8.2.1 Echo. As the name implies, echo in telephone systems is the return of a talker's voice. It is most apparent to the talker himself or herself. Secondarily, it can also be an annoyance to the listener. To be an impairment, the returned voice must suffer some noticeable delay. Thus we can say that echo is a reflection of the voice. The cause of echo is impedance mismatches that might be present any place in the electrical telephone connection. Echo is a major annoyance to the telephone user. It affects the talker more than the listener. Two factors determine the degree of annoyance of echo: its loudness and its length of delay.
8.2.2 Singing. Singing is the result of sustained oscillations due to positive feedback in telephone amplifiers or amplifying circuits. Circuits that sing are unusable and promptly overload multiplex equipment, particularly FDM equipment.
Singing may be regarded as echo that is completely out of control. This can occur at the frequency at which the circuit is resonant. Under such conditions the circuit losses at the singing frequency are so low that oscillation will continue, even after cessation of its original pulse.
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