into fluctuations of current that are transported through the wires of the PSTN.
The distant party's speaker in the handset reconverts this modulated current into audible
sound. In a normal phone call, most of the time one party talks while the other party is
listening. Nevertheless, signals are transmitted constantly in both directions as each party is
able to interrupt the other at any time. This transmission type is also called a full duplex
transmission. Analogue transmission perfectly meets the requirements of human speech
transmission but it is neither noise resistant nor transmission resource efficient.
In the early days of legacy analogue PSTNs, line amplifiers were used to refresh the signal
on its way from the source to the drain. Unfortunately, system noise was amplified too, which
very often led to very poor voice quality.
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| Analogue Speech Transmission |
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