| 摘要 |
PURPOSE:To prevent a command signal from listening by transmitting a synchronizing signal at a transmission rate lower than that of a voice signal, transmitting a code with the band area of the voice signal, and applying muting on a voice circuit when detecting the synchronizing signal on a reception side. CONSTITUTION:In the case of transmitting the command signal CMND via a speech channel in performing speech, the synchronizing signals BSYN and FSYN in the command signal CMND are transmitted at a bit rate of 300bps simultaneously with the voice signal St, and the codes SYID-CTRL are transmitted at a bit rate of 200bps by muting the voice signal St. At a reception circuit 220, the bit rate of the synchronizing signals BSYN and FSYN is 300bps, and they are eliminated by a filter 222. Therefore, no synchronizing signals BSYN and FSYN are sent to an opposite telephone via an amplifier 223 and a line at a succeeding stage. The muting is applied on the amplifier 223 from the start point of the code SYID, then, it is not supplied to a switch circuit 22. |
