Data compression using bit change statistics

Abstract

A method is provided for compressing relatively time invariant binary data, such as speech data in a telephone answering device, using statistical analysis of changes in the data. An original record organized into multiple frames of multiple bits each is used to construct an XORed record of the same number of frames and bits. The XORed record has a base frame with the same bit value pattern as a corresponding base frame of the original record, and remaining frames with bit values given by the outputs of an exclusive-OR operation applied to the bit values of corresponding and prior frames of the original record. The bit positions of the XORed record frame set are analyzed and reordered, according to their bit value change activity and used to construct an output record. The output record may have a base frame with the same bit value pattern as the corresponding reordered XORed record base frame. Other output record frames are established using a compression scheme wherein at least low bit value change subframes of the reordered XORed record frames are compressed by replacing them with shorter bit patterns having a format comprising a first part representing the number of bit changes occurring in the subframe and a second part identifying the location or locations, if any, of those changes. The foregoing procedure is reversed to restore the original record from the output record.