 (AMPS) handoffs are about 100 ms long, and Rayleigh fades typically are much shorter. As we discuss in Sec. 27.1, most test listen- ers reported that 2 percent signal outage was still good call quality. Signal delay is the last quality issue. Very long distance calls from the Americas to Asia often use two satellite links and take over 1 second for the round trip. Humans are not accustomed to conversations with delay, and they get confused when what they hear is 1 or 2 seconds behind what they are saying. Radio conditions for good speech quality are the same for analog and digital radio: strong signal, weak interference, high signal-to-interference (S/I) ratio, and high E/No as described in Sec. 8.2. The nature of the interference is important, too. Analog interference is best when it is smooth. FM with its constant radio envelope, described in Sec. 4.3, eliminates the syllabic nature of AM interference. Digital inter- ference causes bit errors, and these bit errors tend to be bursty, occurring in the same time interval. Statistically speaking, the likelihood of a bit error is greater when other bit errors occur at nearly the same time. Also, voice reproduction is more sensitive to 5While this definition of distortion is not rigorous enough to satisfy audiophiles or electrical en- gineers, it does describe how a system can change a signal for the worse with only a small change in its frequency response. Not only does the pulsing light seem brighter when the average power is the same, it even seems brighter when the peak power is the same. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Engineering Concepts Telephony Engineering Concepts some bits than others. Speech coding and forward error correction can work together to produce the best voice reproduction in a noisy radio environment. 12.3 Reliability and Redundancy Networks hav