Video gate

Claim 1

IN A PPI DISPLAY SYSTEM WHICH PRODUCES RANGE, HORIZONTAL AND VERTICAL SWEEP SIGNALS RSW, XSW AND YSW AND D.C. MARKER SIGNALS XH AND YH, WHICH REPRESENT THE CO-ORDINATES OF A SELECTED POSITION OF THE DISPLAY, A CIRCUIT FOR PRODUCING A POLAR GATE WHICH IS SUBSTANTIALLY OF CONSTANT ANGULAR WIDTH AND SUBSTANTIALLY OF CONSTANT RANGE DEPTH COMPRISING: SIGNAL PRODUCING MEANS CONNECTED TO RECEIVE SAID HORIZONTAL AND VERTICAL SWEEP SIGNALS XSW AND YSW AND SAID D.C. MARKER SIGNALS XH AND YH AND HAVING TWO CONTROLLABLE CONDITIONS; SAID SIGNAL PRODUCING MEANS PRODUCING SIGNALS EQUAL TO -XSW+XH-KYH AND -YSW+YH+KXH IN ONE OF SAID TWO CONDITIONS AND PRODUCING SIGNALS EQUAL TO -XSW+XH+KYH AND -YSW+YH-KXH IN THE OTHER OF SAID TWO CONDITIONS, WHERE K IS LESS THAN ONE; COMPARING MEANS CONNECTED TO RECEIVE AND TO COMPARE THE SIGNALS PRODUCED BY SAID SIGNAL PRODUCING MEANS WITH A REFERENCE SIGNAL AND TO PRODUCE A SIGNAL WHEN THE SIGNALS PRODUCED BY SAID SIGNAL PRODUCING MEANS EQUAL SAID REFERENCE SIGNAL; A SQUARE WAVE PULSE PRODUCING CIRCUIT CONNECTED TO BE ENERGIZED AND TO RECEIVE SAID RANGE SWEEP SIGNAL RSW FOR REPETITIVELY PRODUCING, WHEN ENERGIZED, A SQUARE WAVE PULSE EACH TIME SAID RANGE SWEEP SIGNAL RSW EQUALS A VALUE LESS BY AN INCREMENT DR THAN SAID RANGE SWEEP SIGNAL WAS AT THE TIME SAID SQUARE WAVE PULSE PRODUCING CIRCUIT WAS ENERGIZED; AND A BISTABLE MULTIVIBRATOR HAVING AN INPUT CONNECTED TO SAID COMPARING MEANS, SAID BISTABLE MULTIVIBRATOR CHANGING STATE IN RESPONSE TO EACH OF SAID SIGNALS PRODUCED BY SAID COMPARING MEANS, THE OUTPUT OF SAID BISTABLE MULTIVIBRATOR BEING CONNECTED TO CONTROL THE CONDITION OF SAID SIGNAL PRODUCING MEANS AND TO ENERGIZE SAID SQUARE WAVE PULSE PRODUCING MEANS; WHEREBY THE SQUARE WAVE PULSES PRODUCED BY SAID SQUARE WAVE PULSE PRODUCING MEANS DEFINE A POLAR GATE IN SAID PPI DISPLAY SYSTEM.