JPH0535615B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a confidential communication circuit in a cordless telephone or the like.

Background technology and its problems In general telephones, the telephone cord is
It is connected to a rosette attached to a pillar or wall. Therefore, the range within which the phone can be moved is
This is determined by the length of the telephone cord, which is approximately several meters. However, if you lengthen the phone cord to increase the range of movement, the cord can become tangled or bunched up.

Therefore, in the United States and other countries, a telephone system called a cordless telephone is being considered.

As shown in FIG. 1, this is composed of a transceiver 1 called a handset and a transceiver 2 called a base unit, and a telephone line 3 is connected to the base unit 2. The handset 1 is connected by radio waves, and calls from subscribers are received by the handset 1 in the same way as a general telephone.

Therefore, the subscriber can move the handset 1 freely when making a call, and the telephone cord does not become tangled or tangled.

The distance that the handset 1 can be separated from the base unit 2 (service area) is
It is about 300m. Also, the frequency of radio waves used between handset 1 and base unit 2 is
40MHz band with 25 duplex channels.
Permitted by the FCC.

Thus, according to this cordless telephone,
You can use your phone conveniently.

However, when the handset 1 and the base unit 2 are connected by radio waves in this way, a third party can eavesdrop on the contents of the telephone call.

However, there are several preventive measures against eavesdropping on radio communications, one of which is to invert the spectrum of the audio signals being transmitted and received.

In other words, in this method, if the original positive sound signal Sa has a spectrum as shown in Figure 2A, on the transmitting side, the subcarrier signal Ss as shown in Figure 2B is balanced-modulated with the signal Sa. A balanced modulation signal Sb as shown in Fig. 2C is obtained from this signal Sb, a lower sideband signal Si with an inverted spectrum as shown in Fig. 2D is extracted from this signal Sb by a low-pass filter, and this signal Si is passed through a modulation circuit. On the receiving side, a signal Si is obtained from a demodulation circuit, and this signal Si is processed in the same way as on the transmitting side to obtain the original audio signal Sa.

Therefore, according to this method, the audio signal Sa is transmitted and received in the state of the signal Si, and its frequency spectrum is inverted, so even if it is eavesdropped, the contents cannot be understood. In other words, the secret of the communication contents is maintained. can.

This spectral inversion method can also be applied to cordless telephones to enable confidential communication.

However, with this spectrum inversion method, the handset 1 requires a balanced modulation circuit and a set of low-pass filters to convert the transmitted audio signal Sa to the inverted signal Si, and also converts the received inverted signal Si into the original audio signal. One set of a balanced modulation circuit and a low-pass filter is also required for converting into the signal Sa, and a total of two sets of balanced modulation circuits and low-pass filters are required. Since the base unit 2 is exactly the same, four sets of balanced modulation circuits and low-pass filters are required as a whole, resulting in a significant increase in cost.

Purpose of the Invention This invention attempts to solve these problems.

Summary of the Invention For this reason, the present invention provides a balanced modulation circuit and a low-pass filter for converting the audio signal Sa into an inverted signal Si when sending a call, and an inverted signal Si when receiving a call.
It also serves as a balanced modulation circuit and a low-pass filter to reconvert Si into the original audio signal Sa,
The VOX is used to switch between sending and receiving calls.

Embodiment Incidentally, a cordless telephone is considered to have a built-in clock in its handset 1 and base unit 2, respectively. In other words, such a cordless telephone with a built-in clock not only allows you to know the time, such as when making a call at an agreed time, or when a call is scheduled to arrive at an agreed time. This is very convenient as it allows you to imagine making a phone call at the same time.

In this case, the built-in clock is generally a digital clock using a crystal oscillator, and the frequency accuracy of the crystal oscillator is within ±20 to 30 ppm. By the way, the error of 1 second per day is approximately
The accuracy is 12ppm.

In the example below, we will focus on these points,
A subcarrier signal Ss for spectrum inversion is obtained from the crystal oscillation output of a clock built into a cordless telephone.

That is, FIG. 3 shows an example of the handset 1, and FIG. 4 shows an example of the base unit 2.

In the base unit 1, 111 is a transmitter, 113 is a transmitting circuit including from an FM modulation circuit to a high frequency power amplifier, 121 is a receiving circuit (tuner circuit) including from an antenna tuning circuit to an FM demodulating circuit, and 123 is a receiver. , 132 is a double-balanced modulation circuit, and 133 is a fifth-order low-pass filter.

