Audio Sync issue?

[tb582]

I have a video thats 30fps - if I encode that to 29.97 everything is in syn in the new file. However if I go back to my 30fps file and export only the audio and then use that audio with my 29.97 version the audio drifts out of sync. Can someone explain why that is?

[AlanHK]

I have a video thats 30fps - if I encode that to 29.97 everything is in syn in the new file. However if I go back to my 30fps file and export only the audio and then use that audio with my 29.97 version the audio drifts out of sync. Can someone explain why that is?

Because fps = frames per second
So with a constant number of frames, changing the fps changes the number of seconds proportionately.

However you encoded it to 29.97 presumably stretched the audio to compensate for that.

[tb582]

can you provide a bit more detail of why this happens?

[Gavino]

When you converted the original to 29.97, both video and audio were slowed down by the same amount (about 0.1%), keeping them in sync with each other.
That slowdown means the new video is longer than the original audio, and so no longer in sync with it.