Changing tempo from 25 to 23.976fps: what am I doing wrong?

[flapane]

Speaking in general terms (because I always have this kind of problem), let's assume that I have two versions of the same video (ie. a PAL dvd and a Bluray) and I rip them: one is at 25fps and the other one is at 23.976fps.
Of course they'll have a different duration, but they're basically the same video/movie/whatever.
Let's assume that I want to mux the 25fps audio with the 23.976fps video for some reason.
I can either use besweet (-ota( -r 25000 23976 ) ) or soundstretch directly from besweet, or the newer soundstretch.exe 1.5 with a tempo decrease of 4%.

The audio convertion matches the 25fps audio duration with the 23.976 video (that's good).
However, when I try to mux the new audio file with the 23.976fps video, it slowly looses sync after some minutes, and I manually have to edit it.

Am I missing something?
Thanks

[Roger1972]

Hi,

You can convert the audio also with eac3to, the result is better. http://forum.doom9.org/showthread.php?t=125966
You check the audio delay in a audio editing program. eg. Audacity http://audacity.sourceforge.net/?lang=de or another.
Load the center channels. And compare the channels. Waveform in audacity. For example, a bang. PAL Audio: 00:05:44.345, Blu-ray Audio 00:05:42.340 --> 4345-2340=2005, the delay for PAL Audio is -2005ms. The bang comes later in PAL audio. also the delay is -

[flapane]

Hi,
thanks for the answer, I'll give eac3to a look, the higer conversion quality, the better.
The problem is that the delay grows slowly, and there's a/v sync at the fist minutes of the video. Then I manually (via Goldwave) add small pieces (about 500ms per piece) of cloned waveforms every 10 or 15 minutes.
I can't explain such non-linear delay.

[hello_hello]

I guess if you change the 25fps audio to 23.976, and remux it with the original video but also change it's frame rate to 23.976, and when you play it back there's no audio sync problems then you'll at least know whether the audio conversion is being done correctly.

Alternatively try increasing the 23.976 video to 25 fps and muxing it with the 25 fps audio track unaltered to see if they really do match.

Maybe the 25 fps and 23.976 versions aren't exactly the same?

[flapane]

I guess if you change the 25fps audio to 23.976, and remux it with the original video but also change it's frame rate to 23.976, and when you play it back there's no audio sync problems then you'll at least know whether the audio conversion is being done correctly.

Alternatively try increasing the 23.976 video to 25 fps and muxing it with the 25 fps audio track unaltered to see if they really do match.


Good point, I'll try it shortly!



Maybe the 25 fps and 23.976 versions aren't exactly the same?

For what I can see, they are. A typical scenario comes to my mind: I own the original DVD in my language, and the new restored Blu Ray (not some extended version) has been released in english, such as some old Disney's cartoon. The quality difference is amazing, so I buy the Blu Ray and rip it in order to mux it with the DVD audio track.
So I obtain a video I can see with other people (not everyone understands english). If a/v are synced for the first 10 or 15 minutes, then why they aren't after this point?

I'll try what you suggested in order to see if the audio conversion's been done done correctly.

[flapane]

OT: what if I set the 25fps video flag on mkvmerge? In this way I could mux without editing the audio track. Would it make a noticeable difference while watching it?

[hello_hello]

OT: what if I set the 25fps video flag on mkvmerge? In this way I could mux without editing the audio track. Would it make a noticeable difference while watching it?

I'd imagine not.

If you still can't get them to match maybe check to see if they do in fact have exactly the same number of frames. I'd open them with VirtualDub and skip to the end of each to check the frame count (there's a plugin for opening MKVs called Matroska.vdplugin).

I've tried doing the same thing with 25 and 23.976 fps content in the past and sometimes driven myself crazy trying to match the audio and video. I discovered with some they'd actually been edited differently even though they looked exactly the same. By that I mean one frame missing where a scene switches here, a couple of frames there, and before you know it the audio is way off. Admittedly those were recordings taken from free to air TV so when it comes to BluRay and DVD the same thing may never happen.

Don't forget MKVMerge has a time stretch function. You can use it to slightly stretch the audio or video tracks if the de-sync is gradual. Works better with some audio formats than others. Or try just changing the frame rate marginally, say from 23.976 to 23.975 or 25 to 25.001. That sort of thing...

One other thought, maybe there's a few little gaps in one of the audio files, so when it's demuxed and converted those gaps need to be filled with silence. I'm not too familiar with eac3to but I think it'll do it.