TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY
If the balancing network (N) perfectly matches the impedance of the two-wire line (L),
, and the return loss would be infinite.
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.
This is the return loss across
the band of frequencies from 300 to 3400 Hz.
2. Balance return loss from the point of view of stability.
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 func-
tion 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 facil-
ity, 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 effec-
tively 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.
Remember, for any number divided by zero, the result is infinity.
Called echo return loss (ERL) in North America, but with a slightly different definition.
Recognize this as the CCITT definition of the standard analog voice channel.
From the point of view of stability--for this discussion, it may be called from the point of view of singing.
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).