Digital frequency receiver

Claim 1

A digital frequency receiver connected to a processor for the recognition of multifrequency code signals received in signalling time slots and transmitted to the receiver in digital form by at least one PCM time multiplex link, the receiver including means for modulating the received signals with signals whose frequencies correspond to the code used and which are supplied by a digital generator modulating the signal in each of plural time-slots of the time multiplex link by the sine and cosine components of each code frequency, means for low-pass filtering the modulated signals and means for analysing the energy contained in the spectrum of the signals received in the neighborhood of the code frequency; said receiver comprising: a time base generator synchronized with data on said PCM time multiplex link for supplying in each time slot (BT) phase signals (ba to bd, A) for successively enabling circuits of the receiver; means for testing predetermined conditions respecting energy levels for each time slot using a processor operating in response to said time base generator, said testing means including an energy-analysis unit, a generator control memory and injection and sampling means; and two groups of low-pass filter cells coupled to said testing controlling means each corresponding to one of the sine and cosine components, each cell comprising a recurrence calculation circuit operating based on a finite difference equation and a memory circuit for storing intermediate values which correspond to each time slot and to each frequency.