Handbook for Sound Engineers

546 Chapter 16

side (stereo difference) signal of the MS microphone
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

(16-28)

(16-29)

Solving for the left and right signals,

(16-30)

(16-31)

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. 16-119, or active circuitry. This matrix may be
external to the M/S microphone or built-in.

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. 16-120 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 microphone 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

rigidly 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 respec-

tive 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 sepa-

rate 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 selec-

tion, polar pattern changeover is possible even during

recording.

Figure 16-119. Transformer sum and difference matrix for
M/S microphones.

M^1

2 LR+

=---------------------

S^1

2 LR–

---------------------=

LMS+=

RMS–=

Brn

Red

Vio

Grn

Blu

Blk

Wht

Brn

Red

Vio

Grn

Blu

Blk

Wht

Orn

Yel

Gry

Orn

Yel

Gry

3 2 1 3 2 1

3 2 1 3 2 1

Center

microphone

Sides

microphone

Left

channel

Right

channel

Figure 16-120. Schematic of the AKG C422 coincident

microphone. Courtesy AKG Acoustics, Inc.

Transducer 2

Transducer 1

60 V

12 V

K M D C J B A

MK 42/20

S 42 E

CH. 1

CH. 2