JPH09321714A - Dtmf signal detecting method and detector applied with it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a detector of small circuit constitution, to which a DTMF(dual tone multi frequency) signal detecting method capable of reducing the load on a memory quantity and an arithmetic quantity within a circuit is applied to. SOLUTION: In this detector, a low-pass filter 2 and a high-pass filter 8 divides an inputted signal S to low or high groups, and a retriever 3, a frequency estimating device 4, a retriever 9 and a frequency estimating device 10 retrieve the increase/ reduction of a varying quantity between respective samples of high group band signal and calculates the average of a sampling interval forming a maximum value and a maximum value obtained by retrieval to estimate the frequency. Then based on the result of judging whether the frequency is in a frequency band constituting the low/high group of the DTMF signal, S/N ratio calculators 6 and 12 calculate an S/N ratio of the amplitude estimated by amplitude estimating devices 5, 11 and a sine wave generated by sine wave generators 7 and 13 according to the frequency with one of the low group band signal and the high group band signal and when the calculating result exceeds a prescribed value, a DTMF encoder 15 regards it as the DTMF signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a DTMF (Dual Tone Mu) coding / decoding device (speech codec) for speech or the like used in mobile digital communication equipment.
lti Frequency) D for detecting the signal
The present invention relates to a TMF signal detection method and a detection device using the same.

[0002]

2. Description of the Related Art Generally, a DTMF signal can be regarded as a tone signal for expressing 16 types of codes by combining 4 types of low group frequencies and 4 types of high group frequencies. That is, a DTMF signal is generated by selecting one type of frequency from each of four types of low group frequencies and high group frequencies and synthesizing the selected sine wave of the low group frequency and the selected sine wave of the high group frequency. .
Such DTMF signals are used in mobile communication devices as well as, for example, push button signals of telephones.

On the other hand, as a conventional DTMF signal detection device for detecting a DTMF signal included in an input signal,
For example, the one as shown in FIG.

In this DTMF signal detecting device,
Eight frequencies 697, 770, 852 that make up the signal
941, 1209, 1336, 1477, and 1633
The input signal S _IN is divided into eight bands by eight band pass filters (BPF) 21 to 28 each having a center frequency with respect to [Hz], and each of the eight level detectors 2 is divided into eight bands.
9 to 36.

Of these, the four level detectors 29 to 32 for the low group band detect the power in each band, select the band having the largest power from the low group band, and select the selected low band. If the power level of the group band is higher than a predetermined level, the frequency of the selected low group band is DTM.
The low group detection signal is output as one of the level detection signals by regarding the F signal as the frequency of the low group band. By the way,
If the power level of the selected low group band is lower than the predetermined level, it is considered that the input signal S _IN is not the DTMF signal. Similarly, four level detectors 33-
At 36, the power in each band is detected, the band having the largest power is selected from the high group band, and if the power level of the selected high group band is higher than a predetermined level, the selected high group is selected. The frequency of the band is regarded as the frequency of the high group band forming the DTMF signal, and the high group detection signal is output as one of the level detection signals. Incidentally, if the power level of the selected high group band is lower than the predetermined level, it is considered that the input signal S _IN is not the DTMF signal.

Therefore, these low group detection signal and high group detection signal are sent to the decoder 40, and the decoder 40
Decoding the low group detection signal and the high group detection signal, and outputs the DTMF decode signal as an output signal S _{OU T.}

[0007]

In the case of the above-mentioned DTMF signal detecting device, since eight BPFs and level detectors are required as constituent elements, the circuit scale becomes large and the processing time of data in the circuit and The amount of memory used for it becomes huge.

That is, since the interval between adjacent frequency elements of the DTMF signal is usually 70 to 150 [Hz], a narrow band steep filter is required to divide the input signal into eight frequency bands. If an IIR type filter is used as the BPF, a filter of 20th order or higher must be used. In such a case, if software processing is attempted by a processor or the like, the processing time (calculation amount) of data and the amount of memory used for it become enormous.

The present invention has been made to solve such a problem, and its technical problem is to provide a DTMF signal detection method capable of reducing the burden of the amount of memory and the amount of calculation in a circuit, and a method therefor. The circuit configuration applied is to provide a small-scale detection device.

