| 摘要 |
PURPOSE:To obtain an AFC operation that follows the deviation of the locally generated frequency of a block down converter by controlling a tuning voltage so as to determine a white/black noise contained in the output from a detecting circuit and to equalize the number of peak white noises to the number of peak black noises. CONSTITUTION:Black and white peak detecting circuits 22 and 24 detect black and white level noise signals NB and NW, respectively, and pulse shaping circuits 23 and 25 shape noise signals NB and NW into pulses, respectively. Counters 28B and 28W count black and white peak noises, respectively. Field counter 27F generates an output at every 16 fields, and clears the counters 28B and 28W, and the output then obtained from a comparator 28C is hold by a sample hold circuit 28H. When more than 16 noise pulses in one field are applied from an OR gate 26 to a counter 27P, 2<3>-output from the counter 27P changes from H-L, this change is transmitted to the terminal S of a flip-flop 29F, the flip-flop 29F is set, an output terminal Q becomes H, Q becomes L, the output from the sample hold circuit 28H passes through a switch 32S via a relay 29R and is transmitted to a PLL circuit 31.
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