GB1318499A - Multifrequency receiver - Google Patents

Abstract

1318499 Selective signalling INTERNATIONAL BUSINESS MACHINES CORP 28 Jan 1972 [3 March 1971] 4051/72 Heading H4 K1H3B1 A receiver for decoding multifrequency signals of which each component frequency is taken from a different predetermined group of frequencies has a different channel for each group of frequencies, each channel including a majority circuit arranged to generate a binary signal representative of the amplitude of a majority of a predetermined number of samples of the frequency signal in the channel, and means for determining the rate of zero crossings of a sequence of the binary signals to determine the frequency of the signal in the channel. The input is passed via an A/D converter to a digital transversal filter which is time-multiplexed with the factors required to obtaining the different groups of frequencies. The filter output goes to the channels mentioned. Each channel consists of a majority circuit followed by a frequency detector. The majority circuit consists of a peak clipper, a sampler and a shift register in series, the shift register stages being connected in parallel to a summer the output of which is compared with a reference value to obtain the binary signal. The majority circuit thus eliminates noise. The frequency detector counts clock pulses for 16 periods of the binary signal (which is a square wave), the count being decoded to identify the component frequency. The decoder output from the channels are combined in a final decoder. Confirmation logic responsive to the channel decoders checks the presence of component frequencies according to simultaneous presence or order of appearance criteria, as appropriate.