compared to other source coders); or, 2. If the bit rate is fixed, we get a very large SQNR (compared to other source coders). All the telecommunication engineers nodded their heads in collective agreement with this notion of "good," and SO it was. 4.3.4 Source Decoder: PCM Decoder If you've made it this far, it's very likely that you understand how the source coder, the PCM, transforms an incoming analog signal into a digital one, and how to decide on a "good" PCM. Taking a look at Figure 4.25, we see what happens to the digital signal output by the PCM in the communication system: it's transformed by a modulator, sent across the channel, and picked up by a receiver. Continuing to explore this figure, you can see the receiver hard at work: it tries to reconstruct the original signal X (t). Basi- cally, it's the receiver's job to undo the effects of the transmitter and the channel, as best it can. As you can see in Figure 4.25, a part of what the receiver does is undo the effects of source coding, a process suitably named source decoding, and we'll talk here about how it works (when the source coding is PCM). The source decoder which undoes the effects of PCM is shown in Figure 4.26. The first thing it undoes is the symbol-to-bit mapping, using a bit-to-symbol mapping, which you'll find in Figure 4.26. This device, for each incoming set of bits, recreates the sample, with one of N possible amplitudes, that was output by the quantizer. Symbol-to-bit Source Quantizer Modulator Channel Demodulator Mapper Decoder Undoes PCM effects Transmitter Receiver Figure 4.25 What happens to the PCM signal in the communication system Chapter Four Bit-to-symbol Mapping nothing here undo symbol-to-bit mapper to undo undo sampler effects quantizer effects Figure 4.26 The source decoder for PCM The source decoder would next like to undo the effects of the quantizer, but a quick look at Figure 4.26 shows that it doesn't do that. Let me explain what's going on here. A quantizer, as you know, maps inputs with any amplitude, for example