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Basics of Mid-Side Recording

by

Phil O'Keefe
‎04-02-2013 05:32
PM - edited ‎04-22-2013 03:54
PM

The
theory and practice behind this stereo miking
technique

By Phil O'Keefe

Stereo mic techniques are great for capturing and
creating a natural, or even a hyper-natural sense of width and space. Probably
the two most common techniques are the XY coincident arrangement, and a spaced
pair. But the lesser-known Mid-Side (“M-S”) mic technique has its advantages too
. . . let’s investigate.

THEORY

A lot of people are hesitant to try M-S
recording, maybe because they read about decoders and math formulas and feel
overwhelmed. Don’t worry — we’ll keep it simple.

You’ll need two mics: One cardioid, and one
bi-directional (“figure 8”). Ideally you want two similar mics, but this isn't
essential; experiment with whatever mics you have that meet the polar pattern
requirements.

M-S uses the center or “mid” mic in combination
with the bi-directional mic to achieve stereo. As the cardioid mic points right
at the sound source, it picks up the direct sound, while the off-axis
bi-directional mic picks up the room ambience and reflected sound. M-S stereo
“sum and difference” is just the center mic + the side mic for one channel, and
center mic - the side mic for the second stereo channel, with the center mic
being positive polarity and common to both sides. As the left and right sides
originate from the same mic, but with the phase inverted, collapsing an M-S
recording to mono cancels out the left and right sides from the bi-directional
mic, leaving only the positive polarity signal from the center (cardioid)
microphone. This significant advantage of M-S recordings insures perfect mono
compatibility without any phase issues.

SETTING UP THE
MICS

Aim the cardioid mic directly at the sound
source. As with normal cardioid mic placement, adjust the “aim” to taste; but if
you're a fan of close miking, try moving back a bit further from the source for
M-S recordings.

Next, place the figure-8 mic so that the two
lobes of the pattern are set 90° relative to the cardioid microphone. M-S is a
coincident microphone technique, so you want to get the diaphragms of the two
mics as close together as you can. Fig. 1 shows a Soundelux
(also known as "Bock Audio") E250 (bottom) and ELUX 251 (top) set up as a M-S
pair. The cardioid E250 is pointed at the sound source (in this case, the
camera), while the pattern selector on the ELUX is set to bi-directional; it’s
picking up to the left and right, and its side null point points directly at the
sound source/camera.

Fig. 1: A
Soundelux E250 (bottom) and ELUX 251 (top) set up as a M-S
pair

SETTING UP THE BOARD AND
RECORDING

At your DAW, simply route each mic to its own
preamp, and assign the cardioid mic to a single DAW track. You can either record
the bi-directional mic to two identical, separate tracks of its own and invert
the polarity of one of the two tracks later, or record the bi-directional mic to
only one track and use a decoder plug-in, or clone the single bi-directional
mic's track later and invert the clone track’s polarity — your choice. I
generally record the bi-directional mic to two tracks simultaneously in Pro
Tools, labeled “SIDE+” and “SIDE-,” and insert a Trim plug-in on the “SIDE-”
track to invert the phase. Fig. 2 shows a basic M-S track
arrangement in Pro Tools.

Now group the two “SIDE” tracks so that any
changes you make to the volume level of one will apply to the other track
simultaneously, and pan the tracks hard left and right. As you raise the level
of the side mic tracks, the stereo width will increase; lowering them decreases
it. Being able to adjust the amount of stereo information in the recording after
the fact is one of the big advantages of M-S recordings.

Fig. 2: A basic
Mid-Side track and panning arrangement in Pro Tools. Note the use of the Trim
plugin to reverse the polarity (red arrow) on the "Side -" cloned track

FREE
DECODER RING INSIDE!

So why would you need a decoder or hardware
matrix box? The downside of simply cloning the “side” mic track and inverting
the polarity is that you can’t hear the final results as you position the mics —
you have to record them, then flip the polarity and start playback. By encoding
the sum and difference data from the two mics and recording the result, you can
hear how the stereo field will sound before you track. PAiA has a hardware M-S
matrix box schematic available on their website for you solder jockeys: http://www.paia.com/ProdArticles/msdecwork.htm. For
other DAW users, Voxengo’s free MSED M-S decoder plug-in (Mac AU/VST, Win VST)
works very well; download it from http://www.voxengo.com/downloads/.

M-S might not be used as frequently as some other
stereo techniques, but the perfect mono compatibility and ability to adjust the
stereo width at mixdown make it handy for broadcast production, live recordings
of small ensembles, individual instruments, and small groups of background
vocalists on multitrack music sessions. Give it a try!

Phil O'Keefe is a multi-instrumentalist, recording engineer /
producer and the Associate Editor of Harmony Central. He has engineered,
produced and performed on countless recording sessions in a diverse range of
styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John
McGill, Michael Knott and Alexa's Wish. He is a former featured monthly
columnist for EQ magazine, and his articles and product reviews have also
appeared in Keyboard, Electronic Musician and Guitar Player magazines.

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