1. A method for processing an ordered series of digital signals that each have an amplitude from a finite set of amplitudes consisting of the most-likely-to-occur amplitude and at least one other amplitude, the processing to reduce the amount of data used to represent the digital signals and to form codewords such that the more likely to occur values or sequences of values of digital signals are represented by relatively short codewords and the less likely to occur values or sequences of values of digital signals are represented by relatively long codewords, the method comprising: classifying the series into a particular class of a plurality of classes according to the distribution of amplitudes of the digital signals, each class having a corresponding coding process of forming the codewords for the series; for the particular class of the series, applying the coding process corresponding to the class to form a set of codewords of the series; and adding an indication to the formed codewords to indicate the particular class, such that relatively short codewords are formed to represent values or sequences of values that are relatively more likely to occur, and relatively long codewords are formed to represent values or sequences of values that are relatively less likely to occur.