[0010]

According to the present invention, a DTM
In a DTMF signal detection device for detecting whether or not an input signal including an F signal is the DTMF signal, a band separation means for separating the input signal into a low group band signal and a high group band signal, and a low group band signal A frequency estimator for estimating a frequency by calculating an average of sample intervals having a maximum value and a minimum value, which are obtained by searching an increase / decrease in the amount of change between samples with respect to the high group band signal, and the estimated frequency Band determining means for determining whether or not is included in the low group band or high group band of DTMF, and DTMF determining means for determining whether or not it is recognized as a DTMF signal from the maximum and minimum values and their codes. A DTMF signal detecting device having

In this DTMF signal detecting apparatus, it is preferable that the frequency estimating means is constructed by combining the band determining means and the DTMF determining means.

Further, according to the present invention, one of the above DTMs
In the F signal detection device, the low group band signal and the high group band signal are calculated from the average power and the maximum value and the minimum value of the low group band signal and the high group band signal with reference to the result of the judgment by the DTMF judging means. The amplitude estimation means for estimating the amplitude of the sine wave, the sine wave generation means for generating a sine wave based on the estimated frequency and the estimated amplitude, and the SN ratio of the sine wave and any one of the low group band signal and the high group band signal. SN ratio calculating means for outputting a calculation signal indicating the calculated result, DTMF recognizing means for recognizing the input signal as a DTMF signal if the SN ratio in the calculation signal exceeds a predetermined threshold value, and DTMF according to the recognition result. A DTMF signal detection device having a DTMF output means for outputting, as an output signal, a frequency corresponding to a signal and a sign of amplitude is obtained.

In this DTMF signal detecting device, the DT
It is preferable that the MF coded output means is configured to have a DTMF recognition means.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The D of the present invention will be described below with reference to Examples.
A TMF signal detection method and a detection device to which the TMF signal detection method is applied will be described in detail with reference to the drawings.

First, the outline of the DTMF signal detection method of the present invention will be briefly described. In this DTMF signal detection method, when detecting whether or not an input signal including a DTMF signal is a DTMF signal, a band separation step of separating the input signal into a low group band signal and a high group band signal, and a low group band A frequency estimation step of estimating a frequency by calculating an average of sample intervals having a maximum value and a minimum value, which are obtained by searching for an increase or a decrease in the amount of change between samples regarding the signal and the high group band signal, Band determination step for determining whether or not the selected frequency is included in the low group band or high group band of DTMF, and a DTM for determining whether or not it is recognized as a DTMF signal from the maximum and minimum values and these codes.
The F determination step is included.

Further, in this DTMF signal detecting method, the low group band signal is further calculated from the average power and the maximum value and the minimum value of the low group band signal and the high group band signal by referring to the result of the judgment in the DTMF judging stage. And an amplitude estimation step of estimating the amplitude of the high group band signal, a sine wave generation step of generating a sine wave based on the estimated frequency and the estimated amplitude,
S of sine wave and either low band signal or high band signal
As a result of calculating the N ratio, if it exceeds a predetermined threshold value, a DTMF certification step of certifying an input signal as a DTMF signal, and a sign of a frequency and an amplitude corresponding to the DTMF signal and an output signal according to the certification result are output. And a DTMF output stage.

FIG. 1 is a block diagram showing the basic structure of a DTMF signal detecting apparatus to which this DTMF signal detecting method is applied.

This DTMF signal detecting device comprises low-pass filters (LPFs) 1 and 2, a low group maximum / minimum value searcher 3, a low group frequency estimator 4, and a low group amplitude estimator 5.
, Low group SN ratio calculator 6 and low group sine wave generator 7
, High-pass filter (HPF) 8 and maximum value for high group
A local minimum value searcher 9, a high group frequency estimator 10, a high group amplitude estimator 11, a high group SN ratio calculator 12, a high group sine wave generator 13, and an SN threshold table 14. , DTM
And an F encoder 15.

