All the world’s telephones are individually connected to the rest of the
system by what we call _two-wire local loops_. Speech travels in both
directions on the same wires over these local parts of the circuit. In
other parts of the system, where speech travels farther and must be
amplified, it is carried over four-wire circuits. These consist of two
pairs of wires, one for transmission in each direction. At the junctions
where the two-wire and four-wire parts of the telephone system meet,
specially designed transformers, called _hybrid coils_, are used.
It is impossible to have these junctions between two-wire and four-wire
circuits always in perfect balance, so part of the speech that reaches a
local loop will be reflected back along the path on which it arrived.
Unless a circuit has been specially treated, this reflected speech will
get all the way back to where it started, and the talker will hear an
echo of his own voice. When the circuit is short enough, the echo is
heard almost instantaneously, and is not bothersome. But when the echo
is delayed by a twentieth of a second or more, it can become extremely
annoying, and even temporarily destroy one’s ability to speak
coherently.
Telephone engineers have long been aware that this echo effect was
present on their long-distance circuits, and they have not let it go
unchecked. Devices known as _echo suppressors_ are installed on circuits
that have more than a critical amount of delay. They are placed in a
four-wire part of the circuit, where there is one-way transmission over
each pair. Since incoming and outgoing sounds are using separate paths,
an echo suppressor can attenuate or shut off the return path when speech
is coming in on the other path.
Unfortunately, echo suppressors have effects of their own on
transmission. They may, for example, cut off some speech that should be
getting through, because they can’t distinguish it from echo. Echo
suppressors can be made more sophisticated, but whether they can be made
to operate more successfully than present ones is not clear. And the
problem of adapting them to the long delays of synchronous satellite
circuits will require a great deal of research and development effort.