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Old 17th September 2015, 18:22   #1  |  Link
TheRancher
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Syncing (audio speed change and more)

Hello! I have a 25 FPS video from which I want to extract audio and sync it to a 23.976 FPS video. I managed to do it fairly easily by converting 23.976 FPS video to 25 FPS with MKVToolnixGUI, copying it to MP4 with FFmpeg and loading it in Sony Vegas. From there, I opened the audio I wanted to sync, used the original one as a reference (I stretched it first in Toolnix by 960/1001) and voilà! I synced it just by moving the audio track and splitting it in one place. I changed the video track because I read the following on MKVToolnix's GitHub: "First of all: don't. If you can avoid it then do not do it. Instead apply the opposite to the video and subtitle tracks present." I asked a guy who has experience with this kind of stuff, he said the opposite; do change the audio track (he recommended Sound Forge) and don't touch the video. Whom should I listen to? I feel like modyfing the audio is a better thing to do because I don't want to re-encode the video, and I feel like MKVToolnix's FPS change option is just a workaround, not a real deal. It does work, but it's just a set of commands inside the container.

Anyway, this isn't why I'm posting this thread. I came across 29.970 FPS video and I need to do the same thing as above; sync an audio file from a 25 FPS video. I again tried MKVToolnixGUI to apply FPS change to 25, but the ending file is five minutes longer and there's no way to do anything with it. 23.976 to 25 FPS conversion shortened the video by 59 seconds, so now I'm wondering how I did sync the audio without stretching it or doing anything besides moving and splitting one time. This shit is confusing. Any tips regarding this? I tried MeGUI with AviSynth script and AssumeFPS, it extended the video just like Toolnix. ConvertFPS kept the original duration, which is nice, but as I said earlier, I don't really want to re-encode the video. Is there any other solution? What should I do to an audio file in order to sync it with 29.970 FPS video?

Thank you in advance!
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Old 18th September 2015, 21:14   #2  |  Link
tebasuna51
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- If you don't need any audio attached to the 23,976 fps video the easy way, and best for me, is change the video fps (and convert subs if needed).
But if you need use audio attached to the 23,976 fps video you must enlarge (rate 1001/960) the audio attached to 25 fps video with a audio editor or other tools like eac3to, BeHappy or sox. Don't use the mkvmerge's stretch function.

- With 29.970 FPS video you can't use the change fps in mkvmerge because the video can't be equivalent to the 25 fps video, you must also enlarge (rate 1001/960) the audio attached to 25 fps video.

These options are always to avoid recode the video.
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Old 23rd September 2015, 02:44   #3  |  Link
TheRancher
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You're a pure genius. It finally worked! A guy on VideoHelp recommended me to use AviSynth's TimeStretch function, which I did, but instead of 25 to 23.976 FPS audio conversion, he told me to use 25 to 29.970 FPS (which was a logic thing to do, if you ask me), so the ending file was 5 minutes shorter and unusable. I did what you told me for 29.970 FPS video, stretched the audio for 55 seconds (I used Vegas' Élastique Pro, is that a better method than none?) and voilà! By the way, is stretching an audio for a minute a big deal? I compared the original audio and the stretched one and I don't really hear a difference. It was a whole different story when I stretched it for 5 minutes the first time.

Thank you!
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Old 23rd September 2015, 11:11   #4  |  Link
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Quote:
Originally Posted by TheRancher View Post
(I used Vegas' Élastique Pro, is that a better method than none?)
In order to timestretch audio from 25 fps video to 23.976/29.970 there are two methods than can be used:

1) Timestretch changing the pitch.

When convert NTSC material to PAL many times (many commercial DVD's for instance) preserve the video changing the fps, and convert the audio with the easy way than change the pitch.

Then the correct way to recover the audio is make a timestretch changing the pitch than recover the original pitch of the track.

This method is easy to implement and is safe to use many free soft like eac3to, sox or AviSynth (SSRC or Timestrech(rate)). Of course commercial soft can offer also this method.

2) Timestretch preserving the pitch.

If the conversion NTSC to PAL was made correctly, or the track was dubbed to a foreing languaje using the video at 25 fps, the track have a correct pitch and we need preserve the pitch when timestrech.

This method is not easy and here can be differences between conversion soft. Seems Zplane (Vegas' Élastique Pro) is recommended in some sites but I can't tell you if is the better.

If you want use free soft AviSynth the proper method is Timestretch(tempo) but is know than this old method don't preserve the phase between multichannel tracks (work only with stereo tracks).

Using AviSynth+ or TimeStretchPlugin(tempo) there are a new version than preserve phase and can be used for multichannel tracks.

---------------------

The first choice is select the method to convert.

If 25 fps video is from a DVD PAL and/or voices sound more acute than expected you need the method 1) and any soft can do the job.

If you think than the pitch is correct an want preserve it you need the method 2).
Then your second choice is select the soft to use, maybe a commercial soft is better than free soft here, but I can't tell you this because I work always with free soft if possible.
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