Differing fps for video files, how to sync them?

[veroslav]

Hi!

I have two video files, one is a VOB (something like 720x576, 50fps) and the other is an AVI (Xvid encoded, 25fps). I would like to replace parts of the VOB audio with the corresponding parts from the AVI audio. In order to do this I am using Kdenlive.

The problem is that the two video files have different fps so I am unable to edit them in Kdenlive because they get out of the sync and it is impossible to align the videos correctly. I never had this problem before, as I usually work with VOBs having 25 fps, this is a first one with 50 fps.

My main priority is to keep the video quality of the VOB file intact (no re-encoding).

What would be the right way to sync the fps, to increase AVI fps or to decrease VOB fps? How can I do that? Any tools for Linux/Windows? I tried increasing the AVI fps to 50 with ffmpeg, but it failed due to some error in the encoding (seems the ffmpeg thinks that the AVI is corrupt). I used this command:

ffmpeg -i input.avi -r 50 output.avi

Also, I target a PAL-DVD as the end result. How is this compatible with the VOB file I have (due to 50 fps, normally I work with 25 fps VOBs)? What is the ideal VOB fps for a PAL-DVD? Does it matter?

Any other tips and suggestions on how to attack this problem are much appreciated!

Thank you in advance.

Kind Regards,
Veroslav

[Richard_G]

Where has the VOB come from? 720x576 @25fps is standard DVD resolution but it is interlaced, so 25 frames but 50 fields per second. Is it possible that somehow each field has been converted to a frame?

[veroslav]

Thank you for your fast reply, Richard_G.

I've purchased a few of these DVDs (old sporting events) online and wanted to replace the commentary with the one from the AVIs I made previously from my own VHS-recordings of the same events.

How can I find whether each field has been converted to a frame? If yes, what could I do in that case?

Thanks again.

[smrpix]

Since all you want is the audio from the avi, just demux it to an uncompressed PCM wav file. Then the video framerate won't matter.

[veroslav]

Thank you for your reply, smrpix.

I think this is exactly how I do it at the moment. Here are the detailed steps of what I do:

1. Demux AVI (export audio as WAV)
2. Import VOB and AVI to Kdenlive on separate tracks and split their audios to separate audio tracks (in this case original AVIs mp3 audio)
3. Replace the AVIs original mp3 audio with the demuxed WAV (I do this step as I otherwise get audio out of sync if I work directly with mp3 stream in Kdenlive)
4. Align AVI video with the VOB video in order to know exactly which audio part I need to use from WAV.

However, this is where I get the synchronization issues due to differing video frame rates as I am unable to align the videos correctly for audio cutting.

I am a newbie so I apologize if I am stating something obvious, but I am trying to understand how demuxing to WAV would cure my video synchronization issue? Could you please elaborate a bit more?

Thank you very much!

[smrpix]

Don't import the AVI into your timeline. If you need a sync point, look at each video separately and find a matching moment. Write the time down on a piece of paper.

Add the wav file to the timeline with the vob. Find the time you wrote down on the wav file and sync it to the matching event on the timeline. If there is matching audio (a kick, a cheer) it will be quickly obvious if you're off. Sync to that. If there's no matching "live" audio a few frames this way or that won't make a lot of difference.

[veroslav]

Hi again,

I might have been just a little bit quick when I said that the VOB has 50 fps (I looked up the video info in VLC), however, when I check the video properties in Kdenlive directly, here is what I get:

VOB: MPEG-2 video, 720x576
Frame rate: 25.000000
Scanning: Interlaced

AVI:
MPEG-4, 512x384
Frame rate: 25.000000
Scanning: Progressive

Looking a bit closer at the video content of the both VOB and AVI, it might be that some parts of the footage are missing from the VOB, thus resulting in the clips difference (which I thought was due to the fact that I thought the VOB had 50 fps because that was what VLC showed).

I apologize for the time waste, will look into this more closely and come back if I still have issues.

Regards,
Veroslav

[smrpix]

I kinda guessed that.

[veroslav]

Yeah, sometimes I begin to panic a bit too fast instead of analysing the "issue" in further details Again, I am sorry, I learned my lesson, but I am very happy that I don't need to throw/downrate my DVDs because it is quite a big collection.

Thanks again, I appreciate the effort and the help.

/Veroslav

[smrpix]

You're welcome, best of luck.