TIME DEPENDENT TWO-TO-THREE LEVEL ALTERNATE ENCODING

To provide spectrum compression for non-synchronous data signals of the type that make successive excursions from a predetermined level, there is a method and means for carrying out a modified two-to-three level alternate encoding technique whereby the polarity of the encoded signal is changed or retained from one excursion of the data signal to the next depending on whether the excursions occur within a predetermined period of time or not. To gain the full advantage of this technique in limited bandwidth facsimile systems, the timing is selected so that adjacent runs of white picture elements are encoded with opposite or identical polarities depending on whether the intervening black run contains one or more picture elements, thereby virtually forcing the system to respond to any single, isolated, black picture elements.