In this DTMF signal detector, the input signal S _IN first passes through the LPF 1 to remove the DC component, and then passes through the LPF 2 and the HPF 8 to be divided into a low group band signal and a high group band signal, respectively. . Therefore, the LPF 2 and the HPF 8 together serve as a band separating means for separating the input signal into the low group band signal and the high group band signal. LPF
2 and the HPF8 have a cutoff frequency of the highest frequency band of the low group band (1000 Hz) and the lowest frequency band of the high group band (120 Hz).
It is set to be in the middle of 0 Hz) (for example, 1100 Hz). LPF2 and HPF8 sample the low group band signal and the high group band signal, respectively, at a sampling frequency Fs.
Sampling at [Hz] and encoding, low group band signal,
The high group band signals are encoded and output to the low group maximum / minimum value searcher 3 and the high group maximum / minimum value searcher 9, respectively.

The low group maximum / minimum value searcher 3 is an LPF 2
For the coded low-group band signals from ## EQU3 ## and their sample numbers. Similarly, the high group maximum / minimum value searcher 9 detects the maximum and minimum values and their sample numbers in the encoded high group band signal from the HPF 8. These operations process the input signal for each frame.

In the maximum / minimum value searchers 3 and 9, the number of samples in one frame is N, the sample data is s [n] (where 0 ≦ n ≦ N−1), and the difference between adjacent sample data is set. The increase / decrease is examined by taking x in the relation of x = s [n] -s [n-1]. However, when n = 0, the last sample of the previous frame is input to s [n-1].

Here, when x has a positive value, it is considered that the interval between samples is increasing, and conversely, when it has a negative value, it is considered that the interval between samples is decreasing. Also, when the sign of x changes and the sign of x changes from positive to negative,
That is, the point where the change from increase to decrease is determined as the maximum value, while the point where the sign of x changes from negative to positive, that is, the point where the change from decrease to increase is determined as the minimum value. Further, the maximum value and the minimum value and the sample number giving them are stored respectively.

Since the DC component of the input signal S _IN is cut through the LPF 1, if the input signal S _IN is a sine wave, the positive and negative amplitudes are considered to be symmetrical with respect to the time axis. In such cases, the local maximum lies in the positive region and the local minimum lies in the negative region. If the minimum value is detected in the positive region or the maximum value is detected in the negative region under this condition, it can be estimated that the input is not a sine wave.
The TMF detection process shall be terminated.

Low group frequency estimator 4 and high group frequency estimator 1
At 0, the sample intervals that give adjacent maxima and minima are calculated, and those sample intervals are averaged and doubled, which is the period of the low and high groups in the frame of the input signal S _IN . Further, the sampling intervals of the maximum value and the minimum value are respectively compared with the average value, and when there is a difference exceeding a predetermined value, the frequency of the input signal S _IN is considered to be dispersed and is not regarded as a DTMF signal. .
Incidentally, if the sampling frequency is Fs [HZ],
The sample interval is 1 / Fs [seconds], and the frequency is Fs / a [HZ] when the period is a [sample number].

Therefore, the eight frequencies 697, 770, 852, 941, 1209, 13 forming the DTMF signal are included.
When one of 36, 1477, and 1633 [Hz] is fd, the cycle is Fs / fd [number of samples], so when the calculated cycle a is a = Fs / fd,
The frequency of the input signal S _IN is estimated to be in the band including fd [Hz] and is determined to be the frequency of the DTMF signal.

In the frequency estimators 4 and 10, the DTMF is
The periods corresponding to the frequencies of the low group and the high group of the signal are obtained in advance, the obtained period is compared with the period obtained as described above, and when any one of them is matched, the frequency is DTMF. It is determined that the input signal S _IN is a DTMF signal because it is within the frequency band of the signal. On the contrary, when the low group and the high group do not match any cycle, the frequency estimators 4 and 10 determine that the input signal S _IN is not the DTMF signal. Frequency estimator 4, 10
Is a determination result indicating whether the input signal S _IN is a DTMF signal and a signal indicating the obtained frequencies of the low group and the high group are D
It is sent to the TMF encoder 15.

Therefore, the maximum / minimum value searchers 3 and 9 and the frequency estimators 4 and 10 search for the increase / decrease in the variation amount between the low group band signal and the high group band signal. The frequency estimators 4 and 10 function as frequency estimating means for estimating the frequency by calculating the average of the sample intervals having the obtained maximum value and minimum value, and the frequency estimators 4 and 10 have the estimated frequency in the low group band or high group band of DTMF. It also functions as a band determination means for determining whether or not it is included, and as a DTMF determination means for determining whether or not it is recognized as a DTMF signal from the maximum and minimum values and their codes.

