There is provided an entropy modifier (10, 60) which is operable to encode, alternatively decode, a stream of input data bits having a first entropy for generating corresponding entropy-modified output data having a second entropy, wherein the entropy modifier is operable to process the stream of input data bits to handle groups of mutually similar bits and the value of the first bit, and one or more maximum run lengths of the groups of mutually similar bits. Optionally, the entropy modifier (10, 60) is operable to control one or more maximum run lengths of the groups of mutually similar bits by using at least one escape code.