Tài liệu Communication Basics

Signals:

Communication happens in the form of signals.
Signals are transmission of energy (mechanical, electrical or light) through appropriate media.
A signal that is constant and changed once conveys single information.
The more changes in the signal, the more information that the signal can convey.
For example, a push can mean one thing and removing it can mean another thing.

But how do we send more information than just these two?
We can do that by varying the amplitude, i.e., a strong push or a light push could have two different meaning.
Or, a rate of change of push, i.e. 10 pushes per minute or 1 push per minute could have different meaning too.
Or, even phase, i.e., pushing at a different angle could have different meaning too.

Amplitude, frequency and phase are the attributes of a signal that changes with time.

This pushing, transfer of mechanical energy has a limit. How many different amplitude, rate and phase of pushing can we transmit and distinguish (receive)? God gave us organs like vocal cord which can create higher rate of pushing and ears to sense that.
Higher the rate of pushing the more information it can convey and even the mixing of these different rate could mean different information. And our vocal cord is able to do that as well.
We have even media (air) which will carry that signal far, so we don't have to stay close to communicate. But that has a limit too.

Then we came out with a solution of converting this pressure wave to electrical signals and then transmitting over long distances.

Signals can exists in different forms and transmit over different media. For example, acoustic (pressure waves), electrical, electromagnatic and light signals.