By the way, when the judgment result shows that the input signal S _IN is a DTMF signal, the frequency estimators 4, 1
0 activates the low group amplitude estimator 5 and the high group amplitude estimator 11, respectively.

The low group amplitude signal and the high group amplitude estimator 11 are supplied with the low group band signal and the high group band signal encoded by the LPF 2 and the HPF 8, respectively, and the amplitude estimators 5, 1
When 1 is activated, the maximum value and the minimum value of the sample data of one frame divided into the low group band and the high group band are averaged, and the difference between the maximum value and the minimum value is compared. If this difference is above a certain level, the amplitude estimators 5 and 11 do not regard the waveform as a DTMF signal because the amplitude is unstable, but if this difference is within a certain level, the maximum value is reached. And the value obtained by further averaging the minimum values is set as the amplitude.

On the other hand, if the input signal S _IN is a DTMF signal, the sample data element groups divided into the low group band and the high group band each have a frequency component representing a single sine wave, so that the amplitude estimator 5, Each of 11 calculates the average power of one frame and estimates the amplitude therefrom. The relationship between the average power and the amplitude of the sine wave here is expressed by the following equation (1).

[0031]

[Equation 1]

Each of the amplitude estimators 5 and 11 compares the amplitude obtained by the equation 1 with the amplitude calculated from the maximum value and the minimum value of the sample data element group described above. Also in this case, the input signal S _IN is determined to be the DTMF signal only when the difference between the two is within the predetermined range. Each of the amplitude estimators 5 and 11 sends this determination result to the DTMF encoder 15. Therefore, the amplitude estimators 5 and 11 refer to the result of the determination by the DTMF determining means described above, and determine the low group band signal and the high group from the average power and the maximum value and the minimum value regarding the low group band signal and the high group band signal. It functions as an amplitude estimating means for estimating the amplitude of the band signal.

In any case, when it is determined that the input signal S _IN is not the DTMF signal for the sample data element group of the input frame in any case, each unit immediately interrupts the processing at that time, and Subsequent DTM
The determination regarding the F signal and the estimation of the parameters are not performed. In this way, the amount of processing is reduced by omitting extra processing for the input signal S _IN that is not the DTMF signal.

Up to this point, it has been examined whether or not the frequencies of the low group band and the high group band of the input signal S _IN are in the frequency bands forming the DTMF signal, respectively. It is necessary to check whether or not the frequency is within the frequency allowable range defined by the standard specifications.

Therefore, the low group sine wave generator 7 and the high group sine wave generator 13, the SN threshold table 14, the low group SN ratio calculator 6 and the high group SN ratio calculator 12 are used for actual operation. The sine waves in both bands of the input signal S _IN are compared with the sine waves generated from the sine wave generators 7 and 13 described later.

That is, to the low group sine wave generator 7, the low group frequency estimator 4 and the low group amplitude estimator 5 send out a signal indicating the frequency and amplitude obtained for the low group band of the DTMF signal. It Similarly, the high group sine wave generator 13 includes a DTMF from the high group frequency estimator 10 and the high group amplitude estimator 11.
A signal is emitted showing the determined frequency and amplitude for the high band of the signal. The sine wave generators 7 and 13 generate sine waves as follows from the signals indicating the input frequency and amplitude, respectively. Therefore, the sine wave generators 7, 13
Acts as a sine wave generating means for generating a sine wave based on the estimated frequency and the estimated amplitude.

Specifically, if the frequency corresponding to the estimated frequency band is f and the amplitude is a, the sine wave s1
[N] is expressed by the following equation (2).

[0038]

[Equation 2]

Here, n represents 0 to N, N is the number of samples per frame, Fs is the sampling frequency, φ
Indicates a phase (0 to Fs / f) (one cycle).

The SN ratio calculators 6 and 12 compare the sine wave generated by the sine wave generators 7 and 13 with the input data s [n] of the corresponding band, respectively. The error dl [n] is calculated by the SN ratio calculator 6 for the low group frequency and by the SN ratio calculator 12 for the high group frequency as dl [n] = s [n] -s.
It is calculated by the relation of 1 [n].

Further, the power of the error d1 [n] is calculated,
SN (φ), which is the SN ratio with respect to the phase φ, is calculated by the following equation (3).

