Abstract: Compression and decompression methods for transmitting facsimile images in which the facsimile image is compressed in both vertical and horizontal directions are implemented using two dimensional maximum differences algorithms. Asymmetrical and fractional image compression are provided by controlling the number of lines, i.e., rows or columns, being compressed and by controlling the bit segment length used during each differences determination. The compressed facsimile image is decompressed by selecting a reference bit in the decompressed facsimile image and expanding the bit in both vertical and horizontal directions in accordance with a binary value of bits near the selected reference bit. A transmission channel adapted to serially perform the encoding and decoding functions is also described.

Abstract: A communication system including: a first secure communication terminal for providing analog voiceband data; a first processing circuit connected to receive the secure data from the first secure communication terminal and for converting the received data into secure baseband data, the first processing circuit transmitting the baseband data; a second processing circuit connected to receive the transmitted baseband data from the first processing circuit, and for converting the received baseband data into analog voiceband data; and a second secure communication terminal for receiving the analog voiceband data from the second processing circuit. The communication system is especially applicable for handling secure data transmitted by a STU-III.