 the sounds of speech, especially conso- nants, are heard more clearly. Old radio broadcasts without this sound-shaping tech- nology lost their high frequencies and were difficult to understand. Much of the consonant sound spectrum is outside the 300- to 3300-Hz telephone bandwidth, and yet we have no trouble understanding people in a clear and quiet telephone call setting. In addition to having a pleasant frequency response, the reproduction should have low enough distortion that the voice sounds clear enough. Distortion is the component of the reproduction that does not sound like the signal source. 5 Hearing something other than the telephone call is distracting and annoying. There is one exception: A slight background hiss is actually a good thing because it reassures the listener that the call is still in progress. Very quiet lines get an occasional "Hello? Are you still there? Hello?" because one party or the other is concerned that the call may be quiet because the line is dead. As the noise contour changes from a wide- frequency hiss to a narrow band of frequencies, or as the noise gets louder, it becomes annoying rather than reassuring. Our senses respond to change. As a pulsing light seems far brighter than a steady light, a changing noise level calls attention to itself. A hissing or rumbling sound that comes and goes or changes its frequency content is much more of a problem than steady, unchanging noise. 6 The worst case is when the noise mimics the rhythms of hu- man speech; we call such interference syllabic. The most likely cause of syllabic inter- ference on a telephone call is leakage from another call. Listeners feel that they can al- most understand what is being said on the interfering conversation, even when it synthesized by a machine and has no human speech content other than its rhythm. Short dropouts are not even noticed by human listeners. A gap of 100 ms (one-tenth of a second) is seldom perceived, and 200-ms gaps are a problem only when they are frequent. Advanced Mobile Phone Service