All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Telephony Figure 3-67 PAM samples Figure 3-68 Quantizing the samples discern the shape of the original wave from their collective amplitudes. Imagine also that we have graphed these samples in such a way that the "wave" of sample points meanders above and below an established zero point on the x-axis, so that some of the samples have positive values and others are negative, as shown. The amplitude levels allow us to assign values to each of the PAM samples, although a glaring problem with this technique should be obvious to the careful reader. Very few of the samples actually line up 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 Chapter 3 exactly with the amplitudes delineated by the graphing process. In fact, most of them fall between the values, as shown in the illustration. It doesn't take much of an intuitive leap to see that several of the samples will be assigned the same digital value by the coder-decoder that performs this function, yet they are clearly not the same amplitude. This inaccuracy in the measurement method results in a problem known as quantizing noise, and is inevitable when linear measurement systems, such as the one suggested by the drawing, are employed in coder-decoders (CODECs). Needless to say, design engineers recognized this problem rather quickly, and equally quickly came up with an adequate solution. It is a fairly well-known fact among psycholinguists and speech therapists that the human ear is far more sensitive to discrete changes in amplitude at low-volume levels than it is at high-volume levels, a fact not missed by the network designers tasked with optimizing the performance of digital carrier systems intended for voice transport. Instead of using a linear scale for digitally encoding the PAM samples