itted analog signal; (b) Received analog signal Noise Figure 1.8 (a) Transmitted digital signal; (b) Received digital signal Introducing Telecommunications Here's the key idea. In the digital communication system, even after noise is added, a 1 (sent as +5 V) still looks like a 1 (+5 V), and a 0 (-5 V) still looks like a 0 (-5 V). So, the receiver can determine that the information transmitted was a 0 1. Since it can decide this, it's as if the channel added no noise. In the analog communication system, the receiver is stuck with the noisy signal and there is no way it can recover exactly what was sent. (If you can think of a way, please do let me know.) So, in a digital communication system, the effects of channel noise can be much, much less than in an analog communication system. 1.3.4 Making It Digital A number of naturally occurring signals, such as Monica's speech signal, are analog signals. We want to send these signals from one point, A, to another point, B. Because digital communication systems are SO much better than analog ones, we want to use a digital system. To do this, the analog signal must be turned into a digital signal. The devices which turn analog signals into digital ones are called source coders, and we'll spend all of Chapter 4 exploring them. In this section, we'll just take a brief peek at a simple source coder, one that will turn Monica's speech signal (and anyone else's for that matter) into a digital signal. The source coder is shown in Figure 1.9. It all begins when Monica talks into the telephone, and her vibrations are turned into an electrical signal by the bottom end of the telephone talked about earlier. This electrical signal is the input signal in Figure 1.9. We will assume, as the telephone company does, that all of Monica's speech lies in the frequency range of 100 Hz to 4000 Hz. The electrical version of Monica's speech signal enters a device called a sampler. The sampler is, in essence, a switch which closes for a brief period of time and then opens, closing and opening ma