JPH04347947A - Frequency inverting communication system - Google Patents

Abstract

PURPOSE:To provide a circuit constitution effective against tapping of a center frequency component which is not practically affected by frequency inversion at the time of performing the frequency inversion ciphering processing for the purpose of preventing tapping on a radio communication path between a slave machine and a master machine of a cordless telephone set or the like. CONSTITUTION:In this frequency inversion communication system, a transmission circuit in the slave machine of the cordless telephone set is provided with an amplitude reducing circuit 4 which inverts the frequency of an audio signal to cipher it and reduces the amplitude in the vicinity of the center frequency, and a reception circuit is provided with a circuit 6 which deciphers the signal by frequency inverse conversion and raises the amplitude by an extent corresponding to the amplitude reduction. The signal whose amplitude is sufficiently reduced is transmitted on the radio communication path between the slave machine and the master machine, and not only the effect of frequency inversion ciphering by also more effect against tapping are obtained.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a communication system, and in particular, for example, a frequency inverted frequency communication method for wireless communication between a base unit and a slave unit of a cordless telephone to prevent eavesdropping. Concerning reverse communication method.

0002

2. Description of the Related Art In cordless telephones, wireless communication is carried out between a base unit and a slave unit, but crosstalk or eavesdropping in this wireless communication space has become a problem. As a method to prevent such eavesdropping, for example, if the frequency spectrum of the signal transmitted from the slave unit to the base unit has the characteristics shown in Figure 3 (A), the slave unit temporarily changes the frequency around the center frequency fM/2. The frequency spectrum is inverted and transmitted to the base unit as shown in FIG. 3(B). The same applies when transmitting a signal from the base unit to the slave unit.

0003

Problem to be Solved by the Invention Even with the frequency reversal encryption described above, the signal amplitude (spectrum) does not change in the vicinity of the center frequency. Therefore, if an audio signal near the center frequency is transmitted from the handset to the base unit or vice versa, frequency reversal encryption has no effect on signals in such frequency bands, and eavesdropping can easily occur. There is. Therefore, it is an object of the present invention to solve the above-mentioned problems with a simple circuit configuration that can be easily implemented in a handset of a cordless telephone.

0004

[Means for Solving the Problems] In order to solve the above problems, the frequency components of the transmission signal of the present invention are inverted and transmitted.
In a frequency inversion communication system that inverts and reproduces the frequency components of a received signal, the transmitting side has means for increasing or decreasing the amplitude near the center frequency of the frequency-inverted transmitted signal, and the receiving side has a means for increasing or decreasing the amplitude of the frequency-inverted transmitted signal, and the receiving side has a means for increasing or decreasing the amplitude of the frequency-inverted transmitted signal. It has means for decreasing or increasing the amplitude in the vicinity of the center frequency by an amount corresponding to the above-described increase or decrease.

[0005]

[Operation] On the transmitting side, in addition to inverting the frequency of the transmitted signal and encrypting it, it also changes the amplitude near the center frequency, which does not change with this encryption. The method of changing this amplitude is to increase or decrease the amplitude. On the receiving side, in addition to inverting the frequency of the received signal and decoding it, the inverse operation of the change in the vicinity of the center frequency on the transmitting side is performed. That is,
If the amplitude is increased on the transmitting side, the amplitude is decreased. This amplitude change method is defined in advance between the transmitting device and the receiving device. Lowering the amplitude makes it harder to hear even if eavesdropping occurs, so it is desirable to lower the amplitude on the transmitting side. On the receiving side, the amplitude is increased by an amount corresponding to the amplitude reduction to restore the original amplitude.

[0006]

[Embodiment] As an embodiment of the frequency inversion encrypted communication system of the present invention, FIG. 1 shows a block diagram of a part of a transmitting circuit and a part of a receiving circuit provided in a cordless telephone handset.
Shown below. The base unit is also provided with a transmitting circuit and a receiving circuit similar to that of the slave unit. In the handset of this cordless telephone,
With a local oscillator (LCO) 5 having a carrier oscillation frequency fM in between, the upper circuit becomes part of the transmitting circuit, and the lower circuit becomes part of the receiving circuit.

