Figure 1: An audio signal (top) may be carried by a carrier signal using AM or FM methods.
Categorization for signal modulation based on data and carrier types
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.
A low-frequency message signal (top) may be carried by an AM or FM radio wave.
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.
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.
Illustration of amplitude modulation
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.
Figure 2: Double-sided spectra of baseband and AM signals.
Schematic of 4 baud, 8 bit/s data link containing arbitrarily chosen values
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.

- Amplitude modulation

Examples are amplitude modulation (AM) in which the amplitude (strength) of the carrier wave is varied by the modulation signal, and frequency modulation (FM) in which the frequency of the carrier wave is varied by the modulation signal.

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

9 related topics

Alpha

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.

A refinement of amplitude modulation, it uses transmitter power and bandwidth more efficiently.

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.

The first type of modulation used to transmit sound over radio waves was amplitude modulation (AM), invented by Reginald Fessenden around 1900.

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.

Most radio systems in the 20th century used frequency modulation (FM) or amplitude modulation (AM) to add information to the carrier.

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.

A key consequence of the usual double-sideband amplitude modulation (AM) is that the range of frequencies the signal spans (its spectral bandwidth) is doubled.

A coherer detector, useful only for Morse code signals.

Detector (radio)

A coherer detector, useful only for Morse code signals.
A simple envelope detector
A simple crystal radio with no tuned circuit can be used to listen to strong AM broadcast signals
A ratio detector using solid-state diodes

In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage.

After sound (amplitude modulation, AM) transmission began around 1920, the term evolved to mean a demodulator, (usually a vacuum tube) which extracted the audio signal from the radio frequency carrier wave.

Analog QAM: measured PAL color bar signal on a vector analyzer screen.

Quadrature amplitude modulation

Analog QAM: measured PAL color bar signal on a vector analyzer screen.
Digital 16-QAM with example constellation points
Constellation points for 4-QAM, 16-QAM, 32-QAM, and 64-QAM overlapped
Bit-loading (bits per QAM constellation) on an ADSL line

Quadrature amplitude modulation (QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information.

It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme.

Antenna used for transmission of radio signals

Double-sideband reduced-carrier transmission

Antenna used for transmission of radio signals

Double-sideband reduced carrier transmission (DSB-RC): transmission in which (a) the frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier and (b) the carrier level is reduced for transmission at a fixed level below that which is provided to the modulator.

Categorization for signal modulation based on data and carrier types

Angle modulation

Categorization for signal modulation based on data and carrier types

Angle modulation is a class of carrier modulation that is used in telecommunications transmission systems.

This contrasts with varying the amplitude of the carrier, practiced in amplitude modulation (AM) transmission, the earliest of the major modulation methods used widely in early radio broadcasting.

Long wave radio broadcasting station, Motala, Sweden

Radio broadcasting

Transmission of audio , sometimes with related metadata, by radio waves to radio receivers belonging to a public audience.

Transmission of audio , sometimes with related metadata, by radio waves to radio receivers belonging to a public audience.

Long wave radio broadcasting station, Motala, Sweden
Slovak Radio Building, Bratislava, Slovakia (architects: Štefan Svetko, Štefan Ďurkovič and Barnabáš Kissling, 1967–1983)
Broadcasting tower in Trondheim, Norway
Advertisement placed in the November 5, 1919 Nieuwe Rotterdamsche Courant announcing PCGG's debut broadcast scheduled for the next evening.
Use of a sound broadcasting station
Transmission diagram of sound broadcasting (AM and FM)
AM broadcasting stations in 2006
FM radio broadcast stations in 2006
Worldwide presence of Radio Maria broadcasters.

Radio stations broadcast with several different types of modulation: AM radio stations transmit in AM (amplitude modulation), FM radio stations transmit in FM (frequency modulation), which are older analog audio standards, while newer digital radio stations transmit in several digital audio standards: DAB (digital audio broadcasting), HD radio, DRM (Digital Radio Mondiale).