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command line audio stream selection?

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Peter Wu
command line audio stream selection?
on Sep 26, 2016 at 7:56:30 pm

I have an mxf file from a canon camcorder. It contains 1 video stream and 2 audio streams corresponding to left and right channel. My video editor (an old version of premiere) cannot read this file, so I have to demux it into video and audio files. I have no problem with the video and first audio stream.

To extract the 2nd audio channel, I can do

ffmpeg -i input.mxf -map 0:2 -acodec copy out2.wav

I also read that you can put stream specifier on the option, so i tried

ffmpeg -i input.mxf -codec:a:2 copy out2.wav

This doesn't work. I get the first audio stream. What am I misunderstanding?
I'm trying to understand ffmpeg better because I have more advance operations in mind (e.g. re-package the mxf file into a file that premiere cs4 can read).

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Chris Oram
Re: command line audio stream selection?
on Nov 29, 2016 at 10:48:34 am

Hi Peter,

MXF files are great because of the wealth of information that they can pack in to the metadata to describe the essence it carries however as a result there are a myriad of different ways of carrying audio in MXF files which will impact what is the appropriate mapping for you to output to extract the files you need.

I would recommend that you post the information you get from the source file by just running the -i command and pasting the results here for us to take a full look at.

The -map option is usually enough to allocate the mapping of your output into files, streams and channels (in that order).
e.g. 0:1:0:0:1:0

tip - A regular gotcha for anybody dealing with numbers in this output is to remember that the first item whether it is a file, stream or channel will start from '0'

-Chris Oram
Technology, Design and Development for Moving Image Creators

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