0195136047.pdf

640 SIGNAL PROCESSING

f

0 W

Amplitude

spectrum

f

0 B

H(f)

K

f

θ(f)

− 360 °td

0

Figure 14.1.6Frequency-domain

interpretation of distortionless

transmission whenB≥W.

|H(f)| ∠θ(f)

x(t) y(t)

|Heq(f)| ∠θeq(f)

z(t)

Transmission medium Equalizer

Figure 14.1.7Transmission sys-

tem with equalizer.

so that the equalized output signal is thenz(t)=Kx(t−td), i.e., undistorted, regardless of the

distortion iny(t). For example, for correcting electrical and acoustical frequency distortion, audio

equalizers in high-fidelity systems are used to adjust the amplitude ratio over several frequency

bands. Sometimes, as in audio systems, phase equalization is not that critical since the human

ear is not that sensitive to delay distortion. However, human vision is quite sensitive to delay

distortion. Equalizers are also applied whenever energy storage in a transducer, or some other

part of a signal-processing system, causes linear distortion.

14.2 Modulation, Sampling, and Multiplexing

Modulationis the process whereby the amplitude (or another characteristic) of a wave is varied

as a function of the instantaneous value of another wave. The first wave, which is usually a

single-frequency wave, is called thecarrier wave; the second is called themodulating wave.

Demodulationordetectionis the process whereby a wave resulting from modulation is so operated

upon that a wave is obtained having substantially the characteristics of the original modulating

wave. Modulation and demodulation are then reverse processes.

The information from a signalx(t) is impressed on a carrier waveform whose characteristics

suit a particular application. If the carrier is a sinusoid, we will see that a phenomenon known

asfrequency translationoccurs. If, on the other hand, the carrier is a pulse train, the modulating