This illustration gives an impression of how each channel ‘sits’ in relation to the pulses generated by the individual channel units. For the benefit of the receiving radio unit, there is a synchronisation pulse of 20µsec duration that clearly distinguishes the start of a new cycle and allows the operator to synchronise the receiver equipment for the reception of intelligence over a channel. Each cycle recurs at a frequency of 9 kHz, (a rate of 540,000 times a minute) which for the time offered an unparalleled quality of multi-channel transmission. In order to avoid interference between channels, the pulses themselves are separated in time within the oscillation frequency of the cycle. A combination of these oscillations in succession (as shown by the diagram) gives a more comprehensive idea of how this system transmits data.

Transmission: how are the pulses created?

The pulses which correspond to the audio signals of each channel are generated by a part of the radio known as the ‘pulser’. This unit has eight individual ‘channel units’ responsible for the signals created by each telephone line. A saw-tooth voltage, a fluctuation that is caused by variation in the circuit current of the channel unit is used to control when the pulses are emitted by the circuit. The current of the channel unit is given a positive bias, which, in conjunction with the saw-tooth voltage, restricts the time duration of the pulse to within a narrow range. This is illustrated by the diagram below.

‘A’ represents one saw-tooth of the voltage which is generated by the pulser unit. When in operation, these are repeated at regular time intervals coinciding with the pulses emitted by the other seven channels. At the magnetron, the arrival of the pulse (negative) prevents the anode from drawing a current. The instantaneous rise in anode potential which results starts the magnetron in oscillation. Since the trailing edge of the pulse (see diagram) is always vertical (a result of the instantaneous fall in potential as the capacitors of the channel unit discharge) only the leading edge can be modulated in a way that varies the pulse width.

The width of this pulse i.e. its duration is determined by the extent of positive biasing of the cathode (marked ‘B’ on the diagram). As can be seen in the illustration, variation of this bias will increase or reduce the time duration of the pulse, which is potted by ‘C’. As in the WS10, all of the channels have the same recurrence frequency, approximately 3.5µsec, however modulation of the cathode, in addition to the saw-toothed voltage can produce speech channels which vary in their information carrying capacity.

This instance is illustrated in the following diagram. As can be seen, the period of the saw-tooth voltage remains constant, however a varying positive bias of the cathode, ’B’ has the effect of varying the channel width. It is for this reason that this system of communication is described as ‘pulse-width modulation’.