M4A file editing - insert gap at start of track and save.

[chris2922]

I've just been pointed in the direction of Audacity and am very impressed that the current Beta build can open, edit and export to M4A. Seems a hard feature to come by in a Windows program. I dont have/want iTunes.

I have an M4A file that has been seperated from an MKV video/audio file. The file contains an AAC audio stream (AAC LC - 141 Kbps - 2 Ch - 48 Khz) and i need to edit it.

It is out of sync with its video counterpart by about 0.5 seconds. The best way i can think of fixing this issue is by inserting a 0.5 second gap in front of the audio. Is this possible in Audacity?

Can i save the result back as an M4A file (AAC LC - 141 Kbps - 2 Ch - 48 Khz) without losing the quality/compression of the original?

Hope someone can help - thanks for reading.
Chris.

[steve]

Assuming that you have successfully separated the AAC audio stream and managed to import it into Audacity, then yes you can trim a bit off the beginning and export it. I'm still using Audacity 1.3.5 so I don't know how well AAC is supported, but regarding your other question, Audacity always works on uncompressed audio data, so to edit an AAC file, edit it in Audacity, and then re-compress it as an AAC file will introduce some degree of sound quality loss. It is probably worth testing it on a small section to check how well it works before spending a lot of time on a long project.

It is out of sync with its video counterpart by about 0.5 seconds

It may be worth checking that the fault is with the file and not with the player. If you've not got it already, try the VLC player ( http://www.videolan.org/vlc/ ) it's free, cross platform, and supports a huge number of formats. I have experienced occasions when other players (good quality commercial programs) have not played audio and video in sync, but VLC has managed it flawlessly.