Rca Inc.

Robert A Barnes

Bruce G Marks

Keith R Frantz

A video display apparatus operates at two or more horizontal scan rates. In order to prevent excessive collector voltage from being applied to the horizontal output transistor, a transition circuit turns off the horizontal output transistor when switching between horizontal scan frequencies occurs. The horizontal output transistor is maintained nonconductive until the correct oscillator frequency and supply voltage is coupled to the horizontal output circuit.

Frequency switching circuit for multiple scan rate video display

A video display apparatus having automatic frequency control circuitry for adjusting the frequency of a horizontal oscillator to provide oscillator synchronization with incoming horizontal synchronizing pulses incorporates an interface circuit for controlling the horizontal oscillator frequency response to frequency control output voltage. The interface circuit incorporates a two-transistor switch which provides a current path for charging the horizontal oscillator timing capacitor during a portion of the horizontal interval and a current path for bypassing the horizontal oscillator timing capacitor during another portion of the horizontal interval. The amplitude of the frequency control output voltage will determine the relative length of time that each of the transistors is conducting in order to provide a net increase or decrease rate in the charging of the oscillator capacitor, which in turn adjusts the oscillator operating frequency.

Oscillator-frequency control interface circuit

A resonant degaussing circuit provides rapid recovery for use with computer monitors or video display apparatus where frequent movement of the display apparatus occurs. The degaussing circuit comprises a capacitor in parallel with a degaussing coil. Triggering of an SCR causes the capacitor to discharge, so that it resonates with the degaussing coil to produce the desired AC degaussing current. The SCR is triggered only when the vertical deflection current is substantially zero, in order to eliminate interference of the vertical flux with the degaussing current.

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Bainbridge, PA, United States of America

Robert A Barnes