Framerate conversion: Since when is a second not a second?

[gonzalimator]

I am trying to rip some animation. It's the Dragon's Lair DVD. I am using Smartripper to rip just one specific VOB. I use DVD2AVI to create the d2v from the VOB. DVD2AVI says the stream is 29.97fps.

I am trying to make the video 23.976fps so that the animation is true to the way it was created. I want my animation students to be able to study the clip and understand the timing (how many drawings were used etc.)

I open TMPG and load the NTSCfilm (for the Inverse telecine) template which I modified: bitrate and such unlocked, and I open the d2v. After the encoding video looks great, the audio is out of sync (late) by the end of the video.

Is a second not a second? If I get audio from a 29.97fps source, why is it longer than the video that was converted to 23.976fps? I know there are now fewer FRAMES in the video, but they're being displayed at a slower rate. Shouldn't the audio sync up?

Yes, I already searched the forums but I can't seem to find a clear answer on the 29.97 to 23.976 conversion. I know how to fix the audio track's length but can I avoid this step by extracting the audio correctly (at 23.976fps) in the first place? I need help.

[adam]

Ok to make a long story short, you are incorrectly trying to reverse the telecine process but it really doesn't matter for what you want to do.

The VAST majority of NTSC DVDs actually ARE 24fps. They are encoded at this fps and stored like this on the DVD, but they contain a flag which instructs the DVD player to do a real time telecine as the movie plays. This is an encoding technique. Since you only have to encode 24 frames every second instead of ~30, you save yourself around %20 of your bitrate which has a dramatic effect on quality. This same encoding technique can be used on VCDs and SVCDs as well.

Now if you are encoding SVCDs, VCDs, or DVDs, then you want to preserve this 24fps source, and the way to do this is to use the forced film setting in dvd2avi, which bypasses the pulldown flags and exports the 24fps source. But in your case, this is like killing a fly with a bazooka.

The crucial issue here is what your playback device is for your students, because depending on that, you are either out of luck or already exactly where you want to be.

If your playing this on a tv then there is no way you are going to get 24fps playback unless both the tv and the dvd player are progressive. Regardless of whether you play the DVD directly, or encode at 24fps (23.976fps) in another format, its still going to be played back at 29.97fps because the NTSC standard requires it. The dvd player is going to telecine the movie as it plays.

If you are going to play this on a pc, mostly likely projected on a screen, then you will already get 24fps playback. Software dvd players do not process the pulldown flags because pc monitors are progressive.

Now to answer your actual question, yes 1 sec is still 1 sec so even though you are removing frames each second, the movie is playing slower so the audio sync should not be effected. Basically, the original 24fps source and the telecined 29.97fps output should still have the same playtime, which will match up with the audio. I think your problem was because of the incorrect IVTC method you were using. In order to do an IVTC in TMPGenc you have to use the actual IVTC filter. Simply setting the NTSCfilm template just sets the output fps to 23.976fps. If your source is anything other than this, then it will just add or decimate random frames to make up the difference. Technically this shouldn't affect sync either, but you can't expect perfect precision when you are doing something completely randomly. Actually I think there might have been some other things causing your sync problem as well, such as maybe incorrectly handling the audio offset that most AC3 tracks use, or possibly the fact that you seem to have encoded audio and video together at the same time, which often results in improper multiplexing. As I said before, all these things are probably moot since in all liklihood, there is no reason for you to have to re-encode the DVD at all.

[Cornucopia]

gonzalimator:

how do you know for sure that the wasn't done at 29.97? IIRC, the original was on Laserdisc and may have been created with that in mind by using 30 fps cels.

If you know differently, you can figure out if it is a pulldown problem (difference between 24 fps and 23.976) or if it's a telecine problem (difference between 23.976 and 29.97 fps). you could do the math and see if these differences correlate to the delay in A/V sync.

You really shouldn't change the audio though; it's rarely the culprit in these situations.

Remember, as far as the DVD/MPEG decoder is concerned, all it sees is X# frames of video (played at the rate specified by some flag) and Y# of audio samples (played at the rate specified by some other flag).

HTH,
Scott

[gonzalimator]

Thank you Adam and Cornucopia for such a quick response. This a great community/website.

The final output will be mpeg1 for viewing on PC's. No VCD authoring or DVD players involved.

I will try the Force film setting in DVD2AVI and see if I can avoid the whole IVTC isue entirely. As far I can tell the IVTC filter in TMPG was used correctly. It was checked off in the advanced settings and TMPG took half an hour or so processing the IVTC before encoding the mpeg.

About the audio...should I convert the audio that DVD2AVI created to an uncompressed wav before encoding with TMPG? (maybe that's why the sync is off?) I can also encode the audio and video separately as you've suggested Adam, but I guess I need the answer to this new question first.

You guys are great. Long live dvdrhelp.com!


Gonzalimation for the Nation!

[adam]

Ok gonzalimator, I didn't realize you actually used the IVTC filter. You didn't do anything wrong then, though using forced film would have made things infinitely easier. Also I have not had much luck with TMPGenc's IVTC filter at all.

If your using DVD2AVI to process your video than either set it to decode to wav and then encode that in TMPGenc, or set dvd2avi to demux to AC3 and then use either Head3che or BeSweet to convert to mp2. Then multiplex this mp2 with your video. Personally I think the latter option is higher quality, but neither should really affect the audio sync. If you do use BeSweet make sure you correctly input the audio offset that is in the filename of the AC3 file.

To use forced film in dvd2avi... Load the vob and hit F5 to preview it. If it says 95% or higher film, then forced film will work. If it says anything else than you cannot use forced film.

Too bad your pc at school doesn't have a dvd-rom. It sure would make things simpler.

[Baldrick]

Please post ripping questions in the dvd to forums. I'm moving this one.

[gonzalimator]

Sorry Baldrick... my bad.

We have too many computers to outfit them all with DVD ROMs! Every student in the CG animation program has thier own PC and every two students in the Classical animation program share one PC. We have servers and it's much easier to put an mpeg on one machine and have 75 PC's access it at will.

Thanks again Adam, I'll try the demux to AC3 and then convert with Headac3he.