View source for Physics:Signal modulation

{{Short description|Process of varying one or more properties of a periodic waveform}}
{{About|the electronic method to imprint data on a transmitted carrier wave|other uses of 'Modulation'|Modulation (disambiguation)}}
{{Multiple issues|
{{Technical|date=February 2017}}
}}

{{Modulation techniques}}
[[File:Modulation categorization.svg|thumb|300px|Categorization for signal modulation based on data and carrier types]]

'''Signal modulation''' is the process of varying one or more properties of a periodic [[Waveform|waveform]] in [[Engineering:Electronics|electronics]] and [[Engineering:Telecommunication|telecommunication]] for the purpose of transmitting information.

The process encodes information in the form of a message signal modulated onto a carrier signal to be transmitted.<ref name=tait>{{Cite web |date=2014-10-22 |title=How does modulation work? {{!}} Tait Radio Academy |url=https://www.taitradioacademy.com/topic/how-does-modulation-work-1-1/ |access-date=2024-06-17 |website=Tait Radio Academy}}</ref> For example, the message signal might be an [[Engineering:Audio signal|audio signal]] representing [[Physics:Sound|sound]] from a [[Microphone|microphone]], a video signal representing moving images from a [[Physics:Video camera|video camera]], or a digital signal representing a sequence of binary digits, a [[Bitstream|bitstream]] from a computer. 

Carrier waves are necessary when the frequency of the message is too low to practically transmit<ref name=tait />. Generally, receiving a [[Radio wave|radio wave]] requires a radio antenna with a length that is one-fourth of the wavelength of the transmitted wave.<ref>{{cite web |url=https://www.wimo.com/en/faq/post/what-antenna-length-do-i-need-for-which-frequency |title=General Antennas: What antenna length do I need for which frequency? |first=Rodrigo "Rod" |last=Herrera |date=April 23, 2024 |website=wimo.com |access-date=June 19, 2024 |lang=en}}</ref>  For low-frequency radio waves, wavelength is on the scale of kilometers and building such a large antenna is not practical.<ref name="jg">{{cite book |last1=Gibson |first1=Jerry |title=Principles of Digital and Analog Communications |date=1993 |publisher=Macmillan Publishing Company |location=New York |isbn=0023418605 |page=105}}</ref>

Another purpose of modulation is to transmit multiple [[Communication channel|channels]] of information through a single [[Physics:Transmission medium|communication medium]], using [[Frequency-division multiplexing|frequency-division multiplexing]] (FDM). For example, in [[Engineering:Cable television|cable television]] (which uses FDM), many carrier signals, each modulated with a different [[Engineering:Television channel|television channel]], are transported through a single cable to customers. Since each carrier occupies a different frequency, the channels do not interfere with each other. At the destination end, the carrier signal is demodulated to extract the information-bearing modulation signal.

A '''modulator''' is a device or circuit that performs modulation. A '''demodulator''' (sometimes ''[[Engineering:Detector (radio)|detector]]'') is a circuit that performs demodulation, the inverse of modulation. A [[Modem|modem]] (from '''mod'''ulator–'''dem'''odulator), used in bidirectional communication, can perform both operations. The lower frequency band occupied by the modulation signal is called the ''[[Baseband|baseband]]'', while the higher frequency band occupied by the modulated carrier is called the ''[[Passband|passband]]''.<ref>{{Cite web |date=2023-09-11 |title=What is Modem? |url=https://www.geeksforgeeks.org/computer-networks/what-is-modem/ |access-date=2025-07-20 |website=GeeksforGeeks |language=en-US}}</ref>

Signal modulation techniques are fundamental methods used in wireless communication to encode information onto a carrier wave by varying its amplitude, frequency, or phase. Key techniques and their typical applications  

