JPS58148541A - Privacy telephone circuit - Google Patents

Abstract

PURPOSE:To simplify circuit constitution, to reduce the power consumption and to improve the degree of privacy of a privacy telephone circuit, by constituting an inverted privacy telephone circuit with an operational amplifier, analog switches and capacitors. CONSTITUTION:A voice signal from a voice signal input terminal 31 is applied to an input stage switched capacitor filter 25, the resistance of signal communication is changed with a clock from a clock signal input terminal 35 and the input of the voice signal is controlled. The frequency spectrum band of the sound signal is limited with the capacitors of the filter 25 and the operational amplifier, the output is applied to the analog switches 26-29 for spectrum inversion and non-inversion control, and the switches 26-29 are controlled with a clock frequency-divided at a 2n-frequency-division circuit 34. The sound signal outputted through the combination of the switches 26-29 is inputted to an output stage switched capacitor filter 30, a privacy telephone signal is outputted from a privacy telephone output terminal 32 with the clock from the terminal 33, the circuit constitution is simplified and the power consumption is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention: The present invention relates to a secret speech circuit that inverts the frequency spectrum of an audio signal and transmits the signal.

Background of the technology: A conventional analog type spectrum inversion secret speech device is configured as shown in Fig. 1.
1 Yoshi frequency 11~f! The band-limited audio signal is multiplied by a signal of oscillation frequency /e from the sine wave oscillator 12 in the ring modulator 13, and the spectrum of the audio signal is shifted by the frequency fo. That is, the spectrum of the input audio signal is shown by the curve 14 in FIG. 2A, which is the frequency/.

The spectrum of the output of the ring modulator 13 is multiplied by the signal 15 of the frequency f, as shown in FIG. 2B.

This is the band on both sides of . In order for the inverted spectrum 16 to be located in the band f1 to fm of the original audio signal as shown in FIG. 2B, it is necessary to set 10=/l+/*. For example, if /1-0.5 KHg, /3 KHz is 10-3.3 ff, the ring modulator color output has f.

It also contains the above frequency component 17, and the low-pass filter 18
, the unnecessary component 17 is removed, and a spectrally inverted audio signal 16 is obtained at the output terminal 19.

Prior Art and Problems The principle for obtaining a two-spectrum inverted waveform is the same as described above, but in actual analog equipment, distortion due to cross modulation degrades reproduced audio due to the nonlinearity of the ring modulator 16. It has shortcomings.

Another drawback was that the scale of the circuits and filters for obtaining highly stable sine waves at low frequencies was rather large, making it unsuitable for miniaturization.

In order to eliminate the drawbacks of these analog type spectrum inversion privacy circuits, a digital signal processing type spectrum inversion privacy circuit using digital signal processing technology is known.

FIG. 5 shows an embodiment of a digital signal processing type spectrum inversion secret speech circuit, in which 11 is an audio signal input terminal, 18 is a low-pass filter, 19 is a spectrally inverted audio signal output terminal, 21 is an analog-to-digital converter, 22 is a clock generator, 26 is a spectrum inversion circuit, and 24 is a digital-to-analog converter. The audio signal applied to the audio signal input terminal 11 is analog-to-digital converted by the analog-to-digital converter 21, and the converted digital code string a(kT,) (k is an integer, TO is the sampling period)
The spectrum inversion circuit 23 performs cisvector inversion on the signal, and the digital-to-analog converter 24 converts it back into an analog signal. In this digital signal processing type spectrum inversion secret speech circuit, when the frequency spectrum of the input audio signal is L-fz, the sampling frequency number ft
It is known that when i is chosen to be fs-2Cf1+fz'), a spectrally inverted secret signal can be obtained by inverting the sign of every other digital code string cL(kTO). That is, for the digital code string α(kr,), simply (-1)''
Spectral inversion is performed simply by creating α(kTO).

By applying digital signal processing technology to the spectrum-inverted secret signal in this manner, it is possible to extract a spectrum-inverted secret signal with better quality than an analog spectrum-inverted secret signal circuit.

