Audio long exactly like the video but gradually out of sync...?

[takui]

Hi guys, first post here, hope someone can help me to get rid of this issue...

I've 2 files, one is perfect synched, both audio and video, and another one that's long EXACTLY (or at least...mpc, mediainfo and potplayer tells me that) the first one...now, I've extracted the aac track from the second, and I've tryed to mux it in the mkv of the first file with mkvtoolnix...and the problem is...this second track go out of synch gradually...not much, around 1 second at the end of the movie (near 2 hours of play), but I dunno how to fix it!

I've fixed similiar problems before with eac3to, but in those occasions files were of different lenght...so know I dunno what to do...can someone help me?

sorry for my broken english and thanks in advance!

ps.: both fps seems the same too for the 2 files, for what I can tell...

[jagabo]

Remux with Mkvtoolnix and change the frame rate of the video to account for the sync error at the end. Ie, increase or decrease (you didn't say whether the audio ahead or behind) the frame rate by about 0.014 percent (calculate the exact value based on the sync error and length of the video).

[takui]

Remux with Mkvtoolnix and change the frame rate of the video to account for the sync error at the end. Ie, increase or decrease (you didn't say whether the audio ahead or behind) the frame rate by about 0.014 percent (calculate the exact value based on the sync error and length of the video).

It's a try and error method or can I find the value with some formula?

Plus I can't do as you say because I've 2 audio tracks and one is perfectly synched, if I touch the video, probably I will get desynched the other track.. :/

[jagabo]

Remux with Mkvtoolnix and change the frame rate of the video to account for the sync error at the end. Ie, increase or decrease (you didn't say whether the audio ahead or behind) the frame rate by about 0.014 percent (calculate the exact value based on the sync error and length of the video).

It's a try and error method or can I find the value with some formula?

It's simple math. If the movie is 2 hours long (7200 seconds) and you need to shorten the length by 1 second:

new fps = old fps * 7200 / 7199

Plus I can't do as you say because I've 2 audio tracks and one is perfectly synched, if I touch the video, probably I will get desynched the other track.. :/

Then you'll have to change the length of the audio track. Audacity with the ffmpeg source plugin is suitable for this.

[takui]

Remux with Mkvtoolnix and change the frame rate of the video to account for the sync error at the end. Ie, increase or decrease (you didn't say whether the audio ahead or behind) the frame rate by about 0.014 percent (calculate the exact value based on the sync error and length of the video).

It's a try and error method or can I find the value with some formula?

It's simple math. If the movie is 2 hours long (7200 seconds) and you need to shorten the length by 1 second:

new fps = old fps * 7200 / 7199

Plus I can't do as you say because I've 2 audio tracks and one is perfectly synched, if I touch the video, probably I will get desynched the other track.. :/

Then you'll have to change the length of the audio track. Audacity with the ffmpeg source plugin is suitable for this.

The problem is: of which track? Because the one just synched its ok, and the one with problems its long exactly like the video, for what I can see...

[takui]

edit: ok I've tryed audacity to extend the track lenght with the change time command...now the track is long exactly like the other one, but its still going out of synch gradually :/ btw just for knowledge it was 2 sec short for a 2 hours movie...

what can I try now? :/

[jagabo]

Did you make it 2 seconds longer?

[jagabo]

The problem is: of which track?

The one that's out of sync.

edit: ok I've tryed audacity to extend the track lenght with the change time command...now the track is long exactly like the other one, but its still going out of synch gradually :/ btw just for knowledge it was 2 sec short for a 2 hours movie...

what can I try now? :/

You don't want to make it the same length as the other audio track. You want to extend the length by 2 seconds so that the audio syncs to the video at the end.

[takui]

The problem is: of which track?

The one that's out of sync.

edit: ok I've tryed audacity to extend the track lenght with the change time command...now the track is long exactly like the other one, but its still going out of synch gradually :/ btw just for knowledge it was 2 sec short for a 2 hours movie...

what can I try now? :/

You don't want to make it the same length as the other audio track. You want to extend the length by 2 seconds so that the audio syncs to the video at the end.

I've read the lenght of the other track because it's perfectly synched with the video and I dunno how to read perfectly the lenght of the video file alone (mediainfo dont show seconds, and yamb show different information from players, so I don't know if they are right or not)...from potplayer or mpc the movie is 1:50:50, and the "good" audio track is 1.50.50.741 (from audacity), so I've assume this one to have a correct lenght... and so "bad one" is 2.25 seconds shorter than the good...

So yes, I've extended the lenght of the bad one to match the good one, but its still gradually desynching... just for give it a try, I've synched the good one in the video where the bad one come from, and the problem is the same (obviusly...)

Ah btw, I've tryed to change the video fps too, I've just said its not a solution because I'll probably get the same problem with the track synched, but just to give it a try, changing the fps dont resolve the issue, still getting desynched....

[jagabo]

You don't need to know the exact length of the video track or the other audio track. All you need to know is that after 1:50:50 (6650 seconds) the bad audio track is 2.25 seconds behind the video. So the audio track needs to be stretched so that it's 2.25 seconds longer at 6650 seconds. In Audacity change the speed by -0.0338346 percent (-2.25 / 6650 * 100).

[takui]

You don't need to know the exact length of the video track or the other audio track. All you need to know is that after 1:50:50 (6650 seconds) the bad audio track is 2.25 seconds behind the video. So the audio track needs to be stretched so that it's 2.25 seconds longer at 6650 seconds. In Audacity change the speed by -0.0338346 percent (-2.25 / 6650 * 100).

Ok but I've guessed that difference comparing the other track with this one, otherwise how I can know how much the bad track is behind the video? Btw I've just tryed to adjust the speed with that value but its still gradually going out of synch :/

[takui]

You don't need to know the exact length of the video track or the other audio track. All you need to know is that after 1:50:50 (6650 seconds) the bad audio track is 2.25 seconds behind the video. So the audio track needs to be stretched so that it's 2.25 seconds longer at 6650 seconds. In Audacity change the speed by -0.0338346 percent (-2.25 / 6650 * 100).

Ok but I've guessed that difference comparing the other track with this one, otherwise how I can know how much the bad track is behind the video? Btw I've just tryed to adjust the speed with that value but its still gradually going out of synch :/

ps.: I've donwloaded audacity plus ffmpeg but I don't know how to make them works together, so I'm saving the track in ac3, its a problem?

[takui]

Ok I've tryed VLC too, and for what I can say, the bad track start 1 sec behind at the beginning of the movie, to be 1,5 sec behind at the end...so I've tryed to stretch it with audacity of about 0,5 sec, adding 1 sec of delay too with mkvtoolnix...the result is a track synched at the begin and at the end of the movie, but desynched in the middle...probably its the wrong method... -.-

[takui]

nothing?

[jagabo]

the bad track start 1 sec behind at the beginning of the movie,

You should have mentioned this before.

to be 1,5 sec behind at the end...so I've tryed to stretch it with audacity of about 0,5 sec, adding 1 sec of delay too with mkvtoolnix...the result is a track synched at the begin and at the end of the movie, but desynched in the middle...probably its the wrong method... -.-

Then you've exhausted the possible easy fixes. You'll have to analyze where and how the audio goes out of sync then do what's necessary (stretch, shrink, insert silence, remove sections) to fix it.

[takui]

but both videos are long the same...isn't strange? I think one is only the compressed version of the other....what's strange its that the audio tracks are out of synch....

[jagabo]

but both videos are long the same...isn't strange?

Who cares whether it's strange or not? You still have to deal with it.