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Mid-Side (MS) Mic Recording Basics

Mid-Side (MS) Mic Recording Basics

Posted by Daniel Keller on May 24,
2011 10:51:46 AM PDT

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A Mid-Side microphone setup with an AKG 414 as the
Side mic

and an Audio Technica 4051 as the Mid
mic.

When most people think of stereo recording, the first thing that comes to
mind is a matched pair of microphones, arranged in a coincident (XY) pattern. It
makes sense, of course, since that's the closest way to replicate a real pair of
human ears.

But while XY microphone recording is the most obvious method, it's not the
only game in town. The Mid-Side (MS) microphone technique sounds a bit more
complex, but it offers some dramatic advantages over standard coincident miking.
If you've never heard of MS recording, or you've been afraid to try it, you're
missing a powerful secret weapon in your recording arsenal.

More Than Meets the Ears

Traditional XY recording mimics our own ears. Like human hearing, XY miking
relies on the time delay of a sound arriving at one input milliseconds sooner
than the other to localize a sound within a stereo field. It's a fairly simple
concept, and one that works well as long as both mics are closely matched and
evenly spaced to obtain an accurate sonic image.

One of the weaknesses of the XY microphone technique is the fact that you're
stuck with whatever you've recorded. There's little flexibility for changing the
stereo image once it's been committed to disk or tape. In some cases, collapsing
the tracks into mono can result in some phase cancellation.

The MS technique gives you more control over the width of the stereo spread
than other microphone recording techniques, and allows you to make adjustments
at any time after the recording is finished.

Mid-Side microphone recording is hardly a new concept. It was devised by EMI
engineer Alan Blumlein, an early pioneer of stereophonic and surround sound.
Blumlein patented the technique in 1933 and used it on some of the earliest
stereophonic recordings.

The MS microphone recording technique is used
extensively in broadcast, largely because properly recorded MS tracks are always
mono-compatible. MS is also a popular technique for studio and concert
recording, and its convenience and flexibility make it a good choice for live
recording as well.

What You Need

While XY recording requires a matched pair of microphones to create a
consistent image, MS recording often uses two completely different mics, or uses
similar microphones set to different pickup patterns.

The "Mid" microphone is set up facing the center of the sound source.
Typically, this mic would be a cardioid or hyper-cardioid pattern (although some
variations of the technique use an omni or figure-8 pattern). The "Side" mic
requirement is more stringent, in that it must be a figure-8 pattern. This mic
is aimed 90 degrees off-axis from the sound source. Both mic capsules should be
placed as closely as possible, typically one above the other.

Mid-Side recording signal flow.

How it Works

It's not uncommon for musicians to be intimidated by the complexity of MS
recording, and I've watched more than one person's eyes glaze over at an
explanation of it. But at its most basic, the MS recording technique is actually
not all that complicated. The concept is that the Mid microphone acts as a
center channel, while the Side microphone's channel creates ambience and
directionality by adding or subtracting information from either side.

The Side mic's figure-8 pattern, aimed at 90 degrees from the source, picks
up ambient and reverberant sound coming from the sides of the sound stage. Since
it's a figure-8 pattern, the two sides are 180 degrees out of phase. In other
words, a positive charge to one side of the mic's diaphragm creates an equal
negative charge to the other side. The front of the mic, which represents the
plus (+) side, is usually pointed to the left of the sound stage, while the
rear, or minus (-) side, is pointed to the right.

The signal from each microphone is then recorded to its own track. However,
to hear a proper stereo image when listening to the recording, the tracks need
to be matrixed and decoded.

Although you have recorded only two channels of audio (the Mid and Side), the
next step is to split the Side signal into two separate channels. This can be
done either in your DAW software or hardware mixer by bringing the Side signal
up on two channels and reversing the phase of one of them. Pan one side hard
left, the other hard right. The resulting two channels represent exactly what
both sides of your figure-8 Side mic were hearing.

Now you've got three channels of recorded audio– the Mid center channel and
two Side channels – which must be balanced to recreate a stereo image. (Here's
where it gets a little confusing, so hang on tight.) MS decoding works by what's
called a "sum and difference matrix," adding one of the Side signals—the plus
(+) side—to the Mid signal for the sum, and then subtracting the other Side
signal—the minus (-) side—from the Mid signal for the difference.

If you're not completely confused by now, here's the actual mathematical
formula:

Mid + (+Side) = left channel
Mid + (-Side) = right channel

Now, if you listen to just the Mid channel, you get a mono signal. Bring up
the two side channels and you'll hear a stereo spread. Here's the really cool
part—the width of the stereo field can be varied by the amount of Side channel
in the mix!

