8.5
TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY
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8.5
TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY
8.5.1
Introduction
Long-distance analog communication systems require some method to overcome losses.
As a wire-pair telephone circuit is extended, there is some point where loss accumulates
such as to attenuate signals to such a degree that the far-end subscriber is dissatisfied.
The subscriber cannot hear the near-end talker sufficiently well. Extending the wire con-
nections still further, the signal level can drop below the noise level. For a good received
signal level, a 40-dB signal-to-noise ratio is desirable (see Section 3.2.1 and 3.2.2.4). To
overcome the loss, amplifiers are installed on many wire-pair trunks. Early North Ameri-
can transcontinental circuits were on open-wire lines using amplifiers quite widely spaced.
However, as BH demand increased to thousands of circuits, the limited capacity of such
an approach was not cost effective.
System designers turned to wideband radio and coaxial cable systems where each
bearer or pipe carried hundreds or thousands of simultaneous telephone conversations.
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Carrier (frequency division) multiplex techniques made this possible (see Section 4.5).
Frequency division multiplex (FDM) requires separation of transmit and receive voice
paths. In other words, the circuit must convert from two-wire to four-wire transmission.
Figure 8.17 is a simplified block diagram of a telephone circuit with transformation from
two-wire to four-wire operation at one end and conversion back to two-wire operation at
the other end. This concept was introduced in Section 4.4.
The two factors that must be considered that greatly affect transmission design in the
long-distance network are echo and singing.
8.5.2
Echo
As the name implies, echo in telephone systems is the return of a talker's voice. To be an
impairment, the returned voice must suffer some noticeable delay. Thus we can say that
echo is a reflection of the voice. Analogously, it may be considered as that part of the
voice energy that bounces off obstacles in a telephone connection. These obstacles are
impedance irregularities, more properly called impedance mismatches. Echo is a major
Figure 8.17
Simplified schematic of two-wire/four-wire operation.
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On a pair of coaxial cables, a pair of fiber-optic light guides, or a pair of radio-frequency carriers, one coming
and one going.