| 摘要 |
PURPOSE:To prevent a noise from introducing into a signal by putting a frequency dividing circuit and counting circuit into operation only in horizontal and vertical blanking periods and by stopping them in the other period. CONSTITUTION:Output signal 22 of VCO21 oscillating at frequency fH is inputted to operation control circuit 23 to obtain operation control signal 24 of output frequency fH. With this signal 24, the operation and stop of frequency divider 6 and counter 8 are brought under control. Further, signal 22 is inputted to field detecting circuit 25, which outputs signal 26, having information on the start point of one vertical period of a TV signal, to operation control circuit 27. At the output of circuit 27, operation control signal 28 of vertical frequency fV of the TV signal is obtained and with this signal 28, counter circuit 13 and frequency divider 30 that outputs signal 29 of frequency 2fH by frequency-dividing reference clock 3 by 455 are started and stopped. This control is exercised only in the blanking period and interrupted in the other peiord. |