Further, 141 indicates a detection circuit for detecting the presence or absence of the audio signal Sa.The detection circuit 141 is supplied with the audio signal Sa from the transmitter 111 through the amplifier 112.
A detection signal Sc which becomes "1" when the level is above a predetermined value and "0" when it is less than a predetermined value is extracted, and this signal Sc
4 as a control signal, and the switch circuit 13
1,134 is the state in the figure when Sc="0", "1"
When , the connection is reversed to that shown in the figure.

In addition, 150 indicates a clock circuit of a built-in clock, and a crystal oscillator 1 is connected to an inverter (inverting amplifier) 151A.
51B is connected to form a master oscillation circuit 151, and an oscillation signal _S0 serving as a timekeeping reference is formed.
When this signal S ₀ is supplied to the frequency dividing circuit 152,
The frequency is divided into signals indicating minutes, seconds, etc., and this frequency-divided signal is supplied to the LCD 154 through the drive circuit 153 to display the time.

Note that the frequency of the signal _S0 , that is, the frequency of the oscillator 15
The frequency of 1B is generally 32768 (=2 ¹⁵ ) Hz.

Further, in the base unit 2, 213 is a transmitting circuit, 221 is a receiving circuit, 232 is a double balanced modulation circuit, 233 is a low-pass filter, 241 is a level detection circuit, and 250 is a clock circuit. It is similar to the corresponding circuit. However, the detection circuit 241
The detection signal Sc is supplied as a control signal to the switch circuits 231 and 234, and the switch circuits 231 and 234
1,134. Also, 210
is a coupling circuit that connects the base unit 2 and the telephone line 3.

When transmitting a call, there is an audio signal Sa in the handset 1, and Sc="1", so the switch circuits 131 and 134 are connected in a state opposite to that shown in the figure. Therefore, the audio signal Sa from the transmitter 111 is supplied to the low frequency amplifier 112, and the band is set to 0.3 to 3.2 kHz, for example, as shown in FIG. 2A.
This signal Sa is supplied as a modulation input to the balanced modulation circuit 132 through the switch circuit 131, and the oscillation signal _S0 from the oscillation circuit 151 is supplied to the frequency divider circuit 135 so that it is converted to 1/10 as shown in FIG. 2B. frequency, that is, 3.2768kHz, and this frequency-divided signal is sent to the modulation circuit 132 as a subcarrier signal.
Ss, and the second
A balanced modulation signal Sb as shown in Figure C is extracted,
This signal Sb is supplied to a low-pass filter 133, and the lower sideband signal Si of the signal Sb is extracted as shown in FIG. 2D.

Then, this signal Si is supplied to the transmitting circuit 113 through the switch circuit 134 to be converted into an up channel FM signal Su, and this signal Su is transmitted from the antenna 100 to the base unit 2.

Then, in the base unit 2, when transmitting a call, the antenna 200 receives the signal from the handset 1.
The FM signal Su is received, this signal Su is supplied to the receiving circuit 221, and the signal Si (D in FIG. 2) is taken out. This signal Si causes Sc="1" and the switch circuits 231 and 234 switch to the state shown in FIG. is connected in the opposite state.

Therefore, the signal Si from the receiving circuit 221 is supplied as a modulation input to the balanced modulation circuit 232 through the switch circuit 231, and the subcarrier signal Ss is supplied from the frequency dividing circuit 235, so that the signal Si is
A balanced modulation signal Sb as shown in Figure E is obtained, and this signal Sb is supplied to the low-pass filter 233 to extract the lower sideband signal, that is, the original audio signal Sa (Figure 2A). Switch circuit 234 → low frequency amplifier 222 → coupling circuit 21
0 signal line to the telephone line 3.

On the other hand, when a call is not being transmitted, the receiver is in a receiving state.

That is, when not transmitting a call, the handset 1 receives an audio signal from the transmitter 111.
Since Sa cannot be obtained, Sc="0" and the switch circuits 131 and 134 are in the state shown in the figure.
Since the inverted signal Si cannot be obtained from the receiving circuit 221 in the base unit 2, Sc="0",
The switch circuits 231 and 234 are also in the state shown in the figure.

Therefore, in the base unit 2, the audio signal Sa from the other party (A in FIG. 2) is supplied to the modulation circuit 232 through the signal line of the telephone line 3 → coupling circuit 210 → low frequency amplifier 212 → switch circuit 231, and is balanced. The modulated signal Sb (FIG. 2C) is supplied to the filter 233 to become the inverted signal Si (FIG. 2D), and this signal Si is supplied to the transmitting circuit 213 through the switch circuit 234 and downstream. Channel FM signal Sd and this signal
Sd is transmitted from antenna 200 to handset 1.

