When most people think of stereo recording, the first thing that = comes to=20 mind is a matched pair of microphones, arranged in a coincident (XY) = pattern. It=20 makes sense, of course, since that's the closest way to replicate a real = pair of=20 human ears.
But while XY microphone recording is the most obvious method, it's = not the=20 only game in town. The Mid-Side (MS) microphone technique sounds a bit = more=20 complex, but it offers some dramatic advantages over standard coincident = miking.=20 If you've never heard of MS recording, or you've been afraid to try it, = you're=20 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=20 relies on the time delay of a sound arriving at one input milliseconds = sooner=20 than the other to localize a sound within a stereo field. It's a fairly = simple=20 concept, and one that works well as long as both mics are closely = matched and=20 evenly spaced to obtain an accurate sonic image.
One of the weaknesses of the XY microphone technique is the fact that = you're=20 stuck with whatever you've recorded. There's little flexibility for = changing the=20 stereo image once it's been committed to disk or tape. In some cases, = collapsing=20 the tracks into mono can result in some phase cancellation.
The MS technique gives you more control over the width of the stereo = spread=20 than other microphone recording techniques, and allows you to make = adjustments=20 at any time after the recording is finished.
Mid-Side microphone recording is hardly a new concept. It was devised =
by EMI=20
engineer Alan Blumlein, an early pioneer of stereophonic and surround =
sound.=20
Blumlein patented the technique in 1933 and used it on some of the =
earliest=20
stereophonic recordings.
The MS microphone recording technique =
is used=20
extensively in broadcast, largely because properly recorded MS tracks =
are always=20
mono-compatible. MS is also a popular technique for studio and concert=20
recording, and its convenience and flexibility make it a good choice for =
live=20
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=20 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=20 variations of the technique use an omni or figure-8 pattern). The "Side" = mic=20 requirement is more stringent, in that it must be a figure-8 pattern. = This mic=20 is aimed 90 degrees off-axis from the sound source. Both mic capsules = should be=20 placed as closely as possible, typically one above the other.
How it Works
It's not uncommon for musicians to be intimidated by the complexity = of MS=20 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=20 not all that complicated. The concept is that the Mid microphone acts as = a=20 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=20 up ambient and reverberant sound coming from the sides of the sound = stage. Since=20 it's a figure-8 pattern, the two sides are 180 degrees out of phase. In = other=20 words, a positive charge to one side of the mic's diaphragm creates an = equal=20 negative charge to the other side. The front of the mic, which = represents the=20 plus (+) side, is usually pointed to the left of the sound stage, while = the=20 rear, or minus (-) side, is pointed to the right.
The signal from each microphone is then recorded to its own track. = However,=20 to hear a proper stereo image when listening to the recording, the = tracks need=20 to be matrixed and decoded.
Although you have recorded only two channels of audio (the Mid and = Side), the=20 next step is to split the Side signal into two separate channels. This = can be=20 done either in your DAW software or hardware mixer by bringing the Side = signal=20 up on two channels and reversing the phase of one of them. Pan one side = hard=20 left, the other hard right. The resulting two channels represent exactly = what=20 both sides of your figure-8 Side mic were hearing.
Now you've got three channels of recorded audio=E2=80=93 the Mid = center channel and=20 two Side channels =E2=80=93 which must be balanced to recreate a stereo = image. (Here's=20 where it gets a little confusing, so hang on tight.) MS decoding works = by what's=20 called a "sum and difference matrix," adding one of the Side = signals=E2=80=94the plus=20 (+) side=E2=80=94to the Mid signal for the sum, and then subtracting the = other Side=20 signal=E2=80=94the minus (-) side=E2=80=94from the Mid signal for the = difference.
If you're not completely confused by now, here's the actual = mathematical=20 formula:
Mid + (+Side) =3D left channel
Mid + (-Side) =3D right =
channel
Now, if you listen to just the Mid channel, you get a mono signal. = Bring up=20 the two side channels and you'll hear a stereo spread. Here's the really = cool=20 part=E2=80=94the width of the stereo field can be varied by the amount = of Side channel=20 in the mix!
Why It Works
An instrument at dead center (0 degrees) creates a sound that enters = the Mid=20 microphone directly on-axis. But that same sound hits the null spot of = the Side=20 figure-8 microphone. The resulting signal is sent equally to the left = and right=20 mixer buses and speakers, resulting in a centered image.
An instrument positioned 45 degrees to the left creates a sound that = hits the=20 Mid microphone and one side of the Side figure-8 microphone. Because the = front=20 of the Side mic is facing left, the sound causes a positive polarity. = That=20 positive polarity combines with the positive polarity from the Mid mic = in the=20 left channel, resulting in an increased level on the left side of the = sound=20 field.
Meanwhile, on the right channel of the Side mic, that same signal = causes an=20 out-of-phase negative polarity. That negative polarity combines with the = Mid mic=20 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=20 effect, increasing the signal to the right side while decreasing it to = the=20 left.
What's the Advantage?
One of the biggest advantages of MS recording is the flexibility it = provides.=20 Since the stereo imaging is directly dependent on the amount of signal = coming to=20 the side channels, raising or lowering the ratio of Mid to Side channels = will=20 create a wider or narrower stereo field. The result is that you can = change the=20 sound of your stereo recording after it's already been recorded, = something that=20 would be impossible using the traditional XY microphone recording=20 arrangement.
Try some experimenting with this=E2=80=94listen to just the Mid = channel, and you'll=20 hear a direct, monophonic signal. Now lower the level of the Mid channel = while=20 raising the two Side channels. As the Side signals increase and the Mid=20 decreases, you'll notice the stereo image gets wider, while the center = moves=20 further away. (Removing the Mid channel completely results in a signal = that's=20 mostly ambient room sound, with very little directionality =E2=80=93 = useful for effect,=20 but not much else.) By starting with the direct Mid sound and mixing in = the Side=20 channels, you can create just the right stereo imaging for the = track.
Another great benefit of MS miking is that it provides true mono=20 compatibility. Since the two Side channels cancel each other out when = you switch=20 the mix to mono, only the center Mid channel remains, giving you a = perfect=20 monaural signal. And since the Side channels also contain much of the = room=20 ambience, collapsing the mix to mono eliminates that sound, resulting in = a more=20 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=20 popular in the broadcast world.
Other Variations
While most MS recording is done with a cardioid mic for the Mid = channel,=20 varying the Mid mic can create some interesting effects. Try an omni mic = pattern=20 on the Mid channel for dramatically increased spaciousness and an = extended low=20 frequency response.
Experimenting with different combinations of mics can also make a = difference.=20 For the most part, both mics should be fairly similar in sound. This is=20 particularly true when the sound source is large, like a piano or choir, = because=20 the channels are sharing panning information; otherwise the tone quality = will=20 vary across the stereo field. For smaller sources with a narrower stereo = field,=20 like an acoustic guitar, matching the mics becomes less critical. With = smaller=20 sources, it's easier to experiment with different, mismatched mics. For = example,=20 try a brighter sounding side mic to color the stereo image and make it = more=20 spacious.
As you can see, there's a lot more to the MS microphone technique = than meets=20 the ear, so give it a try. Even if the technical theory behind it is a = bit=20 confusing, in practice you'll find it to be an incredibly useful method = to=20 attain ultimate control of the stereo field in your recordings.
Here are some drum loops made with Mid-Side microphone recording. =
The=20
mics were about 5 feet in front of the kit, head height with the drummer =
in a=20
small room. If you have a DAW, you can download the Mid and Side WAV =
files=20
separately and set up the sum-and-difference matrix =
yourself.
May=20 2011 Studio Basics Mid-Side Microphone Recording by Universal=20 Audio
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