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Detecting horizontal blanking time in cathode ray tube devices

Image Number 4 for United States Patent #5969486.

An apparatus and method are disclosed for generating timing pulses to control various functions in a video monitor including the switching of video amplifiers in cathode ray tube (CRT) devices, the control of the phase and frequency of phase locked loops, etc. Unlike previous methods, where switching is timed and controlled by monitoring the retrace voltage level, the system of the present invention monitors the change in direction of current in the retrace tuning capacitor. Monitoring the retrace capacitor current, provides an extremely accurate method for timing in horizontal deflection circuits. The apparatus of the present the invention comprises a small bead inductor placed in the current path of a horizontal retrace capacitor within the horizontal deflection circuit of the cathode ray tube device. Measuring the voltage across the bead inductor allows a very precise monitoring of the current through the retrace tuning capacitor. By designing the size of the bead inductor as disclosed, precise electrical pulses can be generated and detected during the retrace time of the CRT electron beam. These precise pulses are then used to generate accurate control signals for use within the CRT device. The invention also discloses an apparatus and method that minimizes left side ringing distortion in cathode ray tube video displays. Left side ringing is reduced due to the addition of the bead inductor to the horizontal deflection circuit, which lowers the stray resonant frequency within the horizontal deflection circuit.

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