JPH10112701A - Privacy telephone set - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a privacy telephone set that inexpensively realizes a frequency inverted privacy telephone system in digitally performing a frequency privacy telephone system that is one of analog privacy telephone systems. SOLUTION: Taking notice that a PCM Codes which is used in PCM communication is provided with an encoding circuit and a decoding circuit, an exclusive logic circuit 31 is provided between the digital output and input terminals 12 and 13 of a PCM Codec 2. A voice signal is converted into a digital signal of, e.g. eight bits in the Codec 2 and entered to one of input terminals of the circuit 31. In such cases, when a timing signal of logic '1' is inputted to the other input terminal from a timing generating circuit 32 only when a polarity bit of every other digital signal is fetched to the circuit 31, only the polarity bit is inverted. A voice signal to which privacy telephone processing is applied is outputted by returning a digital signal that is acquired in this way to the Codec 2 and converting it into an analog signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD OF THE INVENTION

[0002]

2. Description of the Related Art When exchanging confidential information using an analog telephone, there is a danger of eavesdropping, so that the audio information may be scrambled to confidentialize the audio information. A frequency reversal confidential system is known as one of the confidential systems used in such a case, and is realized using, for example, an analog technique as follows.

That is, by applying amplitude modulation to an original audio signal to form sidebands on the left and right sides of the carrier signal, an inverted spectrum of the original audio signal can be obtained in the audio band. FIG. 4 is a diagram conceptually showing this state. By passing the inverted spectrum thus obtained through a band-pass filter, an audio signal whose spectrum is inverted can be obtained.

[0004] Also, a method of digitally realizing the frequency reversal confidential system has been studied, and this method will be described. That 0 to F n band of the audio _signal, the sampling frequency 2f _n, the frequency of the carrier modulates and f _n, the modulation by multiplying a carrier signal to the audio signal to obtain a frequency inversion spectrum as described above Then, the carrier signal is represented by cos2πf _n t, and the time t takes a discrete value that is an integral multiple of the reciprocal of the sampling frequency. Therefore, if _n is an integer, the carrier signal is represented by Expression (1).

[0005] _{cos2πf n t = cos2πf n (n} / 2f n) = cosnπ = (- 1) n ... (1) inverted signal I As can be seen from (1) (n) is the carrier to the input speech signal S (n) The signal cos2πf _n (t) = (−
1) Since this is multiplied by ⁿ , it is expressed by equation (2).

I (n) = S (n) (− 1) ⁿ (2) In order to obtain a frequency-inverted spectrum, as shown in FIG. It indicates that it is sufficient to multiply by 1, that is, to reverse the polarity. Conventionally, such processing is performed by a DSP (Digital Si
Gnl Processor).

[0007]

However, in the method using the analog technique, the size of each unit such as a modulation unit and a filter is large, so that the apparatus is large. Also D
The digital processing method using the SP is an expensive device in view of the confidential ability because the DSP is expensive. In order to reduce the size of the secret talk circuit, a dedicated integrated circuit chip may be manufactured.

The present invention has been made under such circumstances, and an object of the present invention is to provide an inexpensive and small confidential apparatus as an apparatus for realizing the frequency inversion confidential system.

[0009]

According to the present invention, in implementing a digital processing method, a PCM codec (Pulse Code Modulation) is provided with both A / D conversion and D / A conversion functions. It is intended to manufacture a low-cost device by using this. That is, the confidential communication device of the present invention is provided with a polarity inversion section for inverting the polarity of every other digital signal between the digital signal output terminal and the digital signal input terminal of the pulse code modulation codec. An analog signal corresponding to voice is input to an analog signal input terminal of a codec, the analog signal is sampled and encoded by a pulse code modulation codec, and the encoded digital signal is output to the digital signal output terminal. , Through the polarity inverting section, to the digital signal input terminal, decode the digital signal, and extract the digital signal from the analog signal output terminal as an analog signal.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on an embodiment shown in the drawings. FIG. 1 is a block diagram showing the configuration of a confidential communication device according to the present invention. In FIG.
A PCM codec usually used in M (Pulse Code Modulation) communication,
The PCM codec 1 has an analog input terminal 11, a sampling circuit 21, a decoding circuit 22, and a digital output terminal 1.
2, a digital input terminal 13, an encoding circuit 23, a low-pass filter 24, and an analog output terminal 14.

A signal inverting section 3 for inverting the polarity of every other digital signal output from the PCM codec 1 is provided between the digital output terminal 12 and the digital input terminal 13. The signal inverting unit 3 includes an exclusive-OR (Exclusive-OR) circuit 31 and a timing generation circuit 32.
One input terminal receives the digital signal from the digital output terminal 12, and the other input terminal receives the timing generation circuit 3
Output signals (timing signals) are input from the digital input terminal 13 respectively.

The timing generation circuit 32 includes an oscillator 3
3 has a function of generating a timing signal to be applied to the exclusive logic circuit 31 and a timing signal to be applied to the PCM codec 1 on the basis of the pulse from 3; therefore, these two timing signals are synchronized. The timing signal applied to the exclusive logic circuit 31 is a logic “1” at a timing when a bit indicating the polarity of the digital signal output from the digital output terminal 12, that is, a sign bit (usually MSB) is taken into the exclusive logic circuit 31. Is output.

The operation of such an embodiment will be described. First, the analog signal (voice signal) corresponding to the voice is P
At CM codec first sampling circuit 21, for example, the bandwidth of the audio signal is a 0 to F _n, the sampling frequency 2
sampled at f _n is (sampled), then encoded by the encoding circuit 22, for example, 8 serial digital signal bit mu-law or A-law is obtained.
Note that μ-law and A-law refer to the companding characteristics of the non-linear quantization in making the S / N _Q (signal-to-quantization noise ratio) close between the small amplitude signal and the large amplitude signal. Kind. In ordinary PCM communication, the encoding circuit 2
2 is a transmission signal. In this embodiment, the digital signal is transmitted through the polarity inversion circuit 3 to the P signal.
The signal is returned to the digital signal input terminal 13 of the CM codec 1.

