Audio out of sync? This could be why!

[Pisces225]

Out of sync audio? This could be why...

When I was first learning to work with video, I, like most people, had audio sync problems. It wasn't due to bad source, bad software, nothing like that. Just a simple fact that I didn't know that caused me hours of aggravation, which would have been avoided had I known this one thing...


If your source is PAL (or 25fps) and you're converting to NTSC (a very common scenario) you must also do a 'framerate' conversion on the sound file as well, or you'll end with sound that seems to start out in sync and very quickly (in just a few minutes of video) go out of sync, with the audio coming before the video by an ever larger margin. I've seen many many wonderful guides on this site leave out this little tidbit and it caused me undue headache.

When you're converting to NTSC like most folks in the USA do you're actually slowing down the video just a tad. If you don't do the same with the sound... out of sync audio.

This wasn't intended to be a guide, just a tip for newbs having audio issues like almost everyone has at one time or another. If you've ever had this issue in the past because you didn't know this little fact, bump this thread for a newb please!

[thecoalman]

Hello Newb.

A second is a second. I haven't done this myself for PAL to NTSC but I understand the concept and have done it for other framerate conversions.. When you convert a 25 FPS to 30 FPS (correctly) it's still a second. When done correctly there's duplicate frames created. You can do it your way and just convert the framerate but as you said you need to decrease the audio. Personally I'd leave the audio as is, and convert the frames correctly.

[manono]

Hi-

When you convert a 25 FPS to 30 FPS (correctly) it's still a second. When done correctly there's duplicate frames created.

If I'm understanding you correctly, I and a lot of other people would disagree with you on that. Creating actual dupe frames will make the result play jerky, especially noticeable during panning shots. It'll also waste bitrate on unnecessary dupe frames. What Pisces225 is obviously talking about is slowing the video to 23.976fps, and also slowing the audio. He didn't say so directly, but I assume he's doing this for DVD, so you then run Pulldown on the 23.976fps MPV so the player outputs 29.97fps.

And, of course, the other way to do it is to keep it at 25fps, resize to 720x480 during the encoding, and then run DGPulldown 25->29.97 on the resulting MPV. That way you don't have to mess with the audio at all (and also avoid creating your unnecessary dupe frames). Those are the correct ways to handle progressive PAL sources for conversion to NTSC.

Who's the Newb?

[thecoalman]

Who's the Newb?

I was drinking a little when I posted that, perhaps I overstepped my knowledge of PAL to NTSC conversion. Anyhow my point was that you don't have to convert the audio length, I do it all the time for converting video for the web converting 30 FPS to 15 or even 10. Audio always remains in sync.

I'll go crawl under a rock now. :P

[Pisces225]

Hi-
What Pisces225 is obviously talking about is slowing the video to 23.976fps, and also slowing the audio. He didn't say so directly, but I assume he's doing this for DVD, so you then run Pulldown on the 23.976fps MPV so the player outputs 29.97fps.

That's exactly right, thanks for being more specific than I was originally.

Also, here's another tidbit I wish I had known that would have saved me undue headache...


There's a bug in the current version of AC3Machine regarding framerate conversions. The "to" and "from" fields are reversed in the GUI as compared to what is actually done by the generated command line. You need to basically reverse the two and convert 'from' 23.976fps 'to 25fps when doing an NTSC conversion with this version of AC3Machine. You can verify this by checking the command line and the results if you don't reverse these values. This GUI flaw (works fine if you reverse values) originally threw me for a loop once or twice as well until I realized this fact.

I use the above method of saving my sound to .wav then converting to .ac3 and remuxing when dealing with my sound nearly always. Saving my sound to .wav via VDubmod, and converting to AC3 seperately (throw in a bad frame check on your converted .ac3 to be extra safe) has, for me personally, resulted in the most accurate sound/video sync, I rarely have sound issues, but it still happens occasionally of course. Other methods of dealing with the sound tracks seemed to result in a lot of sync problems more often. I realize my .wav method may involve a little extra time than is absolutely necessary, but for me has been by *far* the most reliable method of retaining proper A/V sync. I have sound issues so rarely I usually don't even check my A/V sync if the bad framerate check comes out clean. Odd where people cut corners, I know.