US 4,047,223 AGranted

Frequency scanning automatic phase control system

AssigneeZenith Radio Corporation

InventorJudson A. Hofmann

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Abstract

A frequency scanning automatic phase control includes a phase detector, a local oscillator and an APC filter. A coincidence detector includes an AND gate responsive to the horizontal retrace and reference synchronizing pulses. A lock detector also responsive to the retrace and reference sync pulses produces an output signal when a predetermined interval of non-coincidence has existed. An AND gate responsive to both the lock detector and coincidence detector gates a current sink which unbalances the phase detector producing a ramp-like error voltage across the APC filter. A threshold detector coupled to the APC filter activates a discharge circuit also coupled to the filter when the filter voltage exceeds the threshold. The oscillator responds to the variations of error voltage and changes frequency accordingly. Once coincidence occurs frequency variation ceases and is resumed only if non-coincidence returns.

Claims14

Dependent claims

In a television receiver including signal means recovering picture, audio and scansion synchronizing information from a received signal;

a picture tube;
automatic gain control means maintaining the recovered picture information at a substantially constant level, horizontal scansion means producing a high amplitude retrace signal and vertical scansion means developing a raster on said display means and horizontal scansion synchronization means including an oscillator and automatic phase control operative thereon synchronizing the horizontal scansion of said display with said synchronizing information, the improvement comprising:
coincidence means detecting coincidence between said retrace pulse and said scansion synchronization information;
lock detecting means determining the existence of an out-of-sync condition by comparing the relative durations of intervals of coincidence and non-coincidence;
and
configuring means responsive to said coincidence means and said lock detecting means causing cyclic variation of said oscillator frequency only if an out-of-sync condition and non-coincidence simultaneously occur.

3 more elements

Dependencies are inferred from the claim text.