Amplitude Modulation

Abstract

Let’s look at one more application of the multiplication property. This has to do with broadcasting signals over a shared communication channel. This shared channel can be the atmosphere as is the case with broadcast radio and television signals, or it can be a a coaxial or optical cable such as that used to distribute television programs. Basically, any communication medium that is being used by a number of users at the same time. If the signals generated by the users occupy the same frequency band sending them all over the same channel at the same time will cause them all to be distorted. We have to separate them somehow. We can do this by separating them in time, or we could separate them in frequency, or as the Austrian-American actress Hedy Lamar demonstrated you could do both (remember her the next time you use Bluetooth). In this module we take a look at one way of moving the signals into separate slots in the frequency domain. At first sight separating similar signals in frequency does not seem like a viable option because generally the signals we send over a common broadcast medium are similar signals and therefore have similar spectral profiles. However, remember from our discussion of sampling that we can move a signal around in the frequency domain by convolving it with a delta function.