Patent GB-1492310-A (GB)
Title: Digital multifrequency signals receiver
IPC: H04Q1/457
Filed: 1975-12-02
Abstract: 1492310 Signal receivers INTERNATIONAL STANDARD ELECTRIC CORP 2 Dec 1975 [18 Dec 1974] 49404/75 Heading G4H [Also in Division H4] A digital receiver for a multifrequency signal has a digital filter formed by a processor to act as a band-pass filter and as two filters for each possible frequency the first having a wider pass band than the second, the outputs of the possible-frequency filters being rectified, timeaveraged and supplied to a digital recognition device which looks for signal beginning and signal end in turn. A digital filter receives a sequence of digitized samples of a 2-out-of-6 multifrequency signal and includes a special-purpose processor which carries out on a time-shared basis the calculations to simulate a network as in Fig. 1 of secondorder filter cells (shown as squares), cells C1-C4 forming a wide-band-pass filter and the pairs of cells C11-C12, C21-C22,...C61-C62 relating to the 6 possible frequencies respectively, the second cell of each pair having a narrower pass-band than the first. The twelve outputs a<SP>ij</SP> shown (from C11-C62) are rectified (by dropping the sign bit) and time-averaged (over a period T<SP>1</SP>=n1.T where T is the sample period) to obtain quantities a<SP>ij</SP> k  where k represents the period T<SP>1</SP>, and these quantities are then stored in a random-access memory 5 of a digital processor (Fig. 5) which also includes a programme control unit 16 (which also supplies weights and other constants), an ALU 6 with buffer register 8, a ROM 10, a shift circuit 14, an output register 15 and gates 9, 11, 12, 17, The ALU has an output V to specify the sign of the difference when operating as a subtractor, and an output W which is 1 when the result from the ALU is all-zeroes. The processor (Fig. 5) performs the following operations. It operates in a signal appearance recognition phase until a multi-frequency signal is recognized as present, and then in a signal disappearance recognition phase until it is recognized as being absent, and so on. In the signal appearance recognition phase; The quantities a<SP>#</SP> k  are weighted to form quantities A<SP>#</SP> k . In each of k1 consecutive intervals T<SP>1</SP>, the greatest of the six quantities A<SP>#</SP> k  is taken, and the average Mk of these over the k1 intervals is computed and, provided this average is not less than a predetermined constant, two threshold values are obtained by multiplying it by constants. The six quantities A<SP>#</SP> k  are thresholded against each threshold. Two should be above the higher threshold and the rest below the lower, to constitute a multifrequency signal, a signal being recognized as present if the same one is so detected in each of k2 consecutive time intervals T<SP>1</SP> (or in k3 of the k2) and provided a transient state is absent. A transient state is taken to be present if any of the said greatest of the quantities A<SP>#</SP> k  differs proportionately too much from M k . In the signal disappearance recognition phase: The quantities a<SP>#</SP> k  are weighted to form quantities A<SP>#</SP> k . A threshold value is obtained by multiplying the most recent value Mk obtained in the preceding appearance recognition phase by a constant. The quantities A<SP>#</SP> k  are thresholded against this threshold. Provided they are all below it for each of k4 consecutive intervals T<SP>1</SP>, disappearance of the multifrequency signal is taken to have been recognized. The multifrequency code may consist of one of four high frequencies together with one of four low frequencies, and in this case there would be a band-pass filter for each of the two groups of frequencies, and M k  and the transient state test could be based on their outputs instead of on the outputs of the individual-frequency filters (to improve protection against interfering speech signals). Applications to telephone systems are mentioned. A number of different channels could be processed in succession.