The M/S Stereo Technique

The M/S technique employs a mid (M) cartridge that directly picks up the mono sum signal, and a side (S) cartridge that directly picks up the stereo difference signal (analogous to the broadcast stereo subcarrier modulation signal). Although two individual microphones may be used, single-unit M/S microphones are more convenient and generally have closer cartridge placement. Figure 1.116, a Shure VP88, and Fig. 1.117, an AKG C422, are examples of M/S microphones.

Figure 1.118 indicates the pickup patterns for a typical M/S microphone configuration. The mid cartridge is oriented with its front (the point of great-est sensitivity) aimed at the center of the incoming sound stage. A cardioid ( unidirectional) pattern as shown is often chosen for the mid cartridge, although other patterns may also be used. For symmetrical stereo pickup, the side car-tridge must have a side-to-side facing bidirectional pattern (by convention, the lobe with the same polarity as the front mid signal aims 90° to the left, and the opposite polarity lobe to the right).

In a stereo FM or television receiver, the mono sum baseband signal and the stereo difference subcarrier signal are demodulated and then decoded, using a

FIgURE 1.116 Shure VP88 stereo condenser microphone. Courtesy Shure Incorporated.

FIgURE 1.117 AKG C422 stereo coincident microphone. Courtesy AKG Acoustics, Inc.

FIgURE 1.118 MS microphone pickup patterns.

sum-and-difference matrix, into left and right stereo audio signals. Similarly, the mid (mono) signal and the side (stereo difference) signal of the MS micro-phone may be decoded into useful left and right stereo signals.

The mid cartridge signal’s relation to the mono sum signal, and the side cartridge signal’s relation to the stereo difference signal, can be expressed simply by

( )

Solving for the left and right signals,

L=M + S (1.30)

R=M− S (1.31)

Therefore, the left and right stereo signals result from the sum and difference, respectively, of the mid and side signals. These stereo signals can be obtained by processing the mid and side signals through a sum and difference matrix, implemented with transformers, Fig. 1.119, or active circuitry. This matrix may be external to the M/S microphone or built in.

FIgURE 1.119 Transformer sum and difference matrix for M/S microphones.

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In theory, any microphone pattern may be used for the mid signal pickup.

Some studio M/S microphones provide a selectable mid pattern. In practice, however, the cardioid mid pattern is most often preferred in M/S microphone broadcast applications.

The AKG C422 shown in Fig. 1.120 is a studio condenser microphone that has been specially designed for sound studio and radio broadcasting. The micro-phone head holds two twin diaphragm condenser capsules elastically suspended to protect against handling noise.

The wire-mesh grille is differently colored at the two opposing grille sides (light is the front grille side; dark is the rear grille side), thereby allowing the relative position of the two systems to be visually checked. The entire micro-phone can be rotated 45° about the axis to allow quick and exact changeover from 0° for M/S to 45° (for XY stereophony) even when the microphone is rig-idly mounted. The upper microphone cartridge can be rotated 180° with respect to the lower one. A scale on the housing adjustment ring and an arrow-shaped mark on the upper system allows the included angle to be exactly adjusted.

In sound studio work and radio broadcasts, it is often necessary to recognize the respective positions of the two systems from great distances; therefore, two light-emitting diodes with a particularly narrow light-emitting angle are employed. One is mounted in the upper (rotatable) housing, and the other in the lower (nonrotatable) housing. To align the heads, simply have the units rotated until the light-emitting diode is brightest on the preferred axis.

Enclosed within the microphone shaft are two separate field-effect transistor preamplifiers, one for each channel. The output level of both channels may be simultaneously lowered by 10 dB or 20 dB.

The C422 is connected to an S42E remote-control unit that allows any one of nine polar patterns to be selected for each channel. Because of noiseless selection, polar pattern changeover is possible even during recording.

