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Elizabeth Howell. The DTMF signal transceiver circuit apparatus includes a circuit capable of generating and detecting DTMF tones that must be interpreted by the terminal equipment, such as a modem, in order to comply with specifications propagated by the ITU. The circuit of the present invention, in its preferred embodiment, is comprised of a communication network interface; a DTMF generator module for generating the fundamental DTMF tones and summing them together; a DTMF detector comprising a sampling module, a computation module, an analysis module, and a decode module; and a terminal device interface.
The sampling module is adjusted to a frequency adequate to avoid data loss, typically twice the frequency of the largest valid transmitted value.
The computation module of the present invention provides a technical advance by compensating for the phase error introduced into all calculations based on a DFT or non-uniform DFT during the processing of the signals.
As the computation module is no longer using a uniform DFT, typical FFT based processor operation reductions are not available to the computation module, instead the present invention develops a fast algorithm to compute the proposed transformation.
Once the modified DFT is completed the computation module calculates the energy levels at the relevant energy peaks coinciding with the fundamental DTMF tones and their respective second harmonic frequencies. The analysis module takes the results of the DFT transform and the energy level calculation to determine whether a DTMF tone has been observed.
The circuit assumes that a DTMF tone has been detected when an signal energy peak is detected within the ITU signal tolerance range of a low fundamental DTMF frequency and a high fundamental DTMF frequency, and there are no energy peaks found at the corresponding harmonics.
The analysis module must also verify that the signal meet the corresponding ITU requirements for signal strength and duration. Once a DTMF signal has been confirmed the decode module, takes the two fundamental frequencies and translates them into the number, symbol, or letter corresponding to the detected frequencies. This translated value is then passed to the terminal device interface for further use by the modem.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. In order that the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to a specific embodiment thereof which is illustrated in the appended drawings.
Understanding that these drawings depict only a typical embodiment of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:. The present invention is drawn to a DTMF signal transceiver circuit comprising control circuitry, a DTMF generator, and a DTMF detector which may be employed or incorporated into a terminal equipment device, such as a modem, for interfacing with a telephone network.
As describe above, telephone networks impose specific stringent ITU standards upon terminal equipment concerning the generation and detection of DTMF tones. For example, one ITU standard requires that a DTMF signal have a signal duration of at least 23 ms and preferably be maintained for 40 ms. The present invention provides several methods by which the DTMF signal may be detected by increasing the resolution without introducing phase error into the signal results, thereby awarding the false detection of phantom tones while validating proper DTMF signals.
The sampling module is adjusted to a frequency adequate to avoid data loss, typically at least twice the frequency of the largest valid transmitted value.
Generally, this sampling rate is 8, Hz, but 7, Hz would also be acceptable. As the computation module is no longer using a uniform DFT, typical FFT based processor operation reductions are not available to the computation module, instead the present invention develops a fast algorithm to compute the proposed transformation for the fixed relevant frequencies. In the preferred embodiment, the method assumes that a DTMF tone has been detected when an signal energy peak is detected within the ITU signal tolerance range of a low fundamental DTMF frequency and a high fundamental DTMF frequency, and there are no other energy peaks found at the fumndamental tones or at the harmonics corresponding to the detected frequencies.
The analysis module must also verify that the signal met the corresponding ITU requirements for signal strength and duration.
Once a DTMF signal has been validated the decode module, takes the two fundamental frequencies and translates them into the number, symbol, or letter corresponding to the detected frequencies. This translated value is then passed to the terminal device interface for further use by the modem, or attached digital device.
The preferred embodiment rejects requirements of standard DTMF decoders, specifically resolution limitations and the reliance on DFT index values that introduce phase error into the DFT computation. A large N makes the space smaller, providing higher resolution in the frequency domain. However, it decreases the resolution in the time domain and increases computation time.
Suppose the sampling frequency f s , is 8, Hz. With these constants there is no value of N, for which subvalue of the bin k would correspond to Hz. Bin The phenomenon of introducing a phase error is called leakage.
The ITU has chosen the DTMF frequencies in such a way that whatever N and f s are chosen assuming even that we can freely chose f s , k will never be an exact integer. Table 3 demonstrates the introduction of this absolute error into the signal analysis. Occasionally errors will happen due the this leakage, but the number of these errors can be minimized. If these errors are left at a certain threshold, the communications equipment satisfies the Bellcore specification and is commercially viable for sales in the United States and Canada.
For example, as indicated in an article by S. Gay, J. Hartung, and G. A more careful analysis would reveal that non-integer values of k would lead to another error as well. For example, suppose that k is a non-integer:. Compared to the approaches known in the prior art which are based on equation 6. This phase error accumulates for every block of N input samples, even if the DFT is not calculated for this block of N samples. Note that we used [. In the above definitions, f can be any frequency, in particular it can be exactly any of the DTMF frequencies.
Any modification of the DFT is of little merit if there is no efficient way to compute it. A fast recursive algorithm to compute the modified DFT is necessary, as an FFT algorithm cannot be used for the above modification, since they cannot be modified accordingly.
The modification of the DFT is of practical significance, because there is a fast recursive algorithm that can be used for its computation. Then we can write the proposed DFT with phase correction as a scalar product.
Clearly the spectrum X n,m f function of the time index n, the number block m, and the frequency of interest f. We have also equation Thus taking advantage of the shifting window contents, a recursive expression for the DFT can be obtained with O N complexity, as demonstrated equation Ultimately, an efficient algorithm to compute the proposed non-uniform DFT with phase correction is obtained, particularly when only a few samples of the signal spectrum are required.
In conclusion, a modified definition of the DFT has been created. However, it can be used in other applications, where accurate detection of tones is necessary.
Furthermore, a new algorithm for computation of this modified DFT has been developed. This new algorithm is a further improvement over the algorithm provided in equation 2, where only the standard DFT was considered. The new algorithm is also an improvement over equation 1, because the absolute phase error is avoided.
In a recent publication this problem was overcome with the use of generalized or non-uniform DFT. Felder et al. Because the non-uniform DFT, in fact, accepts non-integer values of k, this approach has the potential to detect tones exactly. However, there are two problems with this method. The first problem is algorithmic, because k is a non-integer, the many vast algorithms in the prior art cannot be used directly.
An efficient algorithm to compute the non uniform DFT is not known. The second problem appears as a result of the processing of data in blocks. There are k full cycles of frequency f f in the end samples. When k is not an integer there will be a phase error that will accumulate with every block of N samples. It is the purpose of the present invention to overcome these disadvantages.
In particular, the preferred embodiment discloses a generalization of the DFT which takes the phase error into consideration. The preferred embodiment of the present invention also develops a fast algorithm to compute the proposed transformation.
There are many devices which presently utilized DTMF signals. These devices utilize the DTMF signals to enable a communication line, to identify a particular activity, or to authorize access to date.
Once the signal has been transmitted to the DTMF generator , the encoded signal is transmitted across the PSTN or other communication networks. The DTMF detector receives the signal that was originally generated along with noise that may have been added to the signal during the transmission. As a result of the added noise, the detection portion of the process is often the most complicated.
Furthermore, the ITU recommendations established for analyzing a DTMF signal further complicate the detection process by restricting the type of signals that may be accepted.
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