e. The conversion would take on the fol- lowing representation: 1 101 1101 Where the initial "1" indicates a negative sample, "101" indicates the fifth segment, and "1101" indicates the thirteenth step in the segment. We now have an eight-bit representation of an analog amplitude sample that can be transmitted across a digital network, then reconstructed with its many counterparts as an accurate representation of the original analog waveform at the receiving end. This entire process is known as Pulse Code Modulation, or PCM, and the result of its efforts is often referred to as "toll-quality voice." Alternative Digitization Techniques While PCM is perhaps the best-known, high-quality voice digitization process, it is by no means the only one. Advances in coding schemes and improvements in the overall quality of the telephone network have made it possible for encoding schemes to be developed that use far less band- width than traditional PCM. In this next section, we will consider some of these techniques. Adaptive Differential Pulse Code Modulation (ADPCM) Adaptive Differential Pulse Code Modulation (ADPCM) is a technique that allows toll-quality voice signals to be encoded at half-rate (32 Kbps) for transmission. ADPCM relies on the predictability that is inherent in human speech to reduce the amount of information required. The technique still relies on PCM encoding, but adds an additional step to carry out its task. The 64 Kbps PCM-encoded signal is fed into an ADPCM transcoder, which considers the prior behavior of the incoming stream to create a prediction of the behavior of the next sample. Here's where the magic happens: Instead of transmitting the actual value of the predicted sample, it encodes in four bits and transmits the difference between the actual and predicted samples. Since the difference from sample to sample is typically quite small, the results are generally Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies