Talisman III Dynamic Loop Amplifier Systems (TAL/DLS1/3 & TAL/DLS5/3)
Publication Code TAL-DLS-3_TIG Issue 3
Page 4
2.
WHY INDUCTION LOOPS ?
2.1
Introduction
The problems faced by the hearing-impaired are rarely appreciated by those with normal hearing.
Unfortunately hearing aids give only partial restoration of hearing with the added disadvantage that the built-
in microphone cannot mimic the normal ear/brain ability to differentiate between distant, wanted sounds and
nearby, unwanted noises, and so to distinguish speech clearly from a combination of direct, reflected and
reverberant sounds. For instance a hearing aid user attending a concert or play may turn up the volume
control in an attempt to improve intelligibility, invariably resulting in an increase in interference from nearby
coughs, rustles, etc., sometimes to painful levels.
All these problems can be overcome by feeding microphone signals or the existing public address (PA)
system audio directly to the hearing-aid so that only the wanted sound (without auditorium reverberation,
audience noises, etc.) is reproduced.
This can be achieved by driving audio frequency currents around a large loop of wire to generate a magnetic
field (varying in sympathy with the PA program material) and which could be picked up by suitable
equipment. Specially designed headsets are available from a number of manufacturers such as Beyer,
Danavox and telex which have applications in high-noise environments for paging/life-safety and for silent
reproduction of commentary in museums and art galleries.
However, since 1974 all UK National Health Service hearing aids have had a switch to select M (internal
microphone) or T (telecoil pick-up), the T position being originally intended for use with telephone handsets
(later designs of telephone have proved unsuitable for this use). ‘AFILS’ (Audio Frequency Induction Loop
System) are designed to work efficiently, taking into account the positions and electromagnetic
characteristics of the internal pick-up coils normally fitted to hearing aids.
Now with a suitably installed AFILS the hearing impaired can appreciate and enjoy everything that those with
normal hearing do and, as mentioned above, the same type of system can be used to provide paging/life
safety communication in very noisy areas, secret paging for security staff, disturbance free audio for spoken
commentary systems or individual AFILS for booking offices, bank tellers etc.
2.1
The MILLBANK Dynamic Loop System
The dual purpose design allows for assisted listening of the hearing impaired and for applications such as
commentary systems in museums and art galleries.
The audio requirements for the two applications are different; a hearing impaired user requires tight control
of dynamic range and restricted bandwidth to enhance intelligibility by maximising perceived volume while
limiting maximum signal peaks and frequencies. A non hearing impaired user, however, may require less
dynamic range compression and a wider bandwidth.
The Talisman range of audio induction loop amplifiers is unique in that it provides switch selectable options
to cater for these alternative uses. The bandwidth may be set to either 5KHz or 10KHz high frequency roll off
and the dynamic range compression circuit is switchable either on a peak limiter (to prevent distortion in
wide dynamic range applications) or as a dual time constant compressor reducing the dynamic range to no
more than 9dB peak to average for assisted listening applications.
A combined voltage and current feedback technique is used to optimise drive into a wide range of loop
impedances. This system overcomes the disadvantages normally associated with amplifiers using voltage
feedback only. Radio frequency interference (R.F.I.) is prevented by amplitude limiting and low pass filtering
the signal prior to the driver stage. Unlike current feedback only designs, the TAL DLS output stage cannot
clip when driving inductive loads as voltage feedback takes over when the clipping threshold is approached.
Importantly, the feedback configuration means that the dynamic loop system is stable into highly inductive,
multi-turn loops.
