Circuit and method for detecting a tone signal

Abstract

A tone detector includes at least one circuit (hereinafter “single phase reference matcher”) that not only performs a convolution of an input signal with a reference signal, but also compares the result of convolution with a threshold to determine if there is a match, and if so drives a signal active indicating that tone is present. If there is no match, another circuit (hereinafter “phase shifter”) delays the reference signal by a fraction (e.g. ⅛) of the measuring period, thereby to introduce a phase shift (e.g. &pgr;/8) between the input signal and the reference signal. The “single phase reference matcher” again performs the just-described operation, this time with a delayed reference signal, and repeats the operation as often as necessary (e.g. eight times) to cycle through the entire measuring period, thereby to ensure that tone (if present in the input signal) is detected irrespective of phase, during one of the operations. The smaller the phase shift, the greater the number of times the operation is repeated, and vice versa. A tone detector of the type described herein can include more than one “single phase reference matcher,” each of which performs one of the above-described operations in parallel, thereby to reduce the total time required to perform all operations. The number of samples over a measurement period is selected to minimize the noise, e.g. by selecting a number based on the input frequency and difference in frequency with the nearest adjacent frequency at which another tone may be present.