In the handset 1, the antenna 100 receives the FM signal Sd from the base unit 2.
is received, this signal Sd is supplied to the receiving circuit 121, and the signal Si (FIG. 2D) is taken out.
This signal Si is then supplied to the modulation circuit 132 through the switch circuit 131 to become the signal Sb (E in FIG. 2), and this signal Sb is passed through the low-pass filter 1
The original audio signal Sa supplied to 33 (Figure 2A)
is taken out, and this signal Sa is sent to the low frequency amplifier 12
2 to the handset 123.

Thus, according to the invention, the handset 1
and the base unit 2 are coupled by radio waves, and confidential communication is performed by the spectrum inversion method. In this case, in particular, according to the present invention, the modulation circuits 132, 232 and the filters 133, 233 are used for transmitting. Since it is also used for receiving calls, costs can be reduced.

In addition, the modulation circuits 132, 232 and the filter 1
33, 233 for both sending and receiving calls, the switching is performed by the audio signal from the amplifier 112 in the handset 1 and the base unit 2.
If this is done independently using the audio signal Sa from the amplifier 112 and the audio signal Sa from the amplifier 212, disconnections will occur frequently, but in this invention, the audio signal Sa from the amplifier 112
Since this is performed in conjunction with Sa and the inverted signal Si from the receiving circuit 221, there will be no interruption of communication. In addition, the switching is performed using the audio signal Sa from the amplifier 212 and the inverted signal Sa from the receiving circuit 121.
When using Si, the level of the audio signal Sa sent from the telephone circuit 3 may fluctuate significantly, so the signal Sc may not be formed correctly and the switching may not be performed correctly. In the invention, since the audio signal Sa from the amplifier 112 is used, switching can be performed correctly.

Furthermore, if the frequency of the subcarrier signal Ss for spectrum inversion and forward rotation is shifted by 10Hz or more,
The spectrum of the restored audio signal Sa changes, resulting in unnatural sound quality, but the subcarrier signal
Ss is the oscillation signal of the crystal oscillation circuit 151, 251
Since it is formed from S ₀ , even if the spectrum of the restored audio signal Sa deviates, the deviation will be within ±20 to 30 ppm of the accuracy of the crystal oscillator 151B, that is, within ±0.06 to 0.1 Hz. Yes, this is well within the tolerance of ±10Hz. Therefore, it is possible to make a call with natural sound quality. Moreover, there is no need to adjust for secret stories.

Further, since the crystal oscillation circuits 151 and 251 were originally used for watches, and since they were used, there is no increase in cost, and the present invention is extremely suitable as a practical product for consumer use.

Although the above description deals with a cordless telephone, the present invention can also be applied to a transceiver, a personal radio, a carrier-wave intercom, and the like.

Effects of the invention Confidential conversations can be realized using the spectrum inversion method, and calls can be made with natural sound quality. Furthermore, switching between sending and receiving calls can be performed correctly. Moreover, it is extremely low cost.

[Brief explanation of the drawing]

1 and 2 are diagrams for explaining this invention, and FIGS. 3 and 4 are system diagrams of an example of this invention. 113, 213 are transmitting circuits, 121, 221 are receiving circuits, 132, 232 are balanced modulation circuits, 15
0,250 is a clock circuit.

Claims (1)

[Claims] 1. A balanced modulation circuit, a low-pass filter to which the modulated output of the balanced modulation circuit is supplied and extracts a lower sideband signal from the modulated output, a transmitting circuit, a receiving circuit, and first and second switch circuits; and a detection circuit that detects the presence or absence of an audio signal from the transmitter, and the first and second switch circuits are switched to one contact and the other contact according to the detection output of the detection circuit, and when transmitting a voice, , an audio signal from the transmitter is supplied as a modulation input to the balanced modulation circuit through one contact of the first switch circuit, and an audio signal with an inverted spectrum within the audio band is extracted from the low-pass filter;
This spectrum-inverted audio signal is supplied to the transmitting circuit through one contact of the second switch circuit and transmitted, and when receiving a call, the spectrum-inverted audio signal is demodulated from the receiving circuit within the audio band. The audio signal whose spectrum is inverted is supplied as a modulation input to the balanced modulation circuit through the other contact of the first switch circuit, and the audio signal whose spectrum is normally inverted is taken out from the low-pass filter. and this audio signal is supplied to the receiver through the other contact of the second switch circuit.