CDG), an in- dustry association promoting CDMA and developing standards, has adopted CELP CDMA. As a result, the 13-kbps rate set 2 (RS2) cdmaOne coding scheme is sometimes called CDG encoding. 18.2.2 Vocoders The extreme low-bit-rate end of the speech coding spectrum is the vocoder. The forms of human vocal sounds are modeled and formed into an electronic vocal tract to produce human speech. The vowel and consonant sounds of speech are simulated by a set of source-filter parameters based on a speech-production model. Voiced speech is pro- duced by generating a periodic pulse to simulate the opening and closing of vocal cords, whereas unvoiced speech is produced by exciting the same filter with random noise to simulate air rushing through the vocal tract. The receiver decodes the bit stream by us- ing a speech synthesizer based on the same model. The key to the vocoder is the perceptual model of speech, so vocoders do very poorly in sending nonspeech sound and particularly poorly for complex signals like analog modems and fax machines. Because it forms the sound in its own mathematical simu- lation of a human throat and mouth, the vocoder creates intelligible speech at very low bit rates. The vocoder speech, however, has a mechanical, robot-like quality. The speaker's soul seems filtered out. This is a speech coder that we can get away with for a few seconds without anybody noticing the difference, but extended use of a vocoder is not satisfactory for a wireless telephone system. 18.3 Speech Coding Performance We evaluate speech coders by their mean opinion score (MOS). A large community of human listeners rates the voice quality in a scientific, controlled test process. While it TABLE 18.1 Mean Opinion Score (MOS) Definitions Rating Excellent Downloaded from Digital Engineering Library @ McGraw-Hil (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. Speech Coding Speech Coding TABLE 18.2 Mean Opinion 