| 摘要 |
PURPOSE:To set forcibly the output level of a discriminating circuit to non- signal state, by comparing the automatic gain control voltage with the reference voltage, and driving a transistor with the output of comparison to detect the discontinuation of an input signal. CONSTITUTION:The AGC control voltage of an equalization amplifier 1 which compensates the loss fluctuation of a cable connected to the input side is compared with the reference voltage by a comparator 2. The control voltage is higher than the reference voltage when an input signal is discontinued, and the output of the comparator 2 has a high potential. Thus a transistor TR3 is turned on and saturated. In this case, the bias of a terminal D of an FF4 is set at L in terms of the logic level by a resistance 6 connected to the collector of the TR3. Therefore a terminal Q of the FF4 is set at L despite the noises generated when the input signal is discontinued. Thus an uncertain signal is never delivered. The output of a comparator 4 is set at a low level when an input signal exists, and the TR3 is turned off. Thus the FF4 performs a normal discriminating operation. |
