| 摘要 |
PURPOSE:To keep an excellent stable sidetone preventing characteristic with a prescribed value or over by subtracting a voltage across a four-terminal circuit network inserted between a transmission circuit and a communication circuit through a prescribed circuit network. CONSTITUTION:In regarding a transformer 15 as the four-terminal circuit network, its short-circuit impedance, that is, the impedance between terminals 19 and 20 is not zero, a voltage across the four-terminal circuit network, i.e., a voltage ratio generated across a primary winding 16 and a secondary winding 21 of the transformer 15 differs. Thus, since a sidetone voltage generated at transmission is generated with a voltage ratio different depending on the reception and transmission even at the sidetone balancing state where the sidetone voltage is cancelled in a talking circuit 33, a reception voltage in the talking circuit 33 is extracted from a mixer 27 while being not cancelled at all and then given to a reception amplifier 28. The impedance matching with lines 11, 12 is kept by the transformer 15 and if the impedance of the lines 11, 12 connected to terminals 13, 14 is fluctuated, the sidetone balancing condition of the talking circuit 33 is not affected and the stable sidetone preventing characteristic with a prescribed value or over is obtained. |
