[AAC] Changing pan cleans file

[yetanotherlogin]

Hello

Using ffmpeg, I extracted the audio stream from an FLV file (AAC audio + H264 video) which is almost unlistenable unless I use Audacity to move the pan left or right (ie. moving the L/R button left or right). I have no idea why the original file sounds bad and why panning solves the issue.

What would be the right way to doctor this AAC file before trying to recombine it with the H264 video stream into a new FLV file?

I uploaded the file here: http://dl.free.fr/jOC7Wobmp

Thanks for any hint.

[jagabo]

Try inverting one of the channels. Unfortunately, the music is in phase but the voices are out of phase. Inverting one channel will fix the voices but the music will be screwed up. Probably worth it though. First few minutes after inversion attached.

[yetanotherlogin]

Thanks much. I don't know what in/out of phase means (sine wave with voice and music at the opposite of each other?). I guess the person who uploaded the video screwed up adding music at the beginning.

I used Audacity followed by Effect > Invert, but it didn't do anything. What application should I use to get the same result as you did? And next, will fmpeg be able to recombine the AAC file and the H264 file into a single FLV file?

[jagabo]

I used Audacity followed by Effect > Invert, but it didn't do anything.

First split the stereo track into separate left/right channels.



Highlight only one of the channels. Apply Effect -> Invert. Make them back into a stereo track (the option is just above "Split Stereo Track" in the above menu). Export the file to whatever format you want.

And next, will fmpeg be able to recombine the AAC file and the H264 file into a single FLV file?

Audacity will not mux the audio and video, just export the audio as a file. You'll need to use a muxing tool like mmg or mp4box.

[yetanotherlogin]

Thanks but no go. I must be misunderstanding a step, maybe selecting the track:

1. Open AAC file in Audacity
2. (my file.AAC) > open combo per screenshot above > Split Stereo Track
3. With upper track (left?) preselected, ie. with yellow border, Effect > Invert
4. (my file.AAC) > open combo per screenshot above > Make Stereo Track
5. Play: No change.

I attached a screenshot.

[jagabo]

You need to mark the whole track, not just select it. Click on a bare spot on the little panel at the left, say, on the text that says Right or Left. The background of the entire track will turn dark. Then Effect -> Invert.

[yetanotherlogin]

Thanks, that did it.