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  Multi-frequency tone detectorUSPTO Application #: 20060098806Title: Multi-frequency tone detector Abstract: A multi-frequency tone detector with analysis window (i.e. filter size) chosen such that spectral nulls are located at adjacent frequencies of interest. The decision logic block of the tone detector uses the roll-off characteristics of the filter in conjunction with background noise to determine a threshold pass/fail for any tone that has deviated excessively from its nominal value. The foregoing aspects of the invention result in simple filter design (i.e. reduced order) relative to prior art tone detectors. (end of abstract) Agent: Perry & Partners C/o Keating & Bennett, LLP - Mclean, VA, US Inventor: Dieter Schulz USPTO Applicaton #: 20060098806 - Class: 379395010 (USPTO) Related Patent Categories: Telephonic Communications, Substation Or Terminal Circuitry, Power Control Or Detection Circuit The Patent Description & Claims data below is from USPTO Patent Application 20060098806. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates generally to tone detectors for telephone systems and in particular to an improved tone detector with filter characterized by spectral nulls located to suppress frequencies adjacent the tone to be detected and an improved detection algorithm using roll-off characteristics of the filter and background noise. BACKGROUND OF THE INVENTION [0002] Multi-Frequency (MF) tones are used extensively in telecommunications. Examples of MF tones are DTMF and R2. Typical prior art tone detectors include a pre-filter for suppressing out-of-band frequencies, an analysis filter for determining the energy of the input signal at each multi-frequency tone of interest, and a decision logic block for interpreting the multi-frequency energy values output from the analysis filter and in response performing a pass/fail test for determining the presence or absence of a tone at each multi-frequency tone of interest. [0003] Prior art analysis filters have been implemented using standard digital filtering techniques. The analysis window for such prior art digital filters must be carefully chosen. Rectangular windows are often used for simplicity. However, the leakage of energies in the side lobes of the rectangular window is such that other MF frequencies may be incorrectly detected. Also, fast roll-off characteristics (i.e. high order) are usually imposed in order to fail any tone that deviates excessively from its nominal value. SUMMARY OF THE INVENTION [0004] According to one aspect of the present invention there is provided a multi-frequency tone detector with analysis window (i.e. filter size) chosen such that spectral nulls are located at adjacent frequencies of interest. According to another aspect, the decision logic block uses the roll-off characteristics of the filter in conjunction with background noise to determine a threshold pass/fail for any tone that has deviated excessively from its nominal value. The foregoing aspects of the invention result in simpler filter design (i.e. reduced order) relative to prior art tone detectors, while maintaining high tone detection accuracy. BRIEF DESCRIPTION OF THE DRAWINGS [0005] A preferred embodiment of the present invention will now be described more fully with reference to the accompanying drawings in which: [0006] FIG. 1 is a block diagram of a well known multi-frequency tone detector; [0007] FIG. 2 shows the frequency response of a tone detector filter with rectangular window; [0008] FIG. 3 shows the frequency response of a filter with rectangular window centered at 900 Hz, showing the location of spectral nulls at equally spaced frequencies adjacent 900 Hz, in accordance with the present invention; [0009] FIG. 4 shows filter roll-off for a filter with rectangular window; [0010] FIG. 5 shows the frequency response of a filter with rectangular window of size N=195, centered at 852 Hz, showing moderate suppression of adjacent DTMF signal frequencies, in accordance with an alternative embodiment of the present invention; and [0011] FIG. 6 shows the frequency response of a filter with rectangular window of size N=276, centered at 852 Hz, showing improved suppression of adjacent DTMF signal frequencies, in accordance with an alternative embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0012] FIG. 1 shows a typical tone detector. Pre-filter 1 filters the input signal to suppress out-of-band frequencies. The envelope detector 4 determines whether a signal is present or not and synchronizes