0195136047.pdf

628 SIGNAL PROCESSING

Digital message

source

Digital message

output signal

Analog message

source

Analog message

output signal

Digital

modulator

Digital

demodulator

Channel

encoder

Channel

decoder

Source

encoder

(transmitter side)

(receiver side)

Source

decoder

Analog-to-digital

converter

Digital-to-analog

converter

Noise, interference Channel

Figure 14.1.2Basic elements of a digital communication system.

signal (such as an audio or video signal) must first be converted to a digital signal by an analog-

to-digital (A/D) converter. If no analog message is involved, a digital signal (such as the output

of a teletype machine, which is discrete in time and has a finite number of output characters) can

be directly input.

Encoding is a critical function in all digital systems. The messages produced by the source

are usually converted into a sequence of binary digits. The process of efficiently converting the

output of either an analog or a digital source into a sequence of binary digits is calledsource

encodingordata compression.

The sequence of binary digits from the source encoder, known as theinformation sequence,

is passed on to thechannel encoder.The purpose of the channel encoder is to introduce some

redundancy in a controlled manner in the binary information sequence, so that the redundancy

can be used at the receiver to overcome the effects of noise and interference encountered in the

transmission of the signal through the channel. Thus, redundancy in the information sequence

helps the receiver in decoding the desired information sequence, thereby increasing the reliability

of the received data and improving the fidelity of the received signal.

The binary sequence at the output of the channel encoder is passed on to thedigital modulator,

which functions as the interface to the communication channel. The primary purpose of the digital

modulator is to map the binary information sequence into signal waveforms, since nearly all the

communication channels used in practice are capable of transmitting electric signals (waveforms).

Because the message has only two amplitudes in a binary system, the modulation process is known

askeying.Inamplitude-shift keying(ASK), a carrier’s amplitude is shifted or keyed between two

levels.Phase-shift keying(PSK) involves keying between two phase angles of the carrier, whereas

frequency-shift keying(FSK) consists of shifting a carrier’s frequency between two values. Many

other forms of modulation are also possible.

The functions of the receiver in Figure 14.1.2 are the inverse of those in the transmitter.

At the receiving end of a digital communication system, thedigital demodulatorprocesses the

channel-corrupted transmitted waveform and reduces each waveform to a single number, which

represents an estimate of the transmitted data symbol. For example, when binary modulation is