JPH0446025B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method of transmitting a confidential audio signal.

In general, in wireless communication, the recipient cannot be restricted in principle, so for communications that require secrecy or communications established under a specific contract between the sender and receiver, confidential communication methods using encryption are used. It is valid.

The confidential communication method can be roughly divided into two types: a method in which scrambling is performed on the frequency axis, and a method in which scrambling is performed on the time axis.

The former method includes (1) inverting the frequency, and (2) dividing the audio signal into multiple frequency slots on the frequency axis and replacing these slots according to predetermined rules. , a method of inverting the frequency within one slot, a method of (3), a method of changing the switching rule of the above (2) over time, etc.

Also, as belonging to the latter method, (4), a method of changing the polarity of the sample value of an audio signal according to a certain rule, and (5), a method of dividing the audio signal into frames temporally and changing the sample value within one frame. Method (6): Divide the audio signal into frames as in (5) above, but do not change the order of sample values within one frame, but change the order of frames within several frames. There are methods etc.

By the way, all of the conventional methods described above have various drawbacks as described below. In other words, in method (1), secrecy is very low in that it can be decoded if the receiver has the same demodulator, and in method (2), the number of slots is not too large in terms of circuit scale. Since it is not possible, confidentiality is low. In method (3), as in method (2) above, the number of bandpass filters for dividing into multiple frequency slots is included as many as the number of slots, but the narrower the bandwidth of the slot, the lower the time response. You will end up subtracting . Therefore,
If the slot replacement order is changed, a large amount of noise will be introduced near the end of the frame because the effect of the order replacement in the previous frame remains in the descrambling process. In addition, in methods (4) and (5), the scrambled signal becomes close to white noise and has a wider band than the original audio signal, so if it passes through a band-limited transmission path, distortion will occur in the descrambled audio. There is a drawback. Furthermore, in the method (6), the sample value within one frame does not change, so the bandwidth is less widened, and the distortion during descrambling is smaller than in the method (5) above. If you try to improve the confidentiality of the conversation, it is necessary to increase the number of frames to be replaced, which increases the delay during communication.

This invention has been made in view of the above points, and has a method of frequency shifting the audio, which has higher secrecy than the conventional method described above.Also, since the audio is not divided into slots, there is no connecting part, so the audio To provide a voice confidential signal transmission method that does not impair quality and further improves voice privacy.

Hereinafter, one embodiment of the present invention will be described in detail based on FIGS. 1 and 2.

FIG. 1 shows an example of an encoder in which the present invention is used. In the figure, 1 is an input terminal to which an audio signal is supplied; Two output signals having a predetermined phase difference, for example, a 0° output signal and a -90° output signal, are taken out from the output side of the phase shifting circuit network 2, which is a phase circuit network. 3 and 4 are balanced mixers, and one input terminal is supplied with the above-mentioned 0° output signal and -90° output signal, and the other input terminal is supplied with a frequency of ₀ and a phase of Two SSB carrier signals different from each other (0° and −90°) are supplied.
These ₀ (0 ⁰ ) and ₀ (-90°) SSB carrier signals are obtained by dividing the clock frequency _c from the clock generator 5 to 1/N0 by the frequency divider ₆ to obtain a frequency of _40. After that, the frequency is further divided into 1/4 by frequency divider 7. Therefore, the frequency divider 6 has a frequency division ratio 1/N ₀ set in advance to a predetermined value so that a frequency of ₄₀ is obtained on its output side.

8 is an adder that adds the output signals of the mixers 3 and 4, and 9 is a band waver to which the output signal of the adder 8 is supplied. signal components are removed, and only the LSB wave signal components are extracted at the output side. This LSB wave signal is supplied to one input end of a balanced mixer 10, and a frequency shifting carrier signal having a frequency of ₁ or ₂ , which will be described later, is supplied to the other input end of this mixer 10.

The carrier signals ₁ and ₂ , which are FSK waves, are created as follows. That is, the clock frequency _c from the clock generator 5 is divided to 1/N ₁ by the frequency divider 11 to obtain a signal with a frequency of 2 ₁ on the output side, and the frequency is divided to 1/N ₂ by the frequency divider 12. After dividing the frequency to obtain a signal with a frequency of 2 ₂ on the output side, these signals are switched by a switching circuit 13 to obtain an FSK wave.
This FSK wave is further supplied to a frequency divider 14, where the frequency is divided into 1/2. Therefore, carrier signals ₁ and ₂ , which are FSK waves, are obtained on the output side of the frequency divider 14.
This signal is applied to the other input of mixer 10.

