mall signal on the right is lost in the voltage quantization error. Both in music and in speech, normal audio has loud and soft periods. The audio waves shown in these figures have a loud period Superscript(3)In fact, a CD has a robust error-correction scheme that uses 14 bits to get 8 correct bits, so the total raw bit count is more like 2.5 Mbps. 4We are loathe to say "never" here because somebody, somewhere, somehow probably is using 104,000-bit linear PCM on a telephone link. But we have not heard of anybody using it. 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 Volts Figure 18.5 Logarithmic PCM sampling of an audio signal. followed by a soft period, with voltage about four times higher during the loud part. A factor of 4 in voltage is about 12 dB according to Eq. (1.4). We say that this audio signal has a dynamic range of 12 dB. Our ears care about errors that are proportional to the signal, so we mind the greater relative magnitude of the quantization error during the quiet period more than we mind the same error during the loud period. We can improve the performance during the softer periods with only minor degrada- tion of the loud periods by having more levels near 0 V, as shown in Fig. 18.5. There are two different logarithmic PCM standards, u-law PCM used in DS-0 voice links and A-law PCM used in E-0 voice links. 5 The u-law formula calculates the signal function s(t) from the input function i(t) us- ing Eq. (18.1): ln[1+uli(t)l] (18.1) The input function i(t) goes from -1 to 1. The sgn(x) function is +1 for positive x and - 1 for negative x, so the calculation is symmetric for positive and negative input val- ues. The parameter is set to 255 in the United States. 5 u-Law is sometimes spelled out mu-law and is pronounced "myoo-law." Downloaded from Digital Engineering Library @ McGraw-Hil (www.di