| 摘要 |
PURPOSE:To prevent tapping by using visible rays or infrared rays as an audio signal transmitted between a master machine and a slave machine of a cordless telephone. CONSTITUTION:A light emitting part 6 is driven by the pulse train signal subjected to pulse density modulation or pulse width modulation by the audio signal to be transmitted. Transmission light 16a of the light emitting part 16 is inputted to a slave machine light reception part 20 directly or by indoor reflection and is converted to an electric signal and is demodulated by a pulse demodulating circuit 21 having the integral characteristic and is supplied to a receiver 23. A photodetector 41 in the light reception part of the slave machine is stored in a semispherical transparent vessel 40 coated with a filter which permits only the wavelength of received light to pass through, and a light emitting element 44 of the light emitting part stored in a cylindrical transparent vessel 43 with a light shielding plate 42 between vessels 40 and 43 is arranged on the same axis as the photodetector 41. |
