Handbook for Sound Engineers

1554 Chapter 42

The receiving end of the infrared link is a silicon
photo diode that is reversed biased and produces current
when struck by photons. The light gathering area is
small, 7 mm^2 , but is effectively increased by mounting
it in a collecting lens, Fig. 42-13.

The silicon PIN receiving diode has a maximum
sensitivity at a wavelength of 850 nm. Fig. 42-14 shows
the spectral sensitivity of the eye, IR LED transmitting
diode, IR filter, and receiving diode.

Infrared light behaves much like visible light; it

reflects off of light-colored walls, ceilings, and other

surfaces so a receiver can “see” the signal even without

direct line-of-sight to the emitter. Also, the receiver’s

ultrawide-angle fisheye lens captures direct or reflected

signals from almost any direction.

There is a unique limitation to the use of infrared: it

cannot be used in bright daylight. The infrared light

occurring as a natural part of daylight will override the

lower power-modulated light from the system. The

system can also receive interference from very high-

level incandescent lamps. Partially dimmed incandes-

cent lamps can also be a problem in some situations

because the reduced voltage to the lamps causes a shift

toward red that greatly increases the infrared output of

the lamp. This increase in infrared interference can, on

rare occasions, be a problem where audience down

lights are left at a dimmed setting and the infrared beam

from the system is weak. Deep under a balcony is a

likely trouble spot. When this problem occurs, it is

necessary to dim the lights more or add more emitters to

the infrared system to cover under the balcony.

An infrared system comprises three sections: the

transmitter, the emitter (sometimes both combined in

one unit), and the receiver. The transmitter imparts the

Table 42-2. Technical Specifications for Infrared
Systems

Characteristic Narrow Band Wide Band

No. of channels 12 2

Carrier frequencies 55–535 kHz,

excluding 455 kHz

95 kHz and

250 kHz

Channel spacing 40 kHz 155 kHz

Modulation FM FM

Pre-emphasis 100 μs5 0 μs

Normal deviation ±6 kHz ±35 kHz

Peak deviation ±7 kHz ±50 kHz

Transmitters

Frequency response (3dB)50 Hz–8 kHz 50 Hz–13 kHz

Max. distortion (1 kHz) <1.0% <1.0%

SNR (A weighted) >55 dB >70 dB

Receivers

Frequency response (–3 dB) 50 Hz–8 kHz 100 Hz–9 kHz

Max. distortion 1 kHz) <2.5% <1%

SNR (A-weighted) >55 dB >63 dB

Emitter Panels

Frequency response (–3 dB) 30 Hz–710 kHz 30 Hz–710 kHz

Figure 42-12. Infrared emitter panel coverage patterns.

150

100

50

Single

Channel

Two

channels

Six

channels

Nine

channels

o

o

o

o

o

o

o

o

o

o

Feet

Figure 42-13. Infrared receiving diode.

Figure 42-14. Sensitivity of the eye, IR LED transmitting

diode, IR filter, and receiving diode.

Collecting lens

Filter

Infrared light

PIN diode

100

75

50

25

0

400 600 800 1000 1200Wavelength–nm

Percent

Eye

IR

filter

IR

LED

Receiving

diode