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Apparatus and method for audible signaling tone recognition

  • US 5,311,589 A
  • Filed: 03/31/1992
  • Issued: 05/10/1994
  • Est. Priority Date: 03/31/1992
  • Status: Expired due to Term
First Claim
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1. A method of detecting dual-tone, multi-frequency (DTMF) indicators and single-tone indicators, said indicators being used in a telephone system to represent unique ones of a plurality of possible audible signaling tones, said method comprising the steps of:

  • monitoring digitally sampled signals on a digital channel of said telephone system, said monitoring including collecting a frame having a predetermined number of said samples;

    determining amplitudes for a preselected plurality of frequencies in said digitally sampled signals;

    determining which two of said preselected plurality of frequencies have the highest amplitudes during said frame;

    determining from said two determined frequencies having the highest amplitudes whether a DTMF indicator or a single tone indicator is likely to be present in said signals;

    if it is determined that a DTMF indicator is likely to be present, comparing said two determined frequencies having the highest amplitudes to a plurality of predefined pairs of frequencies, each of said predefined frequency pairs defining one of said DTMF indicators, and selecting as a multi-frequency candidate the DTMF indicator having as its defining frequency pair, the frequency pair matching said two determined frequencies having the highest amplitudes;

    if it is determined that a single tone indicator is likely to be present, comparing a selected one of said two determined frequencies having the highest amplitudes to a plurality of predefined frequencies, each of said predefined frequencies defining one of said single tone indicators, and selecting as a single tone candidate the single tone indicator having as its defining frequency the frequency matching the one selected frequency;

    comparing the amplitude of said digitally sampled signals in said frame to an amplitude threshold dependent in a predetermined fashion upon the indicator corresponding to said candidate;

    determining a cadence of said digitally sampled signals; and

    delivering an identification of said indicator corresponding to said candidate if said digitally sampled signals satisfies said amplitude threshold and has a cadence corresponding to a predefined cadence for said indicator.

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