channel decoding. First, in Figure 6.1 you'll see a device called the channel coder, located right in the middle of the source encoder and the modulator. (It is this device that performs channel coding.) If you look at the input to the channel coder, you'll see that it is a stream of bits, and its output is also a stream of bits. This leads us to an intriguing question. Why would somebody introduce a device that takes in a stream of bits and puts out a stream of bits? To answer this question, we need to look a little more closely at what the channel coder does: It takes each set of k incoming bits and maps it into a set of n outgoing bits, where n is greater than k. The extra n - k bits introduced into the bit stream by the channel coder are added SO that we can detect transmission errors and/or remove transmission errors at the receiver side. TRANSMITTER Source Channel Modulator encoder coder for example, across speech signal for example, for example, channel 101110 Figure 6.1 Introducing the channel coder Chapter Six Now take a look at Figure 6.2, where you'll see a picture of an updated receiver side. This receiver includes, between the demodulator and the source decoder, a device known as the channel decoder. The channel decoder undoes the operation of the channel coder. That is, the channel decoder maps each set of incoming n bits into its best guess on the original set of k bits. Specifically, what it does is use the extra n- k bits introduced at the channel coder to correct/detect any errors that might have occurred during transmission. There are two types of channel coding: block coding and convolutional coding. This chapter will focus on block coding, as we will save convolutional coding for Chapter 7. RECEIVER Demodulator Channel Source decoder decoder received from channel X (t) r(t)=s(t) + n(t) for example, for example, speech 101110 signal Figure 6.2 Introducing the channel decoder 6.1 Simple Block Coding 6.1.1 The Single Parity Check Bit Coder In this simple example of channel coding, called the sin