The switching circuit 13 is switched by a switching control signal K from a code generator 15. For example, K is 0
When K is 1, the switching circuit 13 is connected to the contact a side and outputs a 2 ₁ signal to its output side, while when K is 1, the switching circuit 13 is switched to the contact b side,
It is designed to output ₂₂ signals to its output side. On the input side of the code generator 15, a frequency divider 16 divides the clock frequency _c of the clock generator 5 into 1/1.
A clock signal of _K divided by N _K is supplied.
On the other hand, the code key 17 is configured to set the switching control signal K to the code generator 15 in advance, so that when the clock signal from the frequency divider 16 is input, the code set by the code key 17 is set. Accordingly, a switching control signal K is supplied to the switching circuit 13,
A switching operation as described above is performed.

By mixing the carrier signals ₁ and ₂ , which are the FSK waves obtained in this way, with the output signal from the bandpass filter 9, that is, the LSB wave, in the mixer 10,
Substantially, the LSB wave is modulated by carrier signals ₁ and ₂ , and by passing the output of this mixer 10 through a low frequency filter 18, an audio scramble signal _S0 is extracted at its output side. This audio scramble signal is supplied to one input terminal of an adder 19, and the other input terminal of this adder 19 is supplied with a frequency _P
A synchronizing carrier signal is supplied. This synchronization carrier signal is created as follows. That is, the clock frequency _C of the clock generator 5 is divided by 1/N _P by the frequency divider 20 to generate a synchronizing carrier signal of _P. This _P synchronization carrier signal is supplied to one input terminal of the AND circuit 21, and the synchronization signal generator 22 generates a synchronization signal S in synchronization with the clock 5 to the clock generator. S is supplied to the other input terminal of the AND circuit 21, and the synchronization signal S is essentially used to switch _the synchronization carrier signal of _P.
The synchronizing carrier signal is taken out and supplied to the other input terminal of the adder 19. In addition, the synchronization signal generator 2
The synchronization signal S from 2 is also supplied to the code generator 15 for reset purposes. Therefore, output terminal 2
3, a scrambled audio signal S' ₀ to which _{the P} synchronization carrier signal is added is output.
This scrambled audio signal is transmitted using any transmission method. Furthermore, the frequency divider 11,1
2, 16 and 20 frequency division ratios 1/N ₁ , 1/N ₂ ,
1/N _K and 1/N _P are set in advance so that predetermined frequencies are respectively extracted from the clock frequency _C of the clock generator 5.

Next, the operation of this embodiment will be explained. Now, when the audio signal E _A(t) = Acos at is input to the input terminal 1, this signal is supplied to the phase shift circuit network 2, and the phase shift circuit network 2
Signals expressed by the following equations are taken out from the output side of each.

E _AI(t) = Acos at...(1) E _AQ(t) = Asin at...(2) That is, equation (1) is the output signal with phase 0°, and equation (2) is the output signal with phase -90°. is the output signal of

On the other hand, the clock frequency _C is divided into 1/ _4N0 and supplied to mixers 3 and 4, respectively, ₀ (0°) and ₀ (-
90°) SSB carrier signal is expressed by the following formula.

E _CI(t) = cosω ₀ t ...(3) E _CQ(t) = sinω ₀ t ...(4) Therefore, mixer 3 receives the signals expressed by equations (1) and (3) above. On the other hand, the mixer 4 is supplied with the signals expressed by the above equations (2) and (4), respectively. Then, a signal expressed by the following equation is taken out at the output side of the mixers 3 and 4.

E _AI(t)・E _CI(t) =A/2{cos(ω ₀ +a)t+
cos(ω ₀ −a)t}……(5) E _AQ(t)・E _CQ(t) =−A/2{cos(ω ₀ +a)t
-cos(ω ₀ -a)t}...(6) The signals expressed by equations (5) and (6) are respectively supplied to the adder 8 and added, and then passed through the bandpass converter 9, resulting in the output The LSB on the side is expressed by the following formula.
You get waves.

