Modulation

Categorization for signal modulation based on data and carrier types
A low-frequency message signal (top) may be carried by an AM or FM radio wave.
Waterfall plot of a 146.52 MHz radio carrier, with amplitude modulation by a 1,000 Hz sinusoid. Two strong sidebands at + and - 1 kHz from the carrier frequency are shown.
A carrier, frequency modulated by a 1,000 Hz sinusoid. The modulation index has been adjusted to around 2.4, so the carrier frequency has small amplitude. Several strong sidebands are apparent; in principle an infinite number are produced in FM but the higher-order sidebands are of negligible magnitude.
Schematic of 4 baud, 8 bit/s data link containing arbitrarily chosen values

Process of varying one or more properties of a periodic waveform, called the carrier signal, with a separate signal called the modulation signal that typically contains information to be transmitted.

- Modulation
Categorization for signal modulation based on data and carrier types

50 related topics

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Orthogonal frequency-division multiplexing

Type of digital transmission and a method of encoding digital data on multiple carrier frequencies.

Type of digital transmission and a method of encoding digital data on multiple carrier frequencies.

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Subcarriers system of OFDM signals after FFT

Based on this feedback information, adaptive modulation, channel coding and power allocation may be applied across all subcarriers, or individually to each subcarrier.

Figure 1: An audio signal (top) may be carried by a carrier signal using AM or FM methods.

Amplitude modulation

Figure 1: An audio signal (top) may be carried by a carrier signal using AM or FM methods.
One of the crude pre-vacuum tube AM transmitters, a Telefunken arc transmitter from 1906. The carrier wave is generated by 6 electric arcs in the vertical tubes, connected to a tuned circuit. Modulation is done by the large carbon microphone (cone shape) in the antenna lead.
One of the first vacuum tube AM radio transmitters, built by Meissner in 1913 with an early triode tube by Robert von Lieben. He used it in a historic 36 km (24 mi) voice transmission from Berlin to Nauen, Germany. Compare its small size with above transmitter.
Illustration of amplitude modulation
Figure 2: Double-sided spectra of baseband and AM signals.
Figure 3: The spectrogram of an AM voice broadcast shows the two sidebands (green) on either side of the carrier (red) with time proceeding in the vertical direction.
Figure 4: Modulation depth. In the diagram, the unmodulated carrier has an amplitude of 1.
Anode (plate) modulation. A tetrode's plate and screen grid voltage is modulated via an audio transformer. The resistor R1 sets the grid bias; both the input and output are tuned circuits with inductive coupling.

Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave.

A portable battery-powered AM/FM broadcast receiver, used to listen to audio broadcast by local radio stations.

Radio receiver

Electronic device that receives radio waves and converts the information carried by them to a usable form.

Electronic device that receives radio waves and converts the information carried by them to a usable form.

