ny times a second. When the switch is closed, the electrical speech signal passes through; when the switch is open, nothing gets through. Hence, the output of the sampler consists of samples (pieces) of the electrical input. Monica's speech signal Quantizer Symbol-to-bit Sampler Mapper Figure 1.9 A simple source coder Chapter One As some of you may know (and if you don't, we'll review it in Chapter 4, SO have no worries), we want the switch to open and close at a rate of at least two times the maximum frequency of the input signal. In the case at hand, this means that we want the switch to open and close 2 X 4000 = 8000 times a second; in fancy words, we want a sampling rate of 8000 Hz. After the switch, the signal goes through a device called a quantizer. The quan- tizer does a simple thing. It makes the amplitude of each sample go to one of 256 possible levels. For example, the quantizer may be rounding each sample of the incoming signal to the nearest value in the set {0, 0.01, 0.02, 2.54, 2.55}. Now, here's something interesting. There is a loss of information at the quantizer. For example, in rounding a sample of amplitude 2.123 to the amplitude 2.12, informa- tion is lost. That information is gone forever. Why would we put in a device that intentionally lost information? Easy. Because that's the only way we know to turn an analog signal into a digital one (and hence gain the benefits of a digital communication system). The good news here is engineers (like you and me) build the quantizer, and we can build it in a way that minimizes the loss introduced by the quantizer. (We'll talk at length about that in Chapter 4.) After the quantizer, the signal enters into a symbol-to-bit mapper. This device maps each sample, whose amplitude takes on one of 256 levels, into a sequence of 8 bits. For example, 0.0 may be represented by 00000000, and 2.55 by 11111111. We've now created a digital signal from our starting analog signal. 1.4 Congrats and Conclusion Congratulations-you made it through the first chapter. Just to