[LSB]=Acos(ω ₀ −a)t (7) This LSB wave is supplied to one input end of the mixer 10.

On the other hand, the clock frequency from the clock generator 5
_C is divided into 1/N ₁ and 1/N ₂ by frequency dividers 11 and 12, respectively.
The signals of frequencies 2 ₁ and 2 ₂ obtained by frequency division are switched by a switching circuit 13 to obtain an FSK wave. The switching of the switching circuit 13 is based on the clock frequency _C from the clock generator 5 divided by 1/ _NK by the frequency divider 16, and the code signal K generated by the code generator 15 in accordance with the key code. Switching circuit 1
3 and switch. For example, when K=0, 2 ₁ signals are selected, and when K=1, 2 ₂ signals are selected.

Then, the frequency of the selected 2 ₁ and 2 ₂ signals is divided by 1/2 by a frequency divider 14 and supplied to one input terminal of the mixer 10 as a carrier signal for frequency shifting. If the carrier signal supplied to one input end of the mixer 10 at this time is E _CS(t) , then this
E _CS(t) is expressed by the following formula.

E _CS(t) = (1-K) ccsω ₁ t + Kcosω ₂ t (8) However, in the above equation (8), the relationships among ω ₀ , ω ₁ , and ω ₂ are selected as follows.

Δω _S =ω ₁ +ω ₂ /2−ω ₀ , Δω _K =ω ₂ −ω ₁ /2, Δ
ω _S > Δω _K Therefore, the signal from the bandpass filter 9, i.e.
A signal expressed by the following equation is obtained at the output side of the mixer 10 to which the LSB wave and the frequency shift carrier signal from the frequency divider 14, that is, the FSK wave are supplied.

[LSB]・E _CS(t) = A/2 [(1-K) {ccs(ω ₀ +ω ₁
− a)t +cos(ω ₁ −ω ₀ +a)t}+K{cos(ω ₀ +
ω ₂ -a)t+cos(ω ₂ -ω ₀ +a)t}] ...(9) When this obtained signal is passed through the next stage low-pass filter 18, the output side is as shown in the following equation. A sound scrambled signal S ₀ is obtained.

S ₀ = A/2 {(1-K) cos(Δω _S −Δω _K +a)t+K
cos(Δω _S +Δω _K +a)t}...(10) Next, the clock frequency _C from the clock generator 5
is divided into 1/N _P by the frequency divider 20, and the synchronizing carrier signal _P obtained is switched and taken out by the synchronizing signal S from the synchronizing signal generator 22 in the AND circuit 21, and then sent to the other side of the adder 19. , and is added to the audio scramble signal S ₀ supplied from the low frequency converter 18 to one input terminal of the adder 19 . Therefore, at the output side of the adder 19, that is, at the output terminal 23, a scrambled audio signal _S'0 as expressed by the following equation is taken out.

S' ₀ = A/2 {(1-K)cos(Δω _S −Δω _K +a)t
+ Kcos (Δω _S +Δω _P +a)t}+S・cosωs _P t
...(11) Figure 2 shows an example of a decoder used in the present invention. In the figure, 31 is an input terminal to which a scrambled audio signal is supplied, and 32 is an input terminal to which a scrambled audio signal is supplied. 33 is a synchronous separation circuit for separating the synchronous carrier signal added to the synchronous separation circuit 30, one input terminal is supplied with the scrambled audio signal from the synchronous separation circuit 30, and the other input terminal is supplied with the SSB carrier signal of _0. It is a balanced mixer that is supplied with The SSB carrier signal of ₀ is created in the same manner as in the encoder described above. That is, the clock frequency _C from the clock generator 34 is divided by the frequency divider 35 to 1/4N ₀ (substantially corresponds to the product of the division ratios of the frequency dividers 6 and 7) to _0.
Obtain the carrier signal for SSB. The clock frequency _C of the clock generator 34 is set to exactly match the clock frequency _C of the clock generator 5 of the encoder, and the clock frequency accuracy is, for example, about 50 PPM. The output of the mixer 33 is supplied to a bandpass filter 36, and the output side thereof is
Only the USB component is extracted.

Incidentally, this SSB may be created using a phase shift circuit or the like as in the above-mentioned encoder.

