Data are compressed on a block-by-block basis. A frequency-transformer frequency-transforms input data to generate corresponding groups of coefficient values representing different frequency components. A quantiser operates on the groups of coefficient values to map each of a contiguous plurality of ranges of coefficient values onto respective quantised data values, the extent of each range being controlled by a quantisation factor and the first non-zero range boundary being defined by a range boundary parameter. A detector operates to: (a) trial-encode at least some of the data blocks using a range of at least one quantisation factor and at least a subset of the range of boundary parameter regimes available for use by the quantiser, in order to assess the quantity of compressed data produced with each quantisation factor/boundary parameter regime; (b) trial-decode the trial-encoded blocks using a corresponding quantisation factor but a single predetermined boundary parameter; (c) compare the trial-decoded blocks with the data blocks before trial encoding, in order to assess data errors introduced by the trial-encoding and decoding; and (d) detect whether the relationship between quantity of the data produced by the trial-encoding and data errors introduced by the trial-encoding is substantially monotonic. A quantisation controller responds to the detection that the relationship between the quantity of data produced by the trial-encoding and the data errors introduced by the trial-encoding is not substantially monotonic, to vary the range of boundary parameter regimes available for use by the quantiser.
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