Detection of tones in sampled signals
AssigneeNorthern Telecom Limited
InventorsDavid G. Agnew, Sorin Cohn-Sfetcu
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Abstract
A tone detector in the form of a digital correlator is disclosed in which PCM signal samples are delayed in a RAM for the correlation period. Using PROMs, the products of the current and delayed samples with samples of a sine signal at the frequency to be detected are produced, and their difference is cumulatively summed to produce a first accumulated signal. A second accumulated signal is similarly produced using samples of a cosine signal at the frequency to be detected, and an output signal indicating the presence or absence of the tone is produced from the sum of the squares of the two accumulated signals. The use of memories enables the detector to be readily multiplexed for detection of a plurality of tones on each of a plurality of channels.
Claims9
A method of detecting the presence of a predetermined frequency component in a signal sampled with a sampling period t, each signal sample comprising a plurality of bits, the method comprising:
- delaying the samples of said signal by a predetermined correlation period T to produce delayed signal samples;
- producing samples with a sampling period t of first and second signal sequences representing respective signals at said predetermined frequency having a phase difference other than 0 or .pi., each sample of the first and second signal sequences comprising a plurality of bits;
- cumulatively producing for each signal sample first and second accumulated signals, each of the first and second accumulated signals being equal to the first or second, respectively, accumulated signal produced for the previous signal sample modified in dependence upon the product of the current sample of the first or second, respectively, signal sequence with the difference between the current signal sample and the current delayed signal sample;
- and
- deriving from the first and second accumulated signals an output signal which represents the presence or absence of the predetermined frequency component during the preceding correlation period T.
Dependencies are inferred from the claim text.