Handbook for Sound Engineers

1534 Chapter 41

shutoff, as they may be on the same channel and cause
feedback. Acoustical crosstalk can occur if open back
earphones are used because an adjacent live microphone
can sometimes pick up the interpreted language.
If many languages are used, it may be better to
multiplex the signal rather than use a multiconductor
cable. This system would consist of a central modulator
with up to twelve channels driving a network of active
channel selector units using coaxial cable in a
loop-through configuration. Power for the channel
selectors is provided by power supplies injecting dc into
the network.
Most single cable conference systems with delegate
microphone units incorporate a built-in loudspeaker.
The loudspeaker signal is derived directly from a
common audio line. This simplifies cabling by avoiding
the necessity for a second audio line to drive the loud-
speakers. This does mean that the input and output
signal are on the same line, and would create a closed
loop and feedback unless some means of isolating the
two signals is employed. This problem is overcome by
Auditel with the application of a common mode reverse
audio feed (CMRAF), Fig. 41-6. The technique is based
on selective rejection of large common mode signals.
The output from the microphone preamplifier is a

balanced signal and is extracted in the central unit via

transformer. After signal processing, the loudspeaker

drive signal is injected into the audio pair in common

mode form. Since the loudspeaker drive amplifiers and

delegate units reject balanced signals the two signals

can be carried over the same conductors without inter-

action or without compromising the signal quality.

41.1.4. FM Interpretation Systems

FM products can be used for language interpretation by

connecting a stationary FM transmitter to an audio

system transmitting an FM signal to a portable receiver

for assistive listening and a language interpreter. In

addition to the portable receiver, the interpreters use a

portable transmitter and an over-the-head microphone

and earphone unit. This combination allows them to

hear the audio clearly in an adjoining area while

speaking their translations in a normal tone of voice.

Their translations are sent via FM back to participants’

receivers. It is important to have transmitters and

receivers with multiple channels allowing users to find

clear channels even in a crowded venue with extensive

FM use.

Figure 41-5. Two person interpreter’s terminal.

Booth

No. 1

6-channel dual

interpreters unit

OM-1

floor

module

CM-4 (x3)

channel

modules

Floor

Line splitter channel

0 dB

SI controller

Tape O/P SI interface

0 dB

0dB Line splitters optional

Cable

transmission

system

Infra-red

transmission

system

Induction loop

transmission

system

Language

channels

optional