Patent US 4,096,520

Black and white run length encoding means first convert a binary video input signal having a raster scan format into binary black and white run length message codes which are preselected so that the initial or lead bit of each of those codes is at a given logic level. Ternary encoding means convert the binary message codes into a series of ternary black and white run length message codes which have distinctive amplitude transitions at any intra-scan line code boundaries, and a frequency modulator then frequency modulates a carrier signal in accordance with the ternary codes to provide a passband signal suitable for transmission via a limited bandwidth communications channel. To recover the video signal from the passband signal, the process is reversed.