o We have described an electrical communication system in rather
broad terms based on the implicit assumption that the message
signal is a continuous time-varying waveform.o A third reason may be that digital communication systems are often
cheaper to implement.Digital Communication System
o Alternatively, an analogue source output may be converted into a
digital form, and the message can be transmitted via digital
modulation and demodulated as a digital signal at the receiver.o Analog signals can be transmitted directly over the communication
channel via carrier modulation and demodulated accordingly at the
receiver.o We refer to such continuous-time signal waveforms as analogue
signals and to the corresponding information sources that produce
such signals as analog sources.