Digital tone decoder and method of decoding tones using linear prediction coding
First Claim
1. The method of digitally detecting a tone at at least one predetermined tone frequency in a received signal represented by a frame of sequential digital data samples, the method comprising the steps of:
- calculating an array of Z transform domain coefficients from the digial data samples approximately representing the frame of sequential digital data samples;
evaluating a frequency spectrum represented by the array of Z transform domain coefficients at at least one evaluation frequency in a vicinity of each at least one predetermined tone frequency to obtain a spectral value indication at each evaluation frequency;
testing each said spectral value indication for a presence or absence of a tone in the vicinity of each of the at least one predetermined tone frequency to determine for each predetermined tone frequency whether or not the received signal contains a tone at the predetermined tone frequency; and
providing an indication of whether any of the tones at at least one predetermined tone frequency is in the received signal represented by the frame, and if any of the tones are in the signal, the predetermined frequencies of the tones that are present in the received signal.
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Abstract
An all digital feature based pattern recognition tone detection system distinguishes one or more tones from speech or other background noise and provides extremely reliable, low cost DTMF tone detection. The system includes a digital signal processing system which uses a modified PARCOR signal analysis technique to efficiently calculate inverse filter coefficients defining in functional form a smoothed LPC spectrum from a received frame of digital pulse code data. A Fourier transform provides high resolution evaluation of the LPC data at frequencies of interest to permit evaluation of the spectral response in accordance with known features of an expected tone signal.
70 Citations
40 Claims
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1. The method of digitally detecting a tone at at least one predetermined tone frequency in a received signal represented by a frame of sequential digital data samples, the method comprising the steps of:
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calculating an array of Z transform domain coefficients from the digial data samples approximately representing the frame of sequential digital data samples; evaluating a frequency spectrum represented by the array of Z transform domain coefficients at at least one evaluation frequency in a vicinity of each at least one predetermined tone frequency to obtain a spectral value indication at each evaluation frequency; testing each said spectral value indication for a presence or absence of a tone in the vicinity of each of the at least one predetermined tone frequency to determine for each predetermined tone frequency whether or not the received signal contains a tone at the predetermined tone frequency; and providing an indication of whether any of the tones at at least one predetermined tone frequency is in the received signal represented by the frame, and if any of the tones are in the signal, the predetermined frequencies of the tones that are present in the received signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27)
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28. The method of detecting a tone at each of a plurality of predetermined low group and high group frequencies in a received signal represented by a frame of sequential data samples, the method comprising the steps of:
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calculating from the digital data samples an array of Z transform domain coefficients approximately representing the frame of sequential digital data samples; evaluating a frequency spectrum represented by the array of Z transform domain coefficients at at least one evaluation frequency in a vicinity of each predetermined frequency to obtain a spectral value indication at each evaluation frequency; testing each said spectral value indication in the vicinity of each predetermined frequency to determine whether or not the received signal contains exactly one tone at a predetermined low group frequency and exactly one tone at a predetermined high group frequency and providing an indication of whether the received signal contains a combination of one tone at a low group frequency and one tone at a high group frequency and, if it does, which combination.
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29. The method of detecting telephone dialing tones at predetermined DTMF frequencies in a received signal comprising the steps of:
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reresenting a frame of the received signal with sampled digital data values; calculating linear prediction coding coefficients defining an inverse filter from the digital data values; evaluating the Fourier transform of the inverse filter to obtain spectral values at selected frequencies including at at least one frequency proximate each predetermined DTMF frequency; processing the spectral values at the at least one frequency proximate each predetermined DTMF frequency to determine for each predetermined DTMF frequency whether or not a dialing tone exists; and determining whether exactly one tone has been detected in each of two different frequency ranges and, if so, indicating a character corresponding to the detected tones.
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30. The method of detecting a sinusoidal tone signal of a given tone frequency in a received data signal comprising the steps of:
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digitally sampling the data signal at a predetermined sample rate to produce a sequence of sampled data points; grouping the sequence of sampled data points into a sequence of data frames, each representing a predetermined number of sampled data points; and for each frame in succession evaluating the frame to determine whether or not the sinusoidal tone is present, said evaluating including the steps of; performing time domain analysis of the data frame including the generation of an inverse filter transfer function of the form
space="preserve" listing-type="equation">A(z)=1+SUM {a.sub.i z.sup.-i };
(i=1,2,,P)where z is the Z transform operator; performing frequency domain analysis including a Fourier transform of the inverse filter transfer function to obtain spectral data at at least one frequency in the vicinity of the tone frequency; and evaluating the spectral data to determine the presence or absence of the sinusoidal tone. - View Dependent Claims (31, 32, 33, 34)
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35. The method of detecting at least one sinusoidal tone signal, each of a given frequency, in a frame or sampled data representing a data signal comprising the steps of:
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generating in response to the frame of sampled data a time domain finite polynomial sequence containing information that is indicative of a frequency spectrum of the frame of sampled date; performing a Fourier transform of the finite polynomial sequence to obtain spectral frequency data at at least one frequency in the vicinity of the given frequency of each sinusoidal tone signal; evaluating the spectral frequency data to determine the presence or absence of each of the sinusoidal tone signals.
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36. A digital computer program for evaluating a frame of sampled data representing a received signal to determine the presence or absence of a tone signal in the received signal occurring at a frequency which is one of at least one predetermined tone frequency, the program comprising the steps of:
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calculating from the sampled data an array of Z transform domain coefficients approximately representing the frame of sampled data; evaluating a frequency spectrum represented by the array of Z transform domain coefficients at at least one evaluation frequency in a vicinity of each at least one predetermined tone frequency to obtain a spectral value indication at each evaluation frequency; testing each said spectral value indication in the vicinity of each said at least one predetermined tone frequency to determine for each predetermined tone frequency whether or not the received signal contains a tone at the predetermined tone frequency; and providing an indication of whether the received signal carries a tone at one of the at least one predetermined frequency, and if the received signal does carry at least one tone, the predetermined frequency associated with each tone.
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37. A decoder system comprising:
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a data bus coupled to communicate digital data including sampled data representing a frame of a received signal; and a digital signal processing system including a digital data processor, the digital signal processing system including means for calculating from the sampled data an array of Z transform domain coefficients approximately representing the frame of sampled data; means for evaluating a frequency spectrum represented by the array of Z transform domain coefficients at at least one evaluation frequency in a vicinity of each of at least one predetermined tone frequency to obtain a spectral value indication at each evaluation frequency; means for testing each said spectral value indication in the vicinity of each predetermined tone frequency to determine whether or not the received signal carries a tone at the predetermined tone frequency; and means for communicating over the data bus an indication of whether or not the received signal has a tone at a predetermined tone frequency and if the received signal has a tone at a predetermined frequency, the frequency of each tone. - View Dependent Claims (38, 39, 40)
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Specification