mpedance mismatches. Echo is a major Hybrid Hybrid Two-wire Two-wire Four-wire N = Balancing network Figure 8.17 Simplified schematic of two-wire/four-wire operation. 12 On a pair of coaxial cables, a pair of fiber-optic light guides, or a pair of radio-frequency carriers, one coming and one going. LOCAL AND LONG-DISTANCE NETWORKS 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 the length of its delay. 8.5.3 Singing Singing is the result of sustained oscillation due to positive feedback in telephone ampli- fiers or amplifying circuits. The feedback is the result of excessive receive signal feed- ing back through the hybrid to the transmit side, which is then amplified setting up oscillations. Circuits that sing are unusable and promptly overload multichannel carrier (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 impulse. 8.5.4 Causes of Echo and Singing Echo and singing can generally be attributed to the impedance mismatch between the balancing network of a hybrid and its two-wire connection associated with the subscriber loop. It is at this point that we can expect the most likelihood of impedance mismatch which may set up an echo path. To understand the cause of echo, one of two possible conditions may be expected in the local network: 1. There is a two-wire (analog) switch between the two-wire/four-wire conversion point and the subscriber plant. Thus, a hybrid may look into any of (say) 10,000 different subscriber loops. Some of these loops are short, other are of medium length, and still others are long. Some are in excellent condition, and some are in dreadful condition. Thus the possibility of mismatch at a hybrid can be quite high under these circumstances. 2. 