Changing frame rate of ac3 audio?

[ezekiel999]

Hello,

I have searched through previous posts and have looked at quite a bit of stuff but still can't quite get my audio to sync up. Maybe someone else has had the problem and had an EASY fix for it and I just over looked it. Anyhow here it is ...

I'm converting some TV episodes to low-quality dvd with tmpeg and I wanted to have the audio in AC3 format for compatibility and size reasons. At any rate I converted the video just fine as well as the audio using AC3Machine (w/besweet).

When I author the dvd using TMPEG Author or DvdLabPro or even just a simple multiplex the audio is fine in the beginning but is slowly gets out of sync. Its most noticable starting at about half through the episde and the audio is EARLY by about 1/2 second by the end of the 45 minute episose.

Something that puzzled me is that after running the original wav file through ac3machine the newly created ac3 file was actually about 3 seconds longer than the source? Not sure why but it was for all episodes as well.

So it got me to thinking that maybe the ac3 audio is running at a different frame rate than my video (29.97 fps). After reading several posts and most end up pointing to VirtualDub-MPEG2 for help I figured out that if I did change the video frame rate to 29.978 it sync'd up perfectly with the audio.

So if I'm understand this correctly it means that the audio is at a frame rate of 29.978 instead of 29.97 like the video. Well in VirtualDub-MPEG2 I could export the video to the matching frame rate of 29.978 but it only exports to AVI ... which means I would have to convert it back to mpeg format to be able to be burned to dvd compliant format.

I would think the simplier solution would be change the frame rate of the audio to 29.970 to match the video and cross my fingers and hope it syncs up. I have using the option in ac3machine tried to reecode to a new ac3 audio file at the specified frame rate but it still hasn't worked ... any ideas? Thanks.

[reboot]

Audio doesn't have a framerate.
Demux the audio from your captures.
Encode only the video.
Transcode the audio.
Author, burn.
If you capture at 48khz LPCM (.wav), you could transcode to AC3 in ffmpeggui.
It should stay in sync.
If you're capturing at 44.1khz, the desync comes from resampling.
You can also adjust sync in Goldwave using the Timewarp effect.

[SLK001]

Your problem is that your video isn't being captured at exactly 29.97fps. Your audio, however, is being captured at exactly 48ksps. A DVD authoring program expects the framerate to be exactly 29.97fps and the audio to be at 48ksps. See the problem? If they aren't exact, sync issues abound.

You need to dynamically resample your audio as you capture it to match the frame rate of your video - either slow down or speed up the audio as needed. There are various programs that do this. VirtualVCR is one (and the one I use). Also, your audio capture device has to have the capability to capture at other than standard rates - some do, some don't have this capability. Whether or not your card does is a exercise left up to you.

It is a lot easier to do the resampling on the fly than to do it as a post process.

[ezekiel999]

Thanks for you advice. I'm not capturing the source myself. I have some downloaded episodes and that is what I'm working off of. The original sound source was PCM audio which was 44.1khz if I remember correctly. Unfortunately I no longer have the originals b/c I converted the audio to AC3 long ago and never got around to finishing up the project until recently. I'm sure that resampling from 44.1khz to 48khz has something to do with audio being out of sync.

I just tried to time warp the length of the audio from 44:36 to 44:32 (which is the runtime of the video) and the audio is WAY earlier than it was before applying the time warp.

Going to try increasing timewarp just for kicks to see if for somereason it helps 44:37, 44:38, 44:39, etc. I don't know if using the resampling feature in goldwave will help much because it needs to be 48khz anyways. Thanks again.

[Cornucopia]

Sample Rate Conversion never changes the duration of the soundfile, that's not the problem. It probably has to do with how your video was converted/encoded. Could be...

30fps vs. 29.97 fps encoding
Bad DeInterlacing, if used
Bad IVTC (Inverse Telecine), if used

If, for example, your original AVI was 30fps, but was interpreted as being 29.97, then when you encode to 29.97:
45 min. x 60sec/min x 30 frame/sec = 81000 frames
81000 /60 /29.97 = 45 min + ~4 sec. (slow by that add'l amount)

You may just need to re-encode (or Restream) your video.

Scott