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.
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
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
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. In the more modern network configuration, subscriber loops may terminate in an
analog concentrator before two-wire/four-wire conversion in a PCM channel bank.
The concentration ratio may be anywhere from 2:1 to 10:1. For example, in the
10:1 case a hybrid may connect to any one of a group of ten subscriber loops. Of
course, this is much better than selecting any one of a population of thousands of
subscriber loops as in condition 1, above.
Turning back to the hybrid, we can keep excellent impedance matches on the four-wire
side; it is the two-wire side that is troublesome. So our concern is the match (balance)
between the two-wire subscriber loop and the balancing network (N in Figure 8.17). If we
have a hybrid term set assigned to each subscriber loop, the telephone company (adminis-
tration) could individually balance each loop, greatly improving impedance match. Such
activity has high labor content. Secondly, in most situations there is a concentrator with
from 4:1 to 10:1 concentration ratios (e.g., AT&T 5ESS).
With either condition 1 or condition 2 we can expect a fairly wide range of impedances
of two-wire subscriber loops. Thus, a compromise balancing network is employed to cover
this fairly wide range of two-wire impedances.
Impedance match can be quantified by return loss. The higher the return loss, the better
the impedance match. Of course we are referring to the match between the balancing
network (N) and the two-wire line (L) (see Figure 8.17).
= 20 log