CHAPTER 4

Basic Stereo Techniques

Even if you never intend to record an ensemble larger than a standard rock & roll rhythm section, a good grasp on stereo recording techniques is essential. Stereo miking is commonly used when recording drum kits, pianos, string sections, and Leslies and can certainly be applied to just about any recording situation. So while we wont discuss these techniques in a classical music sense (where a lot of knowledge beyond the scope of this book is a necessity), heres a basic overview of the many stereo miking methods. First of all, stereo miking is an improvement over mono miking because it provides: A sense of the soundeld from left to right. A sense of depth or distance between each instrument. A sense of distance of the ensemble from the listener. A spatial sense of the acoustic environmentthe ambience or hall reverberation.

Types of Stereo Miking

There are four general mic techniques used for stereo recording, each with a different sound and different sets of benets and disadvantages: Coincident pair (including X/Y, M-S and Blumlein)

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Spaced pair Near-coincident pair (the famous ORTF method) Bafed-omni pair or articial head
COINCIDENT PAIR

A coincident pair consists of two directional mics mounted so that their grilles are nearly touching, but with their diaphragms angled apart in such a way that they aim approximately toward the left and right sides of the ensemble. For example, two cardioid microphones can be mounted angled apart, their grilles one above the other. The greater the angle between microphones and the narrower the polar pattern, the wider the stereo spread. X/Y While there are several variations of the coincident pair, the X/Y conguration is the easiest and mostly widely used. X/Y requires two identical directional microphones. Unlike what you may think, the mics are not crossed in an X pattern in this conguration. In fact, the mic capsules are placed as close as possible to one another in a 90-degree angle. (See Figure 58)
Figure 58 Two AKG 451s in an X/Y Conguration

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M-S M-S stands for Mid-Side and consists again of two microphones: a directional mic (an omni can be substituted as well) pointed toward the sound source and a gure-8 mic pointed toward the sides. Once again, the mics are positioned so that their capsules are as close to touching as possible. (See Figure 59)
Figure 59 M-S Miking

M-S is great for stereo imaging, especially when most of the sound is coming from the center of the ensemble. Because of this, its less effective on large groups, favoring the middle voices that the mics are closer to. M-S has no phase problems in stereo, with excellent mono compatibility, which can make it the best way to do room and ambience miking under the right circumstances. In many cases it can sound more natural than a spaced pair. If the source is extra large, sometimes using M-S alone will require too much distance to get the whole section or choir into perspective, so multiple mic locations must be used. If a narrower pickup pattern is required to attenuate hall sound, then a directional mic such as a cardioid or even a hypercardioid will work for the M mic. Just be aware that you may be sacricing the bottom octaves as a result. For best placement, walk around the room and listen to where the instrument or sound source sounds best. Note the balance of instrument to room and the stereo image of the room as well. Once you have found a location, set up the directional mic where the middle of your head was.

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Listening to either of these mics alone may sound okay or may even sound horribly bad. In order to make this system work, the mics output signals need an additional decoding step to reproduce a faithful stereo image. The directional creates a positive voltage all the way around, and the bidirectional mic is mute to the front (of the source, the side of the mic) and creates a positive voltage from anything coming from the left and a negative voltage from anything coming from the right. While you can buy an M-S decoder, you can easily emulate one with three channels on your console. On one channel, bring up the cardioid (M) forward-facing mic. On a second channel, bring up the gure-8 mic (S) on two additional channels, either by multing the outputs or by patching from the insert out of the S channel to the insert in of an adjacent channel. Pan both channels to one side (like hard left), then run a tone down the rst S channel, ip the phase of the second S channel, and bring up the level until the two channels cancel 100 percent. Now pan the rst S channel hard left, the second S channel hard right, balance the cardioid (M) channel with your pair of S channels, and you have your matrix. Another nice feature of the M-S method is that you are able to vary the amount of room sound (or change the focus) by varying the level of the bidirectional S mic. Blumlein Array Developed for EMI in 1935 by famed audio pioneer Alan Blumlein, the Blumlein stereo setup is a coincident stereo technique that uses two bidirectional microphones in the same point and angled at 90 degrees toward each other. This stereo technique will normally give the best results when used at closer distances to the sound source, since at larger distances these microphones will lose the low frequencies. The Blumlein stereo has a higher channel separation than the X/Y stereo but has the disadvantage that sound sources located behind the stereo pair also will be picked up and even be reproduced with inverted phase. (See Figures 60 and 60A)

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Figure 60 A Blumlein Array

Figure 60A Bruce Swedien with a Royer R-121 Blumlein Array

The Stereo Microphone Although not normally thought of as a coincident mic pair, a stereo mic uses two coincident mic capsules mounted in a single housing for convenience. Because of their close proximity to one another, this method may provide the easiest coincident mic setup. (See Figure 61)

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Courtesy of Royer Labs

The Coincident-Pair Technique Has the Following Features

1) 2) 3) 4) Imaging is very good. Stereo spread ranges from narrow to accurate. Signals are mono-compatible. Stereo not as wide as other methods.

