gital communication techniques. But, because some of what currently exists in the world today was built when there was only analog, it is important to have a basic understanding of it. 12.1 Modulation-An Overview An analog communication system, roughly speaking, looks like what is drawn in Figure 12.1. You can see that the information signal x(t) comes in and is mapped by a modulator into a new signal s(t) ready to be sent over the channel. And that is all that happens at the transmitter modulator side-no source coder, no channel coding. At the receiver information signal signal ready to be side, the signal that arrives sent over the channel from the channel is picked up, and it goes through a demodu- lator, which returns it to a best guess of the original informa- tion signal. That's it-no demodulator channel decoder, no source decoder. So, all we'll do in this chapter is study the modulator best guess on x(t) and demodulator. Figure 12.1 An analog communication system 322 Chapter Twelve At the modulator, the input signal x(t) is typically a baseband signal-that is, a signal centered around 0 Hz. You can see this in Figure 12.2(a). The signal you want to send over the channel must be sent around the frequency WC, as seen in Figure 12.2(b). The goal of the modulator is to map the incoming signal at baseband to a signal that is centered around Wc. To do this, the modulator takes a sinusoid at the frequency WC and it shoves x(t) in it as its amplitude, or as variations in its phase or frequency. I can explain that last line better in math, SO: given x(t), the modulator outputs s(t) according to s(t)=A(1)cos(a)(++(i)) (12.1) where the information x(t) is put into either A(t) or O(t). your analog signal must travel the channel in this frequency band Figure 12.2 (a) Your analog signal x(t) in the frequency domain (b) The frequency band over which your signal must be sent 12.2 Amplitude Modulation (AM) One of the options on your radio is AM, shorthand for amplitude modulation, a simple type of modulator. Analog Communi