Patent ID: 6914979

Claim:
A method for determining whether any of a predetermined set of tones present in a plurality of successive frames of digital samples of a received signal falls within a predetermined frequency tolerance, comprising the steps of: obtaining discrete Fourier transform pairs of in-phase and quadrature dot products of said samples and integer multiples of a base frequency, said base frequency being determined by the quotient of the sampling frequency and a multiple of the number of samples in successive ones of said frames; computing the quotients of the highest power ones of said products obtained on successive frames; using said quotients to approximate an arctangent function for ascertaining the phase of frequencies contained in successive frames; computing the phase change for each of said frequencies by subtracting the phase of a previous phase frame from the current frame; and subtracting an expected phase change from said computed phase change of said highest power ones of said products to determine the deviation of an observed tone from said predetermined frequency tolerance.