| 摘要 |
PURPOSE:To make sounds indistinct to prevent them from being received without attenuating the ringing tone, by resonating an inductance in the primary side of a hydrid transformer of a channel signal converting circuit and a capacitor in series with the frequency of the rining tone. CONSTITUTION:When telephone sets 1-1 and 1-2 are hooked simultaneously, a closed loop is formed through a 450Hz oscillator of a channel signal converter 8, and telephone sets 1-1 and 1-2 are in the unnecessary service state. A capacitor C2 is connected in series to a resistance R1 in a channel converting circuit 8-1 to form a series resonance circuit. The value of the capacitor C2 is so determined that the inductance in the primary side of a hybrid transformer 12-1 and series capacity C1.C2/(C1+C2) of capacitors C1 and C2 are resonated in series with a frequency f0=450Hz of the ringing tone. Thus, only the ringing tone f0 passes, and the other audio bands are attenuated. |
