JPS6222294B2 - - Google Patents

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-based spectrum inversion privacy device is configured as shown in Fig. 1, and the audio signal band-limited to frequencies f ₁ to _{f 2} from the audio signal input terminal 11 is generated by a sine wave oscillator. A signal of oscillation frequency f ₀ from 12 is multiplied by ring converter 13, and the spectrum of the audio signal is shifted by frequency f ₀ . In other words, the spectrum of the input audio signal is
This is shown by curve 14 in Figure A, and is multiplied by the signal 15 of frequency f ₀ so that the spectrum of the output of ring converter 13 becomes a band on both sides of frequency f ₀ as shown in Figure 2B. As can be seen from FIG. 2B, in order for the inverted spectrum 16 to be located in the band f ₁ to f ₂ of the original audio signal, it is necessary to set f ₀ =f ₁ +f ₂ . For example, if f ₁ =0.3 KHz and f ₂ =3 KHz, then f ₀ =3.3 KHz. The output of the ring converter 13 also includes a frequency component 17 higher than f ₀ , and a low-pass filter 18 removes the unnecessary component 17 to obtain a spectrally inverted audio signal 16 at an output terminal 19 .

Conventional technology and problems: The principle for obtaining a spectrum-inverted waveform is as described above, but in an actual analog system, distortion due to cross modulation degrades reproduced audio due to the nonlinear nature of the ring converter 13. It has In addition, the scale of circuits such as circuits and filters for obtaining highly stable sine waves at low frequencies becomes 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. 3 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, 23 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 ₀ ) (k is an integer, T ₀ is the sampling period) is The signal is then subjected to spectrum inversion by the spectrum inversion circuit 23, and is again converted into an analog signal by the digital/analog converter 24. This digital signal processing type spectrum inversion secret speech circuit converts the frequency spectrum of the input audio signal into
When f ₁ to f ₂ , the sampling frequency f _s is f _s = 2
It is known that when (f ₁ +f ₂ ) is selected, a spectrally inverted secret signal can be obtained by inverting the sign of every other digital code string a(kT ₀ ). That is, for digital code string a(kT ₀ ), simply (-1) ^K a
Spectral inversion is achieved by simply creating (kT ₀ ).

By applying the digital signal processing technique to the spectrum-inverted secret signal in this way, it is possible to extract a spectrum-inverted secret signal with better quality than an analog type spectrum-inverted secret signal circuit. However, analog-to-digital converters and digital-to-analog converters used for digital signal processing are inherently expensive, and especially analog-to-digital converters and digital-to-analog converters with sufficient precision to process audio signals are extremely expensive. It is. Furthermore, analog-to-digital converters,
Digital-to-analog converters generally consume large amounts of power, so they are also disadvantageous in terms of electrical energy. In this way, the digital signal processing type spectrum inversion confidential circuit is an analog-to-digital converter,
Since it includes a digital-to-analog converter, the price
It has problems in terms of power consumption and accuracy.

Purpose of the invention: In order to eliminate the drawbacks of these analog processing type spectrum inversion confidential signal circuits and digital signal processing type spectrum inversion confidential signal circuits, the present invention consists of an inverting confidential signal circuit consisting only of an operational amplifier, an analog switch, and a capacitor, and a circuit configuration The aim is to obtain a spectrum-inverted secret signal and a secret playback signal by simplifying the process, reducing the price, 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 is a clock signal that can change the resistance value of the audio signal communication path, and can control passing and blocking of the audio signal. An input stage switch capacitor filter consisting of an analog switch, a capacitor, and an operational amplifier; 26 is a first analog switch for spectrum inversion/non-inversion control; 27 is a second analog switch for spectrum inversion/non-inversion control;
8 is a third spectrum inversion/non-inversion control analog switch, 29 is a fourth spectrum inversion/non-inversion control analog switch, and 30 is an output stage switch capacitor filter having the same configuration as the input stage switch capacitor filter 25. It consists of an analog switch, a capacitor, and an operational amplifier. 31 is an audio signal input terminal, 32 is a secret signal output terminal, 33 is a clock signal input terminal, and 34 is 3
Divide the clock applied to 3 by 2n (n is an integer)
This is a 2n frequency divider circuit. The input stage switch capacitor filter 25 and the output stage switch capacitor filter 30 are constructed using a conventionally known general construction method of switch capacitor filters, for example, a bi-cut filter in which basic switch capacitor filters are connected in cascade as shown in FIG. Consists of cascaded switch capacitor filters. In FIG. 5, 35, 36, 37 are operational amplifiers, 38,
39, 40, 41, 42, 43, 44, 45, 4
6, 47, 48 are analog switches, 49, 5
0, 51, 52, 53, 54, 55, 56, 5
7, 58 are capacitors, 59 is a signal input terminal, 6
0 is a signal output terminal. This switch capacitor filter is conventionally known. This circuit is described, for example, in the Journal of the Institute of Electronics and Communication Engineers (1981).
It is described in Figure 6b of Vol. 64, No. 12, page 1295. As is clear from the description in this document, this filter has the function of cascading a sample-and-hold circuit and a second-order transfer function filter circuit.

