LIGHT PEN CIRCUIT AND REMOTE NOISE-IMMUNE PULSE DETECTOR FOR RASTER SCAN CRT DISPLAY SYSTEM

Video telephone stations, which include raster scan cathode-ray tubes, may be connected to each other or to a computing machine by way of a telephone switching system. Each station includes a light pen for providing sensings of the electron beam when video signals are received from the computing machine. The beam sensings are converted to pen pulses, combined with the synchronizing signals of the incoming video signals and the combined signals are transmitted to the computer. In order to distinguish between pen pulses and interference on the signaling link, the light pen aperture is arranged to be sufficient in size to enable the pen to sense an area covering elements of two successive scan lines on the screen, whereby two pen pulses are produced during a field. The computer recognizes a true light probe signal when successive ones of the received pen pulses are separated by an interval of time corresponding to the interval between beam traversals.