Handbook for Sound Engineers

Message Repeaters and Evacuation Systems 1515

40.1 Digital Audio Storage

There are numerous applications for digital audio

storage, that for day-to-day use, and that which is

preserved for future generations. We will first address

the applications and workings of message repeaters as

they are used on a daily basis. Secondly, we will discuss

the specifics of archiving audio information.

40.2 Message Repeaters

Message repeaters have come a long way in 40 years.
The original repeater was a person sitting at a micro-
phone making an announcement over a public address
system at or about the right time. This system had both
advantages and disadvantages. The message could be
changed at any time, and if area switching was avail-
able, different messages could be sent to different areas
in real time. Different messages could not be sent at the
same time. Another disadvantage was requiring dedi-
cated personnel 24 hours a day, seven days a week. In
an emergency, an individual was required to stay at the
microphone and announce in a calm and persuasive
voice—a difficult thing to do at best.
With the introduction of tape recorders, messages
could be prerecorded and played back manually or auto-
matically. Unless a multichannel recorder or multiple
recorders were used, only one message could be played
at one time. To play different recorded messages
required recording them in series and locating the
desired message by fast forwarding the tape, time
consuming in an emergency situation. With the design
of lubricated tape, continuous loop tape recorders were
used. This eliminated the requirement of rewinding but
also meant the message could not be repeated until the
tape had taken its course. Tape stretch and breakage was
always a possibility. Auto-rewind cassette players were
also used but had the same problem of reel-to-reel
machines, they had to be rewound.

The late 1970s brought about the introduction of
digital storage devices using solid state equipment.
Digital message repeaters have the following advan-
tages over the previous systems.

• Reliability.


• Flexibility.


• Solid state reproduction quality.


• User-recordable messages.


• Programmable, often user-programmable, locally or
  remotely.


• Remotely controllable.

40.3 How Message Repeaters Work

The use of digital signal processors (DSPs) has greatly

simplified digital circuitry and made it possible to

design digital message repeaters (see Chapter 39).

A digital message repeater in its simplest form is

shown in Fig. 40-1. In this system, a permanent

message is digitized by the manufacturer and stored in

the digitized message storage circuit. To change the

message requires sending the unit back to the manufac-

turer for reprogramming.

Upon contact closure, the control circuitry transfers

the digitally stored message to the digital to analog

converter (D/A) circuitry where it is changed into an

analog circuit. The analog signal is only approximate at

this time so a filter is used to smooth the waveform and

limit the frequency response. Normally this type of unit

has a frequency response from 300 Hz–3 kHz or the

basic telephone response. The filtered output is then

directed to the audio output circuitry where it is ampli-

fied, made balanced or unbalanced, and matched for the

proper output impedance.

An intelligent digital message repeater, such as an

Instaplay™ by ALARMCO is shown in Fig. 40-2. In

this system, messages can be recorded from a micro-

phone (Mic), Aux. Input (AUX) (e.g., a CD or MP3

player), or standard touch tone telephone (control

Phone). The analog input is then filtered and converted

to a digital signal (A/D conversion circuitry) and stored

(digitized message storage). Several thousand messages

can be stored and individually replaced at any time. By

using flash memory, sound quality is assured by storing

audio data using 16 or 24 bit samples in the Instaplay™.

Audio and programming data may also be downloaded

digitally. With memory and intelligent firmware, each

new recording can be longer or shorter than the original

with all unused memory available for the recording.

To simplify recording from a control phone (either

locally or remotely) [Control Phone and Telephone

Network Interface], prerecorded instructions are stored

in the announcer in digitized form [Command Prompt

Audio Storage]. These instructions guide the user

Figure 40-1. Digital message repeater.

Audio

output

Digitized message

storage

Contact closure

control inputs

Filtering Control circuitry

D to A

conversion

circuitry