Breaking down the M/S technique in simple terms.
Okay, I'm gonna try and do this without vectors, math, "sum and difference" signal theory, and no pictures - except for the pictures we'll draw in our mind. Let's start by imagining a figure 8 ribbon mic and how it works, since the concept is really simple.
A figure 8 ribbon mic basically consists of a piece of thin metal ribbon, hanging in a large magnetic field. The ribbon is fasten at the top and bottom, and a small wire is connected to each end of the ribbon. When there is a sound, it causes the ribbon to move inside the magnetic field; a small voltage is generated, and that's the signal that comes down the cable.
The ribbon is free to move forward and backward, but not side to side. If a sound comes in from the front, it puts out a positive signal. If a sound comes in from the back, it puts out a negative signal. If a sound comes in from the side, it hits both the front and back of the ribbon equally, and no signal comes out.
Okay, now let's put this mic into our mixing board, set all the balance controls straight up, and listen to it.
If we put a singer in front of the mic and another singer in back of the mic, it'll pick up both singers pretty equally, because the front and back of the figure 8 ribbon mic are both open. A singer singing into the side of the mic won't be heard at all.
Okay, is everybody cool on all this stuff so far? Cuz here's where we get a little tricky.
Let's imagine 3 trumpet players spread out across a stage. The player on the left is A, in the center is B, and at the right is C.
At the same time, we're gonna turn our mic sideways, so that the front of the mic is facing the trumpet player (A) standing at the left side of the stage, the back of the mic is facing the trumpet player (C) standing at the right side of the stage, and the side of the mic is facing the trumpet player (B), standing in the middle of the stage. When we listen to the recording, we can hear A and C clearly, and we won't hear B hardly at all, as expected.
OK, here's where the "sleight of hand" magic trick comes in.
Let's run the mic into a Y cable, except one of the ends of the Y cable is wired backwards. This means we'll have a normal positive signal going to one channel, AND a mirror image negative signal going to the other channel.
If you bring up the level of either channel by itself, you'll get a good sound. But if you leave one channel up and bring the other channel up slowly, the ENTIRE signal will eventually disappear when the levels are exactly equal. The plus and minus signals exactly cancel out. But here's where it gets wild.
Stick a cardioid mic on top of the figure 8 and point it towards the center of the stage, and bring up it's level on a third mixer channel, again with the pan control centered. We pretty much hear the middle trumpet now and we hear the two trumpets on both sides of the stage, but softer than the middle one. And the ribbon mic is still silent, so why even bother with it?
Here's the magic trick:
Move the pan control on the first ribbon mic's channel all the way to the left. Move the pan control on the backwards wired channel all the way to the right. Whoa, full, glorious stereo. Kill the middle mono mic and the entire signal disappears again. Bring back the mono center mic and we have full stereo again. What the hell is going on - what's happening?
Think of each trumpet as putting out a little puff of positive air. As the trumpet note from A hits the ribbon the mic faithfully records it to the track that corresponds to that ribbon mic's channel. At the same time it records a mirror image negative to the next ribbon track (since it's wired backwards). It also records a positive puff to the cardioid in the center).
When you add that center cardioid mic, it combines with the ribbon signal from the left, and moves the apparent sound all the way to the left. The trumpet on the right gets the reversed channel's attention and it combines with the center mic to produce a very right hand sounding signal. The center trumpet is only heard by the cardioid so it only comes bact thru the center.
By adjusting the level of the side channels relative to the center mic, you can get any stereo spread you want, from full stereo to perfect mono.
So can you use condenser mics that have a figure eight pattern, or three cardioids, each pointing in a different direction? Yes, as long as you can get the two side channels to null completely, or damn close to it. One mic or side has to face left and the other side/mic has to face right, and the two channels hafta cancel out almost completely. How you do that is up to you. The cardioid mic faces forward and completes the stereo information gathering.
That's how M/S works. I use/like ribbons for the side mic cuz they just have one moving part, no phantom power to worry about, and if the mixer channel has a polarity reverse switch, a simple Y cord will work. And the signal is automatically identical to start with.
Does this about cover it?
(this is a good thing)