Each channel of the microphone incorporates two cardioid diaphragms fac-ing 180° of each other (back to back), Fig. 1.120. Note the 12 Vdc phantom power for the electronics and the 60 Vdc phantom power for the lower transducer (1), insuring that transducer 1 is always biased on. This transducer has a positive

Transducer 2Transducer 1

FIgURE 1.120 Schematic of the AKG C422 coincident microphone. Courtesy AKG Acoustics, Inc.

output for a positive pressure. The second or upper transducer is connected to pin K, which through the S42E, has nine switchable voltages between 0 Vdc and 120 Vdc. When the voltage at K is 60 Vdc, the output of transducer 2 is 0 (60 Vdc on either side of it), so the microphone output is cardioid.

When the voltage at K is 120 Vdc, transducer 2 is biased with 60 Vdc of an opposite polarity from transducer 1 so the output is 180° out of polarity, the mixed output being a figure 8 pattern.

When the voltage at K is 0 Vdc, transducer 2 has a 60 Vdc bias on it with the same polarity as transducer 1. Because the transducers face in opposite directions, when these two outputs are combined, an omnidirectional pattern is produced.

By varying the voltage on K between 0 Vdc and 120 Vdc, various patterns between a figure 8 and an omnidirectional pattern can be produced.

The Shure VP88 stereo microphone, Fig. 1.116, also employs a switchable pat-tern. Figure 1.121 shows the polar response of the mid capsule and the side capsule.

The left and right stereo signals exhibit their own equivalent pickup patterns corresponding to, respectively, left-forward-facing and right-forward-facing microphones. Figure 1.122 shows the relative levels of the mid and side micro-phones and the stereo pickup pattern of the Shure VP88 microphone in the L position with the bidirectional side pattern maximum sensitivity 6 dB lower than the maximum mid sensitivity. The small rear lobes of each pattern are 180° out of polarity with the main front lobes. For sound sources arriving at 0° the left and right output signals are equal, and a center image is reproduced between the loudspeakers. As the sound source is moved off-axis, an amplitude difference between left and right is created, and the loudspeaker image is moved smoothly off-center in the direction of the higher amplitude signal.

When the mid (mono) pattern is fixed as cardioid, the stereo pickup pattern can be varied by changing the side level relative to the mid level. Figure 1.123 shows an M/S pattern in the M position with the side level 1.9 dB lower than the mid level. Figure 1.124, position H, increases the side level to 1.6 dB higher than the mid level. The three resultant stereo patterns exhibit pickup angles of 180°, 127°, and 90°, respectively. The incoming sound angles, which will create left, left-center, center, right-center, and right images, are also shown. Note the changes in the direction of the stereo patterns and the size of their rear lobes.

Taking the directional properties of real microphones into consideration, it becomes clear that the M/S technique provides a higher recording fidelity than the XY technique. There are at least three reasons for this:

The microphones in an XY system are operated mainly at off-axis condi-1.

tions, especially at larger offset angles. The influence of directivity imper-fections is more serious than with MS systems, where the M microphone is aimed at the performance center. This is illustrated by Fig. 1.125.

The maximum sound incidence angle for the microphone is only half that of the 2.

X and Y microphones, although the covered performance area is the same for all microphones. This area is symmetrically picked up by the M microphone,

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but unsymmetrically by the X and Y microphones. The M/S system can supply the more accurate monophonic (M) signal in comparison with the XY system.

The M/S system picks up the S signal with a bidirectional microphone. The 3.

directivity performance of this type of microphone can be designed with a high degree of perfection, so errors in the S signal can be kept particularly small for all directions of sound incidence. The M/S system can supply a highly accurate side (S) signal.

In the M/S technique, the mono directivity does not depend on the amount 4.

of S signal applied to create the stereophonic effect. If recordings are made in the M/S format, a predictable mono signal is always captured. On the other hand, the stereophonic image can be simply influenced by modifying the S level without changing the mono signal. This can even be done during postproduction.

A. Relative levels of the mid and side microphones.

B. Pickup pattern of the system.

MidSide

FIgURE 1.122 Stereo pickup pattern of the Shure VP88 in the L position. Courtesy Shure Incorporated.