M-S Stereo Microphone
Recording

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)

Coffeysounds
Stereo Mic listing -
List of what's out there with prices

Rick
Chins Notes on MS-Matrix Circuts - Detailed info
& schematics.

Turner
Audio: Understanding Stereo Microphone Techniques
& M/S Stereo - Detailed info

Stereo
Mic Techniques for Field Use - Drawings, info
and books on stereo recording.

Full
Compass: Stereo Mic On-Line Catalog - Detailed
listing and prices of stereo mics.

Ideabuzz:
MS Stereo Micing - Text file from a web board
thread on stereo recording.

Ideabuzz: Decoding MS Stereo - Text file from a
web thread on decoding MS stereo.

Rudy
Trubitts Stereo Mic Technique - Good comparison
of stereo mic techniques.

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