[0042]

(Equation 3)

Of the SN (φ) which is the SN ratio for the phase φ found here, the one with the lowest value is the SN of that band.
Put SN1 as a ratio. That is, SN1, which is the SN ratio of the band in this case, is expressed as SN1 = min (SN (φ)).

Next, a method of calculating the SN ratio threshold SN2 stored in the SN ratio threshold table 14 will be described.

When the frequency within the maximum allowable frequency range defined by the standard specifications with respect to the center frequency f of the band is fd, the sine wave s2 [n] with respect to the frequency fd is s2.
It is given as [n] = cos (2πn · fd / FS) + nd.

Here, nd is additional noise generated by multiplying random noise by a constant gain within an allowable range. From the sine wave s2 [n] obtained here and the sine wave s1 [n] obtained by the above equation 2, an error d2 [n] due to frequency deviation and noise is calculated as d2 [n] = s1.
It is obtained according to the relationship [n] -s2 [n]. However, s1 [n] here is a = 1 and φ = 0 in the equation (2).

Next, the power of the error d2 [n] is calculated,
The SN ratio, SN2, is obtained according to the following equation (4).

[0048]

(Equation 4)

S2 [n] = cos (2πn · f)
sinusoidal wave s2 obtained by the relationship of d / FS) + nd
Since [n] has the maximum frequency deviation and additional noise that are allowed as the DTMF signal, the sine wave s2
SN2 as the SN ratio obtained by using [n] is the threshold value of the SN ratio. The SN threshold table 14 sends the SN ratio threshold SN2 stored therein to the SN ratio calculators 6 and 12.

The SN ratio calculators 6 and 12 compare SN1 which is the SN ratio represented by the above-mentioned SN1 = min (SN (φ)) with the SN ratio threshold SN2. When the obtained SN ratio SN1 is higher than the SN ratio threshold SN2, it is considered that the data in that band represents a sine wave having one of the frequencies of the DTMF signal, and therefore, each of the SN ratio calculators 6 and 12 is determined. Determines that the input signal S _IN is a DTMF signal, and if the obtained SN ratio SN1 is lower than the SN ratio threshold SN2, the data in that band represents a sine wave having one of the frequencies of the DTMF signal. The input signal S _IN is not regarded as a DTMF signal.
The SN ratio calculators 6 and 12 send calculation signals representing such determination results to the DTMF encoder 15, respectively. Therefore, the SN ratio calculators 6 and 12 function as SN ratio calculating means for outputting a calculation signal indicating the result of calculating the SN ratio of the sine wave and the low group band signal and the high group band signal.

The parameters (the signals indicating the frequencies of the low group and the high group, the signals indicating the amplitudes of the low group and the high group, and the calculation signal indicating the determination result) obtained as described above are sent to the DTMF encoder 15. If the DTMF encoder 15 indicates that the input signal S _IN is a DTMF signal for all of the sent determination results, the input signal S _IN is the DTM.
Identify as an F signal.

Therefore, in the DTMF encoder 15, the DT
16 DTs corresponding to combinations of frequencies in the low band and the high band are encoded by setting the MF flag.
Select one of the MF frequency codes and select D
The TMF frequency code is output, and the amplitude of each band of the low group and the high group is quantized and output as an output signal S _OUT . Therefore, the DTMF encoder 15 functions as a DTMF certifying means that certifies the input signal S _IN as a DTMF signal if the SN ratio in the calculation signal exceeds a predetermined threshold value, and also responds to the DTMF signal according to the certification result. It also functions as DTMF output means for outputting the frequency and the sign of the amplitude as the output signal S _OUT .

[0053]

As described above, the DTM of the present invention is used.
According to the F signal detection method, the input signal including the DTMF signal is divided into the low group band and the high group band, and the change amount between the samples of the low group band signal and the high group band signal is searched for and found. The frequency is estimated by calculating the average of the sample intervals having the maximum value and the minimum value, and the frequency is D
It is determined whether or not the TMF signal is included in any of the frequency bands forming the low group or the high group, and the frequencies estimated for the low group band signal and the high group band signal with reference to this determination result. And the SN ratio of the sine wave generated based on the amplitude and any of the low group band signal and the high group band signal is calculated, and if the calculation result exceeds a predetermined threshold value, the input signal is a DTMF signal. By considering it, the DTMF detection processing is not performed except when all the conditions of the determination element are satisfied, and thus there is an advantage that the processing amount is reduced as compared with the conventional case. Further, in the DTMF signal detecting device, the burden of the amount of memory and the amount of calculation in the circuit can be reduced, and the circuit configuration can be reduced in size.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of a DTMF signal detection apparatus according to an embodiment to which a DTMF signal detection method of the present invention is applied.