The output of the band wave generator 36 is sent to the balanced mixer 3
7, and the other input terminal of this balanced mixer 37 is supplied with an encoder as well.
Frequency shifting carrier signals ₁ and ₂ , which are FSK waves, are supplied.

This frequency shift carrier signal is created in the same manner as the encoder. That is, the clock generator 34
The clock frequency _C from C is divided into 1/N ₁ and 1/N ₂ by frequency dividers 38 and 39, respectively, and the signals 2 ₁ and 2 ₂ are taken out on the output side, respectively, and these 2 ₁ and 2 ₂ are output.
The switching circuit 40 switches the signal _of
Or take out the carrier signal for frequency shift in _{step 2} . The switching circuit 40 is also switched in the same manner as the encoder. That is, the clock frequency _C of the clock generator 34 is divided by 1/N _K by the frequency divider 42 and then supplied to the code generator 43 as a clock signal of _K.
3, a switching control signal K is taken out to the output side of the code generator 43 according to a code preset by the code key 44, and the switching circuit 40 is switched by this switching control signal. For example, when K is 0, the switching circuit 40 is connected to the contact a side, and the frequency divider 3
On the other hand, when K is ₁ , the switching circuit 40 switches to the contact b side and operates to take out the output of the frequency divider 39, that is, the signal ₂₂ . Note that this switching control signal K is synchronized with the transmitting side. That is, _{the P} synchronization carrier signal separated by the synchronization separation circuit 32 is supplied to the comparator 45 and compared with a reference value, and then supplied to the synchronization signal generator 46, which generates the synchronization signal S. is generated. This synchronizing signal S is supplied to frequency dividers 38, 39, and 42, and is also supplied to a code generator 43 as a reset signal. The reset signal supplied to this code generator 43 is the switching control signal K.
This determines the start time of the

The frequency shift carrier signals ₁ and ₂ obtained in this way are sent to the USB from the bandpass converter 36.
It is supplied to the mixer 37 along with the waves. Then, the output of the mixer 37 is supplied to an output terminal 48 through a low frequency amplifier 47, and a descrambled audio signal is taken out to the output terminal 48.

Next, the operation of this data will be explained. Now, the input terminal 31 is supplied with an input signal as expressed by equation (11) above, and this signal is separated into a synchronization carrier signal _P and a scrambled audio signal in the synchronization separation circuit 32. Ru. This separated scrambled audio signal is sent to the mixer 33
is supplied to one input end of the mixer 3, while this mixer 3
At the other input end of 3, there is a
An SSB carrier signal of ₀ is supplied to the frequency divider 35. Therefore, the above (10) is on the output side of the mixer 33.
The output of multiplying the formula by cosω ₀ t, that is, A/2{(1-K) cos(Δω _S −Δω _K +a) t+Kco
s(Δω _S +Δω _K +a)t} ・cosω ₀ t=A/4 [(1-K) {cos(ω ₀ +Δω _S
−
Δω _K +a)t +cos(ω ₀ Δω _S +Δω _K −a)t}+K{cos(ω ₀ +
An output expressed as Δω _S +Δω _K +a)t +cos(ω ₀ −Δω _S −Δω _K −a)t}] (12) is extracted. When this output is passed through the bandpass filter 36, only the USB component expressed by the following equation is extracted at its output side.

[USB] = A/4 {(1-K) cos(ω ₀ +Δω _S −Δω
_K + a) t + K・cos (ω ₀ + Δω _S + Δω _P + a) t} ... (13) The output signal of this band wave generator 36 is sent to the mixer 37
is supplied to one input end of the mixer 37, while the mixer 37
₁ and ₂ frequency shifting carrier signals selectively extracted as described above are supplied to the other input terminal of the . If the output from the frequency divider 41 supplied to the other input terminal of the mixer 37 is E _CS ', then this E _CS ' can be expressed as shown in the following equation.

E _CS ′(t)=(1-K)cosω ₁ t+Kcosω ₂ t ……(14) Therefore, on the output side of the mixer 37, the product of the output of the bandpass filter 36 and the output from the frequency divider 41, that is, the following equation A signal expressed as is extracted.

