Transformation of a supplied radio frequency or waveform to a signal that is usable.
The little black box that takes the signal from a coaxial cable and outputs to the red/white/yellow RCA cables so you can plug your old VCR(Beta-maxx, anyone?) into your new 92inch 4k curved screen smart TV is transforming the signal, therefore modulating the signal.
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It's the same thing as when over the air TV channels went from broadcasting analog signals(think rabbit ear antenna) and switched to digital format. Analog could be captured with a simple set of antenna that used two wires that were held to the TV via two screws, whereas the new digital signals can only be captured with a digital antenna. Well, not entirely true nowadays. A lot of the newer TV sets can now capture digital radio frequencies without the need for digital antenna.
RF modulator transfers a low frequency signal containing information (called baseband signal) to high radio frequency (carrier) that can be transmitted through the air. We can say also that the carrier is modulated by baseband signal in modulator. There are many modulation types like amplitude, phase or frequency modulation, but all of them finally change some carrier’s parameter accordingly to the baseband information.
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You can ask: why do we need to to increase the frequency of information (like sound, video or digital signals) in modulator? Why cannot we transmit the baseband signal directly from antenna through the air? Yes, we can. However, it is very inefficient way to use a radio spectrum. For example, the frequency range of sound is 20Hz - 20KHz. Theoretically we can transmit this signal from transmitter to receiver directly, but nobody else will be able to transmit another sound signal on the same time, because both signals have the same frequency range! In case we use RF modulator, we can move our sound frequency to one carrier (let’s say 100MHz) and another station can transmit its signal on another carrier (101MHz). Both stations can work fine simultaneously over the air.
RF modulators allow us to transmit and receive many signals on many frequencies in the same time.
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An RF Modolator takes the carrier frequency and impresses the information to be transmitted onto it. There are many forms of modulation, try a search for FCC modulation table. AM (Amplitude Modulation) and FM (Frequency Modulation) are well known because they are popular schemes for car and home radios,. However, there are several forms of these and many more complex modulation ideas.
The world of radio communication started with Morse Code where the signal is turned on and off by a sender pressing a key. The idea of sending speech on a radio signal invited methods to impress a varying amplitude onto the RF carrier.
The progress of the technology advanced greatly when Edwin Armstrong invented the idea of modulating the carrier frequency instead of the carrier amplitude.
There are now many many methods of modulating (and of course demodulating) radio signal.
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Source: Quora
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