[0007] The transmitting circuit is a low-pass filter (LPF).
1, a quadrature modulator 2, an LPF 3, and an amplitude reduction circuit 4. Audio signal INA is input to LPF1, and LOC
Frequency components higher than the carrier frequency of 5 are removed. The quadrature modulator 2 receives the carrier oscillation frequency f from the LCO 5.
A frequency signal of M is applied, and frequency conversion is performed to a carrier-suppressed double-side band signal centered on the carrier frequency. This frequency-converted signal is input to the LPF 3, which passes only the lower sideband component and removes the upper sideband component. As a result, as shown in FIG. 3A, a signal having a frequency component (spectrum) that is inverted from the input audio frequency component is obtained. The amplitude lowering circuit 4 lowers the amplitude of the frequency-inverted signal component by a certain width around the center frequency fM/2. The amplitude reduction circuit 4, specifically, is provided with a notch filter that reduces the amplitude by a predetermined amount at a frequency fM/2, and is centered at the frequency fM/2 at the position indicated by symbol A in FIG. 2(A). In its vicinity, the frequency is inverted, reducing the amplitude of this signal.

[0008] The receiving circuit includes an amplitude increase circuit 6, an LPF 7,
It has a quadrature modulator 8 and an LPF 9. L in the receiving circuit
By PF7, quadrature modulator 8, LOC5, LPF9,
Similarly to the above, the frequency inverse transform decoding process shown in FIG. 3(B) is performed. Prior to this frequency inversion, the amplitude increase circuit 6 increases the amplitude of the received signal by the same amount as the amplitude decrease by the amplitude decrease circuit 4. Similar to the transmitting circuit shown in Fig. 1, the transmitting circuit in the main unit (not shown) lowers the amplitude around the frequency fM/2 and transmits to the receiving circuit in the slave unit shown in Fig. 1. By increasing the amplitude in the amplitude increasing circuit 6 of the receiving circuit, a signal restored to its original amplitude is output to the LPF 7. In this way, the transmitting side of the handset lowers the amplitude by a predetermined amount near the center frequency, and transmits the frequency-inverted and amplitude-reduced signal to the receiver circuit of the base unit. Even if there is eavesdropping, it is difficult to hear. The receiving circuit in the base unit (not shown, but equivalent to the receiving circuit in the handset) increases the amplitude by an amount equivalent to the amount of amplitude decrease in the transmitting circuit of the handset, so the overall cordless phone has a decrease in amplitude. No rise occurs. Considering that the audio signal is frequency inverted and encrypted, it is sufficiently effective against eavesdropping.

The center frequency band amplitude reduction circuit 4 is not limited to the above-mentioned notch filter, but a circuit having the same function as a notch filter can be used. Further, the amplitude increasing circuit 6 may have any circuit configuration as long as it increases the amplitude inversely to the center frequency band amplitude decreasing circuit 4. Furthermore, in the embodiment described above, the amplitude is reduced in the transmitting circuit,
Although the case where the receiving circuit increases the amplitude has been described, the reverse can also be done. In this case, on the transmitting side, the amplitude is increased to a level that is sufficiently large to prevent eavesdropping, thereby making it unbearable to listen to.

Although the above embodiment has been described in connection with a cordless telephone, the present invention is applicable not only to cordless telephones but also to a wide range of communication systems that perform encryption by frequency inversion and decryption by inverse conversion. Applicable.

[0011]

[Effect of the invention] As described above, according to the present invention,
A frequency reversal encrypted communication system that is sufficiently effective against eavesdropping and crosstalk can be provided with a simple circuit configuration.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a transmitting circuit and a receiving circuit of a cordless telephone according to an embodiment of the frequency inversion encrypted communication system of the present invention.

FIG. 2 is a signal waveform diagram showing frequency reversal encryption and frequency reversal decoding in the cordless telephone of FIG. 1;

FIG. 3 is a signal waveform diagram showing conventional frequency reversal encryption and frequency reversal decoding.

[Explanation of symbols]

1, 3, 7, 9...Low pass filter 2, 8... Quadrature modulator 4. Amplitude reduction circuit 5...Local oscillator 6. Amplitude increase circuit

Claims (1)

[Claims] Claim 1: In a frequency inversion communication method in which the frequency components of a transmitted signal are inverted before being transmitted, and the frequency components of a received signal are inverted and reproduced, the amplitude near the center frequency of the frequency-inverted transmitted signal is transmitted to the transmitting side. A frequency characterized in that the receiving side has means for increasing or decreasing the amplitude of the received signal by an amount corresponding to the increase or decrease in the amplitude near the center frequency of the received signal. Reverse communication method.