| 摘要 |
<p>In a video disc player, an FM signal recovered from the disc is subject to frequency demodulation to obtain a composite color video signal. In the composite signal, chrominance information appears in the form of a modulated subcarrier buried in spectrum troughs in the midband of a wider band luminance signal. The composite signal is applied to a modulator to develop amplitude modulated carrier waves, which serve as the "normal" signal input to a 1H delay line. Comb filter circuits respond to the delay line input and output to obtain separated chrominance and luminance signal components, which are suitably processed for delivery to color image display apparatus (e.g., color television receiver). The amplitude modulated carrier wave output of the delay line is also utilized as a "substitution" signal input to the delay line, replacing the normal signal input when a defect detector, monitoring the output of the FM demodulator, indicates the onset of a picture defect. The substitution signal path includes a network providing respectively different amounts of phase shift at the carrier frequency, and at the sideband frequency associated with the color subcarrier. In a variant of the aforesaid arrangement, switching between normal and substitution signals is effected in the video domain. A composite signal from the output of the FM demodulator is normally supplied to the amplitude modulator input, but, upon defect detection, a detected version of the amplitude modulated carrier wave output of the delay line is substituted.</p> |
