 quickly changing input can't be followed by the output, as shown in Figure 4.33(b). Let's see what happens in this case with an increase in 8. Figure 4.34(b) shows us just this. Increasing the 8 allows the output to follow the input more closely, decreasing the amount of overload noise. So, decreasing 8 decreases the granular noise, while increasing 8 helps cut down on the overload noise. Given that, how do telecommunication engineers choose S? Typically, they do one of two things. The pick a value of 8 that they find creates a compromise between the amount of granular noise and the amount of overload noise; or, if they're getting fancy, the value of 8 is updated while the DM is running, increas- ing at times of overload noise and decreasing at times of granular noise. Source Coding and Decoding: Making it Digital 4.4.7 Differential PCM (DPCM) I've talked at length about one type of predictive coder, the DM, and you got through it all. Way to go. Now, I'm going to talk (but, thank goodness, not as much) about a second type of predictive coder, called the differential PCM, or DPCM for short. Being a predic- tive coder, the DPCM is built as shown in Figure 4.27. What makes the DPCM different from the DM is the predicted value, xp, explained in detail in the next section. The Predicted Value The predicted value xp used in the DPCM came about like this. A telecommunica- tions engineer one day had the idea to use the previous K samples to come up with a predicted value. Specifically, he recommended that we get the predicted value accord- ing to the equation (4.54) (4.55) Now, this engineer had two problems to work out. First, as you'll recall (and if you don't, a quick peek at Figures 4.27 and 4.28 will work to remind you), the value x,P is needed at both the predictive coder and the predictive decoder. But, the values of Xn-1' Xn-N used in the x,4 equation are only available at the predictive coder (and aren't available to the predictive decoder). Our engineer, looking at the decoder of Figure 4.28, asked, "What inf