However, analog to digital converters and digital to analog converters used for digital signal processing are inherently expensive. It is on the department side. Furthermore, analog-to-digital converters and digital-to-analog converters generally consume large amounts of power, so they are disadvantageous in terms of electrical energy.

As described above, since the digital signal processing type spectrum inversion privacy circuit includes an analog-to-digital converter and a digital-to-analog converter, it has problems in terms of cost, power consumption, and accuracy.

Purpose of the invention: In order to eliminate the drawbacks of these analog processing method spectrum inversion secret signal circuits and digital processing method spectrum inversion secret speech circuits, the present invention combines the inversion secret signal circuits with operational amplifiers,
This system is constructed with only analog switches and capacitors, and attempts to obtain a spectrum-inverted secret signal and a secret playback signal by simplifying the circuit configuration, reducing cost, and reducing power consumption.

The present invention will be explained in detail below with reference to the drawings.

Embodiment of the invention: FIG. 4 shows an embodiment of the present invention, in which reference numeral 25 can change the resistance value of the audio signal communication path by a clock signal, and can control passing and blocking of the audio signal. analog switch and capacitor.

An input stage switched capacitor filter consisting of an operational amplifier, 26 a first spectrum inversion/non-inversion control analog switch, 27 a second spectrum inversion/non-inversion control analog switch, 28 a fifth spectrum inversion/non-inversion control Control analogs, inch, 29 is a fourth spectrum inversion/non-inversion control analog switch, 30 is an output stage switched capacitor filter, which is composed of an analog switch, a capacitor, and an operational amplifier having the same configuration as the input stage switched capacitor filter 25. configured.

31 is an audio signal input terminal, 32 is a confidential signal output terminal, 3
3 is a clock signal input terminal, and 34 is a 2n division circuit that divides the clock applied to 65 by 2n (% is an integer). The input stage switched capacitor filter 25 and the output stage switched capacitor filter 30 are constructed using a conventionally known general construction method of a switched capacitor filter, for example, a piquad cascade connection type switch in which basic switched capacitor filters are cascaded as shown in FIG. It consists of a capacitor filter. In Figure 5, 35.
36.37 is an operational amplifier, 38, 39, 40, 41
．． 42.43°44, 45, 46, 47, 48
is analog switch 7F-149, 50°51.52,
53, 54, 55, 56, 57.58 are capacitors, 5
9 is a signal input terminal, 60 is a signal output terminal, and a second-order transfer function can be realized with one stage of this basic switched capacitor filter.

The input stage switched capacitor filter 25 in FIG. 4 is a filter for limiting the frequency spectrum band of the input audio signal from fl to fl, and when the sampling frequency is 11, it satisfies the sampling theorem, that is, f8≧2f. It is necessary to select fa such that fa:>2fz in the case of a switched capacitor filter. Furthermore, in this spectrum inversion secret circuit, fa)2f
Under the condition of t, further fs-2t% (/1+fx)
(father-in-law is an integer). This sampling frequency f8
is supplied from the clock signal input terminal 33. FIG. 6A shows the relationship between the signal spectrum arrangement at the output stage of the input stage switched capacitor filter 25 and the sampling frequency f8 when selected in this way. There are several ways to apply spectrum inversion to a signal with the spectrum shown in Figure 6A.
A spectrum-inverted secret signal can also be obtained by multiplying by a signal having the spectrum shown in Figure A. The principle is as follows.

