| 摘要 |
PURPOSE:To ensure the prevention of malfunction and to enable to increase the sensitivity of reception, by monitoring the output state of a tone decoder with an integrator. CONSTITUTION:When a line closing signal Sin is input to the tone decoder input (a), the tone decoder 17 quickly falls down to a low level L and outputs detection signal St. This output is input to an integrator 21, and the output DC voltage value is gradually decreased as shown in (c). Thus, when the level detector 22 detects the scheduled value Eth, a closed signal Sc of low level is output to the line (d) and the contact 14 is accordingly closed and the line is connected. Thus, the sensitivity cannot be so limited with a squelch circuit as in conventional systems and the closing signal Sc is sufficiently stable, when the time constant of the integrator 21 is selected to a sufficient value, even if strong noise N comes in and the decoder 17 responds to it. |
