This invention relates to an automatic frequency control circuit for synchronizing the horizontal deflection current in a television receiver with the horizontal synchronous signal contained in a video signal, such as a broadcast color television signal or a video signal from a video tape recorder.
Conventional automatic frequency control (AFC) circuit includes a voltage-controlled oscillator (VCO) whose frequency is controlled in accordance with a phase error of a flyback pulse (FBP) generated from the horizontal deflection current, with respect to the horizontal sync signal (HSYNC) signal. When the AFC circuit is formed of digital circuits, the phase error is detected with a resolution of the operating clock period. That is, the phase error smaller than the operating clock period could not be detected or corrected.
One way of increasing the accuracy of the phase error detection is to increase the frequency of the operating clock. However, to attain an accuracy adequate for horizontal synchronization in a television receiver, the frequency of the oscillating clock needs to be extremely high, and producing such a high frequency is practically impossible.
Another problem relates to compromise between the response and stability. To increase the response (promptness) in eliminating the phase error, the time constant of an integrator included in the AFC circuit may be reduced. This however lowers the stability, and the AFC circuit is susceptible to jitter or noises, and the stability of the displayed image is lowered.