A portable battery-powered AM/FM broadcast receiver, used to listen to audio broadcast by local radio stations.
A modern communications receiver, used in two-way radio communication stations to talk with remote locations by shortwave radio.
Girl listening to vacuum tube radio in the 1940s. During the golden age of radio, 1925–1955, families gathered to listen to the home radio receiver in the evening
A bedside clock radio that combines a radio receiver with an alarm clock
Symbol for an antenna
Symbol for a bandpass filter used in block diagrams of radio receivers
Symbol for an amplifier
Symbol for a demodulator
Envelope detector circuit
How an envelope detector works
Block diagram of a tuned radio frequency receiver. To achieve enough selectivity to reject stations on adjacent frequencies, multiple cascaded bandpass filter stages had to be used. The dotted line indicates that the bandpass filters must be tuned together.
Block diagram of a superheterodyne receiver. The dotted line indicates that the RF filter and local oscillator must be tuned in tandem.
Block diagram of a dual-conversion superheterodyne receiver
Guglielmo Marconi, who built the first radio receivers, with his early spark transmitter (right) and coherer receiver (left) from the 1890s. The receiver records the Morse code on paper tape
Generic block diagram of an unamplified radio receiver from the wireless telegraphy era
Example of transatlantic radiotelegraph message recorded on paper tape by a siphon recorder at RCA's New York receiving center in 1920. The translation of the Morse code is given below the tape.
Coherer from 1904 as developed by Marconi.
Experiment to use human brain as a radio wave detector, 1902
Magnetic detector
Electrolytic detector
A galena cat's whisker detector from a 1920s crystal radio
Marconi's inductively coupled coherer receiver from his controversial April 1900 "four circuit" patent no. 7,777.
Radio receiver with Poulsen "tikker" consisting of a commutator disk turned by a motor to interrupt the carrier.
Fessenden's heterodyne radio receiver circuit
Unlike today, when almost all radios use a variation of the superheterodyne design, during the 1920s vacuum tube radios used a variety of competing circuits.
During the "Golden Age of Radio" (1920 to 1950), families gathered to listen to the home radio in the evening, such as this Zenith console model 12-S-568 from 1938, a 12-tube superheterodyne with pushbutton tuning and 12-inch cone speaker.
De Forest's first commercial Audion receiver, the RJ6 which came out in 1914. The Audion tube was always mounted upside down, with its delicate filament loop hanging down, so it did not sag and touch the other electrodes in the tube.
Block diagram of regenerative receiver
Circuit of single tube Armstrong regenerative receiver
Armstrong presenting his superregenerative receiver, June 28, 1922, Columbia University
Hazeltine's prototype Neutrodyne receiver, presented at a March 2, 1923 meeting of the Radio Society of America at Columbia University.
Block diagram of simple single tube reflex receiver
The first superheterodyne receiver built at Armstrong's Signal Corps laboratory in Paris during World War I. It is constructed in two sections, the mixer and local oscillator (left) and three IF amplification stages and a detector stage (right). The intermediate frequency was 75 kHz.
A Zenith transistor based portable radio receiver
A modern smartphone has several RF CMOS digital radio transmitters and receivers to connect to different devices, including a cellular receiver, wireless modem, Bluetooth modem, and GPS receiver.

Modulation is the process of adding information to a radio carrier wave.

The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal amplitude or power. It consists of a signal (C) at the carrier wave frequency fC, with the modulation contained in narrow frequency bands called sidebands (SB) just above and below the carrier.

Carrier wave

The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal amplitude or power. It consists of a signal (C) at the carrier wave frequency fC, with the modulation contained in narrow frequency bands called sidebands (SB) just above and below the carrier.

In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information.

Acoustic coupler modems used a telephone handset as the audio medium, with the user dialing the desired number and then pressing the handset into the modem to complete the connection. These systems generally operated at a speed of 300 bits per second.

Modem

Computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio.

Computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio.

Acoustic coupler modems used a telephone handset as the audio medium, with the user dialing the desired number and then pressing the handset into the modem to complete the connection. These systems generally operated at a speed of 300 bits per second.
Collection of modems once used in Australia, including dial-up, DSL, and cable modems.
TeleGuide terminal
The original 300-baud Hayes Smartmodem
USRobotics Sportster 14,400 Fax modem (1994)
V.34 modem implemented as an internal ISA card
V.34 data/fax modem as PC card for notebooks
Dial-up modem bank at an ISP
The Novation CAT acoustically coupled modem
A PCI Winmodem soft modem (on the left) next to a conventional ISA modem (on the right)
DSL modem
Cable modem
A bluetooth radio module with built-in antenna (left)
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An ONT providing data, telephone and television service
Null modem adapter

A modem transmits data by modulating one or more carrier wave signals to encode digital information, while the receiver demodulates the signal to recreate the original digital information.

A variety of radio antennas on Sandia Peak near Albuquerque, New Mexico, US

Radio

Technology of signaling and communicating using radio waves.

Technology of signaling and communicating using radio waves.

A variety of radio antennas on Sandia Peak near Albuquerque, New Mexico, US
Radio communication. Information such as sound is converted by a transducer such as a microphone to an electrical signal, which modulates a radio wave produced by the transmitter. A receiver intercepts the radio wave and extracts the information-bearing modulation signal, which is converted back to a human usable form with another transducer such as a loudspeaker.
Comparison of AM and FM modulated radio waves
Frequency spectrum of a typical modulated AM or FM radio signal. It consists of a component C at the carrier wave frequency f_c with the information (modulation) contained in two narrow bands of frequencies called sidebands (SB) just above and below the carrier frequency.
Satellite television dish on a residence
Satellite phones, showing the large antennas needed to communicate with the satellite
Firefighter using walkie-talkie
VHF marine radio on a ship
Parabolic antennas of microwave relay links on tower in Australia
RFID tag from a DVD
Satellite Communications Center Dubna in Russia
Communications satellite belonging to Azerbaijan
Military air traffic controller on US Navy aircraft carrier monitors aircraft on radar screen
ASR-8 airport surveillance radar antenna. It rotates once every 4.8 seconds. The rectangular antenna on top is the secondary radar.
Rotating marine radar antenna on a ship
A personal navigation assistant GPS receiver in a car, which can give driving directions to a destination.
EPIRB emergency locator beacon on a ship
Wildlife officer tracking radio-tagged mountain lion
US Air Force MQ-1 Predator drone flown remotely by a pilot on the ground
Remote keyless entry fob for a car
Quadcopter, a popular remote-controlled toy

