A Blast From The Past
Mid and Side microphone recording techniques revisited.
One of the many aims of HSR is to introduce readers to as many aspects of recording and the techniques involved as possible. Here is a variation on the simple 'crossed pair' stereo microphone technique called M-S or Mid and Side.
M-S recording has its origins in the Alan Blumlein era of the 1930s when stereo recording was first being investigated. You will generally need to do some DIY recording to appreciate the technique but there is now a unit available from Central Recording Services for the modest sum of £68 which makes it comparatively easy to set up a couple of microphones in this interesting configuration.
Two mics are used, one of which must have a 'figure of eight' polar pattern, but the second mic can have any response. The obligatory figure of eight is arranged sideways on to the sound source and the second mic mounted symmetrically to it and as close as possible. One should imagine the second mic immediately above the figure of eight and pointing forward ie. to the mid of the sound stage. The previously mentioned CRS unit takes the outputs of these mics, after suitable pre-amplification, and creates a 'stereo' signal with advantages over a conventional crossed pair recording.
Firstly, the stereo width can be varied by a rotary control, effectively by altering the contribution of the figure of eight mics' pickup response. Secondly, the use of a 'mid' or forward facing mic overcomes problems inherent in many ordinary crossed pair microphone arrangements. Centre stage sounds, for example, are not 'off-axis' which is often the case with directional crossed pairs where the directional information is first order and relies on the mechanical and acoustic labyrinth arrangements in the mics concerned.
There is a third operational advantage in using M-S techniques. If one records the Mid and Side signals themselves on site, one can later make the cross mix decisions between the signals picked up on each microphone back in the control room in far more favourable monitoring conditions. On site, the M-S unit can simply be placed in the monitoring chain after the inputs to the tape recorder, which is taking the M and S signals on its two channels.
The choice of microphone type for the Mid signal affects the nature of the derived stereo crossed pair. If another figure of eight-type mic is used, the stereo pair will consist of crossed figure of eight patterns with variable angle or stereo width. If an omnidirectional mic is employed the result is a pair of cardioid responses at 180 degrees to each other.
This creates the most natural perspective as it is second order, that is to say, the sound image does not rely on the mics' mechanical and acoustic labyrinth design for the centre stage pickup and hence is free of the high frequency blurring that can occur. If a cardioid response mic is used for the Mid information, facing forward, a cardioid pattern with a fairly wide angle can be simulated. As before, the stereo width of the image is controllable by using the stereo/mono rotary control of the CRS unit.
The CRS device is mains powered, all electronic and utilises high quality, low noise op amps. Inputs and outputs are XLR and quarter inch jacks offering balanced and unbalanced configurations respectively. There's a ground lift switch to cater for the occasions when a hum loop may occur when interconnecting the device to other equipment.
The unit as it stands is excellent value for money though the microphones used in the system must be fed first of all through a suitable pre-amp as the unit is strictly line in and line out. CRS, I hear, are developing an electronically balanced mic pre-amp unit which would obviously overcome this deficiency.
For more info on the Matrix Unit contact Central Recording Services at (Contact Details).
Feature by Mike Skeet
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