Can I preview and then merge tracks with an offset?

[dpkonofa]

Sorry if this is a simple question but it's my first time using LC for this. I have 2 video files that were recorded at the same time/place using Final Cut's new Live Multicam feature. Unfortunately, one of the videos is missing audio due to a mic issue so I want to overwrite the audio of the bad video with just the audio from the good video and, ideally, do this losslessly (aka just a remux).

In LC, I've loaded one of the videos (File A) in LC and used the tracks menu to remove the audio and then used "Include more tracks from other file" to do the opposite for the video file with the audio I want (File B).

Now for the issue:
For some reason, LC sees file A (the video file) as being 1 second longer than file B (the file with the audio I want) as you can see in the first screenshot and yet neither MacOS nor Final Cut (or any other software I've thrown it at for that matter) sees the same thing.

This isn't a problem as I have some wiggle room at the beginning and end to cut/trim but I can't figure out a way to actually preview the resulting video in LC before I export and I can't figure out a way to offset the audio being merged in (which I would need the preview to determine). I do have the audio track enabled in the "Subtitle"/Video/Audio Track section (Sorry, not sure what that's called). If I attempt to view the waveform (either mini or normal), I get nothing displayed except a message to "Zoom in more to view waveform".

So, when I go to preview the file, I'm seeing the video but I'm not hearing the audio from the 2nd file. Additionally, if I just chance it and export, the resulting file seems to be off by a few frames when compared to the audio.

How can I preview the end result of video from file A and audio from file B and adjust an offset so that I end up with 1 file that has the video from file A and the audio from file B but without it being out of sync?

[dpkonofa]

Just adding some context - it looks like ffmpeg does support offsets so I'll probably end up doing this via the CLI directly but it would be awesome if LC added this or could do this (already or in the future).

https://superuser.com/questions/538031/what-is-difference-between-ss-and-itsoffset-in-ffmpeg

[mifi]

Hi. There isn't any way to preview the result in LosslessCut when adding multiple files. However because exporting is usually pretty fast, that could be a workaround. (export and then look at the result).
There also isn't yet any way to offset the audio/video tracks, so you would currently have to do it in this way:

1. cut off the the beginning of the audio or video track the amount that you want to offset and export it as a file
2. open the exported track and merge it with the other corresponding track from the original file.

See also #1887 and #216

[dpkonofa]

@mifi - Do you have any insight into why it's showing different lengths for the files despite the fact that they are the same length and were filmed with Final Cut's Live Multicam? I think that's the main issue at heart here. If the files weren't reporting as different lengths in LC, I wouldn't need to offset anything since they should be in sync already.

[mifi]

That could be either due to one of:

1. QuickTime duration has been rounded differently to the nearest second compared to LosslessCut (so they both show up as :53 but in reality they are not exactly the same as evident by number of frames in your screenshot 147404f vs 389147f).
2. LosslessCut (ffmpeg) calculates duration differently from QuickTime and other tools.