Versatile video crt display

Abstract

A cathode ray tube display system for displaying data and/or pictorial images in an undistorted manner over the entire display screen. Programmable video polarity circuits add versatility to the system, enabling the cathode ray tube to be driven by any one of a variety of different drive circuits and effectively eliminating any skewing or delay experienced by the control signals as a result of the particular drive circuitry employed. Versatile deflection of the cathode ray tube electron beam at low power levels is achieved through the use of deflection circuitry employing a feedback circuit for achieving linearity and a totem pole circuit employing a coupling diode for achieving rapid, automatic flyback during the retrace portion of the deflection cycle. Automatic sizing of the display generated by the cathode ray tube (CRT) is obtained regardless of changes in the operating frequency of the horizontal and vertical sync signals employed to control the cathode ray tube. The sizing circuitry employs solid-state timer circuits, preferably of the integrated circuit type, which generate the necessary horizontal and vertical sweep circuits regardless of frequency changes experienced by the input sync signals. A comparison circuit for use with the sizing circuit utilizes the logarithmic characteristics of semiconductor devices to provide a simple inexpensive circuit for comparing the peak level of a first sawtooth against a reference level for automatically adjusting the slope of an output sawtooth deflection signal, in accordance with the peak voltage of the input sawtooth, whose frequency is a function of the sync signals. A distortion-free display over the entire CRT display screen is obtained through the use of a dynamic focus circuit which developes a dynamic focusing voltage for the cathode ray tube which is derived from the horizontal and deflection voltage sawtooth signals and is independent of the TV timing signal. The dynamic focus circuit employs low cost and yet reliable semiconductor devices. The circuitry for developing the dynamic focusing voltage includes amplifier circuits which produce differential outputs which are independently adjustable to properly focus the beam trace and thereby assure the generation of a distortion-free display. An inverted or noninverted display may be selected in a simplified manner through the employment of the aforementioned programmable video circuits which enable the CRT to be driven by any one of a variety of different combinations of control circuitry without introducing skewing or delay problems.