Dual tone multifrequency signal detection and identification methods and apparatus
First Claim
1. The method of detecting a dual tone multifrequency pulse in an applied signal, said pulse comprising at least two tones having frequencies which are respectively approximately equal to the frequencies of two reference tones, said method comprising the steps of:
- bandpass filtering the applied signal using first and second bandpass filters, the pass band of said first filter including the frequency of a first of said reference tones, and the pass band of said second filter including the frequency of a second of said reference tones;
determining the envelope of the output signal of each of said filters;
monitoring the smoothness of the envelope associated with each of said filters over a predetermined first time interval in order to determine whether each envelope satisfies a predetermined smoothness criterion; and
if the envelopes associated with both of said filters satisfy said smoothness criterion, then for the envelope associated with each of said filters, comparing the envelope value during a predetermined second time interval which is prior to said first time interval to envelope values subsequent to said second time interval in order to produce an indication that said pulse has been detected if said subsequent envelope values exceed said envelope value during said second time interval by at least a predetermined amount for the output signal of at least one of said filters.
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Accused Products
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.
28 Citations
32 Claims
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1. The method of detecting a dual tone multifrequency pulse in an applied signal, said pulse comprising at least two tones having frequencies which are respectively approximately equal to the frequencies of two reference tones, said method comprising the steps of:
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bandpass filtering the applied signal using first and second bandpass filters, the pass band of said first filter including the frequency of a first of said reference tones, and the pass band of said second filter including the frequency of a second of said reference tones; determining the envelope of the output signal of each of said filters; monitoring the smoothness of the envelope associated with each of said filters over a predetermined first time interval in order to determine whether each envelope satisfies a predetermined smoothness criterion; and if the envelopes associated with both of said filters satisfy said smoothness criterion, then for the envelope associated with each of said filters, comparing the envelope value during a predetermined second time interval which is prior to said first time interval to envelope values subsequent to said second time interval in order to produce an indication that said pulse has been detected if said subsequent envelope values exceed said envelope value during said second time interval by at least a predetermined amount for the output signal of at least one of said filters.
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2. The method of detecting a dual tone multifrequency pulse in an applied signal, said pulse comprising at least two tones having frequencies which are respectively approximately equal to the frequencies of the two reference tones, said method comprising the steps of:
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bandpass filtering the applied signal using parallel first and second complex bandpass filters having mutually exclusive pass bands, the pass band of said first filter including the frequency of a first of said reference tones, the pass band of said second filter including the frequency of a second of said reference tones, and each of said filters producing a real and an imaginary representation of the portion of the applied signal which is within the pass band of that filter; using the real and imaginary representations produced by each of said filters to determine an envelope which defines the portion of the applied signal which is within the pass band of that filter; monitoring the smoothness of the envelope associated with each of said filters over a predetermined first time interval in order to determine whether each envelope satisfies a predetermined smoothness criterion; and if the envelopes associated with both of said filters satisfy said smoothness criterion, then, for the envelope associated with each of said filters, comparing a minimum envelope value during a predetermined second time interval which is prior to said first time interval to an average of the envelope values during a predetermined third time interval which is al least partly concurrent with said first time interval in order to produce an indication that said pulse has been detected if said average exceeds said minimum by at least a predetermined amount for the portion of the applied signal within the pass band of at least one of said filters. - View Dependent Claims (3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
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17. Apparatus for detecting a dual tone multifrequency pulse in an applied signal, said pulse comprising at least two tones having frequencies which are respectively approximately equal to the frequencies of two reference tones, said apparatus comprising:
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first and second bandpass filters for bandpass filtering the applied signal, the pass band of said first filter including the frequency of a first of said reference tones, and the pass band of said second filter including the frequency of a second of said reference tones; means for determining the envelope of the output signal of each of said filters; means for monitoring the smoothness of the envelope associated with each of said filters over a predetermined first time interval in order to determine whether each envelope satisfies a predetermined smoothness criterion; and means for comparing the envelope value associated with each of said filters during a predetermined second time interval which is prior to said first time interval to envelope values subsequent to said second time interval in order to produce an indication that said pulse has been detected if the envelopes associated with both of said filters satisfy said smoothness criterion and if said subsequent envelope values exceed said envelope value during said second time interval by at least a predetermined amount for the output signal of at least one of said filters.
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18. Apparatus for detecting a dual tone multifrequency pulse in an applied signal, said pulse comprising at least two tones having frequencies which are respectively approximately equal to the frequencies of two different reference tones, said apparatus comprising:
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parallel first and second complex bandpass filters for filtering said applied signal, said filters having mutually exclusive pass bands, the pass band of said first filter including the frequency of a first of said reference tones, the pass band of said second filter including the frequency of a second of said reference tones, and each of said filters producing a real and an imaginary representation of the portion of the applied signal which is within the pass band of that filter; means for using the real and imaginary representations produced by each of said filters to determine an envelope which defines the portion of the applied signal which is within the pass band of that filter; means for monitoring the smoothness of the envelope associated with each of said filters over a predetermined first time interval in order to determine whether each envelope satisfies a predetermined smoothness criterion; and means responsive to a determination that the envelope associated with both of said filters satisfy said smoothness criterion for comparing a minimum envelope value for each filter during a predetermined second time interval which is prior to said first time interval to an average of the envelope values for that filter during a predetermined third time interval which is at least partly concurrent with said first time interval in order to produce an indication that said pulse has been detected if said average exceeds said minimum by at least a predetermined amount for the portion of the applied signal within the pass band of at least one of said filters. - View Dependent Claims (19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32)
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Specification