| 摘要 |
PURPOSE:To prevent tapping by a fellow passenger or a hidden microphone by outputting other DTMF signal not corresponding to an operated key as a confirming tone of key operation when a dial key is operated and a password is sent. CONSTITUTION:When a talking channel is open and a dial key 51 is depressed to send a password to an opposite party, its key output is fed to a forming circuit 55 from a control circuit 40. Then a DTMF signal Sb having a frequency corresponding to a depressed key (shown in mark O) during a period T1 and a frequency (shown in mark X) not corresponding to the key is formed, extracted as an output signal Se and fed to a speaker 35 during the succeeding period To. Thus, the confirming tone of th key is outputted from a speaker 35. |
