| 摘要 |
<p>The device transmits secret information from a centre to a number of subscribers by using single coaxial transmission cable. Sound is modulated at about 12 volts using an operational amplifier. The microphone is polarised at 12 volts and its voltage is applied across a capacitor to the amplifier input. The capacitor, together with a series resistance forms a high filter, to eliminate 50Hz component. For code transmission, the receiver, on receiving an address modulation pulse generates a control code and compares it with the control code received. If the two are identical the control code is effective to allow receipt of the transmitted signals. Information transmitted can be analogue or digital.</p> |
