Handbook for Sound Engineers

544 Chapter 16

indicates that the stereo image is affected by geometric
compression effects.

Fig. 16-114 shows the XY properties of cardioids.
The recording angle can be altered between 90° and
180°. Again, in-phase reproduction at the correct side is
maintained for all directions of sound incidence. As all
reproduction curves touch the upper edge of the graph,
full stereo width is available at all offset angles. The
individual curvatures indicate deviations from the ideal
geometrical reproduction performance which would be
represented by a straight line. Downward bends indicate

image compression at the base edges, whereas upward

bends indicate expansion. The lower graph shows that

compression effects occur at frontal sound pick-up for

offset angles above 30°, whereas expansion occurs at

smaller offset angles. Best reproduction linearity is

performed at offset angles around 30°. The reproduction

of back sound is always affected by angular compres-

sion. Extreme compression effects occur where the

curves touch the upper edge of the graph. The reproduc-

tion is then clustered at one of the loudspeakers.

16.7.3 The ORTF Technique

A variation on the basic XY coincident technique is the

ORTF technique. The initials ORTF stand for Office de

Radiodiffusion Television Francais, the French govern-

ment radio network that developed this technique. The

ORTF method uses two cardioid microphones spaced

7 inches (17 cm) apart and facing outward with an angle

of 110° between them, Fig. 16-115. Because of the

spacing between the transducers, the ORTF method

does not have the time-coherence properties of M/S or

XY micing.

16.7.4 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

Figure 16-112. Stereophonic localization.

Figure 16-113. XY properties of wide-angle cardioids.

30 o

20 o

10 o

0 o

0 10 20 30

Decibels

In phase In phase

90 o

60 o

30 o

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30 o

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0 o^30 o^60 o^90 o^120 o^150 o^180 o

90 o

60 o

o

30 o

15 o

XY

Figure 16-114. XY properties of cardioids.

In phase In phase

90 o

60 o

30 o

0 o

30 o

20 o

10 o

0 o

0 o 30 o^60 o^90 o 120 o 150 o 180 o

90 o

75 o

o

o

o

XY

15 o

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o