Abstract: The present invention relates to frequency generation of digital type, said frequencies being able to take values included in the 0.4000 Hz band. The invention consists in using the periodicity of the sine function on (0,2 .pi.), in making samples correspond to each frequency f, each sample being represented by a triplet P, Q, S, P representing the rank, Q being a function of the quadrant to which said sample belongs on (0,2 .pi.) S the sign, in memorizing 1/4T samples of the sine wave of frequency 1 Hz on (0,.pi./2), T being the sampling period, and in causing to correspond to any sample Y.sub.N indicative of the frequency f (Y.sub.N =sin 2.pi.P T) a sample of triplet (P, Q, S) of rank Nf chosen from the 1/4T samples, said correspondence being biunivocal. The invention is applicable to the generation of tones, to inter-exchange signalling in particular.

Abstract: The present invention relates to a digital frequency recognition device for a receiver of multifrequency PCM signals, and consists essentially in forming characteristic values V(f.sub.i) for each frequency f.sub.i, in forming characteristic values V(f.sub.i.+-.l) for adjacent frequencies of the code in question, in comparing the value V(f.sub.i) with the values V(f.sub.i.+-.l), the result of said comparison making it possible to deduce whether said frequency f.sub.i is effectively recognized. The invention is applicable to the signalling between automatic telephone exchanges or between subscribers and telephone exchanges.