Truncated run length encoding with a modular variable length code format

Abstract

A fractionally difference modulated binary video signal having a raster scan format and containing some scan lines of difference modulated picture elements and other scan lines of unmodulated picture elements is compressed by a truncated run length encoder which converts black terminated white runs into run length message codes, non-terminating black picture elements into individual message bits, and unterminated white runs into end-of-line message codes. No distinction is made between the difference modulated and unmodulated picture elements while generating the run length message codes and the individual message bits, but different end-of-line message codes are generated in response to the unterminated white runs of difference modulated and unmodulated picture elements, thereby providing a scan line-by-scan line indication of whether the picture elements are difference modulated or not. A block length multiple plus run length residue code format is employed for encoding the black terminated white runs, whereby runs in excess of a predetermined block length are represented by a block length multiple code plus, for any runs which are not integer multiples of the block length, run length residue codes. Shorter black terminated white runs are represented merely by run length residue codes.