Hellohellololo

When Aunt Sue and Uncle Jeff finally come to visit, their first desire is letting the stay-at-homes know they made it to Alaska. So you call the folks Outside, and when they answer, the next thing you hear is your own voice double-talking: "Hihi therere, it'stss meme." You've been hit by the dreaded phone echo.

There's a quick fix: hang up, call the operator and explain what happened, get a new connection. The odds are very good that the new line will work well.

The surprising thing is not that you had the problem but that another try cleared it up---and that's not an insult to the phone company. It really is one of the wonders of modern technology that so many calls go through echo-free. On a long-distance call from the far north, our voices are transmitted electronically via a geostationary communications satellite---that is, one that stays in a fixed position relative to the earth below. To be geostationary, a satellite must be in an orbit 35,800 kilometers (a bit over 22,000 miles) above the earth's surface. Even though the communications signal is zipping up to the satellite at the speed of light, it takes more than a quarter of a second for it to get up there and back down again. That much delay wouldn't usually bother the person to whom you're talking, but if your own voice comes back to you, it arrives with a distracting lag. Each syllable carries its own echo. And it's in the nature of the connection for your voice to come back; the solution is to keep you from hearing it.

Before satellites, telephone companies generally controlled the echo with what amounted to a voice-operated switch. While you spoke, that switch kept the incoming line turned off, so you couldn't hear your voice returning. Of course, if the person to whom you spoke tried to butt in, you couldn't hear that either. Such switches, called echo suppressors, thus left something to be desired even when transmission delays were shorter.

With the longer delays in satellite transmissions, the suppressors just couldn't do the job. The next step was a genuine echo canceler, a device with a job so complex that it seemingly should fill a whole room.

It doesn't have to work with sound itself, but with encoded sound: the telephone turns voice signal into digital code. (It's surprising enough to think that on-off electrical patterns can package and recreate a voice so well that individual vocal characteristics starting in Alaska can be perfectly identifiable to a listener in Seattle, or Hong Kong for that matter, but that's another story.)

The echo canceler works by estimating what electrical characteristics an echo will have, then subtracting those characteristics from the incoming signal. In effect, it sets up feedback loops based on the outgoing code, and these cancel the electrical impulses bouncing back that would turn into an echo in your telephone's earpiece. It's the reverse of the kind of feedback generated when a microphone is set too close to a speaker and enhances the sound so that it produces a deafening squeal. With an echo canceler working properly, you can hear any interruptions from the person on the other phone, but you can't hear the delayed sound of your own voice.

The first of these echo cancelers came along in the 1970s, and they were fairly big--about the size and weight of a standard refrigerator. By 1980, Bell Laboratories had shrunk these useful widgets onto a single tiny chip. That mighty chip contains the equivalent of over 34,000 transistors. It takes that many to register and analyze the outgoing code and set up the necessary feedback to cancel the incoming echo, all too swiftly to hear. We should forgive its occasional lapses; otherwise, we might never think of the amazing work it does.