FIG. 2 is a block diagram showing a basic configuration of a conventional DTMF signal detection device.

[Explanation of symbols]

 1, 2 Low-pass filter (LPF) 3 Maximum / minimum value searcher for low group 4 Frequency estimator for low group 5 Amplitude estimator for low group 6 SN ratio calculator for low group 7 Sine wave generator for low group 8 High-pass filter (HPF) 9 Maximum / minimum value searcher for high group 10 Frequency estimator for high group 11 Amplitude estimator for high group 12 SN ratio calculator for high group 13 Sine wave generator for high group 14 SN ratio threshold Table 15 DTMF encoder

Claims (6)

[Claims] 1. An input signal including a DTMF signal is the DTM.
A band separation step of separating the input signal into a low group band signal and a high group band signal when detecting whether or not it is an F signal;
A frequency for estimating a frequency by calculating an average of sample intervals having a maximum value and a minimum value obtained by searching for an increase or a decrease in the amount of change between respective samples regarding the low group band signal and the high group band signal. An estimation step, a band determination step of determining whether or not the estimated frequency is included in a low group band or a high group band of DTMF, and the maximum value and the minimum value and their signs are recognized as the DTMF signal. And a DTMF determination step of determining whether or not the DTMF signal is detected. 2. The DTMF signal detection method according to claim 1, further comprising: referring to the result of the determination in the DTMF determination step, the average power and the maximum value of the low group band signal and the high group band signal, An amplitude estimation step of estimating the amplitudes of the low group band signal and the high group band signal from the minimum value; a sine wave generation step of generating a sine wave based on the estimated frequency and the estimated amplitude; As a result of calculating the SN ratio of the sine wave and any one of the low group band signal and the high group band signal, if the predetermined threshold value is exceeded, the input signal is recognized as the DTMF signal; D which outputs the frequency and the sign of the amplitude corresponding to the DTMF signal as an output signal according to the authorization result
A method for detecting a DTMF signal, comprising: a TMF output step. 3. An input signal including a DTMF signal is the DTM.
In a DTMF signal detecting device for detecting whether or not it is an F signal, band separation means for separating the input signal into a low group band signal and a high group band signal, the low group band signal and the high group band signal Frequency estimation means for estimating the frequency by calculating the average of the sample intervals having the maximum value and the minimum value, which are obtained by searching the increase and decrease of the variation between the respective samples, and the estimated frequency is D
Band determination means for determining whether or not it is included in the low group band or high group band of TMF, and DTMF determination for determining whether or not it is recognized as the DTMF signal from the maximum value and the minimum value and their codes. And a DTMF signal detecting device. 4. The DTMF signal detecting apparatus according to claim 3, wherein the frequency estimating unit is configured to include the band determining unit and the DTMF determining unit. 5. The DTMF signal detecting apparatus according to claim 3, further comprising: referring to the result of the determination made by the DTMF determining means, the average power and the maximum value regarding the low group band signal and the high group band signal. And an amplitude estimating means for estimating the amplitudes of the low group band signal and the high group band signal from the minimum value, and a sine wave generating means for generating a sine wave based on the estimated frequency and the estimated amplitude. SN ratio calculating means for outputting a calculation signal indicating a result of calculating the SN ratio of the sine wave and any one of the low group band signal and the high group band signal, and the SN ratio in the calculated signal has a predetermined threshold value If it exceeds, the input signal is changed to the DT
A DTMF signal detecting device comprising: a DTMF certifying means for recognizing as an MF signal; and a DTMF output means for outputting, as an output signal, a sign of a frequency and an amplitude corresponding to the DTMF signal according to the qualifying result. 6. The DTMF signal detection apparatus according to claim 5, wherein the DTMF encoding output means is the DTMF.
D characterized by being configured with an accreditation means
TMF signal detector.