the signal edges of the signal with the operation of the analysis filter bank 3. The analysis filter bank consists of multiple filters, one at each MF frequency of interest. These filters calculate the energy of each frequency. The energy calculator 5 computes the total energy of the signal after pre-filtering. The total energy combined with the results of the analysis filter bank 3 is fed to a decision logic block 7 that determines whether a valid tone is present or not. The decision block 7 may perform various tests, such as minimum tone level, twist (if it is a dual tone), maximum frequency deviation, etc. As discussed in greater detail below, according to one aspect of the invention a new method is implemented in decision logic block 7 to determine frequency deviation from a nominal value by using the roll-off in the filter bank 3 combined with background energy. [0013] The analysis filters 3 are implemented as DFT filters using a modified Goertzel algorithm, one for each MF (e.g. R2) frequency. The Goertzel filter is centered at the R2 frequency. [0014] According to the preferred embodiment a rectangular window was chosen for the DFT (Goertzel) filter. The choice of a rectangular window results in reduced complexity relative to other filter types. For example, pre-processing the data with either a raised cosine or Kaiser window has a considerable MIPS impact. As indicated above, however, the leakage of the energies in the side lobes can be significant in rectangular windows (especially if two valid frequencies to be detected have a relative twist). Thus, according to an aspect of the invention, the filter size is chosen in such a way that the other MF frequencies fall in its spectral nulls, thereby greatly reducing the leakage effect. [0015] R2 is a special case of MF communication. The six R2 signaling frequencies are situated at 540 Hz, 660 Hz, 780 Hz, 900 Hz, 1020 Hz and 1140 Hz for the backward direction and at 1380 Hz, 1500 Hz, 1620 Hz, 1740 Hz, 1860 Hz and 1980 Hz for the forward direction. Thus, in each direction the frequencies are 120 Hz apart. A window of size N has nulls situated at frequency intervals Fs/N, as shown in FIG. 2. Consequently, for R2 signaling spectral nulls are needed at 120 Hz intervals. [0016] In order to position the nulls at 120 Hz intervals, and assuming a sampling frequency Fs=8 kHz, a window size of 200 samples is chosen for the filter (the minimum window size for nulls at 120 Hz is N=Fs/120 Hz, which is 66.67, such that the smallest integer window is thus N=200). [0017] For example: If the frequency of interest is at 900 Hz, the filter is centered at 900 Hz using a window of 200 samples. Spectral nulls are therefore located at each of the other MF frequencies, i.e. 540 Hz, 660 Hz, 780 Hz, 1020 Hz and 1140 Hz. FIG. 3 shows the R2 example for a frequency of 900 Hz. It will be noted that the other R2 frequencies fall exactly on the spectral nulls. [0018] For non-equally spaced frequencies (such as DTMF), the approach is not as straight forward as for the equally spaced frequency case. Nonetheless, the windows for each frequency detector (i.e. filter) may be sized in such a way that the closest frequency falls into a null. On the other hand, the window may be chosen such that the interference of the other frequencies is minimized. [0019] For example, if the frequency of interest is the DTMF frequency at 852 Hz., the nearest adjacent frequency is 770 Hz (i.e. separated by 82 Hz). Choosing a window size of N=195 places nulls at intervals of 41 Hz, as shown in FIG. 5. The other DTMF frequencies (697 Hz, 941 Hz) do not fall on the spectral nulls, but are only suppressed by 24.8 dB and 23 dB respectively. However, if the window size is chosen to be N=276 as shown in FIG. 6, none of the DTMF frequencies fall on spectral nulls. Nonetheless, the frequencies are suppressed by 25.3 dB, 24.8 dB and 32.6 dB respectively, thereby improving the overall suppression. Continue reading... Full patent description for Multi-frequency tone detector Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Multi-frequency tone detector patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Multi-frequency tone detector or other areas of interest. ### Previous Patent Application: Programmable audio alert system and method Next Patent Application: Method and apparatus for inclusive echo canceller testing Industry Class: Telephonic communications ### FreshPatents.com Support Thank you for viewing the Multi-frequency tone detector patent info. 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