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162 AUSTRALIAN RADIO — THE TECHNICAL STORY

transmitted via frequency division multiplex equipment during the
late 1940s and early 1950s.

With the mechanically-driven switches of the Murray telegraph
multiplexing system replaced with high speed electronic switching
circuits, the same methods were later applied to the transmission of
telephony. This was achieved by replacing the continuous waveform
of the AF signal by a series of equally spaced pulses, eachone having
an amplitude equal to that of the waveform at that instant. This
comprises, in effect, a series of samples of the signal.

It can be shown mathematically that for good reproduction of the
AF signal there must be at least two such samples within the period of
the highest frequency in the waveform. On this basis, a telephone
channel extending over the range 300 to 3400 Hz requires a pulse
train with a repetition rate of 6800 pulses per second. In practice 8000
pulses per second are used. Thus, when the pulses for say 24 AF
channels are interleaved, the result is a train of very short pulses at a
high repetition rate; in this case 192 kHz. The technology for
handling such pulses was developed in connection with radar during
World War 2 and it is therefore not surprising that multi-channel
systems using time division multiplexing were developed for military
purposes before the war was over.

During the late 1940s, the need for multi-channel point-to-point
communications in Australia was increasing rapidly and STC
received a contract from the PMG’s Department for the supply of
two 24-channel TDM systems to assist in meeting this need. The
equipment produced locally was based ona US military design which
used the pulse time modulation (PTM) system, in which the
amplitude of the AF waveform is represented not by the amplitude of
a pulse but by its time displacement from a fixed reference position.

Although the original intention was to carry out only relatively
minor adaptation of the US design, the stringent requirements of the
PMG specification, particularly with regard to interchannel
crosstalk, led in the end to a complete redesign. This was a major
effort in an area which at that time was in the forefront of the state of
the art. Its execution absorbed a total of 80 man-years which, even by
today’s standards, represents quite a large engineering task. The
supervising engineer responsible for the project was R. R. (Bob)
Long; he was assisted by L. G. (Lawrie) Thomas, P. W. (Phi!)
Humphries and John Fieguth.

An interesting sidelight of this development is that, although the
US system carried 24 telephone channels, the local system was
designed for only 23. The reason for this was that the PMG
Department intended that the system ultimately should be able to use
three channels in parallel giving a sampling rate of 3 x 8 = 24 kHz to
transmit a broadcast programme channel with a bandwidth of 10
kHz. However, to do this without distortion, the three channels
would have to be equally spaced in time.

MULTIPLEXED COMMUNICATION SYSTEMS 163

It will be evident that, in addition to the pulses which carry the AF
signals, another set of pulses is required in a TDM system for
synchronisation of the demodulators; otherwise, there is no way of
identifying the individual channels. If there are 24 AF channels, the
total number of pulses in each frame (1/8000 sec.) is 25. Since this
number is not divisible by three, it is not possible to find a set of three
equally spaced pulses within the frame. The solution is to reduce the
number of AF channels to 23, giving a total of 24, which is divisible by
3.

The local design covered all facets of the equipment, including
audio frequency, power supply and RF systems. It even included a
specially designed oscilloscope for viewing and adjusting the pulse
trains.

The system operated in the 1700 to 2300 MHz band with a peak
power of 50 watts generated by a 2C43 disc-seal lighthouse triode
valve. The valve division of STC actually manufactured quantities of
these valves locally for the PTM project. Although the Australian-
made valves were subsequently superseded by imports, it is
interesting to note that the local electronics industry, spurred on by
the war, was much closer to the leading edge of technology in those
days than it is now.

Two systems were placed in service by the PMG’s Department in
the early 1950s on the Sydney-Wollongong and Dandenong-
Warragul routes, where they operated successfully for fifteen years.

Encouraged by the performance of the equipment operated by the
PMG’s Department, the Australian Army placed orders in the late
1950s for a quantity of the same type of equipment to provide multi-
channel facilities between major radio transmitting and receiving
installations and associated control centres in New South Wales and
Victoria. The most noteworthy difference between the PMG and
Army PTM systems was the replacement of the complex 24-phase
transformer by a resistance/capacitance network to determine the
relative timing of the channels. This design change simplified
manufacture and greatly improved the long-term stability of the
system.

7.2 Frequency Division Multiplexing

At about the same time as STC received the contract for the TDM
system described above, the PMG’s Department awarded a contract
to AWA for a multi-channel FDM system in which the composite
FDM signal was transmitted by frequency modulation of a radio
carrier.

Since multi-channel FDM signals were already produced by
channelling equipment used on wire circuits, this system was much
more compatible with the existing open-wire carrier and carrier-on-
cable multi-channel systems. History has shown that wide band