[USB]・E _CS ′(t)=A/4[(1-K) ^2/2 {c
os(ω ₁ +ω ₀ +Δω _S −Δω _K +a)t+cos(ω ₀ −ω ₁
+Δω _S −Δω _K +a)t}+K ² /2{cos(ω ₂ +ω ₀ +Δω _S
−Δω _K +a)t+cos(ω ₀ −ω ₂ +Δω _S +Δω _K +a)t
]...(15) Then, when the output of the mixer 37 is passed through the wave reducer 47, a descrambled signal, that is, the original audio signal, as expressed by the following equation is taken out at the output terminal 48.

[[USB]・E _CS ′(t)] _LPF = A/8 {((1-K
) ²・cos(ω ₀ −ω ₁ +Δω _S −Δω _K +a)t+K ²・cos(ω ₀ −ω ₂ +Δω _S +Δω _K +a
)t}...(16) Here, ω ₀ −ω ₁ =−(Δω _S −Δω _K ), ω ₀ −ω ₂ =
−(Δω _S +Δω _K ), the above equation (16) is expressed as the following equation.

[[USB]・E _CS ′(t)] _LPF = A/8 {(1-K) ²・
cosat+K ² cosat}=A/4cosat...(17) Here, the band expands by (Δ _S + Δ _K ), but this spread is about 10%, which is sufficient for privacy, and a relatively narrow band privacy system can be realized. .

As described above, according to the present invention, the signal formed by the product of the audio signal and the carrier signal of a predetermined frequency is
A voice secret signal is generated by multiplying the LSB wave or USB wave with a carrier signal whose frequency changes over time, and the LSB wave or
By generating a carrier signal for multiplication with the USB wave and transmitting it by adding it to the audio secret signal, when demodulating this audio secret signal, the audio scramble can be easily solved and the original audio signal can be reproduced. can do. In addition, since the frequency of the carrier signal for product with the LSB wave or USB wave is coded so that it can be changed freely, this code can be changed on the transmitting and receiving sides as necessary to further improve the performance. Voice privacy can be achieved.

Further, in the above-described embodiment, a case has been described in which the synchronization signal is added to the audio signal, but the synchronization signal of the video signal may be used, for example, when combining with a video signal and performing scrambling.

Furthermore, in the above embodiment, the LSB is determined by multiplying the audio signal and the SSB carrier signal of ₀ on the encoder side.
The wave is obtained, and on the decoder side, it is multiplied by the audio scramble signal and the SSB carrier signal, which is almost the same as ₀ above.
The case where the USB wave is obtained has been described, but the encoder side may obtain the USB wave and the decoder side may obtain the LSB wave.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing an embodiment of an encoder used in the present invention, and FIG. 2 is a system diagram showing an embodiment of a decoder that operates in the present invention. 2 is a phase shift network, 3, 4, 10, 33, 37 is a balanced mixer, 5, 34 is a clock generator, 6, 7, 11, 12, 14, 16, 20, 3
5, 38, 39, 41, 42 are frequency dividers, 8, 19
is an adder, 9 and 36 are band wave generators, 13 and 40 are switching circuits, 15 and 43 are code generators, and 17 and 4 are
4 is a code key, 18, 47 is a wave reducer, 2
2 and 46 are synchronization signal generators, 32 is a synchronization separation circuit, and 45 is a comparator.

Claims (1)

[Claims] 1. A lower sideband (LSB) wave or an upper sideband (USB) wave is formed by the product of an audio signal and a carrier signal of a predetermined frequency, and the frequency is changed based on a predetermined frequency change rule. forms a carrier signal whose frequency is changed as appropriate, and the product of the lower sideband (LSB) wave or upper sideband (USB) wave and the carrier signal whose frequency is changed as appropriate based on the predetermined frequency change rule. Thus, a voice confidential message signal is formed, a synchronization signal is added to the voice confidential message signal and transmitted, and the transmitted voice confidential message signal is received, the synchronization signal and the voice confidential signal are separated, and the voice confidential message signal is transmitted. Demodulating the original audio signal by multiplying the confidential signal and a carrier signal whose frequency is appropriately changed according to the frequency change rule, and changing the frequency of the carrier signal for the product with the received confidential signal. A method of transmitting a confidential voice signal, characterized in that the timing is set based on the received synchronization signal.