Mid/Side is a different way of encoding stereo audio signals, which more commonly are represented by 2 discreet channels (Left and Right). Mid Side was conceived by Alan Blumlein in the 1930’s and is used for the cutting of stereo vinyl records and the transmission of FM radio. Mid/Side encoding can be used in the sound engineering world for the mixing and mastering of stereo audio material. Mid/Side can easily be derived from a stereo pair by using the following formula:
Mid= (Left + Right) - 3dB
Side=(Left - Right) – 3dB
MS is a feature that is included in a variety of plugins, it is however quite simple to create a Mid/Side encoding matrix with the aid of a physical or virtual mixing desk. Given any stereo signal the left and right components should be summed to create the mid component. For the Side the polarity of the Right channel should be inverted before summing.
One of the main advantages presented by Mid/Side is that it give a certain amount of control over the width of the stereo signal, and allows the processing of the mid and side channels separately enabling the user to for example: center an unstable stereo bass by applying High Pass filtering to the Side channel. Most stereo recording will exhibit a non-phase coherent reverb and therefor will be prominent in the side channel. It should be possible to reduce or enhance the width of natural reverb by balancing the levels of mid and side channels. The character of the recorded reverb can also be manipulated by applying EQ to the side channel.
Once Mid Side processing has been applied it will be necessary to return the processed signal to regular stereo. This can be achieved by using the following formula:
Left=(Mid + Side) - 3dB
Right=(Mid – Side) -3dB
By monitoring via the decoded signal it becomes possible to develop a more intuitive use of Mid/Side processing as the effects of any processing are apparent within a familiar frame of reference.
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