Mid-Side decoding

I have Audacity 2.0.5 installed on a computer running Win Vista Home Premium. Nuances of version and OS are probably irrelevant to my question, which is, Does Audacity have an M-S decoding function?

I record in M-S format so it is fundamental that any editing software I use should be able to convert M-S to stereo. The basic operations are-

Left channel=M+S and Right channel =M-S.

It is often done with 3 mixer channels in hardware or software. There is nothing particularly special about the mixer channels except that at least one of them needs a phase invert function on its input, and that this channel, and another, have to be fed with the same input signal. We get subtraction (M-S) by summing/mixing the two signals in antiphase.

Anyone had any experience in doing this operation in Audacity?


To invert a track, use “Effect > Invert”.
You may also need to split / join stereo tracks: http://manual.audacityteam.org/o/man/splitting_and_joining_stereo_tracks.html

You can also download Steve’s channel-mixer plug-in.
One of the presets (e.g. left to center) is what you want.

Available here: Missing features - Audacity Support
Installation instructions: Missing features - Audacity Support

Steve and Robert, thanks for your prompt and helpful responses. For the plugin to do what it is capable of tends to indicate that it is doing sum and difference operations on the stereo signals and finally restoring them to stereo. This suggests that M-S decoding is possible.


The plug-in is written in a scripting language called “Nyquist”.
Nyquist plug-ins are plain text files, so can easily be edited in any plain text editor (such as NotePad or NotePad++)

You can read about Nyquist plug-ins here: http://wiki.audacityteam.org/wiki/Nyquist_Plug-ins_Reference
and we have a forum board specifically for things to do with Nyquist: http://forum.audacityteam.org/viewforum.php?f=39

Why does there need to be the phase difference between the two sides? Don’t they cancel each other?