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.
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.
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.
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 side band.
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.
