Muscio4

Recognised text

Text below was produced by optical character recognition and may contain errors. Check it against the original.

166 AUSTRALIAN RADIO — THE TECHNICAL STORY

The design of the modulator is such that in each case the carrier
frequency is suppressed and a band-pass filter selects only the lower
sideband for transmission. At the receiver the missing carriers are
reinserted from local sources and in the demodulators the stacked
channels are translated back to the three separate channels each
occupying the band 300 to 2800 Hz as originally applied to the
equipment at the sending end of the line. As shown in Fig. 30, a pilot
tone within the frequency band but between the channels is
transmitted from each end to permit the use of automatic gain control
to allow for variations in line losses.

Since the basic equipment required for each channel at each end of
the circuit involves a modulator and a demodulator, the shortened
descriptive term “modem” came into general use at an early stage and
has been applied widely in quite a number of areas of
communications.

With the individual 300 to 2800 Hz telephone channels located at
intervals ranging from 3.1 to 4 kHz apart in a three-channel
configuration, the need for highly-stable steep-slope filters is
immediately evident. The use of stable master oscillators to provide
the carrier supplies is also of paramount importance.

As the need for telephone channels continued to increase,
development of twelve-channel open-wire carrier telephone
equipment was undertaken. The basic principles employed were
substantially the same as in the less complex system already in wide
use. In the twelve-channel system, however, the high frequency end of
each speech channel was extended from 2800 to 3400 Hz and
provision was also made for an out-of-band signalling tone at 3825
Hz. Thus for twelve channels and more, the individual speech
channels were stacked at 4 kHz intervals.

The thinking of design engineers and system planners was
apparently conditioned to such an extent by the number of three-
channel systems already in use or in the planning stages that the
frequency range occupied by the more elaborate twelve-channel
system was located above the existing three channels, i.e. within the
range 36 to 143 kHz as shown in Fig. 30. The four-slightly different
frequency ranges shown as NA, SA, NB and SB are those
recommended by the International Telephone and Telegraph
Consultative Committee (CCITT) to minimise system-to-system
crosstalk where a number of 12-channel systems are operated on
adjacent pairs on an open-wire route. As will also be noted from the
drawing, two pilot tones are used for automatic control of gain and
equalisation.

The first twelve-channel open-wire carrier telephone systems,
operating in conjunction with existing three-channel systems, were
placed in operation on the Sydney-Melbourne service in 1939. These
early equipments were manufactured in Australia by STC from sub-
assemblies produced in the US. After the war, partial local
manufacture of equipment designed by STC in London was

MULTIPLEXED COMMUNICATION SYSTEMS 167

commenced in Sydney, the local content being increased gradually
until, by the beginning of the 1950s, complete local manufacture had
been achieved. This situation continued until about the middle 1960s
when the requirements for open-wire equipment began to be phased
out in favour of the more extensive use of carrier-on-cable and
carrier-on-radio systems.

7.4 Carrier-on-Cable Systems

During the period of expansion of communication facilities made
possible by open-wire carrier telephone equipment, development was
also proceeding on multi-channel systems for use on various types of
cable, including coaxial cable. In fact, the requirements for
additional channels increased at such a rate that buried or suspended
cables, either simply multi-pair, quad, coaxial or multi-coaxial,
progressively became the standard means of establishing new routes
or expanding old ones. ——

In the absence of some of the constraints imposed upon open-wire

line systems, the basic speech channels for cable systems are
translated into frequency bands rather lower than those used in open-
wire systems as shown in the top portion of Fig. 30. Under these
circumstances, a complete 24-channel system can be accommodated
within the range 12 to 108 kHz, the basic twelve-channel group being
placed between 60 and 108 kHz in accordance with CCITT
recommendations. ;

Although developed and placed in service prior to the drafting of
these recommendations, some details of an earlier carrier-on-cable
system supplied to the PMG’s Department are interesting. In 1938,
an order was placed with Philips for a system designed to provide
seventeen telephone channels on each pair of a multi-pair cable.
During the progress of the contract, the Philips engineer responsible
for planning and installation arrived in Australia only to learn shortly
afterwards that Holland had been invaded and the equipment already
manufactured in Eindhoven had been lost. However, the need was
urgent and, after some negotiation between customer and supplier, it
was decided that a separate department would be set up by Philips
under the leadership of the engineer from Eindhoven, T. S. (Tom)
Skillman, to manufacture the equipment in Australia. In 1941 the
group thus formed became a separate organisation known as
Communication Engineering Pty Limited.

The scheme worked very well with the result that the entire order
had been fulfilled and all sections of the system were fully operational
by 1943. In each set of equipment the seventeen telephone channels
were accommodated within the carrier frequency range of 4 to 72
kHz. No pilot tones were used for control purposes but provision was
made at each repeater for adjustment of gain with major temperature
variations throughout the year.