Modulation
:
For the musical use of "modulation", see modulation (music).:
"Modulated" redirects here, for Electrical Engineering use of "modulated", see module (programming).Modulation is the process of varying a
carrier signal in order to use that signal to convey
information. The three key parameters of a sinusoid are its
amplitude, its
phase and its
frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. There are several reasons to modulate a signal before transmission in a medium. These include the ability of different users sharing a medium (multiple access), and making the signal properties physically compatible with thepropagation medium. A device that performs modulation is known as a
modulator and a device that performs the inverse operation of demodulation is known as a
demodulator. A device that can do both operations is a
modem (a contraction of the two terms).
In
digital modulation, the changes in the signal are chosen from a fixed list (the
modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a
constellation diagram.
In
analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.
Modulation is generally performed to overcome signal transmission issues such as to allow
* Easy (low loss, low dispersion)
propagation as
electromagnetic waves
*
Multiplexing — the transmission of multiple data signals in one frequency band, on different
carrier frequencies.
* Smaller, more
directional antennas
Carrier signals are usually high frequency
electromagnetic waves.
* Angular modulation
**
Phase modulation (PM)
**
Frequency modulation (FM)
*
Amplitude modulation (AM)
**
Single-sideband modulation (SSB, or SSB-AM), very similar to
single-sideband suppressed carrier modulation (SSB-SC)
**
Vestigial-sideband modulation (VSB, or VSB-AM)
*
Sigma-delta modulation (∑Δ)
Any form of digital modulation necessarily uses a
finite number of distinct signals to represent digital data.
* In the case of
PSK, a finite number of phases are used.
* In the case of
FSK, a finite number of frequencies are used.
* In the case of
ASK, a finite number of amplitudes are used. This is very similar to
pulse code modulationEach of these phases, frequencies or amplitudes are assigned a unique pattern of
binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the
symbol that is represented by the particular phase.
These are the general steps used by the
modulator to transmit data:# Accept incoming digital data;# Group the data into symbols;# Use these symbols to
set or
change the phase, frequency or amplitude of the reference signal appropriately;# Pass the modulated signal on for further processing, such as filtering and channel-coding, before transmission.
At the receiver, the
demodulator# Is passed the de-filtered and de-channel-coded signal;# Determines its phase, frequency or amplitude;# Maps the phase, frequency or amplitude to its corresponding symbol;# Translates the symbol into its individual bits;# Passes the resultant bit stream on for further processing such as removal of any error-correcting codes.
As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.
The principal classes of modulation are:
*
Phase-shift keying (PSK)
*
Frequency-shift keying (FSK) and
audio frequency-shift keying (AFSK)
**
Minimum-shift keying (MSK)
**
Gaussian minimum-shift keying (GMSK)
**
Very minimum-shift keying (VMSK)
*
Amplitude-shift keying (ASK) and its most common form,
on-off keying (OOK)
*
Quadrature amplitude modulation (QAM) a combination of PSK and ASK
*
Continuous phase modulation (CPM)
*
Trellis coded modulation (TCM) also known as
trellis modulation*
Polar modulation like QAM a combination of PSK and ASK, but using a different circuit architecture
MSK and
GMSK are particular cases of
continuous phase modulation (CPM). Indeed, MSK is a particular case of the sub-family of CPM known as
continuous phase-frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signalling).
Often incorrectly referred to as a modulation scheme,
orthogonal frequency division multiplexing (OFDM) usually takes advantage of one of the digital techniques. It is also known as
discrete multitone (DMT). When OFDM is used in conjunction with
channel coding techniques, it is described as
Coded orthogonal frequency division multiplexing (COFDM). OFDM is strictly a
channel access method and not a modulation scheme.
These are hybrid digital and analogue techniques.
*
Pulse-code modulation (PCM)
*
Pulse-width modulation (PWM)
*
Pulse-amplitude modulation (PAM)
*
Pulse-position modulation (PPM)
*
Pulse-density modulation (PDM)
*The use of
on-off keying to transmit
Morse code at
radio frequencies is known as
continuous wave (CW) operation.
*
Adaptive modulation*
Wavelet modulation*
Electrical resonance *
Order of the modulation*
Types of radio emissions*
Communications channel*
Channel access methods
*
Channel coding*
Line code*
Telecommunication*
Modem*
RF modulator*
Codec*
ring modulation*
"Data Encoding Techniques" and
"Specifications for Data Encoding" discuss the various encoding techniques that have been used with various types of Ethernet.
*
" Modulation Trainer Kits" - Digital communication trainer kits PCM, DPCM, ASK, FSK, PSK, DPSK, QPSK ...
