Mid/Side (M/S)

General

• M/S is the same Concept that is used when Combining a Un-Directional Microphone with a Bi-Directional Microphone in Order to Capture a Stereo Field During Recording
• i.e. M/S Recording Employs 2 Mics in the Same Spot, the “Mid” Mic (Typically Cardioid) is Pointing at the Sound Source and the “Side” Mic (a Figure Eight Pattern Mic) is Pointing to the Left by 90° 
• so the 2 Mics are Pointing Perpendicular to Each Other while the “Mid” Mic Records the Direct Image of a Sound at the Null Spot of the Figure Eight Side and the “Side” Mic Records the Indirect Sounds via the Figure Eight Side 

– Mid (Centre) = Contains All the Info that Appears on Both, the Left and Right, i.e. Sum or Frequencies that are the Same on Both Sides, What is Present Equally on Both Channels

– Side = Contains the Info that Differs between the Left and Right, i.e. Difference or Frequencies that are Different on Both Sides, you can also Say “Left + Right Phase Inverted Right”

• Mid / Side can be also Explained as Mid = Sum of Left and Right and Side = Difference of Left and Right

• Notice that Technically Speaking, Both Mid and Side are Mono Channels

• when Mastering a Stereo Record they will need to Encode an Existing Stereo Field into Mid (M) and Side (S) Components so they can be Processed or Mixed separately (then Decode back to Stereo)

• Plugins that Provide M/S Technique basically Separate the L/R Signal into Centre Elements and Side Elements 

Encoding

M = L+R

• to Create the Mono Component simply Combine the Left and the Right Channels in Phase

S = L-R

• to Create the Stereo Component Combine the Left and the Right Channel that is Opposite in Polarity

• by Doing this you are Removing the Elements of the Stereo Field that are Common between the 2 Channels (since they are 180 Degrees out of Phase), leaving only the Stereo Portion of the Image

Decoding

• Decoding M and S Signal is the same Process used by M-S Matrix Systems and can also be accomplished by using 3 Faders of a Console

1) Send the M Signal to Fader 1 = Panned Center

2) Send the S Signal to Fader 2 = Panned Left

3) Send the S Signal to Fader 3 = Panned Right but with Reversed Polarity

4) Finally Decrease both Levels, Left and Right Channel by 1/2 (6dB) so that the Level is the same as the Original

Result will be:

Left Channel = M+S or Substituting the Formula above (L+R) + (L-R) = 2L

Right Channel = M-S or (L+R) – (L-R) = L+R – L+R = 2R