Muxing 2 bluray with different fps: change audio speed or video speed (for quality)?

[incredibilePino]

Hello, everytime I need to mux different bluray with different fps, should I modify the speed of audio or the fps of the video? the goal is to preserve the quality as much as I can, and I don't know what is better to do and why. I know:
1) changing the audio speed can alter the pitch and it can't be done without re-encoding the file (for video is possible to avoid re-encoding)
2) changing the video fps should be avoided because for big changes (23.9 to 25) can completely alter the product and in animation is a bad practice because anime/cartoon animation is highly correlated to the speed between a frame and another

I don't know if my motivations are good, what is the best option in these situations? And more important: why?

[videoh]

What are the two FPS rates you are dealing with?

[gonca]

Are you talking about muxing the video of one blu ray with the audio of a different blu ray?

[incredibilePino]

What are the two FPS rates you are dealing with?

it's just an example, I need a sort of general rule (I know it's subjective, but it can be a generalization in my opinion). The most common cases for me are: muxing audio of a bluray of my region, with the video of a foreign bluray. So my language BD is often 24fps or 25fps, and the foreign BD is 23.9, but it can be the opposite.

In your opinion is it better to stretch the video or the audio (in order to preserver quality), and why?

[incredibilePino]

Are you talking about muxing the video of one blu ray with the audio of a different blu ray?

Yes, an example can be:
I have 2 BD of a Ghibli movie, an European BD in my native language and a Japan BD with a bit better video (nothing shocking, but all communities prefer the JAP BD when we talk about some Ghibli movie, and they have a little [3-4%] more bitrate) than the BD of my native language. So I have to choose:

1) should I stretch the EU BD audio or change fps of the JAP BD video? What is the less destructive action in these situations?

2) is it worth to get a better video quality in exchange for an audio/video stretch, or should I give up the JAP video and use my language BD in order to preserve the quality as much as I can (so avoiding any modification of fps/speed) but loosing a better video?

[gonca]

it's just an example, I need a sort of general rule (I know it's subjective, but it can be a generalization in my opinion). The most common cases for me are: muxing audio of a bluray of my region, with the video of a foreign bluray. So my language BD is often 24fps or 25fps, and the foreign BD is 23.9, but it can be the opposite.

In your opinion is it better to stretch the video or the audio (in order to preserver quality), and why?
You can't really stretch video, you must duplicate frames, I believe
When doing this you must make sure sync won't be lost

[Emulgator]

I would stretch audio to keep video at its intended pace.
25p speedup is +4,16%, not my cup of tea.
Almost all films suffer if sped up.
Downside: you need a very good audio resampler.

[incredibilePino]

1) I can use ffmpeg "atempo" or sox, I don't know if you consider it a good choice, I suppose yes
2) anyway for the video I'd change FPS in a lossless way, changing the timestamp, so without encoding the video. Why this is bad in your opinion? It seems not noticeable (I tested 24fps->25fps)

[tebasuna51]

If you don't need audios from the best video source (warning: the bitrate must be corrected to the new duration to know if is really best, a 4% can be only by the diferent duration) the best option is change the fps (lossless operation) of the foreing video (without recoding).

Of course if is a VFR video you need correct also the timestamps.

[incredibilePino]

If you don't need audios from the best video source (warning: the bitrate must be corrected to the new duration to know if is really best, a 4% can be only by the diferent duration) the best option is change the fps (lossless operation) of the foreing video (without recoding).

Of course if is a VFR video you need correct also the timestamps.

Why are you sure it's the best option? This is my main doubt since weeks. I read on videhelp (one of the latest thread "Mux 2 bluray with different fps: change audio or video speed (for quality)?" on Bluray Ripping section) that devices like players/monitors, handle videos with losslessly changed fps in a destructive way (on-the-fly interpolation).

Cit: Losslessly changing the frame rate will prevent any quality loss of the video or audio. Any recompression of video or audio with any lossy codec will result in some loss of quality.
The issue then becomes how your playback devices deal with different frame rates. For example most 50 Hz displays will convert lower frame rates to 50p by simple duplication. A 25 fps source will be converted to 50 fps by displaying every source frame twice. But that won't work with a 24 fps source as the result would be 48 fps, not 50 fps. So most source frames will be displayed for the duration of 2 output frames, but every 12th will be displayed for 3 output frames. So every 12 input frames becomes 25 output frames, 24 input frames becomes 50 output frames. That creates 2 little jerks every second. It will be obvious in smooth panning shots or other smooth motions. In such a case you may decide that a small loss of audio quality is preferable to getting slightly jerky video.

[tebasuna51]

I can't understand your argument. Correct me if my example is wrong:

I have a movie 25 fps in spanish language (duration 1000 sec.) and the same movie at 23.976 fps in other language (duration 1042.7 sec) with best video.
My playback device must be ok for 25 fps.

If I slowdown the spanish language and use the 23.976 video I have two problems the audio and the video.
If I use the spanish language and play the 23.976 video at 25 fps all is ok, we don't need any recode or aproach.

[incredibilePino]

My playback device must be ok for 25 fps.
?

So I don't know when a playback device is good for a certain fps. I use MPV player (archlinux) on a good Dell U2715H LCD. In my country (italy) there are 23.9fps, 24fps and 25fps bluray. I suppose they are all good for my device

[tebasuna51]

In my country (italy) there are 23.9fps, 24fps and 25fps bluray. I suppose they are all good for my device

Also here (spain), then what is the problem to play the foreing video at 25 fps instead 23.976?

A player show a video image 40 ms (if 25 fps) instead 41.708 ms (if 23.976). No problem.