| 摘要 |
PURPOSE:To always secure a constant phase relation between a subcarrier and a horizontal synchronous signal and ensure a stable extraction of a color frame signal, by adding a simple circuit consisting of a counter and a switching circuit. CONSTITUTION:In case no input VBS signal exists, a switching circuit 12 is controlled by the output of an input detecting circuit 8. The output of a 1/455 counter 13 is applied to a synchronous signal generator 11 through the circuit 12 to set a 1/910 counter inside the generator 11. Thus the phase relation is made constant between the horizontal synchronous signal of the generator 11 and the output signal of the counter 13. At the same time, the frequency signal of a 4fsc oscillator 4 is divided by 1/2 counters 5 and 6 to be turned into a subcarrier SC of frequency fsc. The 2fsc frequency signal of the output of the counter 5 is divided down to 1/455 by the counter 13 to be turned into a signal of horizontal synchronous frequency H, and a constant relation is always secured for this signal with the subcarrier SC of the counter 6. Then a stable synchronous coupling is carried out when the input VBS exists to ensure a stable extraction of a color frame signal. |
