This is pretty easy to visualize, thanks to this gif from Wikipedia. With frequency modulation (FM) you make the wave slower or faster to add a signal to it. With amplitude modulation (AM) you make the carrier wave taller or shorter to add a signal to it. Two of the easiest forms of modulation are amplitude modulation and frequency modulation. Each one doesn’t interfere with the others because they are all on different carriers. This way the whole signal stays intact and you can have as many different signals as you have carriers. When you’re talking about modulating a video or audio signal it’s done using a carrier. We take the basic signal and we mathematically add it or subtract it to a much more basic signal. One would be the sound of the lead guitar. If we go back to the song analogy, one would be the sound of the drums. We take a bunch of different signals and we do something special to each one of them, something that makes it stand out. Now, if you were a robot you could concentrate so well that nothing else would even enter your mind. If you concentrate hard enough you can mentally block out everything else. When you hear a song you’re taking in the whole song, but if you have a trained ear you can listen to just the drums or just the vocals. The best way I can think of is if you think about music. The problem is that even with the best diagrams, it’s hard to explain. Without it there would only be one TV station. Modulation is a trick that lets you put more than one signal on a wire or over the air. Modulation is a trick that takes this signal and only lets it occupy a portion of the “space” on a wire. When you look at an unaltered video and audio signal, it travels through a wire in such a way that nothing else can travel on that wire. In order to understand what a modulator is, you have to understand what it does. Modulators are still around but they’ve grown up and gotten a lot fancier. This was sort of mandatory if your TV didn’t have A/V inputs and a lot of them back then didn’t. You used a device back then called an “RF Modulator” that put the output from the game system on channel 3. In fact if you think of them at all you probably think back to your old game system. Unless you’re in a commercial building or a hotel, you probably don’t think about modulators.
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