deep inside framerate conversion

[hypercube]

can anybody explain to me why a 25fps movie soundtrack at 44100Hz
must be time stretched to fit in the same movie at 24fps ?

there is a similar stuff in BeSweet named "NTSC to PAL audio convertion".

in my mind, the movie duration is independent of video framerate.
and video framerate is independent of audio framerate. So what are
theses stuff ?

in my case, I record french version of totoro from Videotape: 25fps
I have also the japanese version from DVD: 24fps

[sterno]

The most common way to convert between 24fps and 25fps is just to run the video at the speed you want. By just changing the playback speed you avoid having to add or drop frames (which means you avoid jerkiness), but you change the playing time of the movie slightly. If you change the length (time) you have to adjust the audio to match or it will start out in sync and gradually get farther and farther off throughout the movie.

If you were to convert a 25fps movie to 24fps by just dropping 1 frame every second you wouldn't need to change the audio length, but a lot of scenes wouldn't play smoothly.

[EddyH]

Because the video is showing the same number of frames, but at a different speed, so the playing time is fractionally different.

If its a downloaded or otherwise AVI copy of the japanese one (i assume you're doing something im thinking of doing - applying the non-disney version dub to a nice new dvdrip.. except here its english not french ) then you may get away with just changing the speed of that to 25fps without having to reencode it at all.

If however you wish to keep the jap video intact or there are other reasons... load french audio into e.g. Cooledit. Change playback speed to 23976hz (or 24000hz if it's actually 24.000fps). Re-Sample to 25000hz. Put speed back to 44100hz (and save). The sound will now play slightly slower (and ever so slightly lower pitched) and in time with the slower frame rate.

[hypercube]

in fact I found some information about this pb.
PAL movie are produced from 24fps movie simply
accelerating playing. So audio is quicker than original.
My pb is more complex because my french audio source
(videotape) is not regular. so a simple timestretching
do not work.

thanks for you help.