Audio change fps

[machine head]

How can i convert an audio track from 29.97 fps to 25 fps without it sounding slowed down, or is it not possible? Thanks

[SaSi]

Look for guides about changing DVD from NTSC to PAL and also relevant postings with specific questions. Reading the guides will help you understand the process and the individual postings help you refine it.

[FOO]

Audio doesn't have frames per second.

[machine head]

Hi,
I have done as you sugested and done some successful conversions from 23.97 to 25, but when i try from 29.97 to 25 i'm stuck.

[SaSi]

Audio doesn't have frames per second.

You are correct, but there's more to it.

In uncompressed sampled audio (like WAV), the information is audio samples at a given data rate. For example, 48kHz sampling at 16 bit means you have 48.000 samples, each 2 bytes, for every second.

This audio certainly doesn't have frames per second.

(You could conceive every 48,000 words to be a frame and have 1 fps, of consider 8,000 words to be a frame and have 6 fps - but it's all in your mind - the audio is just samples.).

Even if you encode to MPEG-1, the basic idea, of constant data flow (samples per second) remains the same.

The problem is to synchronize audio with Video. If you have an audio stream multiplexed with video within an AVI, changing the video frame rate (by just changing a constant) will make the video run faster or slower and audio go out of sync.

To change the video from 29,97 to 25, you need to do drop certain frames so that frame rate goes down. Normally, this should cause irregular motion, but as I've read, it won't because NTSC video frame rate is generated from films (which are 24fps) by repeating certain frames. Therefore the inverse process is just removing those copied frames, dropping the framerate.

The trick is how to synchronize the audio with the video.

Search the forum for "Pulldown" and "inverse telecine". It should give you some information on how it's properly done - I think

[adam]

Hi,
I have done as you sugested and done some successful conversions from 23.97 to 25, but when i try from 29.97 to 25 i'm stuck.

The process is identical. When NTSCfilm (23.976fps) is converted to NTSC (29.97fps) the actual playback speed doesn't change because instead of speeding up the video, new frames are created to increase the fps. Audio synced to 23.976fps video is identical to audio synced to 29.97fps video.

NTSC runs 4% slower than PAL, whether the video is NTSCfilm or NTSC. So even though your fps is 29.97fps, you'd actually want to speed up the audio rather than slow it down when converting to 25fps. If I were you I'd use the NTSC->PAL option in BeSweet to do the conversion for you, and if you aren't comfortable with this program, just do the audio conversion however you did your NTSCfilm->PAL audio conversion before.

[flaninacupboard]

Adam,
i think you've misinterpreted him here.

Machine head. Have you encoded the file at 29.97 and tried playing it on your DVD player? EVERY dvd player i have seen can output NTSC, or pseudo-NTSC, so you should have no problems playing it back at all.

[adam]

His post seemed pretty clear to me, and I don't see how my post doesn't address his problem. He converted his video from 29.97fps to 25fps and needs to adjust the audio to match.

He then said he successfully converted 23.976fps video to 25fps in the past, and managed to convert the audio correctly as well.

Well, in regards to audio, the adjustment is identical, so I'm just saying all he needs to do is repeat the process he did before with his audio.

If he follows my advice his audio will sync with his newly encoded video, and that seems to be all he wants to do.