0195136047.pdf

706 COMMUNICATION SYSTEMS

LPF

0–4.2 MHz

mY(t) fY(t)

Matrix LPF

Color camera

0–1.6 MHz 2.0–4.2 MHz

mI(t) sI(t) fI(t)

LPF BPF

BPF

B

G

R

B

G

R

0–0.6 MHz 3.0–4.2 MHz

Color-carrier

oscillator

Visual-carrier

oscillator

Horizontal sweep rate

Audio

Vertical (field) sweep rate

Aural carrier

Cameras for R, G, B

Color filters for (R, G, B)

Sync

generator

Visual

modulator

Transmitted

signal sTV(t)

Standard

AM signal

DSB signal

DSB

DSB

Composite

baseband fc(t)

Frequency

modulator

LSB

filter

VSB

VSB

2

−π

2

÷^455

2

÷^525

3.579545 MHz

mQ(t)

mR(t)

mG(t)

mB(t) sQ(t) fQ(t)

ΣΣ

+

+

+

++

+

Figure 15.2.28Color TV transmitter in a TV-transmitting station.

• fI(t): Nearly a VSB signal, when the DSB signal is filtered by the BPF of passband 2–4.2
  MHz to remove part of the USB in the DSB.


• fQ(t): DSB signal that is produced when the other chrominance signal modulates a quad-
  rature-phase version of the color subcarrier. This DSB signal passes directly through the
  BPF with passband 3–4.2 MHz without any change.


• fY(t): Filtered luminance signal ofmY(t)byanLPF.


• fc(t): Composite baseband waveform by addingfY(t),fI(t),fQ(t), and sync pulses. This has
  a bandwidth of about 4.2 MHz and modulates a visual carrier by standard AM.
  The standard AM signal is then filtered to remove part of the lower sideband. The resulting
  VSB signal and the audio-modulated aural carrier are added to form the final transmitted signal
  STV(t). Figure 15.2.29 illustrates the spectrum of a color television signal.
  Acolor television receiveris shown in Figure 15.2.30 in block diagram form, indicating only
  the basic functions. The early part forms a straightforward superheterodyne receiver, except for
  the following changes:


• The frequency-tuning local oscillator is typically a push-button-controlled frequency syn-
  thesizer.


• IF circuitry in television is tuned to give a filter characteristic required in VSB modulation.
  The filter shapes the IF signal spectrum so that envelope detection is possible. The output
  of the envelope detector contains the composite visual signalfc(t) and the frequency-modulated
  aural carrier at 4.5 MHz. The latter is processing in a frequency demodulator to recover the
  audio information for the loudspeaker. The former is sent through appropriate filters to separate