1. Field of the Invention
The invention relates to displays such as cathode ray tube (CRT) displays, particularly with respect to generating the raster deflection control signals therefor.
2. Description of the Prior Art
Cathode ray tube displays for conveying television picture information are ubiquitous in present day technologies. For example, complex avionics systems utilize cathode ray tube displays to replace many formally mechanical flight instruments as well as to additionally provide other complex display information to the operator. Generally, the data is provided in the form of a television raster scan or in the form of calligraphic stroke written symbology. Such avionics systems require the presentation of the television raster images with accurately positioned stroke written symbols overlayed thereon. Modern aircraft instrumentation of this type is frequently referred to as a "glass cockpit". Such avionics systems often utilize a plurality of television sensor inputs with diverse video formats to be displayed on a plurality of display devices. Each display processing channel must synchronize its operation to the diverse timing information present in the television input signals. Specifically, horizontal and vertical synchronizing pulses permit a picture image to be properly placed up a CRT screen. Each of the television signal inputs can vary in its timing characteristics adversely affecting the positional accuracy of the presentation. The positional accuracy problem is further exacerbated in modern complex avionics systems because of the large number of distinct input and output signal characteristics.
The signals required to drive the CRT displays comprise primarily a video or intensity electrical signal, the instantaneous amplitude of which translates into a proportional intensity on the CRT screen and a pair of periodic sawtooth deflection control signals. The deflection signals are denoted as horizontal or X and vertical or Y deflections and comprise electrical signals with amplitudes resulting in proportional deflections from the CRT screen center in the respective horizontal and vertical directions.
In order to provide a set of signals of the correct characteristics, resettable integrators are utilized to provide the linear deflection sweeps. Conventional deflection generators are, however, subject to inaccuracies in the generation circuitry. These inaccuracies result from the timing variations discussed above as well as because of drift in component characteristics due to environmental conditions such as variations in temperature.
Several methods are utilized in the prior art to overcome these inaccuracies. Firstly, overscanning may be utilized wherein the picture size is expanded until the image size significantly exceeds the screen size. Although overscanning eliminates objectionable blank areas at the CRT screen edges and permits the deflection generation characteristics to drift without noticeably disturbing the image, overscanning is unacceptable for avionics applications, particularly in military systems, since a loss of picture information and positional accuracy occurs in the offscreen area.
A second prior art solution involves precisely adjusting the characteristics of each deflection generator. This solution requires the use of high precision and temperature stable electrical components to generate a precisely placed image. Each of the X and Y deflection signal generators is precisely adjusted by the use of signal offset and gain adjustment trimming mechanisms. This approach is cumbersome for multiple sensor applications since a new adjustment value must be provided for each sensor input. The solution becomes inordinately complex when a large number of systematic timing variations must be accommodated. In the prior art, individual sensor variations are accommodated by attempting to design and match an analog deflection network to each of a number of input characteristics by utilizing analog switching means.