A method of converting n-bit information words into m-bit code words, and the other way round, is described. The code words have a limited disparity. For every information word, two code words are assigned to a group of information words, which code words can be derived from one another by inversion. A choice between these two code words is made to limit the digital-sum-value so as to obtain a d.c. free code. In order to obtain a further limitation of the digital-sum-value within the code words, these code words are derived from each other by inversion and reversal.
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1. A method of transmitting information, which comprises converting n-bit information words into m-bit code words before transmission and reconverting said m-bit code words into n-bit information words after transmission, and which further comprises for converting consecutive n-bit information words into m-bit code words with a limited maximum disparity .+-.d prior to transmission, where n, m and d are integers which comply with n<m and d<m, in such a way that a digital-sum-valve taken over all the preceding code words at the beginning of a code word remains limited to a range which is bounded by a first and a second value, selecting the following code word, at least with respect to the polarity of a disparity thereof, as a function of said digital-sum-value over all the preceding code words so as to ensure that said following code word cannot cause an increase of the absolute value of said digital-sum-value, for which purpose said method comprises assigning a pair of code words to at least a first group of possible n-bit information words, the code words of said pair having opposite disparities with an absolute value d and being the bit-by-bit inverse of one another for each associated information word, characterized in that for limiting the instantaneous digital-sum-value to a range which is bounded by a third and a fourth value, which third and which fourth value are situated outside the range bounded by the first and the second value in such a way that the spacing between the second and the fourth value is smaller than the spacing between the third and the first value, said method comprises assigning first and second code words with a disparity +d and -d, respectively, to at least a part of the first group of information words, which second code words for every associated information word are the bit-by-bit inverses of the first code words whose transmission sequences have been reversed, said first code words having been selected from at least that group of code words which, in conformity with said selection rule, remain within the range which is bounded by the third and the fourth value, while the corresponding bit-by-bit inverted code word does not remain with said range and after reversal of the transmission sequence does remain with said range.
2. A method as claimed in claim 1, characterized in that after transmission of the code words, said method comprises checking whether the code words exhibit a disparity +d or -d, and converting directly or after bit-by-bit inversion and reversal of the transmission sequence, depending on the polarity of the disparity, code words belonging to said pair of the first group of information words.
3. A method as claimed in claim 1 or 2, wherein the maximum disparity .+-.d is equal to the minimum possible disparity and is unequal to zero, so that the digital-sum-value taken over all the preceding code words at the beginning of a code word remains limited to either the first value or the second value, which values are spaced from each other, and the first code words which belong to the first group of information words cause said digital-sum-value to vary from the first value to the second value, while the associated second code words cause said digital sum value to vary from the second value to the first value, characterized in that said method further comprises selecting from the first code words to encode, an information word of the first group if the digital-sum-value exhibits the first value at the beginning of the code word, and selecting the bit-by-bit inverse of the code word whose transmission sequence has been reversed if the digital-sum-value exhibits the second value at the beginning of the code word.
4. A method as claimed in claim 3, wherein the maximum disparity .+-.d is equal to .+-.2 and code words of zero disparity are assigned to a second group of information words, characterized in that said method further comprises selecting said code words independently of the digital-sum-value at the beginning of the code word, which do not exceed the third value when they vary from the first value to the first value, and which do not exceed the fourth value when they vary from the second value to the second value.
5. A method as claimed in claim 4, characterized in that n=8 and m=10 and the third value is situated at a spacing 2 from the first value and the fourth value is situated at a spacing 1 from the second value.
6. An encoding device for converting n-bit information words into m-bit code words with a limited maximum disparity .+-.d prior to transmission, where n, m and d are integers which comply with <m and d <m, in such a way that a digital-sum-vaue taken over all the preceding code words at the beginning of a code word remains limited to a range which is bounded by a first and a second value, in which the following code word is selected, at least with respect to the polarity of the disparity, as a function of said digital-sum-value over all the preceding code words so as to ensure that said following code word cannot cause an increase of the absolute value of said digital-sum-value, for which purpose a pair of code words is assigned to at least a first group of possible n-bit information words, the code words of said pair having opposite disparities with an absolute value d and being the bit-by-bit inverse of one another for each associated information word, characterized in that said encoding device comprises means for determining the digital-sum-value taken over all the preceding words; means for converting the information words of the first group; and means for inverting and reversing the code words obtained by converting information words of the first group if this is required by the digital-sum-value thus determined.
7. A decoding device for converting m-bit code words into n-bit information words, said m-bit code words having been formed by said encoding device as claimed in claim 6, characterized in that said decoding device comprises means for determining the disparity of the code words received;
- means for inverting and reversing the code words which correspond to information words of the first group if this is required by the disparity found; and
- means for converting the code words corresponding to information words of the first group.