in the case where a two - wire circuit forming a trunk or a subscriber line of the two - wire system is converted by a hybrid circuit to a four - wire circuit of sending and receiving paths to form a transmission line between subscribers , if an impedance variation on the two - wire side is large , a leak from sending path to receiving path occurs and is amplified to cause singing . to eliminate the singing , a circuit for detecting the impedance fluctuation of the two - wire side is inserted in the two - wire circuit , and switching means inserted in the four - wire circuit is activated by an impedance detecting signal of the detecting circuit to electrically open the four - wire circuit , thereby preventing the singing phenomena . this method does not involve the use of an expensive attenuator which is troublesome to design for the prevention of singing .