Investigation of an Analog Technique to Decrease Pentracking Time in Computer Display

Abstract

Many modern digital computer systems contain cathode-ray tube display equipment to facilitate man-machine communications. Through the use of a display and a light-sensitive pen, graphical material can be directly inserted into the computer by using the pen to control the position of the electron beam at the face of the CRT-a process called pen tracking. Beam position is continually sampled by the computer, permitting continuous display of the material being sketched. In present digital pen-tracking techniques, a tracking pattern (usually a cross) with a substantial number of points is generated on the face of the CRT and the binary response of the pen to the individual points of the pattern is employed to calculate pen position. The large number of pattern points, and the phosphor decay time associated with each, yield a typical tracking cycle of 500 to 1000 microseconds. Since the cycle must be repeated about 100 times per second, 5 to 10 percent of display time is consumed. To reduce the time required by the tracking operation, an analog technique employing a four-point tracking pattern is proposed in this study, in which the amplitude response of the pen to corresponding pairs of points is used to determine the position of the pen relative to the center of the pattern. To study the method, one channel of the proposed two-channel analog tracking system was designed, constructed, and coupled to the horizontal channel of a high-speed computer display console. To avoid the phosphor-decay limitation, an experimental "Beam" pen capable of detecting the electron beam rather than the phosphor luminescence is employed. The system included a pattern generator, sample-and-hold gates, difference amplifier, envelope detector and noise filter, and a threshold-logic analog-to-digital converter. The time required to generate the tracking pattern and develop the binary equivalent of the horizontal distance separating pen and pattern center is only 25 microseconds. Tracking is generally satisfactory, but some anomalies were noted, apparently due to the characteristics of the experimental pen being used. It is concluded that the analog technique is feasible for improving the speed of pen tracking, but recommended that further studies be made of the limitations inherent in the method.