Sequential block interleave coding of two-tone facsimile data

Abstract

A new method of compression, called sequential block interleave coding (SBIC), is presented which features a fixed-rate output and exhibits a strong immunity to transmission errors. The method encodes block-pairs of binary image data over a field of blocks without the need for codebooks. By initially scrambling blocks the field size requirement can be considerably decreased, thereby reducing the propagation effect of transmission errors. In its lossless form the implementation is quite straightforward, requiring primarily storage operations and simple identification of block-pair states. For better noise immunity, a lossy form of coding is preferred with encoding distortion taking the form of lost black elements. For the set of test images employed, the results of simulation have shown that it is possible to obtain a bit rate reduction of 38% (CR = 1.6) using 8-pel blocks, while good intelligibility can be maintained in the decoded image in the presence of transmission errors whose average error rate may be as high as one in 32 bits. Additional compression may be obtained by cascading the SBIC technique with block-to-block coding (giving 50% bit rate reduction for the test images), at the expense of increased sensitivity to transmission errors.