on input syntax. Which is why it can be a bugger to use. But it is awesome.

It doesn't add any. The video simply runs at a different speed. That why I had a problem with subtitles from a DVD version of the film (25FPS) and a blu ray video version at 24FPS. The subtitles slowly got ahead of the video until at the end they were a full 4 minutes out.

So it's running slightly faster, not enough to notice?

Exactly. The only way you'd notice would be to run my copy and the original (unaltered) version side by side. I haven't tried it, but i reckon you'd see them going out of sync with each other quite quickly, say after 30 seconds. But if you watched one after the other they'd appear identical. Imagine a fairly substantial piece of action, say, somebody enters a bar, buys a drink and sits at a table. The scene takes half a minute. You're never going to notice a 1 second(ish) difference spread over that entire scene.

It only happened because the original movie Les choses de la vie was shot on film at 24FPS and the PAL DVD was encoded at 25FPS (which, incidentally is because the PAL TV standard was developed to match the 50Hz power supply common in Europe. In the US it was 30FPS to match 60Hz power, but colour TV meant they ended up with 29.97FPS, which is a long story). In the US and other places that use NTSC, the difference between film at 24FPS and NTSC at 29.97 is too great to simply speed up the playback, so they do have to add in images, but these images (fields as opposed to frames, because the video is interlaced) are added to slow the FPS down to 23.976 (don't ask).