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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.

• 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.
• We refer to such continuous-time signal waveforms as analogue
signals and to the corresponding information sources that produce
such signals as analog sources.
• Analog signals can be transmitted directly over the communication
channel via carrier modulation and demodulated accordingly at the
receiver.
• We call such a communication system an analogue communication
system.
Digital Communication System
• 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.
• There are some potential advantages to transmitting an analog signal
by means of digital modulation.
• The most important reason is that signal reliability is better controlled
through digital transmission than through analog transmission.
• In particular, digital transmission allows us to regenerate the digital
signal in long-distance transmission, thus eliminating effects of noise
at each regeneration point.
Digital Communication System
• In contrast, the noise added in analog transmission is amplified
along with the signal when amplifiers are periodically used to boost
the signal level in long-distance transmission.
• Another reason for choosing digital transmission over analog is that
the analog message signal may be highly redundant.
• With digital processing, redundancy may be removed prior to
modulation, thus conserving channel bandwidth.
• A third reason may be that digital communication systems are often
cheaper to implement.