Coded data generation or conversion – Digital code to digital code converters – Coding by table look-up techniques
Patent
1987-12-30
1990-06-26
Shoop, Jr., William M.
Coded data generation or conversion
Digital code to digital code converters
Coding by table look-up techniques
341 51, 341 55, H03M 700
Patent
active
049375741
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to data conversion, in particular to a data conversion method performed in apparatus comprising two complementary units, the first of which converts a block of n data input bits to a block of n data output bits and the second of which performs the inverse of the conversion effected by the first unit.
The invention is defined in the claims appended hereto, to which reference should now be made.
BRIEF SUMMARY OF THE INVENTION
It is anticipated that a data conversion method and apparatus embodying of the invention will be used in the field of broadcasting where the conversion apparatus used at the receiving end of a data transmission link is likely to be replicated many times. It is, therefore, desirable that the second unit, which performs the "inverse" conversion should be made as simple and inexpensive as possible. Any consequential complication to the transmission or forward conversion unit which may result can be accepted.
A simple and inexpensive "inverse" conversion apparatus may be constructed using the data conversion method of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
A data conversion method and apparatus in accordance with the invention will now be described, by way of example, with reference to the drawings, in which:
FIG. 1 is a block diagram of a data conversion unit in accordance with the invention; and
FIG. 2 is a block diagram of a complementary data conversion unit for use with the unit of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As mentioned above, it is envisaged that the first and second data conversion units should be used at the input and output ends of a data transmission link. The unit 2 shown schematically in FIG. 1 is intended to be used at the output end of the data link and thus carries out the "inverse" data conversion process.
The unit 2 comprises an input data buffer 10 coupled to the input of a conversion input register 13 by means of a three-way multiplexer 12. A control-word register 11, which holds the next version of a control word used to control the conversion performed by the unit 2, can also be connected to the conversion input register 13 by means of the multiplexer 12.
The conversion input register 13 feeds a bit-permutation matrix 14 whose outputs are coupled to eight identical substitution look-up tables 20 to 27. The outputs of the look-up tables 20 to 27 are connected to a conversion output register 30 which is connected in turn to an output data buffer 32 and, through the multiplexer 12 to the conversion input register 13.
The input data buffer 10 holds a 64 bit input data block received from a data link (not shown). The conversion input register 13 is fed via the three-way multiplexer 12 with a block of 64 bits of data from one of three sources; the input data buffer 10, the control word register 11 or with data recirculated from the conversion output register 30, according to the stage of the conversion process which has been reached. At the beginning of the conversion process, the 64 bit input data block received from the data link held in the input data buffer 10 is fed to the conversion input register 13 and its output is then applied to the bit-permutation matrix 14 which effectively rearranges the order of the data bits from the conversion input register 13. The bit-permutation matrix 14 can be defined by a table which defines, for each output bit, the bit-number of the corresponding input bit. Thus for a 64 bit input block this table will contain 64 six-bit numbers. Unless the conversion process is to be a one-way function, the table must contain all possible six-bit numbers (0 to 63 inclusive) with each value appearing only once throughout the 64 locations.
The output bits from the bit-permutation matrix 14 are then split into groups of eight bits, each group then being used as the address input to one of the eight identical substitution look-up tables 20 to 27. There is one look-up table for every eight bits in the input data block. The eight identical substitution look
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British Broadcasting Corporation
O'Connell Robert F.
Shoop Jr. William M.
Young Brian K.
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