Abstract: Apparatus for transmitting and receiving information blocks, particularly video information, is arranged so that on receiving a corrupted information block it stores a first receiver buffer address at which the data block should be stored if subsequently successfully repeated in a later information block. The apparatus keeps track of the time taken to receive a requested repeat information block and on receiving the later information block at the predicted time, and on determining that it is uncorrupted, stores the data in the receiver buffer at the stored buffer address. Such apparatus can allocate received, repeated and non-repeated data blocks to their correct relative positions within the receiver buffer without the need for numbering of the data blocks on transmission giving a consequential reduction in the size of the data blocks being transmitted.

Abstract: Values, such as a set of coefficients in a transform-based video coder are to be transmitted sequentially in any one of a number of possible preset orders. The order to be selected is the one that results in the largest number of consecutive zeros (or low values) at the end of the sequence. Values are supplied (in an arbitrary sequence) with addresses, in parallel to a bank of assessment sections--one for each preset order--each of which translates the addresses into addresses representing a position in the respective order and records the highest for which the associated value is non-zero. The outputs of the sections are then compared to identify the lowest, and hence the optimum order of transmission.

Abstract: A coder assembles bursts of blocks of image data (e.g. transform coded coefficients) and associated overhead data into a sequence of data which are then transmitted. Both the transform coded coefficients and the overhead data are initially stored in separate FIFO memory stores (21), (22). The blocks of transform coded coefficients have a `last coefficient` flag accompanying the last coefficient, and the overhead information contains a unique word SWITCH, after the header information but before the `end-of-block` flag. The `last coefficient` flag and the SWITCH word are recognized by a toggle controller (29) which controls the reading of the stores so that the coefficient data and overhead data are selectively passed to a buffer (30) in a desired sequence. The buffer (30) is followed by a variable length coder (31) which supplies the output of the coder at a constant bit rate.

Abstract: A coder assembles bursts of blocks of image data (e.g. transform coded coefficients) and associated overhead data into a sequence of data which are then transmitted. Both the transform coded coefficients and the overhead data are initially stored in separate FIFO memory stores (21), (22). The blocks of transform coded coefficients have a `last coefficient` flag accompanying the last coefficient, and the overhead information contains a unique word SWITCH, after the header information but before the `end-of-block` flag. The `last coefficient` flag and the SWITCH word are recognized by a toggle controller (29) which controls the reading of the stores so that the coefficient data and overhead data are selectively passed to a buffer (30) in a desired sequence. The buffer (30) is followed by a variable length coder (31) which supplies the output of the coder at a constant bit rate.