The input stage switch capacitor filter 25 in FIG. 4 divides the frequency spectrum band of the input audio signal into f ₁
If the sampling frequency is f _s , it is necessary to select _{f s so} that it satisfies the sampling theorem, that is, f _s ≧ 2f ₂ .
In the case of a switch capacity filter, f _s
≫2f ₂ is usually selected. Further, in this spectrum inversion secret speech circuit, it is assumed that f _s =2n (f ₁ +f ₂ ) (n is an integer) under the condition that f _s ≫2f ₂ . This sampling frequency f _s is supplied from the clock signal input terminal 33. When selected in this way, the signal spectrum arrangement and sampling frequency f at the output stage of the input stage switch capacitor filter 25
The relationship between _s is shown in Figure 6A. The switch capacitor filter shown in FIG. 5 is represented by an equivalent circuit as shown in FIG. 6a of the above-mentioned document, and its transfer function H is expressed by the following equation.

H= _-v2 / _v1 = _α0 + _α1Z ^-1 + _α2Z ^-2 /1
-β ₁ Z ^-1 -β ₂ Z ^-2 The Z transformation parameter Z can be written as Z=exp(j2π/f _s f) for the frequency f and the sampling frequency f _s , so it is a periodic function of the frequency f _s becomes. Therefore, when a signal having spectral components from frequency f ₁ to f ₂ is input to this switched capacitor filter, a signal with a spectrum as if the input signal was amplitude-modulated every f _s is output, and among them,
If harmonic components of 2 f _s or more are removed, the result is as shown in Figure 6A. 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 a signal with a spectrum of . 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 obtained by multiplying each component of the spectrum _shown in FIG. 1) It is a spectrum expressed by all combinations of products of f ₀ , . . . with each sine wave. These spectra overlap one another, but if the period of the stacked square waves is chosen so that f ₀ = f ₁ + f ₂ = f _s /2n, each spectral component is separated as shown in Figure 6C. . here,
In this way, the case of f _s =2nf ₀ is handled. As is clear from Fig. 6C, if the product signal is passed through a low-pass switch filter with a cutoff frequency _f0 ,
The spectrum inversion waveform shown in Figure D can be extracted. In other words, as mentioned earlier, the switch capacitance filter used here operates with sampling of the frequency _fs , so its output includes the basic passband below _f0 as shown in Figure 8. is accompanied by a spectrum that is an integer multiple of f _s . F indicates a pass range. Therefore, if harmonic components of 2 f _s or more are ignored in the output of this switched capacitor filter, the spectrum waveform shown in FIG. 6D is obtained. By the way, the product of the audio signal and the rectangular wave shown in Figure 7 is the audio signal 1/
It is replaced by repeating amplitude inversion and non-inversion with a period of (2f ₀ ) seconds. This is clear from the properties of the rectangular wave in FIG. As described above, for the pass frequency band f ₁ to f ₂ in which the sampling frequency f _s of the switch capacitor filter should be limited, f _s = 2n.
(f ₁ + f ₂ ), (n≫1), and 1/
The amplitude is repeatedly inverted and non-inverted every 2f ₀ = 1/2 (f ₁ + f ₂ ) seconds, and the frequency of the inverted/non-inverted signal is limited by a low-pass switch capacitor filter with a cutoff frequency of f ₀ . In this case, a spectrally inverted waveform can be obtained.

The spectrum inversion operation every 1/(2f ₀ ) second explained in detail above is carried out by the spectrum inversion/non-inversion control analog switches 26, 27, 28, 29 shown in FIG.
It will be held in Its operation is as follows.
First, from time 2n/(2f ₀ ) seconds (n is a positive integer) to (2n
+1)/(2f ₀ ) seconds, the first analog switch 26 and the fourth analog switch 29 are closed, and the second analog switch 27 and the third analog switch 28 are opened, in this case the input stage switch The audio signal from the capacitor filter 25 is sent to the output stage switch capacitor filter 30 at the subsequent stage without having its amplitude inverted. Then (2n+1)/
Between (2f ₀ ) and (2n+2)/(2f ₀ ) seconds, the second
The analog switch 27 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 audio signal from the input stage switch capacitor filter 25 is amplitude inverted and sent to the output stage switch capacitor filter 30. In this way, only four analog switches 26, 27, 28,
Spectral inversion can be achieved by controlling 29. Analog switch 26, 27, 28, 2
The clock for opening and closing 9 is a 2n frequency dividing circuit 34.
Supplied by The output stage switch capacitor filter 30 in FIG. 4 is a low-pass filter for removing unnecessary waves in FIG. 6C, and the cutoff frequency is
f ₀ . As already explained, the spectral arrangement of the signal passed through the low-pass switched capacitor filter of frequency f ₀ is as shown in FIG. 6D.

