Analog Transmission

Until 1960, analog transmission was the only form of transmission in telecommunication networks. Today, the telecommunication network is mostly digital, except for the subscriber lines.

This section outlines some basic aspects of the transmission of analog signals in telecommunications. In this section, the term signal refers to information (speech or voiceband data) exchanged between subscribers during a call (as opposed to signaling, which is the subject of this book).

1.4.1 Analog Circuits [4,5]

An analog signal is a continuous function of time. Telecommunication started out as telephony, in which a microphone (or transmitter, mouthpiece) produces an electrical analog signal, whose variations in time approximate the variations in air pressure produced by the talker's speech. A receiver (or earpiece) reconverts the electrical speech signal into air-pressure variations that are heard by the listener.

The pressure variations of acoustic speech are complex, and not easily described. "Average" acoustic speech contains frequencies from 35 Hz to 10,000 Hz. Most of the speech power is concentrated between 100 Hz and 4000 Hz. For good-quality telephony, only the frequencies between 300 and 3400 Hz need to be transmitted. Analog communication channels in the network, which historically have been designed primarily for speech transmission, therefore accommodate this range of voiceband frequencies (Fig. 1.4-1).

Speech Band

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