Dual tone multifrequency signal detection and identification met

Details Dual tone multifrequency signal detection and identification met Dual tone multifrequency signal detection and identification met

1990-12-11

1993-10-26

Dwyer, James L.

Telephonic communications

Centralized switching system

Switching controlled in response to called station...

379386, 328138, H04M 300, H03D 300

Patent

active

052573095

ABSTRACT:
Dual tone multifrequency ("DTMF") signal pulses are detected and identified by subjecting the signal being analyzed to complex bandpass filtering for each of the two DTMF frequency bands. The envelope of the outputs of each of these filters are determined and subjected to a succession of different tests including a ripple or smoothness test to ensure adequate smoothness, a ratio test to ensure the occurrence of a step function, a twist test to ensure the proper amplitude ratio between the two bands, and a minimum energy test to ensure that the signal has sufficient energy. If the signal passes all of the detection tests, then the actual DTMF signal is identified by using the real and imaginary parts of each complex bandpass filter output to compute an associated complex phase angle, and comparing that phase angle to the corresponding phase angles for valid DTMF tones.

REFERENCES:
patent: 4689760 (1987-08-01), Lee et al.

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