A data compaction system and apparatus is disclosed which, in the preferred embodiment, includes a high speed compaction controller utilizing both read only storage and read-write storage. A compaction device according to the present invention could then be placed upon both ends of a transmission line, the data received by a compaction unit at one end of the line from whatever apparatus wished to transmit data on the line, the data compacted within the compaction unit according to the present invention, the data transmitted on the line to a compaction unit on the other end of the line, the data decompacted, and the data provided to whatever apparatus wished to receive the data. Data received by the compaction device according to the present invention in a fixed length, fixed number base, coded manner would then be compacted by altering the expression of the data, as by altering the number bases in which the data is expressed and by switching between number bases. Thus, it has been found that expressing the data as a string of characters of varying lengths and varying number bases, that is characters not all expressed in the same number base, shortens the overall length of the data transmitted. This is true even if the length of certain characters may be increased by the techniques according to the present invention. Also, prior character and prior record comparisons according to the present invention significantly enhance the compaction ability of the present invention.