less tele- phone system has no obvious way of telling a true lost call from a user terminal shut- ting down during a call, perhaps from a weak battery, or a user becoming frustrated with lousy sound and ending a call prematurely. 12.2.3 Sound quality Call sound quality may be measured subjectively, but most of us agree on what consti- tutes a good-sounding call.1 4 The sound should be pleasant, a faithful reproduction of the source without being strident or muffled. Speech should be intelligible so that we know what the person is saying. The background should be quiet. The connection should be continuous, with dropouts kept short and infrequent. The call should have minimum delay so that the conversing parties do not get confused. Unpleasant sound reproduction makes subscribers less anxious to use their tele- phones and makes less money for telephone service providers. In the world of high- fidelity audio reproduction, we measure frequency response to ensure that each fre- quency in the musical audio band is played back in proportion to how it was recorded. In telephones, we compromise this objective by emphasizing higher frequencies to make speech clearer, but listeners will find severe frequency-response errors objection- able. By emphasizing certain frequencies, we can improve the telephone's ability to 4The first step in measuring quality is to define the multiple aspects of quality from the user's perspective. Once these elements are defined, engineers can find ways to capture the right data and make appropriate measurements. 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 Key Telephony Concepts communicate human speech. Research on the sound shapes (formants) of speech goes back to the 1930s at Bell Telephone Laboratories, and telephones are designed with de- liberate frequency-response deviations so that