Here, the operation of the polarity inversion circuit 3 is shown in FIG.
In other words, for every other digital signal of, for example, 8 bits output from the PCM codec 1, a bit (sign bit) indicating the polarity of the audio signal in the digital signal is an exclusive logic circuit. 31 is input to the timing generation circuit 3
The logic of the timing signal input from 2 to the exclusive logic circuit 31 becomes “1”, and at other timings it becomes “0”. Therefore, the output of the exclusive logic circuit 31 is “1” if the sign bit (bit indicating whether the binary code is a positive value or a negative value, usually the most significant bit) of every other digital signal is “1”. It becomes "0", and if it is "0", it becomes "1".

On the other hand, when a bit signal other than the sign bit of every other digital signal is input to the exclusive logic circuit 31, the logic of the timing signal input from the timing generation circuit 32 to the exclusive logic circuit 31 is " 0 "
Therefore, if the logic of the digital signal input to the exclusive logic circuit 31 is “0”, it becomes “0”, if it is “1”, it becomes “1”. The logic of the sign bit does not change, and the logic of the bit signal corresponding to the data does not change. That is, the polarity inversion circuit 3 inverts the polarity of every other digital signal.

The digital signal output from the exclusive logic circuit 31 is taken into the decoding circuit 23 from the digital signal input terminal 13 of the PCM codec 1, where the digital signal is decoded into an analog signal. This analog signal is passed through the low-pass filter 24 and output from the analog signal output terminal 14 as a scrambled (secretly processed) audio signal. The operation of each circuit in the PCM codec 1 is based on the timing generation circuit 3
2 is performed based on the timing output from

By combining the PCM codec 1 and the polarity inverting circuit 3 in this manner, an audio signal in which the spectrum of the original audio signal has been inverted can be obtained, and the frequency inversion privacy method can be digitally realized.
Then, instead of using a DSP, a PCM codec is used and a simple polarity inversion circuit may be added. Therefore, since it can be configured using several inexpensive IC chips, it is possible to achieve a significant cost reduction as compared with the case where a DSP is used.

FIG. 3 shows a communication apparatus to which the embodiment of the present invention is applied. In this communication device, two PCM codecs 4A and 4B and two exclusive logic circuits 5A and 5B are used, and a single timing generation circuit 32 performs a process of applying a confidential speech on the transmission side and a process of applying a vocalized speech signal. It implements both the undoing process. That is, on the transmitting side, the speech signal is passed through the PCM codec 4A, the exclusive logic circuit 5A, and the PCM codec 4A in the order described above, and the speech signal is subjected to confidential processing. Codec 4B → Exclusive logic circuit 5B
→ The voice signal subjected to the confidential processing is returned to the original by passing through the PCM codec 4B in this order. The processing performed on the receiving side is the same as that on the transmitting side, and the PCM codec 4B
If the polarity of the digital signal output from the other side is inverted every other one in accordance with the digital signal whose polarity has been inverted on the transmission side, the digital signal returns to the original digital signal.

According to the configuration shown in FIG. 3, it is possible to realize both the confidential processing and the processing for restoring the audio signal that has undergone the confidential processing, and the cost can be reduced.

[0020]

According to the present invention, when implementing the frequency reversal confidential system, it is realized by digital processing, so that the size can be reduced. Since the digital processing is performed by using the PCM codec, it is simple. The circuit can be produced at low cost.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing an embodiment of the present invention.

FIG. 2 is a time chart showing how the polarity of a digital signal is inverted in the embodiment of the present invention.

FIG. 3 is a block circuit diagram illustrating a main part of an example of a communication device to which the present invention is applied.

FIG. 4 is an explanatory diagram showing a method for realizing the frequency inversion confidential system using analog technology.

FIG. 5 is an explanatory diagram showing a method for realizing the frequency inversion confidential system using digital technology.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 PCM codec 11 Analog signal input terminal 12 Digital signal output terminal 13 Digital signal input terminal 14 Analog signal output terminal 21 Sampling circuit 22 Encoding circuit 23 Decoding circuit 24 Low-pass filter 3 Polarity inversion circuit 31 Exclusive logic circuit 32 Timing generation circuit 33 Oscillator 4A, 4B PCM codec 5A, 5B Exclusive logic circuit

Claims (2)

[Claims] 1. A pulse code modulation code, and a polarity inversion provided between a digital signal output terminal and a digital signal input terminal of the pulse code modulation code for inverting the polarity of every other digital signal. An analog signal corresponding to voice is input to an analog signal input terminal of the pulse code modulation codec, and the analog signal is sampled and encoded by the pulse code modulation codec, and encoded. A digital signal input from the digital signal output terminal to the digital signal input terminal through the polarity inverting section, and the digital signal is decoded and taken out from the analog signal output terminal as an analog signal. . 2. The logic circuit according to claim 1, wherein when the logic of the input signal is different, one of the logic signal “1” and the logic signal “1” of the logic “1”
A logic circuit for outputting (“0”) and a timing generation circuit, wherein one of the input terminals of the logic circuit has the pulse code modulation code.
A digital signal from the Dick and an output signal of the timing generation circuit are input to the other input terminal, respectively, and the timing generation circuit changes the polarity of the digital signal input to one input terminal of the logic circuit. The one logic signal "1"("0") is output when the indicated bit is input, and the other logic signal "0"("1") is output when the other bit is input. 1
Secret device as described.