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US5353345A
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Method and apparatus for DTMF detection
Assignee: International Business Machines Corp
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Filing: 1992-10-14
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Granted: 1994-10-04
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Expires: 2012-10-14
Abstract
Abstract A DTMF tone is detected through the tracking of two tunable filters HAVING coefficients within so-called tunnels each limited by thresholds derived from the tone frequency components to be detected. In and out-band energies E1(n) and E2(n) are used to further validate tone detection.
Inventors
- 👤 Claude Galand
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-28 22:01
CPC Classifications
H; H04; H04Q; H04Q1/00; H04Q1/18; H04Q1/30; H04Q1/44; H04Q1/444; H04Q1/46; H04Q1/45; H04Q1/457; H04Q1/4575
Claims Analysis
Total Claims: 9
Claim 1
Independent Claim
201 words
A method for detecting a Dual Tone Multi Frequency (DTMF) signal in a received sampled input signal x(n) derived from a band-limited signal s(n), said tone including two predefined single frequency components being taken respectively within one out of two groups of predefined frequencies, said method including: setting a so-called tunnel for each one of said group frequencies, each tunnel being limited by an upper and a lower threshold derived from said one of said group frequencies; feeding the input signal x(n) into a first filtering means splitting x(n) frequency bandwidth into two bands each including one of said predefined groups of frequencies and deriving therefrom signals x1(n) and x2(n) respectively; feeding each said x1(n) and x2(n) into an adaptive digital filter each including one varying coefficient k1(n) and k2(n) respectively; tuning each of said adaptive filters and tracking said filter coefficients k1(n) and k2(n), said tracking including comparing each of said filter coefficients k1(n) and k2(n) with said upper and lower thresholds corresponding to one of said set tunnels to determine if said filter coefficients k1(n) and k2(n) each remain within one of said set tunnels for a minimum predefined time length; whereby a tone is being detected and identified.
Claim 2
Independent Claim
201 words
A method for detecting a Dual Tone Multi Frequency (DTMF) signal in a received sampled input signal x(n) derived from a band-limited signal s(n), said tone including two predefined single frequency components being taken respectively within one out of two groups of predefined frequencies, said method including: setting a so-called tunnel for each one of said group frequencies, each tunnel being limited by an upper and a lower threshold derived from said one of said group frequencies; feeding the input signal x(n) into a first filtering means splitting x(n) frequency bandwidth into two bands each including one of said predefined groups of frequencies and deriving therefrom signals x1(n) and x2(n) respectively; feeding each said x1(n) and x2(n) into an adaptive digital filter each including one varying coefficient k1(n) and k2(n) respectively; tuning each of said adaptive filters and tracking said filter coefficients k1(n) and k2(n), said tracking including comparing each of said filter coefficients k1(n) and k2(n) with said upper and lower thresholds corresponding to one of said set tunnels to determine if said filter coefficients k1(n) and k2(n) each remain within one of said set tunnels for a minimum predefined time length; whereby a tone is being detected and identified.
Claim 3
Dependent Claim
35 words
A method for detecting a DTMF tone signal according to claim 1, wherein said upper and lower thresholds are derived from cos 2π f(i), with f(i) being the frequency of one of said predefined frequencies.
Claim 4
Dependent Claim
26 words
A method for detecting a DTMF tone signal according to claim 2, wherein said tracking is performed for a predefined number of signal x(n) sampling intervals.
Claim 5
Dependent Claim
47 words
A method for detecting a DTMF tone signal according to claim 3, wherein said tracking for a predefined number of sampling intervals is terminated if either of said filter coefficients k1(n) and k2(n) is outside said corresponding one of said set tunnels for a predefined timeout period.
Claim 6
Independent Claim
29 words
A method for detecting a DTMF tone signal according anyone of to claims 1 through 4, wherein filter coefficient tracking operations for both groups of frequencies are performed sequentially.
Claim 7
Independent Claim
31 words
A method for detecting a DTMF tone signal according anyone of to claims 1 through 4, wherein the filter coefficient tracking operations for both groups of frequencies are performed in parallel.
Claim 8
Dependent Claim
57 words
A method for detecting a DTMF tone signal according to anyone of claim 1 through 4, further including a validation step involving determining an in-band (E1) energy and an out-of-band (E2) energy associated with said, band-limited input signal after performing a low-pass filtering operation and checking said in-band and out-of-band energies versus a pair of pre-defined thresholds.
Claim 9
Dependent Claim
72 words
A method for detecting a DTMF tone signal according to claim 7, wherein said energy validation checking step includes testing that a ratio R of the in-band energy to the out-of-band energy of the band-limited input signal, after reducing each of said in-band and out-of-band energies by an idle noise energy, is greater than a pre-specified threshold TH1 and that the input signal energy E1 is greater than a pre-specified threshold TH2.
References Cited (Backward Citations)
USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
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| 3.5 Years (Year 4) | 1997-10-03 | 1998-04-04 | 1998-10-04 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 2001-10-03 | 2002-04-04 | 2002-10-04 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 2005-10-03 | 2006-04-04 | 2006-10-04 | $7700.00 | Window Closed / Lapsed |