GB1529506A - Frequency-selective signal receiver - Google Patents

Abstract

1529506 VF receivers INTERNATIONAL STANDARD ELECTRIC CORP 11 May 1976 [16 May 1975] 19320/76 Heading H4K [Also in Division G4] A frequency-selective receiver compares stored absolute times of the zeros of the input signal with corresponding times from a counter for a reference signal, the product of the two signals being integrated on the basis of the comparison. The receiver disclosed is for recognizing telephone voice-frequency dialling information consisting of 1-out-of-4 "high" frequencies plus 1-outof-4 "low" frequencies. A high pass filter passes the high frequency via a limiter to a device which measures the times of a series of zero-crossings using a clock-driven counter and stores them. These times are used to determine which of the 4 frequencies is present, assuming that one is, to control a counter to provide the zero-crossing times appropriate to this frequency. These go to a comparator together with the stored times for the input signal, the larger comparand in each comparison being gated to a store, the difference between successive entries to this store being taken, successive such differences being summed with alternating sign. The comparator controls the counter and the input signal time store, the effect being to correlate the input frequency signal with the reference, i.e. integrate their product. Another correlation is done using the reference phasedisplaced by 90 degrees. The results of the two correlations are each squared and then added together. If the sum exceeds a threshold, one of 4 outputs is marked corresponding to the reference frequency used. Similar arrangements, starting with a low pass filter, deal with the low frequency, to mark one of 4 outputs. The two 1-out-of-4 markings are converted to a 1-out-of-n marking. The receiver can be made using integrated circuits.