The frequency spectrum arrangement of the rectangular wave in FIG. 7 is as shown in FIG. 6B. Therefore, the spectrum of the signal obtained by multiplying the audio signal having the spectrum shown in FIG. 6A by the rectangular wave shown in FIG. 7 is calculated by multiplying each component of the spectrum shown in FIG.
This is a spectrum expressed by all combinations of products of r..., (27L-1)fo,... with each sine wave. Such spectra overlap one another in a step, but if the period of the superimposed rectangular waves is selected so that fo-f1+f2-fII/2n, each spectral component is separated as shown in FIG. 6C. Here, we will deal with the case of fa-2nfo in this way. As is clear from FIG. 6C, if the product signal is cut off and passed through a low-pass switched capacitor filter with the same wave number of 10, the spectrum-inverted waveform shown in FIG. 6 can be extracted. By the way, the product of the audio signal and the rectangular wave shown in FIG. 7 can be replaced by repeating amplitude inversion and non-inversion of the audio signal at a period of 1/(2fo) seconds. This is clear from the properties of the rectangular wave in FIG. As described above, the pass frequency bands f1 to f! in which the sampling frequency/a of the switched capacitor filter should be limited! fs −2n (/1
+, I'2), (n)1) and '/2 for the signal passed through the input stage switched capacitor filter.
fo = '/2 Amplitude inversion every (fl12) seconds
By repeating non-inversion and band-limiting the inverted/non-inverted signal using a low-pass Swiss capacitor filter with a cutoff frequency of 10, a spectrum inverted waveform can be obtained.

As explained above in detail, the spectrum inversion operation every 1/(210) seconds is performed by the spectrum inversion/non-inversion control analog switches 26, 27, 28, and 29 shown in FIG.

Its operation is as follows. First, time 2”;/(2
j'o ) seconds (% is a positive integer) to (2n+1)/(2
10) Close the first analog switch 26 and the fourth analog switch 29 and open the second analog switch 27 and the third analog switch 28 for a period of seconds. In this case, the audio signal from the input stage switched capacitor filter 25 is sent to the subsequent output stage switched capacitor filter 3o without having its amplitude inverted.

Then from (2%+1)/(2fo) seconds to (2'rL+2
)/(210) seconds, the second analog switch 2
7 and the third analog switch 28 are closed, and the first analog switch 26 and the fourth analog switch 29 are opened.

In this case, the input stage switched capacitor filter 25
The output audio signal is amplitude inverted and sent to the output stage switched capacitor filter 30. In this way, only 4
By controlling the analog switches 26, 27, 28, and 29, cis vector inversion can be realized.

A clock for opening and closing the analog switches 26, 27, 28, and 29 is supplied by a 21s frequency divider circuit 54. The output stage switched capacitor filter in FIG. 4 is a low-pass filter for removing unnecessary waves in FIG. 6C, and has a cutoff frequency of 10. As already explained, the spectral arrangement of the signal that has passed through the low-pass switched capacitor filter at frequency 10 is as shown in Figure 6.

In order to obtain the final spectrum-inverted secret signal from a signal having the spectrum shown in Figure 6, it is necessary to completely remove signal components above the frequency. Therefore, strictly speaking, it is necessary to incorporate an analog low-pass filter into the output stage switched capacitor filter 30. However, since the sampling frequency /s is usually sufficiently high, signal components of frequencies f1 and above are sufficiently attenuated due to the zero-order hold effect and do not appear at the secret signal output terminal 32. Therefore, no analog low-pass filter is required. Also, if the sampling frequency /s is not high enough, an analog low-pass filter is required, but the sampling frequency of the switched capacitor filter is selected so that 210<fe holds even in such cases. Therefore, a first-order or second-order loose analog filter is sufficient. It is easy to incorporate such a filter into the output stage of the switched capacitor filter and integrate it with the switched capacitor filter.

It is clear that the original audio signal can be obtained by applying the spectrum-inverted secret speech signal again to the above-mentioned spectrum-inverted secret speech circuit.

One of the features of the embodiments of the present invention is an input stage switched capacitor filter, an output stage switched capacitor filter, and four first to fourth filters located between the input stage switched capacitor filter and the output stage switched capacitor filter. Simplification of multi-circuit configuration by integrating all analog switches and 25 frequency divider circuits 2
The aim is to reduce prices and power consumption.

