, 6.345, to an output with only one of N possible amplitudes. For example, for input 6.345, the output is 6. So, when a value of 6 is made available to the source decoder, it has no way of knowing exactly what input came into the quantizer. Was the input 6.001? How about 6.212? All these inputs would create an output of 6. Looking again at Figure 4.26, you'll see a low-pass filter (LPF), which is used to undo the effects of sampling. That's because, as you'll recall, and you can check back if you don't, that the effects of the sampler are totally undone by a low-pass filtering. So there you have it. In summary, the source decoder for PCM is made up of two parts, a piece that undoes the symbol-to-bit mapping, followed by a part that removes the sampling effects. 4.4 Predictive Coding Pulse code modulation, while popular, is not the only type of source coding out there in the telecommunication world today. With a clear definition of what constitutes a good source coder, telecommunication engineers set out to make really good source coders. This chapter shares some of what they found, which we'll call predictive coding. Source Coding and Decoding: Making it Digital 4.4.1 The Idea Behind Predictive Coding I'll explain the idea behind the special type of source coding called predictive coding with the help of Figure 4.27. This figure uses discrete-time signal notation, and I'll explain that as I go through a step-by-step description of the figure. At a glance, Figure 4.27 shows a striking similarity to Figure 4.23 (the PCM), with only one main differ- ence: there is a subtraction after the sampler. I'll take you in for a close look and we'll see what's going on. First, we've got a signal coming in, which we'll call X (t). It goes right into the sampler, which outputs samples of the incoming x(t). In Figure 4.27, we use the notation Xn to indicate the nth sample output by the sampler. Here's where something new and exciting happens. Rather than pass this Xn right to the quantizer, we first do a subtraction. Imagine tha