| 摘要 |
PURPOSE:To avoid malfunction due to a voice and noise by transmitting another frequency signal different from a multi-frequency signal at the same time when the multi-frequency signal is transmitted and allowing the reception side to recognize a correct multi-frequency signal based on the signals. CONSTITUTION:A signal of low group frequency, a signal of high frequency and a signal of 400Hz are transmitted from a telephone terminal device T. The signal of low group frequency is inputted to a detection circuit DET via a band pass filter BPF-L, limiter LIM(L) and band pass filters BPF(L1)-BPF (L4), converted into a DC component and the result is inputted to AND gates G1-G4. Similarly the signal of high group frequency is inputted to AND gates G5-G8. The frequency signal of 400Hz is inputted to the AND gates G1-G8 via a selection amplifier circuit 3F, a band pass filter BPF and a detection circuit DETa to open the AND gates G1-G8. Thus, only when the 400Hz signal exists, the multi-frequency signal is recognized. |
