JPS61225994A - Multi-frequency signal receiver - Google Patents

Abstract

PURPOSE:To identify accurately a multi-frequency signal by providing a filter circuit between a transmission line and a circuit receiving and decoding the multi-frequency signal to reduce the effect of the 3rd-order distortion. CONSTITUTION:A DTMF signal incoming via the transmission line 2 is led at first to a filter circuit 6, where the frequency component of the tertiary distortion is attenuated, and the result is fed to a DTMF signal receiver 4, in which the signal is received and decoded. Although the low frequency (fL=941Hz) component is attenuated by nearly 1.8dB in the filter circuit 6, the tertiary distortion 674Hz is attenuated by 4.9dB, which is larger than the said 1.8dB, resulting in that the distortion factor of the low group frequency fL is improved by 3.1dB.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to a DTMF (Dual To
The present invention relates to an improvement of a multi-frequency signal receiving device for receiving and decoding multi-frequency signals and the like.

[Technical background of the invention]

In recent years, systems have been developed that use DTMF signals generated by operating push-button dial keys on telephones to reserve tickets, remotely control electrical equipment, and the like. FIG. 5 shows a schematic configuration of the system. In the figure, a telephone 1 faces another telephone 3 via a transmission line 2, and this telephone 3 has a DTMF signal receiver 4.
are connected in parallel, and an electronic computer 5 is connected to this DTMF signal receiver 4. Note that, for example, when the telephone 1 is a mobile device, the transmission path 2 includes a radio line 21, a transceiver 22, a mobile switching center 23, and a central office 26, as shown in FIG.
, a central station 27, a centralized station 28I3, and a terminal station 29.

In such a configuration, when the push button dial key of the telephone 1 is operated after the line is connected between the telephone 1 and the telephone 3, the DTMF signal generated by this operation is transmitted to the transmission line 2.
The signal is sent to the other party's DTMF signal receiver 4 via the DTMF signal receiver 4, where it is received and decoded, and then supplied to the computer 5, where it is used, for example, to reserve a seat on a train.

[Problems with background technology]

However, if the wireless system in such a system includes an IDC circuit to limit excessive signal levels, or if a nonlinear circuit is present on the transmission line 2, third-order distortion of the DTMF signal may occur on the transmission line 2. occurs and this third-order distortion is D
This is not preferable because it may have an adverse effect on the signal identification operation of the TMF signal receiver 4. For example, the push button dial key "Masu" has a low group frequency fL of 9411 as shown in the table.
h.

The high group frequency f, is 12091h, and these frequencies give 72ft, -fu-673)b, 2fo -ft
A third-order distortion of -1477- occurs. When the level of these third-order distortion components becomes large, the DTMF signal receiver 4 causes erroneous detection, which sometimes makes it impossible to perform accurate reservations and the like.

[Object of the Invention] The present invention has been made in view of the above-mentioned drawbacks, and an object of the present invention is to provide a multi-frequency signal receiving device that can accurately identify multi-frequency signals by reducing the influence of third-order distortion. do.

[Summary of the invention]

In order to achieve the above object, the present invention provides a filter circuit between a transmission line and a circuit that receives and decodes a multifrequency signal, and this filter circuit eliminates third-order distortion of the multifrequency signal generated on the transmission line. Among them, a predetermined frequency component is attenuated to enlarge the signal level difference with the multi-frequency signal.

[Embodiments of the invention]

FIG. 1 is a schematic diagram of a telephone system equipped with a multi-frequency signal receiving device according to an embodiment of the present invention, and the same parts as in FIG. 5 are given the same reference numerals.

The DTMF signal receiver 4 as a multi-frequency signal receiver is configured as shown in FIG. 2, for example.

That is, the DTMF signal Is is introduced into a high group filter 43 and a low group filter 44 via an input amplifier 41 and a dial tone filter 42.
3.44, each frequency is determined. The judgment outputs of these filters 43 and 44 are led to a digital detection circuit 47 via zero/zero detectors 45 and 46, respectively. This digital detection circuit 47 identifies the frequency of the input DTMF signal by counting the period of the input signal for each high group and low group using a high speed clock, and supplies the identification output to the code converter 48 . The code converter 48 converts the identification output into a code suitable for the computer 5, and the signal converted by the code converter 48 is sent to a tri-state buffer 49 which opens and closes according to signals from the computer 5. is output to the computer 5 via.

Note that 51 is a staying logic circuit that controls the operations of the digital detection circuit 47 and code converter 48;
53 is a bias circuit, and 53 is a clock generation circuit.

Now, as described above, a filter circuit 6 is provided upstream of the TMF signal receiver 4, which together with the DTMF signal receiver 4 constitutes a multi-frequency signal receiving apparatus. The filter circuit 6 is, for example, an active type high-pass filter having a second-order Butterworth characteristic using an operational amplifier 61 as shown in FIG. 3, and has a cutoff wave number fc of approximately 80.
It is set to 0H2. FIG. 4 shows its filtering characteristics.