In radio communication, used in radio and television broadcasting, cell phones, two-way radios, wireless networking, and satellite communication, among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by modulating the radio signal (impressing an information signal on the radio wave by varying some aspect of the wave) in the transmitter.

Illustration of the spectrum of AM and SSB signals. The lower side band (LSB) spectrum is inverted compared to the baseband. As an example, a 2 kHz audio baseband signal modulated onto a 5 MHz carrier will produce a frequency of 5.002 MHz if upper side band (USB) is used or 4.998 MHz if LSB is used.

Single-sideband modulation

Illustration of the spectrum of AM and SSB signals. The lower side band (LSB) spectrum is inverted compared to the baseband. As an example, a 2 kHz audio baseband signal modulated onto a 5 MHz carrier will produce a frequency of 5.002 MHz if upper side band (USB) is used or 4.998 MHz if LSB is used.
Frequency-domain depiction of the mathematical steps that convert a baseband function into a single-sideband radio signal.
A Collins KWM-1, an early Amateur Radio transceiver that featured SSB voice capability
VSB modulation

In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves.

Spectrum of a baseband signal, energy E per unit frequency as a function of frequency f. The total energy is the area under the curve.

Baseband

Spectrum of a baseband signal, energy E per unit frequency as a function of frequency f. The total energy is the area under the curve.
Comparison of the equivalent baseband version of a signal and its AM-modulated (double-sideband) RF version, showing the typical doubling of the occupied bandwidth.

In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies.

Constellation diagram example for BPSK

Phase-shift keying

Constellation diagram example for BPSK
Constellation diagram for QPSK with Gray coding. Each adjacent symbol only differs by one bit.
Conceptual transmitter structure for QPSK. The binary data stream is split into the in-phase and quadrature-phase components. These are then separately modulated onto two orthogonal basis functions. In this implementation, two sinusoids are used. Afterwards, the two signals are superimposed, and the resulting signal is the QPSK signal. Note the use of polar non-return-to-zero encoding. These encoders can be placed before for binary data source, but have been placed after to illustrate the conceptual difference between digital and analog signals involved with digital modulation.
Receiver structure for QPSK. The matched filters can be replaced with correlators. Each detection device uses a reference threshold value to determine whether a 1 or 0 is detected.
Timing diagram for QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown at the top, and the total combined signal at the bottom. Note the abrupt changes in phase at some of the bit-period boundaries.
Signal doesn't pass through the origin, because only one bit of the symbol is changed at a time.
Difference of the phase between QPSK and OQPSK
Timing diagram for offset-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note the half-period offset between the two signal components.
Dual constellation diagram for π/4-QPSK. This shows the two separate constellations with identical Gray coding but rotated by 45° with respect to each other.
Timing diagram for π/4-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note that successive symbols are taken alternately from the two constellations, starting with the "blue" one.
Constellation diagram for 8-PSK with Gray coding
BER comparison between DBPSK, DQPSK and their non-differential forms using Gray coding and operating in white noise
Differential encoding/decoding system diagram
Mutual information of PSK over the AWGN channel

Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave).

Example of QPSK carrier recovery phase error causing a fixed rotational offset of the received symbol constellation, X, relative to the intended constellation, O.

Demodulation

Extracting the original information-bearing signal from a carrier wave.

Extracting the original information-bearing signal from a carrier wave.

Example of QPSK carrier recovery phase error causing a fixed rotational offset of the received symbol constellation, X, relative to the intended constellation, O.
Receiver structure for QPSK. The matched filters can be replaced with correlators. Each detection device uses a reference threshold value to determine whether a 1 or 0 is detected.

There are many types of modulation so there are many types of demodulators.