Crosfield Electronics Limited

Ian W Rodgers

Rupert Leslie Goodings

Peter F Ross

A method and apparatus for decoding a digital codeword of a codeword system comprising digital codewords of varying length in which no short codeword forms the prefix of a longer codeword in the same system comprises detecting with means (26) whether the codeword has a predetermined prefix, the predetermined prefix being shorter than any codeword containing it. Further means (26) are provided for decoding only the suffix of the codeword or the entire codeword respectively according to whether or not the predetermined prefix is detected.

Codeword decoding

A method and apparatus for coding signals representing a stream of digital data groups (for example run lengths) for transmission or storage, each group representing a numerical quantity, is described. The signals are supplied via input and modify registers (4,6) and an encode latch (11) to a set of encode PROMs (12). The PROMs (12) generate for each data group a code representing the group, the code comprising a terminator codeword and, where necessary, one or more make-up codewords. The codewords are based on one or more multiplicative modifiers. The codewords are fed in parallel to output registers (13,14) and then serially shifted out of the output registers for subsequent transmission, storage, or further processing.

Data coding

A method and apparatus for detecting digital codewords or predetermined portions of digital codewords in a stream of codewords is described. The apparatus comprises a shift register (21) having a length at least one bit greater than the length of the longest codeword or predetermined codeword portion to be detected; and means including an inverter (24) for loading the shift register (21) with the logical complement of the leading bit of the stream of codewords. A clock (25) is provided to cause data in the data stream to be serially shifted into the shift register (21). The contents of the shift register after each shift are then used to address one or more PROMs (26) to determine whether the data in the shift register (21) upstream of the first change between data in the shift register of one type and data in the shift register of the complementary type comprises a codeword or a predetermined codeword portion.

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Cambridge, United Kingdom

Ian W Rodgers