Merging .AAC files?

[killerteengohan]

I have 2 separate AAC-LC .MP4 audio files I want to merge.

Everytime I use the append feature in MKVmerge it throws the audio out of sync where it got appended at. If I had a 30 second audio clip and 21 minute audio clip and appen the 21 minute to the 30 second audio, it plays perfectly for the first 30 seconds then the audio goes out of sync the more and more the video goes along after the append.

Can anyone suggest a good program for merging/combining AAC-LC .MP4 files so that I can mux it in MKVmerge without this audio issue the Append is always giving me?

[hello_hello]

The audio you're appending..... where did it come from? Another video? If so, is the frame rate the same for each video? Although even if they are, there's no guarantee they'll sync. Sometimes video which appears the same is slightly different. One frame at the end of a scene less here, another there, and the audio appears to be wandering out of sync.

If you appended the audio and the second part is constantly out of sync (the audio sync doesn't change), then you'd possibly need to trim the audio or apply a delay etc to sync it, but it shouldn't go progressively further out of sync simply because you appended it. There's likely another reason for that happening.

[killerteengohan]

Yes its the same frame rate. I just cut the opening, the episode and the ending theme from the same source into 3 separate parts. When I append them, the more I append, the more they go out of sync and the more it goes out of sync as more get appended.

I made sure the starting and ending frames of the cuts were correct, so theres no overlapping or missing frames.


30 second opening plays fine
20 minute episode when appended, goes a small bit out of sync as if it needs a delay added. i tried the delay but then it made the later parts go out of sync.
40 second ending when appended goes even more out of sync almost as if it needs a delay again.

Adding delays fixes it a bit but after about 6 minutes go by you can tell its going out of sync slowly again.

Ive had this issue with more than one source and I have tried it several times.

[sneaker_ger]

Mkvmerge is designed in a way that no desyncing should happen even if there are overlapping frames. There's likely something wrong with your cutting process you've applied before merging. If in doubt mux video+audio together as a first step and see if every part is in-sync. Only after that append those parts.

[hello_hello]

Yes its the same frame rate. I just cut the opening, the episode and the ending theme from the same source into 3 separate parts. When I append them, the more I append, the more they go out of sync and the more it goes out of sync as more get appended.

What's happening in-between? I assume you're not just splitting the source and then appending the parts together again? Are you extracting the audio or re-encoding any of it?

Adding delays fixes it a bit but after about 6 minutes go by you can tell its going out of sync slowly again.

Initially it sounded like the audio is just losing sync where it's appended, but that sounds like it's a gradual thing which in theory shouldn't happen if the audio is in sync for each MKV before they're appended. Is that the case?

If you're extracting the audio after splitting and/or converting it, how are you appending it together again? If you've split the MKV containing the video, try adding the appropriate audio to each one and saving them as new MKVs. Check the audio sync for each. If it's not right try applying an appropriate audio delay and remuxing. When you have three individual MKVs with synced audio, append them. The audio should stay in sync. If that's not the way you're doing it, give it a try.

I'm not sure why you're splitting/appending exactly, but what happens if you extract the original audio (without splitting it) and adding it to the MKV's containing the video while appending them. If it goes out of sync it can't be because the audio was split, as it wasn't.

[killerteengohan]

I just ended up editing the audio with Goldwave and adding in silence where necessary to keep everything in sync.