Effects of the Invention: As explained above, by configuring the spectrum inversion secret speech circuit with an operational amplifier, an analog switch, and a capacitor, the secret speech circuit can be realized with a simpler circuit configuration than the analog system. Since a circuit composed of an operational amplifier, an analog switch, and a capacitor is extremely easy to integrate, it can be manufactured as a single chip IC including the low-pass filter necessary for the input/output stage. In addition, it does not require an analog-to-digital converter or a digital-to-analog converter unlike the digital signal processing method spectrum inversion secret circuit, and it also requires an operational amplifier and an analog switch.

A circuit consisting of a capacitor can be manufactured as a CMO5IC, which is characterized by low power consumption. Therefore, it is more effective than the digital processing method in terms of power consumption and cost.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional analog type spectrum inversion signal processing circuit, Fig. 2 A and E are spectral diagrams for explaining its operation, and Fig. 3 is a conventional digital signal processing type spectrum inversion signal processing circuit. A block diagram showing the circuit; FIG. 4 is a block diagram showing an example of the confidential communication circuit according to the present invention; FIG. 5 is a block diagram showing the configuration of a basic secondary switched capacitor filter; FIGS. 6 A, B, C, D is a spectrum diagram for explaining the present invention in detail, and FIG. 7 is a rectangular waveform diagram. 11: Audio signal input terminal, 12: Sine wave oscillator, 15:
Ring modulator, 14: Spectrum of input audio signal, 15
: Product sine wave frequency, 16: Spectrum of the audio signal whose spectrum has been inverted, 17: Spectrum of unnecessary waves generated when the spectrum is inverted, 18: Low-pass filter, 19 Varnish vector inverted audio signal output terminal, 21: Analog/digital Converter, 22: Clock generator, 26: Spectral inversion circuit, 24: Digital to analog converter, 25
: Input stage switched capacitor filter, 26, 27° 28.29 Analog switch for varnish vector inversion/non-inversion control, 50: Output stage switched capacitor filter,
31: Audio signal input terminal pair, 62: Audio signal output terminal pair, 33: Clock signal input terminal, 34: 2fi5
+Ji111 circuit, 55, 56.57: operational amplifier,
68° 39.40, 41, 42, 43, 44, 45.4
+5, 47, 48: Analog switch, 49, 50,
51, 52, 53, 54, 55, 56, 57, 58:
Capacitor, 59: Signal input terminal pair, 60: Signal output terminal pair Patent applicant: Japan Telegraph and Telephone Public Corporation No.
1 Figure 3 Figure 4 Figure 5 - Total

Claims (1)

[Claims] 'An input stage switched capacitor filter consisting of an analog switch, a capacitor, and an operational amplifier that controls passage and cutoff of the audio signal by varying the resistance value of the audio signal communication path according to a clock signal, and the input stage switched capacitor filter. An output stage switched capacitor filter consisting of an analog switch, a capacitor and an operational amplifier having the same configuration as the above, and first to fourth 4'' analog filters located between the input stage switched capacitor filter and the output stage switched capacitor filter. a switch, the input terminal of the first analog switch and the input terminal of the second G analog switch among the four analog switches are coupled to one terminal of the output terminal pair of the input stage switched capacitor filter. , the input terminal of the fifth analog switch among the four analog switches and the input terminal of the fourth analog switch are coupled to the other terminal of the output terminal pair of the input stage switched capacitor filter; The output terminal of the first analog switch and the output terminal of the fifth analog switch are coupled to one terminal of a pair of input terminals of the output stage switched capacitor filter, and the output terminal of the second analog switch and the output terminal of the fourth analog switch are coupled to one terminal of the input terminal pair of the output stage switched capacitor filter. and the output terminal of the analog switch is coupled to the other terminal of the input terminal pair of the output stage switched capacitor filter,
In order to take in a clock signal supplied from the outside and operate the input stage switched capacitor filter and the output stage switched capacitor filter, a 2n frequency divided clock is generated by a frequency dividing circuit that divides the frequency of the clock signal by 23 (n fi integer). A secret communication circuit characterized in that four analog switches, first to fourth, located between the input stage switched capacitor filter and the output stage switched capacitor filter are operated by a signal.