Vertical synchronizing signal detector

In a vertical synchronizing signal detector comprising:

• (a) a synchronizing signal source for outputting a composite synchronizing signal including a horizontal synchronizing signal and a vertical synchronizing signal, each synchronizing signal having a reference level;
• (b) a pulse source for generating a clock pulse of a frequency higher than that of the horizontal synchronizing signal;
• and
• (c) counting means having an input terminal for counting the number of pulses supplied to said input terminal during each recurrent period of horizontal synchronizing signal and for producing a detection signal when the counted number exceeds a predetermined value;
• the improvement which comprises:
• (d) latch means supplied with said composite synchronizing signal and said clock pulse for sampling the level of said composite synchronizing signal at a predetermined incidence of each said clock pulse and holding the thus-sampled level till the next sampling at the predetermined incidence of a succeeding clock pulse, so that a latched signal whose presence substantially corresponds to the presence of the synchronizing signals at the reference level is obtained;
• (e) gate means connected between said pulse source and said input terminal of said counting means for allowing said clock pulse to pass therethrough during the presence of said latched signal output from said latch means and for preventing said clock pulse from passing therethrough during the absence of said latched signal.

A vertical synchronizing signal detector comprising:

• (a) reference pulse generator means supplied with a horizontal synchronizing signal included in a composite synchronizing signal for generating a horizontal reference pulse which is synchronized with and in a predetermined phase relationship with said horizontal synchronizing signal, and a multiplied pulse obtained by multiplying said horizontal reference pulse;
• (b) latch means supplied with the composite synchronizing signal and the multiplied pulse for producing a latched pulse whose level is corresponding to a level of the composite synchronizing signal at every sample time which is either of commencement and termination of the multipled pulse and is held till change in the level of the composite synchronizing signal occurs at a subsequent sample time, so that the thus-produced latched pulse has a length which substantially corresponds to that of the horizontal and vertical synchronizing signals;
• (c) AND means for producing a logic product of the latched pulse and the multiplied pulse;
• and
• (d) counter-discriminator means for counting the number of the multiplied pulse output from said AND means during each period of time of repetition of said horizontal reference pulse and for generating a detection signal whenever the counted number exceeds a predetermined value.