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fixing audio in MPEG 2 multiplexed file

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William Thomas
fixing audio in MPEG 2 multiplexed file
on Feb 2, 2012 at 8:34:28 pm

For some legal video work, I have been asked to fix some low volume audio in a gigantic .mpg file made up of MPEG 2 video and Mpeg 2 audio. I have demuxed the file saving the video part, and exporting the audio as an .aif file. I fixed the audio and now want to convert it to MPEG 2 audio and re-multiplex the video with the repaired audio. This needs to be rendered or transcoded or saved to a .mpg extention to be able to work in a legal software. I am on a Mac, but I am open to learn how this can be done on a PC.

How's the best way to accomplish this audio repair? In case it matters, the audio had low volume across 6 hours of the file, except for a 2 min section where it was adequately loud volume. The objective was to bring the volume up as loud as possible without clipping throughout and to make less noticeable the abrupt bump in volume (and later drop in volume).

Bill


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Michael Rampe
Re: fixing audio in MPEG 2 multiplexed file
on Feb 2, 2012 at 9:12:47 pm

[William Thomas] "This needs to be rendered or transcoded or saved to a .mpg extention to be able to work in a legal software."

The syntax is:

$ ffmpeg -i input.m2v -i input.aif [codec options] output.mpg

This will only work if the video has no audio. Otherwise you will need the map command to set the streams.

Michael


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stephen dixon
Re: fixing audio in MPEG 2 multiplexed file
on Feb 3, 2012 at 12:02:57 pm

As far as fixing the audio, you might want to use audio compression (not to be confused with file sizes). Putting it simply, Audio compression turns down the volume on the loud parts and brings up the volume of the quiet parts. I don't think ffmpeg has audio compression filters, but Audacity (free) certainly does.

Just to extend on William's post, to use the map command you use "-map input:stream" so for your situation: -map 0:0 -map 1:0 to map the first stream of the first file and first stream of the second file (the count starts at 0, so 0:0 is first stream of first file). If you want to know which streams are which just typing "ffmpeg -i yourfile.mov" will display all the file info including the streams, without doing anything.

As far as converting it to MPEG, the ffmpeg.org FAQ is a good starting point:


-stib


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