A color television receiver is shown in Figure 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 synthesizer.

• 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 signal fc(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 out signals fY(t) and [fI(t) + fQ(t)], which is further processed by two synchronous detectors in quadrature to recover SI(t) and SQ(t).An appropriatematrix combines fY(t), SI(t), and SQ(t) to yield close approximations of the originally transmitted mR(t), mG(t), and mB(t). These three signals control the three electron beams in the picture tube.

The output of the envelope detector is also applied to circuits that separate the sync signals needed to lock in the horizontal and vertical sweep circuits of the receiver. The bursts of color carriers are isolated such that a PLL can lock to the phase of the color carrier, and thereby provide the reference signals for the chrominance synchronous detectors.