0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 699

Preemphasis

KHp(ω)

Preemphasis

KHp(ω)

DSB

modulator

2

38-kHz

oscillator

(a)

(b)

SCA

FM

modulator

19-kHz pilot

fL(t)

fR(t)

FM demodulator BPF Product LPF Deemphasis Hd(ω)

Deemphasis Hd(ω)

23–53 kHz 0–15 kHz

38-kHz subcarrier

Replaced by phase-locked loop

or

Local-carrier recovery

Synchronous detector

NBPF × 2 multiplier

19 kHz

LPF

0–15 kHz

sFM(t)

fd(t) f 2 (t)

f 1 (t)

f 1 (t)

f 2 (t)

f 3 (t)

fs(t)

fd(t)

2 fR(t)

2 fL(t)

AB

Phase detector LPF

÷ 2

38-kHz VCO

AB

f 4 (t)

sFM(t)

f 3 (t)

Σ

ΣΣ

+

+

+ +

+

+

+

−

Σ

Σ

+

+

+

−

Figure 15.2.19(a)FM stereo transmitter.(b)FM stereo receiver.

The most bandwidth-efficient analog communication system is the SSB SC system with a
transmission bandwidth equal to the signal bandwidth. In bandwidth-critical applications, such
as voice transmission over microwave and satellite links and some point-to-point communication
systems in congested areas, this system is used widely. When transmission signals have a
significant dc component, such as image signals, SSB SC cannot be used because it cannot
effectively transmit direct current. A good compromise is the VSB system (with its bandwidth
slightly larger than SSB and a capability for transmitting dc values), which is widely used in
television broadcasting and some data-communication systems. When bandwidth is the major