MSTools
The MS technique makes it possible to apply effects only to the mid (M) or the side signal (S) of a stereo recording. In this way you can (for instance) let an EQ work on the M to give a voice more air without affecting the cymbals (that are panned to the side) too much or you can use a compressor to affect only the M-elements like kick, bass, voice. You also can adjust the ratio of M to S and there are many other situations where MS processing makes sense. Of course the input signal must be mono compatible. Since you can't use the MS technique on a normal stereo recording with left (L) and right (R) channel the following tools will make it possible to use your favorite plugins for this purpose too.
LRtoMS and MStoLR
These two plugins should always be used in conjunction as insert effects. The idea is to convert the stereo signal to MS format (with LRtoMS, left channel carries the Mid signal and right channel the side signal) - then apply your effect - and convert it back to normal stereo format (with MStoLR). Thus the plugins have to be routed in a chain. Since there are plugins that let you handle both channels separately and others that only work on both channels there are two situations to consider:
(1) With plugins that can be used on the left an right channel separately (i. e. the Elemental Audio EQs) you can build up the chain as described above and you are done. The left channel of the effect works on the mid signal and the right channel on the side signal (see picture).
(2) With plugins that always work on both channels you will have to duplicate the track - pan one track fully left and the other fully right - and send them to a group. Insert LRtoMS to both channels and MStoLR to the group - now you can use your effect as usual. On the left panned channel it works on the mid signal - on the right panned channel it affects the side signal. In the group channel all is converted back to LR (see picture).
Of course you will have to watch for latency as with other processing too but there is nothing else to consider. The easiest way ist to apply the effect to both channels with "light" settings on that channels where you don't want it to do audible changes and deal with latency in the groups but there are other ways like track delay too. With future versions of audio software it may not be an issue anymore. (Please note: There are no parameters - the "unused" labeled parameter you see is only a "dummy" and has no influence.)
MSadjust
This is an easy to use effect. It converts LR to MS format - let you adjust the ratio between M and S - and converts back to LR format.
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