I needed to record some background ambients for this project (alt.sex) and wanted them to be in stereo, to give the soundtrack a little wider sound than just all the voices panned in the center. In the past, if I wanted to do stereo, I'd just get a couple of omni mics and cross them in an "x" pattern and away I'd go. I guess the deal with the "X" pattern is they generally end up about as wide as someone's head, and that's how one normally hears stereo sounds in real life.

One of the "problems" of recording stereo this way is that you *CAN* get phasing problems by having a sound hit one mic a few milliseconds before the other side does. (it *SOMETIMES* cancels out certain frequencies and you end up with thin, tinny or strange sounding audio.) The other problem is that you can get a "deader" spot in the direct middle of your recording because the center portion is where the sound drops off (or is off axis) from the mics. Since none of the mics are pointing towards the center portion of your audio field, you won't be getting a good picture of what's there. (even though SOMETHING will be recorde. Enter M-S Stereo. (Mid-Side stereo) I'm gonna try 'n explain it as best as I think I know how.

How it's done.

You use a cardiod (preferably something like a Sennheiser MKH-40, I've been using my hyper-cardiod MKH-50) microphone along with a figure-8 microphone (I have an AKG SE300 power module/CK 94 mic head combo) and stack them one on top of each other. The figure-eight microphone records signals directly out to the sides (180 degrees in the shape of a figure-eight) and your cardiod focuses on what's going on in the middle. You record both mics separately onto a DAT. What you have now is something that sorta sounds like stereo, but everything in the middle comes out of one speaker, and everything from the sides comes from the other.

You now have to MATRIX the signal. (fear not, you won't be popping out of a pod with goo all over you) I'm gonna try 'n explain how I think it's done. Check some of the links at the bottom of this page to read what is actually going on. When you matrix a pair of M-S signals, you take the figure-eight WIDE audio and double it, flip the phase of one of these channels and combine both with the mono center channel of your cardiod mic signal. What you end up with is a super wide stereo field of audio that has good definition in the middle along with the sides.

The tricky part to understand is how you get TWO signals out of one (mono) figure-eight microphone. A figure-eight microphone has one sound recording diaphragm within it, but it's pointing perpendicular/out to the sides of this microphone. To record audio, the diaphragm goes "back and forth" when sound hits it from either direction. Sound from one side pushes it one way, sound from the other side pushes it back. Once you've doubled this signal, you flip the phase of one side, or "take whatever sound that pushed the diaphragm in one direction, and extract it from whatever pushed the diaphragm back the other way." Both signals are said to be "out of phase" from each other and will cancel each other out if you sum the two together as a mono signal. (similar problem that we had with the "X" mic configuration, but it works with what we're doing because we're taking two entirely different sound sources, an extreme left and right, and playing them back through stereo speakers, and not just one speaker per se.)

So why should I bother with M-S recording?

1. What you end up with is a completely mono compatible signal. The figure-eight signal completely disappears when you play everything through a mono system. (like...A TV SET!) You won't end up with funky, out-of-phase background audio which could completely disappear on you. You'll at least have a good signal of what the middle microphone was point at.

2. You end up with audio which appears in the rear channel of a surround sound system! Because of the phasing that is going on with M-S matrixing, it causes surround systems to think that the signal is part of the rear channel. I'm not referring to the Dobly 5:1 channel or DTS surround, but Dolby Pro-Logic set-ups. This can either be good or bad. If you DON'T want the signal to come out the rear speakers, you have to mix everything on a surround sound capable mixing system. Since I'm a pathetic indie guy with no budget to do a full-blown surround mix, I can FAKE like I did a surround mix by using a M-S matrixed audio!

3. You have more options after the fact. You can decide how wide the audio spread should be after you've recorded your sound. (This is done during the Matrixing.) Also...if for some reason you DON'T want to use a full 180 degree field of audio, but just want was happening in front of you, you CAN only use the cardiod mic and nothing else. A good example of this would be your recording sound effects in the field, you come upon something like a cricket, and you want the sound of the cricket and NOT the entire forest around you. You have the option of only using the middle cardiod signal. If you've been recording everything with an "X" configuration, you'll have to go with a wide signal or nothing.

So how do I do this Matrixing? (without waking up in a puddle of goo with wires attached to your body)

In the past, you had to shell out a couple of hundred bucks for a little box that took both signals and did it's magic before you sent it into your mixer. The other way was to do the split/reverse phase/pan and mix with the middle on a mixing board. (You had to have something that could flip the phase of a channel, some mixers could do this, or you can get a little doo-hickey phase shifter plug.)

Since I digitally transfer everything into a computer anyways, the easiest and cheapest way is to use a plug-in for a sound program. I use Peak (an audio application on the Macintosh) and the Hyperprism VST Matrix M-S plug-in. I'm pretty sure that there are a couple of different plug-ins that do M-S matrixing, but this is the one that I've "found" first.

What other options are there for recording M-S stereo audio?

The CHEAPEST way to record M-S stereo is to use the Shure VP-88 stereo microphone. This mic is a piece of shit. I bought one online and thankfully I was able to send it back. The thing is heavy and huge and sounds nice, but the side channel is COMPLETELY NOISY. I'm talking about "what IS that? TAPE HISS coming out of my mic?" Unless you want to record a rock concert or drum cymbals, avoid this mic.

All the other mics seem to start around a grand and go WAY up. Check out the list of links at the bottom of the page. Since I already had the MKH-50, I used that and bought an AKG Blue Line figure-eight mic. There aren't too many figure-eight microphones out there. (Note: I did do tests with an AKG C414 large diaphragm mic. It has a figure-eight pattern built into it. I got it to do M-S stereo with my MKH-50, but it sure was awkward going out into the field with that set-up!) One of the best ones is the Sennheiser MKH-30. This will put you back about $1300. The AKG Blue Line mics are modular, so you swap the heads and use it for other things. My AKG Se 300/CK 94 only cost me $500. (The figure-eight capsule is the most expensive one they have.) You can easily buy a whole M-S AKG Blue Line rig for about $800. The CK94 figure-eight head seems to be nice 'n quiet, pretty close to my MKH-50 in noise level. If you want to record birdies chirping and wind blowing through the trees, you'll need a pair of quiet mics.

One last thing, the handy-dandy mic clips which hold the two micrphones together are:

Sennheiser MZD 30 dual mic clip or the AKG H300 dual mic clip. Both cost about $60. The AKG H300 rotates so you can use it either in an X-Y pattern or an M-S set-up. I think the MZD 30 is designed to be used inside of their Sennheiser zeppelin housing. I think it has little hooks so you can attach it with rubber bands to the inside of the zeppelin/blimp.

Quick update 12/25/05:

I noticed that audio recorded like this does some weird things when played back in a surround sound environment. Check it out if you have the chance. Seems to do some weird phasing between the mics which screws with the older surround matrix. (Dolby Pro Logic)