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 (administration) 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).
If the balancing network (N) perfectly matches the impedance of the two-wire line (L), then ZN = ZL, and the return loss would be infinite.13
We use the term balance return loss (Ref. 5) and classify it as two types:
1. Balance return loss from the point of view of echo.14 This is the return loss across the band of frequencies from 300 to 3400 Hz.15
2. Balance return loss from the point of view of stability.16 This is the return loss between 0 and 4000 Hz.
"Stability" refers to the fact that loss in a four-wire circuit may depart from its nominal value for a number of reasons:
• Variation of line losses and amplifier gains with time and temperature.
• Gain at other frequencies being different from that measured at the test frequency.
(This test frequency may be 800, 1000, or 1020 Hz.)
• Errors in making measurements and lining up circuits.
The band of frequencies most important in terms of echo for the voice channel is that from 300 Hz to 3400 Hz. A good value for echo return loss for toll telephone plant is 11 dB, with values on some connections dropping to as low as 6 dB. For further information, the reader should consult CCITT Recs. G.122 and G.131 (Refs. 5, 6).
Echo and singing may be controlled by:
• Improved return loss at the term set (hybrid).
• Adding loss on the four-wire side (or on the two-wire side).
• Reducing the gain of the individual four-wire amplifiers.
The annoyance of echo to a subscriber is also a function of its delay. Delay is a function of the velocity of propagation of the intervening transmission facility. A telephone signal requires considerably more time to traverse 100 km of a voice-pair cable facility, particularly if it has inductive loading, than it requires to traverse 100 km of radio facility (as low as 22,000 km/sec for a loaded cable facility and 240,000 km/sec for a carrier facility). Delay is measured in one-way or round-trip propagation time measured in milliseconds. The CCITT recommends that if the mean round-trip propagation time exceeds 50 msec for a particular circuit, an echo suppressor or echo canceler should be used. Practice in North America uses 45 msec as a dividing line. In other words, where echo delay is less than that stated previously here, echo can be controlled by adding loss.
An echo suppressor is an electronic device inserted in a four-wire circuit that effectively blocks passage of reflected signal energy. The device is voice operated with a sufficiently fast reaction time to "reverse" the direction of transmission, depending on which subscriber is talking at the moment. The block of reflected energy is carried out by simply inserting a high loss in the return four-wire path. Figure 8.18 shows the echo path on a four-wire circuit. An echo canceler generates an echo-canceling signal.17
13Remember, for any number divided by zero, the result is infinity.
14Called echo return loss (ERL) in North America, but with a slightly different definition.
15Recognize this as the CCITT definition of the standard analog voice channel.
16From the point of view of stability—for this discussion, it may be called from the point of view of singing.
11 Echo canceler, as defined by CCITT, is a voice-operated device placed in the four-wire portion of a circuit and used for reducing near-end echo present on the send path by subtracting an estimation of that echo from the near-end echo (Ref. 7).
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