1. A method of converting a number (r) into a signed binary representation in m&plus;1 digits (r′ m,..., r′ 0 ) from its binary representation in m bits (r m−1,..., r 0 ), characterised in that it consists of: a) adding in the binary representation of the number (r) to be converted a bit (r m ) of rank m, a bit (r −1 ) of rank −1 and a bit (r −2 ) of rank −2 and initialising the said bits to the value 0, b) using at least one calculation variable (b) and c) initialising to the value 0 a bit of rank m (b m ) of the said variable, d) performing a conversion loop, for i equals m up to i equals 0, comprising the following operations: (1) applying as inputs (e 1, e 2, e 3 ) of first calculation means ( 30 ) the bit of rank i (b i ) of the calculation variable (b), and the bits of ranks i−1 and i−2 (r i−1, r i−2 ) of the binary representation of the number to be converted, the said first means supplying as an output the bit of rank i−1 (b i−1 ) of the calculation variable (b), performing an arithmetic operation equivalent to the default value of the arithmetic sum divided by two of the bits applied as inputs (e 1, e 2, e 3 ), (2) applying as inputs (e 1 ′, e 2 ′, e 3 ′) of second calculation means ( 31 ), the bits (b i, b i−1 ) of rank i and i−1 of the calculation variable (b), and the bit (r i ) of rank i of the binary representation of the number to be converted (r), the said calculation means performing an arithmetic operation equivalent to the multiplication by minus two of the bit of rank i of the calculation variable (b), and the arithmetic sum of the preceding result with the other two bits applied as an input, in order to supply as an output the digit of rank i (r′ i ) of the signed binary representation.