s basic sound then goes through a synthesis filter with three parameters called pitch lag L, pitch gain b, and pitch spectral lines i. The output of this goes through a fi- nal sound cleanup, a spectral postfilter and gain control. Since the CDMA coders uses variable bit rates, there are different parameter coding schemes for the different speech data rates. The lowest rates use a simplified algorithm appropriate for the case where the subscriber is not speaking. The CELP encoder is more complicated than PCM-based encoders because it must determine the values of I, G, L, b, and i to send over the CELP speech coding link. As 8 Although one of the authors (Kemp) has met some computer technicians who are capable of re- producing modem signals with their mouths, just as some auto mechanics can illustrate the sounds of a car in distress. 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. Speech Coding Key Data Concepts in the case of ADPCM, the encoder has a complete decoding system within it, and it compares the output of the decoder with its own input to refine the signal being sent. The encoder computes line spectral pairs (LSPs) and pitch spectral lines i values. Then it uses an analysis-by-synthesis optimization employing a weighted error measure in a closed-loop feedback system to find the best values for pitch lag L and gain b. Finally, the selection of codebook index I and loudness G are made with a similar optimization based on i, L, and b values. CELP is significantly more complex than ADPCM, and it uses a great deal more com- puter memory. Most of the computational complexity is in the encoding stage, where the optimization computations occur. Also, CELP is decidedly more tuned to the human voice than the PCM coding schemes by putting more information in its prearranged codebooks rather than in its bit stream. The CDMA development group (