t you could somehow magically predict the value of the sample Xn , creating a predicted value x, Well, in fact, as I'll show you a little later, we can create just such a signal with the magic of engineering. What happens in Figure 4.27 is, once the sample Xn is generated, a predicted value x,P is immediately created, and subtracted from Xn. The output for an input sample Xn is the sample E the error between the actual sample value Xn and the predicted value x, Quantizer Symbol-to-bit sampler Mapper Main difference between predictive coder and PCM Figure 4.27 The predictive coder The error sample En=xx-x, (and not the actual sample value xn) now enters into the quantizer. The quantizer maps this error sample to an output sample made up of one of N possible amplitudes. We'll call its output E Finally, each of the quantized samples is turned to a set of bits by a simple mapping device called the symbol-to-bit mapper. It works in just the same way as described in the PCM section. 4.4.2 Why? You might be saying, "That's all fine and dandy, I follow you, but why would anyone want to use this device instead of the PCM?" I'll take some time out here and answer that important question. Chapter Four Let's say we've got a way to get a really good predicted value x,", , one that's really close to X In that case, the error signal E, = x,-x," is a very small number close to 0. Now, consider this: use a quantizer that, for each input sample E, , outputs a sample with an amplitude that takes on one of two possible levels, either -8 or +8, where 8 is a very small number close to 0. This creates two things that telecommunication engineers get excited about: (1) because the quantizer input E was a small number close to 0 to begin with, the error introduced by the quantizer (E,-É,) is small, and we get a large SQNR; (2) also, because the quantizer only creates two possible output levels for each sample, the number of bits per sample is low; this leads to a low bit rate. In a word, WOW-using the predictive coder, it may well be poss