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. Experimenting With Pure Delay Although we don’t know how good echo suppressors can get, we do know that a long circuit with the best possible suppressors could never be _better_ than a circuit of the same length that had no echos. This brings us back to the problem of how serious the effect of delay alone is on conversations. If the delay in a synchronous satellite system, even without any echo, made conversation all but impossible, there would be little point in developing echo suppressors for such satellites.