Character Display on a CRT

Character fonts on raster scanned display devices (e.g., the home television receiver) are usually represented by arrays of bits and displayed as a matrix of black and white (or color) dots. In VIDEOTEX systems, due to the limitations on bandwidth, number of scanning lines, and the interlace of the broadcast television standard, each frame is allowed to contain about 21 rows of 40 characters each; each character occupying a rectangular area of 12 picture elements by 20 lines. The character sets that have traditionally been used in VIDEOTEX systems, have not used the multilevel display capabilities of the television receiver. Due to interlace, this results in flicker at every sharp color vertical transition. In this note, we show how the multilevel capabilities of a television receiver can be used by nonlinearly filtering the characters generated by a conventional VIDEOTEX character generator such that each vertical transition is made less sharp. Although the characters are more blurred due to filtering, the apparent resolution of the display is increased allowing smaller fonts to be used. Flicker is reduced to a large extent, thus, decreasing the fatigue due to prolonged viewing of the display. A font occupying 9X16 rectangular area per character is proposed. It results in about 65 percent more characters per ´page-screen´ compared to the present VIDEOTEX system. The hardware required for such nonlinear filtering is minor, since, storage of multilevel intensities of the characters is not required due to local processing at the receiver.