The source decoder is shown in Figure 4.28. It works to undo the effects of the predictive coder in reverse order. It begins with a bit-to-symbol mapper, a device which undoes the effects of the symbol-to-bit mapper in the source coder. Next, it would like to have a device that undoes the effects of the quantizer; but since there is no such device (as we saw in section 4.3.4), it moves on. The next effect to try to undo is the subtraction of x,'s at the encoder. This is undone at the source decoder by adding back the value x, Finally, the effects of sampling are undone by a low-pass filter; and voila, you've got a source decoder which undoes predictive coder effects. Bit-to-symbol received Mapper after being sent across channel undoes symbol-to-bit nothing here to undo removal of undo sampler effects mapper undo quantizer effects predicted value Figure 4.28 The decoder for a predictive coder Source Coding and Decoding: Making it Digital Now, here's a very important point. To make the source decoder described above (and seen in Figure 4.28) possible, telecommunication engineers are always very careful to make sure that any signal xp they remove during source coding is a signal that can be created and added back at the receiver side during source decoding. In the next sections, we'll explore two predictive coders, one per section. We'll detail (among other things) how these devices get x, and how they use only x, values that can be created and added back at the receiver during source decoding. 4.4.4 The Delta Modulator (DM) The Delta Modulator is the first of two predictive coders that we'll talk about in this book. Being a predictive coder, it works in just the way we saw in Figure 4.27. All we'll do here is detail how this system creates its xp, and understanding that, we'll explore in a bit of mathematical detail its inner workings. How the DM creates an xp This predictive coder creates the predicted value x, using a really simple idea: If you sample an incoming signal very quickly, then a good estimate of the c