Audio FPS

[yaang]

Hey guys,

I'm really confused about this i got warned in another forum because i said audio files don't have FPS values. Then i searched around and saw all this audio fps conversion topics around. If i understand correctly changing the fps value of a video file doesn't change it's time length (as in seconds) so how can a audio file be out of synch because of this ? I hope i won't get a warning or a ban for this...

[Baldrick]

If you change the fps in the video it WILL change its time length so you must also stretch the audio. Like several pal dvd releases are from 24fps sources and converted to 25fps, 4% shorter.

[yaang]

Ok thanks, I thought the time length stayed the same and the number of frames changed instead. Thanks again for the fast response

[Gavino]

It really depends on how you change the video frame rate.
If you just change the rate stored in the header, for example, the number of frames is unchanged so the duration will be different. But there are other ways (eg using Avisynth) that involve adding or removing frames to keep the duration the same - in that case, no audio change is required.

[Chris K]

You're right when you say that a audio file has no framerate.

Compressed audio like ac3 and mp2 is stored in blocks of a fixed length that have no relation to video frame rates althought they sometimes are called frames.

The confusion is mostly caused because some audio stretching tools allow to enter a from/to framerate value.
These tools don't have any knowlege of framerates but are interested in the "difference" of the two given numbers. They calculate in percents how much value x differs from value y and then stretch/shrink the audio duration by that percentage.

There isn't any audio stretching tool on which you can throw an arbitrary audio file and say make it 25 fps.