Why should the signal be modulated in communication transmission? What are the main modulation methods?

Taking digital signals as an example, the spectrum of high and low level signals is very wide, so it is difficult to have a channel suitable for long-distance transmission of digital signals. RS485 is already a very long distance, exceeding 1 km. Another problem is that baseband signals are transmitted in the same channel, and the frequency spectrum overlaps, which is easy to interfere. If they are modulated on different carriers, they can be transmitted simultaneously.

The modulation of analog signals is divided into amplitude modulation.

And angle modulation

Two kinds.

Angle modulation includes frequency modulation and phase modulation.

AM includes:

Traditional amplitude modulation (am),

Suppress carrier double sideband amplitude modulation (DSB),

Single sideband modulation (SSB),

Residual sideband modulation (VSB).

The modulation of digital signals includes:

Amplitude keying (ASK), frequency shift keying (FSK), phase shift keying (PSK)

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