Coded data generation or conversion – Digital code to digital code converters – To or from 'n' out of 'm' codes
Patent
1986-12-10
1989-05-09
Shoop, Jr., William M.
Coded data generation or conversion
Digital code to digital code converters
To or from 'n' out of 'm' codes
341103, 341 86, 455608, H04B 900
Patent
active
048293008
DESCRIPTION:
BRIEF SUMMARY
It is common practice in line transmission systems to code data which is transmitted. This line coding serves a number of purposes. Firstly, digital repeaters must be able to extract a clock signal from the transmitted data and thus it is advantageous to transmit sequences of digits having frequent transitions and to avoid long sequences of like digits. Line coding also helps to detect errors in transmission and to restrict the frequency spectrum of transmitted signals so that supervisory signals can be transmitted. It is also useful to ensure that as far as possible the characteristics of the transmitted signals match those of the total medium.
In optical fibre transmission systems binary codes have proved advantageous and in particular the balanced disparity a, (a+1) Binary codes (a odd) are much used. Code and decode operations can be quite complex, particularly at the decoder, and especially at high information rates, of the order of GBit/s obtainable with optical transmission systems. Commercial logic is not yet available for dealing with this, In the past, the coding and decoding of information has been achieved using look-up stables constituted by ROMs. This is expensive and generally impractical in the case of high bit rates.
In accordance with one aspect of the present invention, a method of generating signals representing a plurality (p) of n bit words corresponding to respective input signals comprises continuously generating first signals representing m basic words whereby the p words are the same as or are cyclic rearrangements of the n bits of the m basic words, detecting the input signals, determining the one of the p words corresponding to each detected input signal, and selecting the appropriate first signal, if necessary after recycling, to consitute an output signal corresponding to the determined one of the p words.
In accordance with a second aspect of the present invention, apparatus for generating signals representing a plurality (p) of n bit words corresponding to respective input signals comprises a basic word generator for generating first signals representing m basic words whereby the p words are the same as or are cyclic rearrangements of the n bits of the m basic words, control means for determining the one of the p words corresponding to each input signal and for generating corresponding control signals, and selection means responsive to the control signals for selecting the appropriate first signal, if necessary after recycling, to constitute an output signal corresponding to the determined one of the p words.
The invention makes use of a particularly suitable codeword system based on the balanced disparity approach in which a majority of the codewords have the same number of digits of each logical type. In such a codeword system, a variety of different codewords can be generated simply by cycling the digits of the basic words.
Preferably, recycling is achieved by selectively delaying and sampling the first signals.
This method of recycling does not require expensive logic and conveniently can be implemented by providing the selection means as a plurality of delay units for imposing time delays on the first signals before the first signals reach corresponding sampling means, the sampling means being responsive to the control signals to pass a selected one of the first signals to a common outport port.
This method of achieving recycling is particularly applicable in the case where the first signals comprise optical signals enabling a large proportion of the coding to take place in the optical domain.
By using the recycling property, m will be less than p thereby simplifying the apparatus and reducing its cost and complexity.
In one example, in which the bits of at least some of the basic words are logically complemental, as hereinafter defined, the basic word generator includes one or more logic units for generating the complemental basic words.
A pair of basic words are hereindefined as "logically complemental" if each bit of one word is the logical complement of the correspo
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British Telecommunications
Romano Gary J.
Shoop Jr. William M.
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