SPACED PAIR

With the spaced pair technique, two identical mics are placed several feet apart, aiming straight ahead toward the musical ensemble. The mics can have any polar pattern, but the omnidirectional pattern is the most popular for this method. The greater the spacing between mics, the greater the stereo spread. (See Figure 62)
Figure 62 Spaced Pair Diagram

If the spacing between mics is too far apart, the stereo separation seems exaggerated. On the other hand, if the mics are too close together, there will be an inadequate stereo spread. In addition, the mics will tend to favor the center of the ensemble because the mics are closest to the center instruments. 64 The Recording Engineers Handbook

Courtesy of Royer Labs

Figure 61 Royer SF-12 Stereo Microphone

In an attempt to obtain a good musical balance, the mics are usually placed about 10 or 12 feet apart, but such spacing results in exaggerated separation. One solution is to place a third microphone midway between the original pair and mix its output to both channels. That way, the ensemble is recorded with a good balance, and the stereo spread is not exaggerated. The spaced pair method tends to make off-center images relatively unfocused or hard to localize. In addition, combining both mics to mono sometimes causes phase cancellations of various frequencies, which may or may not be audible. The advantage with spaced miking is a warm sense of ambience in which concert hall reverberation seems to surround the instruments and, sometimes, the listener. Another advantage of the spaced mic technique is the ability to use omnidirectional microphones. An omni condenser mic has more extended low frequency response than a unidirectional condenser mic and tends to have a smoother response and less off-axis coloration.

The Spaced Pair Technique Features

Off-center images are diffuse. Stereo spread tends to be exaggerated unless a third center mic is used. Provides a warm sense of ambience. Phasing problems possible.

The Decca Tree A variation of the spaced pair is the Decca Tree, which is essentially a spaced pair with a center mic connected to a custom stand and suspended over the conductor. (See Figure 63) Decca Records, which had a long tradition of developing experimental recording techniques, including surround sound and proprietary recording equipment, developed the Decca Tree as a compromise between the purist stereo pair and multi-mic arrays for orchestral recording by Decca engineers in 1950s. Apart from an individual engineers choice of mic, it remains unchanged to this day. It is still in use in lm scoring/classical orchestral and opera
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recording as it produces a very spacious stereo image with good localization.

Figure 63 The Decca Tree

The classic Decca Tree setup uses three Neumann M50s arranged in a triangle 10 to 12 feet above the conductors position, although the spacing varies with venue and size of ensemble. The left mic is panned left, the right mic panned right, and the center mic panned to the center with additional mics (spot mics or sweeteners) used over violins (usually panned to the left) and cellos (usually panned to the right) and harp/timps and soloist. Distance between mics depends on the size of the ensemble. For orchestra, the left and right mics are 8 to 10 feet apart, with the center about 6 to 7 feet in front of the left-right axis. It is a little-known fact that Decca has used (and maybe still uses) mics other than the M50 on the tree. In particular, Decca has used M49s and KM56s on the tree, but modern substitutions include the TLM 50, M150, Brauner VM-1, and the DPA 4003s with APE spheres.
NEAR-COINCIDENT PAIR

The most common example of the near-coincident method is the ORTF system, which uses two cardioids angled 110 degrees apart and spaced seven inches (17 cm) apart horizontally. (ORTF stands for Ofce de Radiodif-fusion Television Franaise French Broadcasting Organization.) This method tends to 66 The Recording Engineers Handbook

provide accurate localization; that is, instruments at the sides of the orchestra are reproduced at or very near the speakers, and instruments halfway to one side tend to be reproduced halfway to one side. ORTF provides a much greater sense of space, due to time/phase differences, because the capsules are as far apart as your ears. (See Figure 64)
Figure 64 An ORTF Setup Using AKG 451s

The Near-Coincident Pair Advantages and Disadvantages

Sharp imaging. Accurate stereo spread. Provides a greater sense of air and depth than coincident methods. Wide image and depth of Blumlein without as much of the reverberant eld.

BAFFLED OMNI PAIR

A bafed omni pair, whether using two mics or a dummy head, is simply trying to emulate the way our ears are placed on our heads and, therefore, the way we hear. In this method, a bafe separates two omnidirectional mics by a few inches. The bafe is a hard disk covered with absorbent foam (as in the Jecklin disk, Figure 65). Or, the bafe is a hard sphere with the mics ush-mounted on opposite sides (as in the Schoeps spherical mic, Figure 66).
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Figure 65 Jecklin Disc

Figure 66 Schoeps KFM360 Spherical Mic

With the bafed omni pair, the level, time, and spectral differences between channels create the stereo images. The omni condenser mics used in this method have excellent low-frequency response. 68 The Recording Engineers Handbook

Courtesy of Schoeps GmbH

Courtesy of Josephson Engineering

Also falling into this category is the dummy head such as the Neumann KU-100. (See Figure 67)
Figure 67 Neumann KU-100 Dummy Head

The Bafed Omni Pair Technique Features

Sharp images Accurate stereo spread Excellent low-frequency response

For more information on stereo recording: http://www.kellyindustries.com/microphones/stereo_miking_ techniques.html http://www.turneraudio.com/tech/stereomic.html http://www.music.columbia.edu/cmc/courses/g6630/stereomic. html http://www.tape.com/Bartlett_Articles/stereo_microphone_ techniques.html http://www.josephson.com/mictech.html http://www.audio-technica.com/using/mphones/worship/miking. html http://www.csun.edu/~record/stereo/ http://www.nucleus.com/~lockwood/nc_vs_ms.html http://www.dpamicrophones.com/eng_pub/index.html
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Courtesy of Neumann USA