Until the 1970s, most the telephone calls were transmitted as electric signals corresponding to the vibrations of the voice. These are known as analogue signals because they are analogues – similar in structure – to the sound. Electrical interference in the transmitting circuits can distort voices.
After the 1970s, the analogue system began to be replaced by a digital system which cuts out most interference and distortion. The analogue electrical signals from the microphone are changed to binary numbers in electronic circuits at the exchange and transmitted in coded form.
To do this, the wave heights of the electric current are measured thousands of times every second. The measurement is expressed as a sequence of the digits one and zero. Current is then converted to a series of pulses – a flow for 1 and a break in-flow 0 – representing the wave measurements. This is known as pulse-code modulation (PCM). As each pulse is very short, the pulses of one telephone message can be interleaved between the pulses of others.
This technique of time multiplexing allows 32 simultaneous calls to be sent along a single pair of wires, or thousands of messages to be sent at once along the same coaxial cable.
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