How to sync and AC3 audio with the video in a lossless way?

[Daimroc]

Hello:

I would like to sync an AC3 audio with a video, but I don't know well how to do it, and I would like to do it in a lossless way, if it is possible.

I have checked that the audio fit to the destionation video in length, the only problem is that the video, that is an episode of a serie, has some black transitions. This transitions are different in the video in the video in which I take the audio and the destination video.

For that, the first process that I think I have to do it is to know where to remove or add silences. For that, I convert boths audios (the audio that I want to sync and the audio of the video in which I want to add the synchronized audio) to a stereo wav, open then in a audio editor and sync it manually.

Is this the correct way, or are there a better way than do it manually?

Once I know where to cut and where to add silences, I was thinkin to use EAC3To or FFMPEG to cut the audio in parts, add create audios for the silences and then join them in a new file.

In this case, my doubt it is if to cut or add silences to a AC3 it is a lossless process or it is a lossy process because it is needed to decode the audio and reencode again? Although if I use the same parameters to reencode, I would lose quality if I have to reencode.

And another question it is how much accurate it is to cut and add silences with AC3To or FFMPEG. I mean, if in the audio editor I see that I have to cut for example 847ms, with EAC3To or FFMPEG, would I cut this length or it would have some range of accurate?



So in sumary, I would like to know a general guides to know how to sync the audio with the video and how could I do it in a lossless process.



Thanks.

[davexnet]

AC3 is a lossy format, so any modification will require a re-encode. Just keep a high bitrate for the new file for best results

[ProWo]

You can use Mpeg Video Wizard DVD for this.
Load the video in the top line and the Ac3 audio in the third line. Right-click on the video stream, select Audio, Show Audio peaks.
Do the same for the AC3 stream.
Then you can use the peaks to compare both streams and adjust, shorten, cut and move the audio stream accordingly.
When you are finished, select the video track and delete it so that only the modified AC3 stream is left.
Then click on Save.


[Attachment 75937 - Click to enlarge]

[pandy]

AC3 use fixed packet size so lossless operation can be only with this type of accuracy - ProWo recommended good way to achieve success but you need to verify at the end if muxing was done properly - small few tens ms error is probably not perceivable ( golden rule: +- time of field/half frame period is OK).

[Daimroc]

Thanks so much for the advises, it is a good base to start to see.

I will try the Mpeg Video Wizard application. Is it freeware application?


Thanks so much.

[Quint]

If it is only a delay, then there is also this nice, old, and simple small program to cut AC3 losslessly:
https://www.videohelp.com/software/delaycut

[pandy]

PTS (presentation time stamps) can be manipulated by ffmpeg in various fashion too https://superuser.com/questions/982342/in-ffmpeg-how-to-delay-only-the-audio-of-a-mp4-...verting-the-au