We will consider modulation and demodulation using an example in which a person's voice is sent.
If you talk to a person who is several kilometers away, the content (information) will not reach the other party however loud you speak. In this case we might consider transmission media such as radio waves, wires, light and the like, but voice itself vibrates the air with sound waves so sending that alone will not work. In this case, it is possible to change the sound to an electric signal using a microphone and send that signal. The electrical signal is a continuous (analog) representation of the proportional strength of the sound. The following two methods are available for transmitting the signal using radio waves or wire.

1 Sending the analog quantity as it is
2 Numerically converting the signal (digitizing it) and sending it as a digital quantity.

FM and AM radio use the first method, while mobile phones and BS digital broadcasting use the second method. At the receiving end, if the signal received is digital, it is converted into an analog quantity or voltage, and uses a speaker to produce sounds based on the strength of the voltage.

Let's consider sending radio wave data
Analog or digital information signals (called base band data) cannot simply be sent as they are through space. It is necessary to combine the base band data with a carrier frequency that is sufficiently high to pass through space as a radio wave. Converting the electric signal including the original information into a signal that is appropriate to the transmission path (in this case radio waves) in this way is called modulation. So modulation systems comprise analog modulation and digital modulation.

1 Analog modulation systems
Analog modulation systems include AM, FM, PM and so on. They modulate the carrier using an analog method.

2 Digital modulation systems
How is data that is originally a numerical value transmitted? There is apparently no other method of sending it than as a digital quantity (signal). Digital modulation directly shifts the high frequency parameters of the radio equipment using the base band data (digital quantity). There are may kinds of digital modulation systems such as FSK, MSK, CPFSK, GMSK, GFSK, ASK, PSK, DBPSK, DQPSK, QPSK, BPSK, multi-value QAM, OFDM, CCK and so on.

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What is the Modulation and Demodulation?

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