In order to obtain the final spectrum-inverted secret signal from the signal having the spectrum shown in _FIG . Therefore, strictly speaking, it is necessary to incorporate an analog low-pass filter into the output stage switch capacitor filter 30. However, since the sampling frequency f _s is usually high enough, the frequency
Signal components of f ₂ or higher are sufficiently attenuated and do not appear at the secret signal output terminal 32. An analog low pass filter is therefore not required. Also, if the sampling frequency f _s is not high enough, an analog low-pass filter is required;
Since the sampling frequency of the switched capacitor filter is selected so that 2f ₀ >>f _s holds, a first-order or second-order gentle filter is sufficient as the analog switch. It is easy to incorporate such a filter into the output stage of a switch capacitor filter and integrate it with the switch capacitor filter.

In addition, the spectrum-inverted secret signal is again
It is clear that the original audio signal can be obtained by applying it to the spectrum inverted secret speech circuit described above.

One of the features of the embodiments of the present invention is an input stage switch capacitor filter, an output stage switch capacitor filter, and first to fourth filters located between the input stage switch capacitor filter and the output stage switch capacitor filter. The aim is to simplify the circuit configuration, reduce price, and reduce power consumption by integrating all four analog switches and the 2n frequency divider circuit.

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, it is possible to realize a secret speech circuit with a simpler circuit configuration than an analog system. Since the circuit consisting of an operational amplifier, analog switch, and capacitor is extremely easy to integrate, it can be manufactured as a single chip IC including the low-pass filter required for the input/output stage. In addition, it does not require analog-to-digital converters and digital-to-analog converters unlike digital signal processing type spectrum inversion confidential circuits, and the circuit consisting of operational amplifiers, analog switches, and capacitors has low power consumption. It can be manufactured as a CMOS IC.
Therefore, it is more effective than the digital processing method in terms of power consumption and cost.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a conventional analog type spectrum inversion signal processing circuit, Fig. 2 A and B are spectrum diagrams for explaining its operation, and Fig. 3 is a spectrum inversion signal processing circuit of a conventional digital signal processing method. 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 switch capacitor filter; FIGS. 6A, B, and C. , D is a spectrum diagram for explaining the operation of the present invention,
FIG. 7 is a rectangular waveform diagram, and FIG. 8 is a diagram showing the output spectrum of the switched capacitor filter. 11: Audio signal input terminal, 12: Sine wave oscillator, 13: Ring modulator, 14: Spectrum of input audio signal, 15: Product sine wave frequency, 16: Spectrum of spectrally inverted audio signal, 1
7: Spectrum of unnecessary waves generated when spectrum is inverted, 18: Low-pass filter, 19: Spectrum-inverted audio signal output terminal, 21: Analog-to-digital converter, 22: Clock generator, 23: Spectrum inversion circuit, 24: Digital・Analog converter, 25: Input stage switch capacitor filter, 26, 27, 28, 29: Spectrum inversion・
Analog switch for non-inverting control, 30: Output stage switch capacitor filter, 31: Audio signal input terminal pair, 32: Audio signal output terminal pair, 33: Clock signal input terminal, 34: 2n frequency dividing circuit, 3
5, 36, 37: operational amplifier, 38, 39, 4
0,41,42,43,44,45,46,4
7, 48: Analog switch, 49, 50, 5
1, 52, 53, 54, 55, 56, 57, 5
8: Capacitor, 59: Signal input terminal pair, 60:
Signal output terminal pair.

Claims (1)

[Claims] 1 An input signal having spectral components from frequency f ₁ to f ₂ is sampled with a clock having a frequency of 2n (f ₁ + f ₂ ), (n is an integer) and is output between the first output terminal and the second output terminal. a first switched capacitor filter that outputs an output to the first switched capacitor filter, which has the same configuration as the first switched capacitor filter, is driven by the same clock, and inputs a signal between the first input terminal and the second input terminal. A second switch capacitor filter and one terminal of a first analog switch and a second analog switch are connected to the first output terminal of the first switch capacitor filter, and one terminal of the first switch capacitor filter is connected to the first output terminal of the first switch capacitor filter. Connect one terminal of a third analog switch and a fourth analog switch to the second output terminal of the filter, and connect the other terminal of the first and third analog switches to the second output terminal of the capacitor filter. Connect the other terminals of the second and fourth analog switches to the other input terminal of the second switch capacitor filter to one input terminal, and connect the first and fourth analog switches to the second and fourth analog switches. The second analog switch is paired with a third analog switch and alternately conducts every 1/2 (f ₁ + f ₂ ) using a clock obtained by dividing the sampling clock by 2n, thereby inverting the spectrum of the input signal. The secret circuit to do.