Why It Works

An instrument at dead center (0 degrees) creates a sound that enters the Mid
microphone directly on-axis. But that same sound hits the null spot of the Side
figure-8 microphone. The resulting signal is sent equally to the left and right
mixer buses and speakers, resulting in a centered image.

An instrument positioned 45 degrees to the left creates a sound that hits the
Mid microphone and one side of the Side figure-8 microphone. Because the front
of the Side mic is facing left, the sound causes a positive polarity. That
positive polarity combines with the positive polarity from the Mid mic in the
left channel, resulting in an increased level on the left side of the sound
field.

Meanwhile, on the right channel of the Side mic, that same signal causes an
out-of-phase negative polarity. That negative polarity combines with the Mid mic
in the right channel, resulting in a reduced level on the right side.

An instrument positioned 45 degrees to the right creates exactly the opposite
effect, increasing the signal to the right side while decreasing it to the
left.

The Mid mic acts as the center channel,

while the Side mic signal creates the stereo
ambience.

What's the Advantage?

One of the biggest advantages of MS recording is the flexibility it provides.
Since the stereo imaging is directly dependent on the amount of signal coming to
the side channels, raising or lowering the ratio of Mid to Side channels will
create a wider or narrower stereo field. The result is that you can change the
sound of your stereo recording after it's already been recorded, something that
would be impossible using the traditional XY microphone recording
arrangement.

Try some experimenting with this—listen to just the Mid channel, and you'll
hear a direct, monophonic signal. Now lower the level of the Mid channel while
raising the two Side channels. As the Side signals increase and the Mid
decreases, you'll notice the stereo image gets wider, while the center moves
further away. (Removing the Mid channel completely results in a signal that's
mostly ambient room sound, with very little directionality – useful for effect,
but not much else.) By starting with the direct Mid sound and mixing in the Side
channels, you can create just the right stereo imaging for the track.

Another great benefit of MS miking is that it provides true mono
compatibility. Since the two Side channels cancel each other out when you switch
the mix to mono, only the center Mid channel remains, giving you a perfect
monaural signal. And since the Side channels also contain much of the room
ambience, collapsing the mix to mono eliminates that sound, resulting in a more
direct mix with increased clarity. Even though most XY recording is mono
compatible, the potential for phase cancellation is greater than with MS
recording. This is one reason the MS microphone technique has always been
popular in the broadcast world.

Other Variations

While most MS recording is done with a cardioid mic for the Mid channel,
varying the Mid mic can create some interesting effects. Try an omni mic pattern
on the Mid channel for dramatically increased spaciousness and an extended low
frequency response.

Experimenting with different combinations of mics can also make a difference.
For the most part, both mics should be fairly similar in sound. This is
particularly true when the sound source is large, like a piano or choir, because
the channels are sharing panning information; otherwise the tone quality will
vary across the stereo field. For smaller sources with a narrower stereo field,
like an acoustic guitar, matching the mics becomes less critical. With smaller
sources, it's easier to experiment with different, mismatched mics. For example,
try a brighter sounding side mic to color the stereo image and make it more
spacious.

As you can see, there's a lot more to the MS microphone technique than meets
the ear, so give it a try. Even if the technical theory behind it is a bit
confusing, in practice you'll find it to be an incredibly useful method to
attain ultimate control of the stereo field in your recordings.

Here are some drum loops made with Mid-Side microphone recording. The
mics were about 5 feet in front of the kit, head height with the drummer in a
small room. If you have a DAW, you can download the Mid and Side WAV files
separately and set up the sum-and-difference matrix yourself.

May
2011 Studio Basics Mid-Side Microphone Recording by Universal
Audio

Tags: Daniel
Keller, Recording,
Microphone
Technique, Mid-Side
Recording

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Darren Wolff said
Thanks! Got
it! (at last) ....now to have a play with my H6 accordingly! :-)
Jason Cox
said
I've read how MS works 100 times, (this article more than 3 times) and
it just sinked in. :) sweeeeet
Jesse Michael Howard said
So when I do
this and flip the phase I get a drastically higher volume on either the left or
the right depending on which side I phase. Is that normal?
Dikshit Sharma
said
thanks :)
Alexander Kretov said
Thanks, excellent explanation!

이치고 said
오옷 짱이네요오
Moises Bernal said
La verdad que ha sido una
técnica que apenas la conocía ni practicado y de la que he sacado un gran
provecho. Antes montaba un par de micros AB sobre la batería y la verdad hacía
que no saliera un sonido compacto con el bajo... a partir de ahora creo que será
una de las técnias que más usare, aún así lo importante de todo esto es seguir
aprendiendo e inventando.
Maria Golubina said
yep!

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