With such a configuration, the DTMF signal arriving via the transmission path 2 is first introduced into the filter circuit 6, where the frequency component of third-order distortion is attenuated, and then introduced into the DTMF signal receiver 4. The signal is then received and decoded by the receiver 4 as described above. Now, if we focus on the DTMF signal of the push-button dial key "Homare" which is currently in question, the low group frequency fs--941- of this push-button dial key "Masu" is the filter circuit 6.
is attenuated by approximately 1.88, but the third-order distortion (2 ft, -fu=673
．． As a result, the distortion rate of the low group frequency fL is improved by 3.1 dB. That is, if the standard for third-order distortion of the DTMF signal receiver 4 is D, then the transmission line 2
If the third-order distortion generated by the signal is approximately D-3 or less, it is possible to receive the low group frequency of the push button dial key "搏".

Therefore, the problem that this low group frequency fL is erroneously determined as the low group frequency of another push button dial key is greatly reduced. In other words, this corresponds to being able to relax the standards for third-order distortion of the DTMF receiver 4.
This allows the design conditions and circuit configuration of the DTMF signal receiver 4 to be simplified. By inserting the filter circuit 6, the DTM of the push button dial key "3" where the low group frequency is the lowest and the high group frequency is the highest.
The expansion of the received signal level difference between the low group frequency and the high group frequency of the F signal may become a problem, but in this case, the expansion of the level difference of the DTMF signal passing through the filter circuit 6 is 4.
．． This level of expansion satisfies the characteristics of the DTMF signal receiver 4 and poses no particular problem.

Note that the present invention is not limited to the above embodiments. For example, in the above embodiment, the case was explained in which the distortion of the low group frequency of the push button dial key "Masu" was reduced.
If distortion at high group frequencies is a problem, a low-pass filter with a cutoff wave number of approximately fc=1.2K)lx may be used in the filter circuit. Furthermore, if it is necessary to simultaneously reduce each third-order distortion of the low group and the high group, a band-pass filter that passes only the frequency band of 800 tb or more and 1.2 centigrade or less may be used as a filter circuit. Roughly speaking, in the above embodiment, a case has been described in which an active type filter having a second-order Butterworth characteristic is used as the filter circuit, but a first-order filter using a resistor and a capacitor may also be used. In addition, the types of multi-frequency signals, the structure of the multi-frequency signal receiver, the structure of the system to which the multi-frequency signal receiver is applied, etc. can be modified in various ways without departing from the gist of the present invention.

〔Effect of the invention〕

As detailed above, according to the present invention, a filter circuit is provided between a transmission line and a circuit for receiving and decoding a multifrequency signal, and this filter circuit eliminates third-order distortion of the multifrequency signal generated on the transmission line. The present invention provides a multifrequency signal receiving device that can reduce the influence of third-order distortion and accurately identify multifrequency signals by attenuating predetermined frequency components and expanding the signal level difference between the multifrequency signal and the multifrequency signal. can be provided.

[Brief explanation of drawings]

1 to 4 are for explaining a multi-frequency signal receiving device according to an embodiment of the present invention, FIG. 1 is a schematic configuration diagram of a telephone system to which the device is applied, and FIG. 2 is a DTMF
Figure 3 is a circuit block diagram showing the configuration of a signal receiver, Figure 3 is a circuit diagram of a filter circuit, Figure 4 is a diagram showing the filtering characteristics of the same circuit, Figure 5 is a diagram showing a conventional DTMF signal receiving device. FIG. 6 is a schematic diagram showing an example of the configuration of a transmission path. 1.3... Telephone, 2... Transmission line, 21... Radio line, 22... Transmitter/receiver, 23... Mobile switching center,
26... General station, 27... Central station, 28... Central station, 29... Terminal station, 4... DTMF signal receiver, 5
... Computer, 41 ... Input amplifier, 42 ... Dial tone filter, 43 ... High group filter, 44.
...Low group filter, 45°46...Zero cross detector,
47... Digital detection circuit, 48... Code converter, 49... Tri-state buffer 7.51... Staying logic circuit, 52... Bias circuit, 53...
... Clock generation circuit, 6... Filter circuit.

Claims (1)

[Claims] In a multi-frequency signal receiving device that receives and decodes a multi-frequency signal sent from a transmitting side via a transmission line, there is a gap between the transmission line and a circuit that receives and decodes the multi-frequency signal. What is claimed is: 1. A multi-frequency signal receiving device, comprising: a filter circuit that attenuates a predetermined frequency component of third-order distortion of the multi-frequency signal to increase a signal level difference between the multi-frequency signal and the multi-frequency signal.