| 摘要 |
PURPOSE: To prevent wire tapping by a third party by a simple circuit by providing a first FM modulation part and a second FM modulation part on a transmission system and providing a first FM demodulation part and a second FM demodulation part on a reception system. CONSTITUTION: A frequency of 95kHz is inputted from a local oscillator 5 to a mixer circuit 4, the frequency is subtracted from the center frequency 100kHz modulated in a first FM modulation part 3 and an FM sound signal of 5kHz is outputted. This signal is inputted in a second FM modulation part 6, FM modulations are performed to the signal by carrier waves of 380MHz and 250MHz in the case of a host machine and in the case of a slave machine, respectively, and the signal is transmitted via a transmission antenna 7. Namely, because the transmitted radio waves are modulated by first carrier waves (100kHz) and, subsequently, the signals are the ones that FM modulations are performed again to the signals for which frequency conversions to 5kHz are performed by second carrier waves, these transmitted radio waves become meaningless signals even if the radio waves are demodulated by the other receiver. A receiver executes demodulation by the same frequency, so that the privacy at the time of a call is secured. |
