Cathode ray display intensity control circuit

Claim 1

A dynamic intensity control circuit for use in a display system employing a cathode ray tube having means producing an electron beam, a screen on which the beam impinges, means for deflecting the beam across said screen to produce a visible trace, a grid for controlling the intensity of the beam, and an input signal for providing a control signal to said grid, comprising, a. means responsive to said input signal coupled to said grid for generating an unblanking voltage of variable amplitude whereby said beam impinges on said screen with an intensity proportional to the amplitude of said unblanking voltage, said unblanking voltage having an on-to-off time in response to said input signal, b. means for sampling duration of on-to-off time of said unblanking voltage, and c. means responsive to said sampled unblanking voltage for varying the amplitude of said unblanking voltage in proportion to the on-to-off time of the unblanking voltage to vary the intensity of said beam whereby the brightness of said beam is maintained substantially constant, d. said amplitude varying means including a network having a capacitor coupled to said unblanking voltage generating means arranged to be charged with a voltage in proportion to the sampling duration of on-to-off time, and a path in said network for discharging said capacitor through biasing means coupled to said unblanking voltage generating means for varying the amplitude of said unblanking voltage in proportion to said sampled duration, e. said network including a resistance network for a charge path and a discharge path of said capacitor, said charge path comprising a resistor of value R1 ohms and said discharge path comprising a resistor of value R2 ohms, the voltage on said capacitor being proportional to the ratio R1/ (R1 + n R2), where n is the percent of on-time of said unblanking voltage, f. said network further including a fixed resistor and an adjustable resistor in the discharge path of said first mentioned capacitor, the respective resistance values of said resistors being selected so that the capacitor voltage characteristic substantially coincides with the grid-to-cathode characteristic of said cathode ray tube, the value of said fixed resistor being selected to define the maximum unblanking voltage amplitude whereby a constant beam brightness is maintained for varying duty cycles of said unblanking voltage.