Patent ID: 6351226
Filing Date: 2002-02-26
Classification: H04N

Abstract:
Data compression apparatus in which data blocks are compressed on a block-by-block basis, said apparatus comprising:(i) a frequency-transformer for frequency-transforming blocks of data to generate corresponding groups of coefficient values representing respective frequency components of said data blocks; (ii) a quantiser operable on said groups of coefficient values to map each of a contiguous plurality of ranges of coefficient values onto respective quantised data values, an extent of each range being controlled by a quantisation factor and a first non-zero range boundary being defined by a range boundary parameter; said quantiser being operable to vary said boundary parameter between data blocks by selecting from a range of boundary parameter regimes; (iii) a detector operable: (a) to trial-encode at least some of said data blocks using a range of at least one quantisation factor and at least a subset of said range of boundary parameter regimes available for use by the quantiser, in order to assess a quantity of compressed data produced with each quantisation factor/boundary parameter regime; (b) to trial-decode said trial-encoded blocks using a corresponding quantisation factor; (c) to compare said trial-decoded blocks with said data blocks before trial encoding, in order to assess data errors introduced by said trial-encoding and decoding; (d) to detect whether said relationship between quantity of data produced by said trial-encoding and data errors introduced by said trial-encoding is substantially monotonic; and (iv) a quantisation controller operable in response to a detection that said relationship between quantity of data produced by said trial-encoding and data errors introduced by said trial-encoding is not substantially monotonic, to vary said range of boundary parameter regimes available for use by said quantiser.