, they designed and employed a nonlinear scale that is weighted with much more granular- ity at low volume levels-that is, close to the zero line-than at the higher amplitude levels. In other words, the values are extremely close together near the x-axis and get farther and farther apart as they travel up and down the y-axis. This nonlinear approach keeps the quantizing noise to a minimum at the low-amplitude levels where hearing sensitiv- ity is the highest, and allows it to creep up at the higher amplitudes, where the human ear is less sensitive to its presence. It turns out that this is not a problem, because the inherent shortcomings of the mechan- ical equipment (microphones, speakers, the circuit itself) introduce slight distortions at high-amplitude levels that hide the effect of the nonlinear quantizing scale. This technique of "compressing" the values of the PAM samples to make them fit the nonlinear quantizing scale results in a bandwidth sav- ings of more than 30 percent. In fact, the actual process is called com- panding, because the sample is first compressed for transmission, then expanded for reception at the far end, hence the term. The actual graph scale is divided into 255 distinct values above and below the zero line. In North America and Japan, the encoding scheme is known as u-Law (Mu-Law); the rest of the world relies on a slightly dif- ferent standard known as A-Law. There are eight segments above the line and eight below (one of which is the shared zero point); each segment, in turn, is subdivided into 16 steps. A bit of binary mathematics now allows us to convert the quan- tized amplitude samples into an eight-bit value for transmission. For the sake of demonstration, let's consider a negative sample that falls into the 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. Telephony Telephony thirteenth step in segment fiv