Abstract: A multimedia transaction processor for facilitating the sale of multimedia material, the apparatus comprises a media server operable to store multimedia material from at least one vendor, and to store meta data representing the content of the multimedia material and data identifying the vendor providing the multimedia material, the meta data and the identifying data being stored in association with the multimedia material. The apparatus includes a communications processor connectable, via a communications link, to one or more data processing systems and is operable to receive via the communications link data indicative of a request for multimedia content from one of the buyers.

Abstract: A system for watermarking and transferring watermarked material comprises a transaction server, first and second clients, first apparatus for applying a perceptible watermark to the material and second apparatus for removing the watermark. The server, clients and first and second apparatus are linked by one or more communications networks.

Abstract: An apparatus for recording video data with a multi-level arrangement of error correction coding. Input video data is demultiplexed so as to form a plurality of data streams each having a plurality of data blocks. A stream error correcting code is generated for each of the plurality of data streams, which is adaptable to allow correction of a data block. The respective data blocks and stream error correcting code of each data stream is distributed or arranged in a predetermined manner. The distributed data blocks and stream error correcting code of each data stream are recorded on recording tracks of a recording medium by n recording heads such that only either a data block or a stream error correcting code of a respective data stream is recorded at corresponding positions along n adjacent recording tracks of the recording medium.

Abstract: Video data compression apparatus in which successive images are encoded, comprises a data quantity allocator for allocating respective quantities of output encoded data for two or more complementary areas of each image; and an encoder for encoding each area of each image of the group of images in accordance with the quantity of output encoded data allocated to that area.

Abstract: A video data compression system in which input data representing at least portions of a picture can be encoded by generating motion data representing image motion between the picture and at least two different sets of one or more reference pictures. The system is operable to compare test data indicative of the sum of the quantity of motion data and quantity of error data required for encoding with respect to the sets of reference pictures, select the set of reference pictures for which the test data indicates the lowest sum, and use the selected set of reference pictures to encode the input data.

Abstract: A data compression system decorrelates input data into sets of frequency component data. Different sets of frequency component data are entropy encoded using different coding tables matched to the data values characteristic of the sets of frequency component data they are used to compress. Matching the coding tables to the sets of frequency component data improves compression. The order in which the data values from within each set of frequency component data is fed to the entropy encoder is varied to exploit any intra-set correlation.

Abstract: A data compression apparatus is described which applies a dynamically variable degree of quantisation Q.sub.s to input data so as to control blocks of output data to have a predetermined maximum size. The dynamic control can be carried out on an inter-block basis or an intra-block basis. An overflow detector is provided for inserting an overflow indicator within blocks of compressed data to indicate when overflow has occurred.