Principles of Synchronous Digital Hierarchy

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4

Line Coding and Digital Modulation

In the previous chapter, we have seen how an analog signal is converted into

a digital signal. One such signal, after sampling and pulse code modulations

(PCM), may look like that shown in Figure 4.1.

Each sample of the analog signal is thus converted into an eight-bit

sequence of pulses (or the absence of them), called PCM. In other words,

each signal is “coded” into a xed-length bit stream. Since this coded signal

is required to be sent to another place using a transmission line, the signal

code has to be imposed on the transmission line. This scheme of coding the

signal is called “line codi ...