treating two channels frequency response differently
The mid-side (M-S) stereo technique is one of the two formats of “intensity stereo,” that is, stereo in which spatial localization is determined by the differences in the intensity of a sound wave as it arrives in phase at a coincident pair of microphones. Intensity stereo relies completely on the directional characteristics (polar patterns) of the microphone pair to produce this effect, since only intensity differences and not phase differences exist between the channels for any single source arriving at a coincident pair.
(Source: “M-S Stereo: A Powerful Technique for Working in Stereo” by Wesley L. Dooley and Ronald D. Streicher)
The most common situations where M/S (aka M-S aka mid-side) techniques are getting applied are:
- during the recording process when mid-side microphonie is utilized
- on the 2bus during the late mixing or mastering stage
- in audio restauration situations
I’m skipping the mid-side microphonie stuff here and just recommend “A More Realistic View of Mid/Side Stereophony” by Trevor Owen de Clercq which is available online.
While the M/S signal is obtained in a natural fashion during microphone stereophony, in most other cases stereo (L/R) encoded signals are available. In this case and to obtain the advantages of M/S a conversion is necessary.
The conversion is dead easy and is accomplished via a sum-and-difference matrix network, where typically the mid signal is the sum (M = R + L) and the side signal is the difference (S = R – L). The other way around is that simple as well and there are plenty of tools available which handle this stuff for us in the digital audio workstation.
Once a stereo source is encoded in M/S, the door is open to treat mid- and side content individually. For example, this allows for selective correction of some problems encountered on location such as out-of-phase low-frequency noise from the environment. In this case the side channel, which contains the majority of this information, can be passed through a high-pass filter to reduce such unwanted low-frequency content, and this can be done without any alteration of the mid content.
Some other typical mixing/mastering targets ideally achieved in the M/S domain are:
- assuring mono compatibility
- stereo widening and increasing depth perception
- attenuating or emphasizing the signals room information
- increasing intelligibility of voice in a mix
This is accomplished mostly by performing alterations of both channels frequency and dynamic response. The tools to be used just need to have separate controls per channel as long as M/S encoding/decoding is done externally before and after the plugin, other tools already offer internal M/S handling to make things easier: The BaxterEQ is an example for a M/S frequency shaper and Density MKII is an example for a M/S dynamics compressor.
Variety Of Sound 
Personally I have never had any luck with M/S in mastering. I always ended up with a ‘mono’ sounding track so to speak. Once I went back to regular L+R = stereo it always sounded better. I did like how you can clean up the low end nicely in MS though.
Try this simple approach if you wan’t your master to sound reasonably wider: use only cutting for mid channel and only boosting for side channel (except HPF), and you should work listening to each channel separately. As a final adjustment turn them both on and set their relative balance with stereo width control. Try to create continuous stereo image, without hole in the middle.
It depends on source material which technique is adequate (M+S or L+R), both most modern production is better suited for mid/side.
Thanks for the tip man! I am unclear about one thing you said.. (“As a final adjustment turn them both on and set their relative balance with stereo width control.”) – Are you referring to using a stereo imager or just balacing the volume of the Side channel vs the Mid…
Thanks for this post about this awesome but often overlooked microphone technique! Please mention the importance of using a decoder to complete the effective results of set up.
I think it’s important to understand that one of the microphones used in mid-side must be a figure eight!
I’ve recently recorded some drum parts using M/S as the overheads. The goal was to allow flexibility later on. So, during the verses of the song, the kit sounds “wider” and during the chorus narrows down. It’ll be months before it’s finished, but I’ll share a streaming link when it’s ready and people can tell me what they think of the effect. Great article!
Its something which personally I rely on alot, Affords me so much more control if and/or when I require it
I also use M/S quite extensively. It’s a shame that it is talked about and perceived of as an effect or a psychoacoustic trick when in fact Mid and Side (Sum and Difference) are components of stereo audio and perception on equal footing with the components of Left and Right. Very nice article.
I love MS processing! I use it for my field recordings to enhance spatiality, in mixing and in mastering. Cutting lows on side is most common – it focuses soundstage.
I have once noticed, when processing one of my field recordings, that when I used non-lin-phase EQ on the side channel – the stereo image was skewed, there were some problems with imaging. When I switched that EQ to linear phase mode, stereo image was OK.
And now e.g. Brainworx EQ is not linear phase, and so is BaxterEQ – I am confused 🙂
Thanks for posting the article I love using my sound analyzer software on my computer to look at different tones!
Exactly those informations that I see here are really helpful when it comes to late phase of mixing . Some of tracks are getting pushed in a cue and dynamics , so they seems to be more energetic .