Handbook for Sound Engineers

Interpretation and Tour Group Systems 1537

diode incorporates a collecting lens which bends the

infrared light to hit the diode, Fig. 41-9. Increasing the

number of diodes increases the IR intensity and the area

of coverage multiplies by the number of diodes in the

panel. The diodes have a continuous operating life of

100,000 h before the light output diminishes to 70% of

its original value.

Depending on the size of the room, its shape, and

surface characteristics, a single small radiator or

multiple large radiators may be required. Auditel

Systems Limited states that the number of radiators

required for a SNR of >40 dB can be calculated using

the following equation:

(41-1)

where,

Table 41-2. Technical Specifications for Infrared
Systems

Characteristics Narrow Band

System

Wide Band

System

No. of channels: 12 2

Carrier frequencies: 55 kHz–535 kHz

(excl. 455 kHz)

95 kHz and

250 kHz

Channel spacing: 40 kHz 155 kHz

Modulation: FM FM

Pre-emphasis: 100 μs 50 μs

Normal deviation: ±6 kHz ±35 kHz

Peak deviation: ±7 kHz ±50 kHz

Transmitters:

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

Max. distortion at 1 kHz: <1.0% <1.0%

Signal/noise (A weighting): >55 dB >70 dB

Receivers:

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

Max. distortion (at 1 kHz): <2.5% <1.0%

SNR (A weighting): >55 dB >63 dB

Emitter Panels:

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

Figure 41-8. Light emitting diode (LED). Courtesy

Sennheiser Electronics.

Collecting lens

Filter

Infrared light

PIN diode

Figure 41-9. Infrared transmission characteristics.

Time

Time

Time

Time

Infrared light with amplitude modulated subcarrier.

AF signal

Infrared light with frequency modulated subcarrier.

(^0100) 200 300 400 1 kHz
0 100 200 300^400 1 kHz
1 channel wideband (swing ± 40 kHz).
2 channel wideband (swing ±40 kHz).
0 40
20 60
80 200 300 1 kHz
100
Multichannel narrowband (swing ±7 kHz).
N area m
2 unumber of channels
D
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