gitalengineeringlibrary.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 Volts Figure 18.6 Reconstructing the audio wave from logarithmic PCM. The A-law formula calculates its signal function s(t) using Eq. (18.2): (18.2) s(t)=sgn[i(t) = 1 + In A' 0 s li(t)l s 1 As in the earlier equation, the input function i(t) goes from -1 to 1. The value A = is typically used in Europe. When we put the logarithmic PCM bits back into audio output, we see better resolu- tion of the soft passage in Fig. 18.6 than we saw earlier in Fig. 18.4. The logarithmic PCM schemes, both u-law and A-law, allow 8 bits per sample to work as well as 13-bit linear PCM. These 64,000-bit-per-second schemes are full-rate PCM telephone links.6 The A-law function s(t) is logarithmic for li(t) I VI 1/A and linear for li(t)l > 1/A. A-law provides better signal performance than u-law at higher levels at the expense of some performance at lower levels.7 7 Telephone communications between u-law systems and A-law systems must have conversion routines in their transmission paths. 6The PSTN in the United States reserves 1 bit per sample, 8000 bits per second, for its own sig- naling, so DS-0 speech coding is usually 7 bits per sample, 56,000 bits per second. 7Is this because Europeans talk louder or more consistently loud than Americans? 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 18.1.3 Adaptive differential pulse code modulation (ADPCM) While logarithmic quantization brings our requirement down from 13 to 8 bits per sam- ple, we can do better by using differential coding. Instead of encoding the voltage for each sample, we can make a prediction of the next sample based on the recent sample history and encode the difference between the act