US 4,539,646 AGranted

Tone detection arrangement

AssigneeU.S. Philips Corporation

InventorsPaul J. Stein, Rodney W. Gibson

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Abstract

A multi-tone sequential call signal detector in which a microcomputer is used to detect the tones of a received signal by simulating digitally the response of analogue tuned circuits. The detector includes a limiter for producing a square wave signal from a tone signal, a differentiator for producing interrupt pulses, and a microcomputer which responds to the interrupt pulses to activate a visual display and a call lamp of an output circuit when it detects all the received tones of the signal. The microcomputer performs an algorithm by which it measures the period between successive interrupt pulses, determines which tone frequency has this period, starts a number count which represents the "rise" response of a tuned circuit for that tone frequency, and determines when the number count reaches a threshold number which represents the resonant condition of the tuned circuit.

Claims7

Dependent claims

A method for detecting the tones of an electrical multi-tone signal comprising a plurality of sequential tone bursts of predetermined frequencies comprising the steps of:

(a) generating clock pulses;
(b) producing an interrupt pulse in respect of each zero crossing in at least one sense of the multi-tone signal;
(c) counting the number of clock pulses occurring in the period between each interrupt pulse and the preceding interrupt pulse to produce a first number which is representative of this measured period;
(d) comparing said first number with predetermined numbers which are respectively representative of the periods of all the different possible tone frequencies which the multi-tone signal can have, to obtain a tentative detection of the particular tone frequency to which said measured period corresponds;
(e) generating in respect of each tentative detection of a tone frequency a number count commencing with a selected number, and thereafter incrementing the relevant number count by a given number each time the tentative detection of the tone frequency concerned is obtained;
(f) after incrementing a number count, comparing it with a threshold number to obtain a confirmed detection of the tone frequency concerned when the number count exceeds said threshold number;
(g) decrementing by a fixed fraction the values of the number counts subsisting in respect of all of the tone frequencies, after each comparison of any number count with said threshold number in which the threshold number is not exceeded;
(h) providing a recognition of the multi-tone signal when a confirmed detection of all of the tone frequencies thereof in succession has been obtained.

5 more elements

Dependencies are inferred from the claim text.