Scores of Speech Coding Systems Coding scheme Score 64-kbps u-law CDG 13 kbps for cdmaOne 32-kbps ADPCM 8-kbps EVRC Stationary AMPS, 26 dB S/I 8-kbps IS-96 13-kbps GSM 2400-bit vocoder 50 km/h AMPS, 17 dB S/I is expensive and time-consuming to have a large number of listeners evaluate a speech coding system, it does give statistically valid comparisons. The MOS is defined in In- ternational Telecommunications Union-Telecommunications Standardization Sector (ITU-T) Specification P.800. The quality of reproduced speech is ranked by the listen- ers on a scale from 1 to 5, as shown in Table 18.1. Table 18.2 shows a range of speech coders and their MOS values ranging from the very high quality of full-rate DS-1 u-law encoding at 4.3 down to a marginal Advanced Mobile Phone Service (AMPS) channel in a moving vehicle at 2.3. It is noteworthy that 8-kbps enhanced variable-rate codec (EVRC) has the same high MOS of 4.1 as 32-kbps ADPCM. Speech coding technology keeps getting better sounding with more compli- cated and sophisticated algorithms. 18.4 Conclusion In this chapter we have explored the coding schemes used most commonly in landline and wireless telephony. The ability of the 13-kbps CDG CELP standard and, more re- cently, 8.55-kbps EVRC to provide sound quality nearly as good as standard u-law PCM is significant to the cellular industry. Providing essentially equal quality in less than one-quarter the bit rate per call means that each call takes less than one-quarter the bandwidth of the carrier than would be needed if the standard PCM coding scheme were used. One-fourth the bandwidth per call translates directly into a factor of 4 in- crease in the call capacity of the cellular carrier. In the next chapter we will turn our attention to the methods of transporting digital data over networks that were first developed within the information technology indus- try and are now used for both computer data transmission and telephony. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary