Mid-Side Microphone Technique
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What Is the Mid-Side Microphone Technique
The Mid-Side microphone technique is a stereo recording technique that allows you to record two channels of audio 'in the field' (or studio) while still allowing some flexibility of 'changing the microphone position', at least in a simulated way back in the studio. Imagining the recording as a live performance on a stage may make it easier, so put yourself there.
In case you mix-down to stereo in a live situation, Mid-Side still gives you the comfort of changing the width of your stereo field (like physically changing the microphone angles in a X/Y-setup) remotely from the control room. That is why Mid-Side is widely used in TV/broadcast or radio drama recording situations.
The Mid-Side microphone technique is considered a coincident stereo technique because both microphones, for all intents and purposes, are in the same location. This means that the stereo picture is created by the loudness or intensity of each sound in the left or right channels, but not by a time delay.
Setup
The "Side" microphone must be a figure-8 microphone, and should normally be as closely matched as possible to the Mid. A pair of multi-pattern LDCs or a set of similarly designed SDCs like those from Schoeps, MBHO, or Neumann and others will work. It is possible to use radically different microphones, but for the sake of defining the technique, we will keep them the same. The coherence of the microphones is less important when the source is small and directly in front of the M microphone so the S microphone is mostly picking up reflections. The microphones should be well matched for larger sources (like piano) where the M and the S microphones will share source information and therefore panning information. If the microphones are not well matched, the tone quality of the source will change across the stereo image.
The Side microphone must be a figure-8 and should be placed as close as possible to the same point in space as the Mid microphone. Usually they are positioned one above the other. But the Side mic doesn't aim the same direction as the Mid microphone, it aims 90º off-axis. Usually it's good to work out a standard like always facing the front of the figure-8 to the left, so the rear is aimed to the right and the microphone rejects the middle of the stage.
How it works
Record those two channels live and bring them home.
Theory
Remember that figure-8 microphones are "positive" on the front side and "negative" on the back. That is, a wave that "pushes" the front of the mic creates a + voltage and a wave that "pulls" the front creates a - voltage. But for sounds arriving from the rear, a "push" moves the same direction as a front-side "pull" and so creates a negative voltage, while a "pull" from the rear creates a positive.
Here's where it gets fancy.
Let's say I take that cardioid Mid mic and add it to the Side mic - just bus them to the same bus on the console. Well, if you were to look at the polar plots of the two patterns and simply add up the numbers you get at each º around the graph, something interesting happens. (Remember the figure 8 is aimed to the left and is positive on that side and negative on the right.)
Well, as you add the Mid to the Side around the left side of the graph, the signals add together and draw a nice round pattern. At 0º the Mid microphone is full power, but the Side is null, at 270º the Side is full but the Mid is down -6dB, at 180º both microphones are null. But around the right side of the microphone the Side mic is 'incorrect' compared to nature, and therefore cancels the Mid, so you get much less signal on the right side.
In fact, if you draw out all the numbers, you end up with a hypercardioid microphone aimed 63.45º to the left of center.
At this point, add the two mics together and you get a hypercardioid signal aimed better than 45º off to the left.
Then you can take a copy of the Mid and Side and repeat the process - only this time, invert the signal from the Side microphone. Now the rear of the figure-8 microphone is phase-correct with nature while the front is wrong, or on your graph, the right side is positive and the left is negative. Go through the math this time and you again create a hypercardioid pattern aimed 63.45º to the right! That's crap.
Mixing
While you have created an XY pair, as you increase or decrease the volume of the Mid microphone in the mixdown, you create a narrower or wider stereo spread, respectively. Starting from that 126.9º wide hypercardioid pair, you can increase the Mid and as you get to a 70:30 Mid:Side ratio, you get nearly cardioid mics spread at an 81.2º angle. Reduce the Mid to a 30:70 ratio and you approach a pair of figure-8 mics spread 155.8º.
So yes, you get an XY pair, but a very special XY pair that can be repositioned wider & narrower, and with different patterns... AFTER the performance is over!!
Variations
Omni Pattern Mid Microphone
If you make the Mid microphone an omni pattern, then when you decode it the spacing is fixed at a 180º spread - that is, the resulting patterns aim due left and right no matter the volume. But this time, as you adjust Mid vs. Side volume the pattern shifts from cardioid when the microphones are equal, to wide cardioid when there is more Mid, to hypercardioid when there is more Side.
Figure-8 Mid Microphone
On the other hand, if you make the Mid microphone a figure-8, then when you decode the patterns will still be a pair of figure-8s. But this time when you adjust the Mid vs. Side volume you will change the angle from 90º-wide (45º off-axis) when they are equal and either narrower or wider with the Mid increasing or decreasing - but still figure-8s.
Resources
The Mid Side Stereo Microphone Technique By Joseph Lemmer has sample wav files that you can put into your DAW to test the Mid Side Technique without any set up.
