Modulation is the process of encoding information onto a carrier signal by varying its amplitude, frequency, phase or other properties. This allows the signal to be transmitted efficiently over long distances. Common modulation types include amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM). Modulation is used in radio, television, Wi-Fi and other wireless communications to transmit data or audio on an electromagnetic carrier wave.

1. modulation

2. What is modulation?
Modulation is the process of converting data into radio waves by adding
information to an electronic or optical carrier signal. A carrier signal is
one with a steady waveform -- constant height, or amplitude, and
frequency.
How modulation works
Information can be added to the carrier by varying its amplitude,
frequency, phase, polarization -- for optical signals -- and even quantum-
level phenomena like spin.
Modulation is usually applied to electromagnetic signals: radio waves,
lasers/optics and computer networks. Modulation can even be applied
to a direct current -- which can be treated as a degenerate carrier wave
with a fixed amplitude and frequency of 0 Hz -- mainly by turning it on
and off, as in Morse code telegraphy or a digital current loop interface.
The special case of no carrier -- a response message indicating an
attached device is no longer connected to a remote system -- is called
baseband modulation.
Modulation can also be applied to a low-frequency alternating current --
50-60 Hz -- as with powerline networking.

3. What are the types of modulation?
There are many common modulation methods, including the following, which is an
incomplete list:
Amplitude modulation (AM): The height (i.e., the strength or intensity) of the
signal carrier is varied to represent the data being added to the signal.
Frequency modulation (FM): The frequency of the carrier waveform is varied to
reflect the frequency of the data.
Phase modulation (PM): The phase of the carrier waveform is varied to reflect
changes in the frequency of the data. In PM, the frequency is unchanged while the
phase is changed relative to the base carrier frequency. It is similar to FM.
Polarization modulation: The angle of rotation of an optical carrier signal is varied
to reflect transmitted data.
Pulse-code modulation: An analog signal is sampled to derive a data stream that is
used to modulate a digital carrier signal.
Quadrature amplitude modulation (QAM): Uses two AM carriers to encode two or
more bits in a single transmission.

5. Radio and television broadcasts and satellite radio typically use
AM or FM. Most short-range two-way radios -- up to tens of miles
-- use FM, while longer-range two-way radios -- up to hundreds or
thousands of miles -- typically employ a mode known as
single sideband (SSB).
More complex forms of modulation include phase-shift
keying (PSK) and QAM. Modern Wi-Fi modulation uses a
combination of PSK and QAM64 or QAM256 to encode multiple
bits of information into each transmitted symbol.
PSK conveys data by modulating the phase of the carrier signal by
varying the sine and cosine inputs at precise times. PSK is used
widely for wireless LANs, RFID and Bluetooth communications.
The demodulator determines the phase of the signal received and
translates it back to the symbol it represents.

6. Why use modulation
The carrier wave used by radio frequency (RF) transmissions
doesn't carry much information itself. To include speech or data,
another wave has to be superimposed on the carrier wave, thus
changing the shape of the carrier wave. The process of doing so is
called modulation. To transmit sound, the audio signal must first
be converted into an electric signal, using a transducer. After
conversion, it is used to modulate a carrier signal.
Analog vs. digital
Modulation schemes can be analog or digital. An analog scheme
has an input wave that varies continuously like a sine wave. In
digital modulation scheme, voice is sampled at some rate and
then compressed and turned into a bit stream, and this in turn is
created into a particular kind of wave which is then superimposed
on the carrier signal.

7. Modulation and demodulation
Modulation is the process of encoding information in a transmitted signal, while
demodulation is the process of extracting information from the transmitted signal. Many
factors influence how faithfully the extracted information replicates the original input
information. Electromagnetic interference can degrade signals and make the original
signal impossible to extract. Demodulators typically include multiple stages of
amplification and filtering in order to eliminate interference.
A device that performs both modulation and demodulation is called a modem -- a name
created by combining the first letters of MOdulator and DEModulator.
A computer audio modem allows a computer to connect to another computer or to a
data network over a regular analog phone line by using the data signal to modulate an
analog audio tone. A modem at the far end demodulates the audio signal to recover the
data stream. A cable modem uses network data to modulate the cable service carrier
signal.
Sometimes a carrier signal can carry more than one modulating information
stream. Multiplexing combines the streams onto a single carrier -- e.g., by encoding a
fixed-duration segment of one, then of the next, for example, cycling through all the
channels before returning to the first -- a process called time-division multiplexing (TDM).
Another form is frequency-division multiplexing (FDM), where multiple carriers of
different frequencies are used on the same medium.
In another form, wavelength-division multiplexing (WDM) modulates multiple laser
wavelengths/frequencies on long-haul fiber links to increase the total available
bandwidth.

8. Why use modulation in communications?
Multiple carriers of different frequencies can often be
transmitted over a single media, with each carrier being
modulated by an independent signal. For example, Wi-Fi uses
individual channels to simultaneously transmit data to and from
multiple clients.
A carrier signal is used to reduce the wavelength for efficient
transmission and reception. Because the optimum antenna size
is one-half or one-quarter of a wavelength, an audio frequency
of 3000 Hz would need a wavelength of 100 km and a 25-
kilometer antenna. Instead, using an FM carrier of 100 MHz,
with a wavelength of 3 meters, the antenna would only need to
be 80 cm long.