'''Types of Signal Modulation'''<ref>{{Cite web |last=Technology |first=Electrical |date=2019-01-29 |title=Types of Modulation - AM, FM, PM Modulation & Modulated Signals |url=https://www.electricaltechnology.org/2019/01/types-of-modulation.html |access-date=2025-07-20 |website=ELECTRICAL TECHNOLOGY |language=en-US}}</ref><ref>{{Cite web |date=2024-07-11 |title=Modulation Schemes, Coding Rates, and 4G/5G Data Speeds |url=https://www.waveform.com/a/b/guides/modulation-coding-speeds |access-date=2025-07-20 |website=Waveform}}</ref><ref>{{Cite web |date=2020-10-05 |title=Wireless Fundamentals: Modulation |url=https://documentation.meraki.com/MR/Wi-Fi_Basics_and_Best_Practices/Wireless_Fundamentals%3A_Modulation |access-date=2025-07-20 |website=Cisco Meraki Documentation |language=en}}</ref><ref>{{Cite web |title=Modulation Methods {{!}} Electronics Basics {{!}} ROHM |url=https://www.rohm.com/electronics-basics/wireless/wireless_what3 |access-date=2025-07-20 |website=www.rohm.com}}</ref>
{| class="wikitable"
|+
!Category
!Modulation Type
!Key Features
!Example Uses
|-
|'''Analog Modulation'''
|AM
|Varies Amplitude of Carrier
|AM Radio
|-
|
|FM
|Varies Frequency of Carrier
|FM and Two-way Radio
|-
|
|PM
|Varies Phase of Carrier
|Analog TV and Satellite
|-
|'''Digital Modulation'''
|ASK
|Amplitude represents binary data
|RFID, Optical Comm
|-
|
|FSK
|Frequency shift encodes data
|Modems, Bluetooth
|-
|
|PSK
|Phase of carrier encodes bits
|Wi-Fi, Satellite
|-
|
|QPSK
|4 phase states: two bits per symbol
|DVB, LTE
|-
|
|QAM
|Both amplitude and phase vary; can transmit multiple bits
|Cable TV, Wi-Fi
|-
|
|OFDM
|Multiple carriers, each modulated separately
|4G/5G, Wi-Fi
|-
|'''Pulse Modulation'''
|PWM
|Pulse width represents amplitude
|Motor Control, Audio
|-
|
|PPM
|Pulse position within a time slot represents data
|Optical Comm., Radar
|-
|'''Spread Spectrum'''
|DSSS
|Signal bandwidth spread using code sequence
|CDMA, GPS
|-
|
|FHSS
|Carrier hops between different frequencies
|Bluetooth, Military
|}

* Amplitude Shift Keying (ASK): Varies the amplitude of the carrier signal to represent data. Simple and energy efficient, but vulnerable to noise. Used in RFID and sensor networks.<ref name=":0">{{Cite web |title=Modulation Methods {{!}} Electronics Basics {{!}} ROHM |url=https://www.rohm.com/electronics-basics/wireless/wireless_what3 |access-date=2025-07-20 |website=www.rohm.com}}</ref>

* Frequency Shift Keying (FSK): Changes the frequency of the carrier signal to encode information. Resistant to noise, simple in implementation, often used in telemetry and paging systems.<ref name=":0" />
* Phase Shift Keying (PSK): Modifies the phase of the carrier signal based on data. Common forms include Binary PSK (BPSK) and Quadrature PSK (QPSK), used in Wi-Fi, Bluetooth, and cellular networks. Offers good spectral efficiency and robustness against interference.<ref name=":0" />
* Quadrature Amplitude Modulation (QAM): Simultaneously varies both amplitude and phase to transmit multiple bits per symbol, increasing data rates. Used extensively in Wi-Fi, cable television, and LTE systems.<ref name=":0" />
* Orthogonal Frequency Division Multiplexing (OFDM): Splits the data across multiple, closely spaced sub-carriers, each modulated separately (often with QAM or PSK). Provides high spectral efficiency and robustness in multipath environments and is widely used in WLAN, LTE, and WiMAX.<ref name=":0" />
Other advanced techniques:
* Amplitude Phase Shift Keying (APSK): Combines features of PSK and QAM, mainly used in satellite communications for improved power efficiency.
* Spread Spectrum (e.g., DSSS): Spreads the signal energy across a wide band for robust, low probability of intercept transmission.
In '''analog modulation''', an [[Engineering:Analog signal|analog]] modulation signal is "impressed" on the carrier. Examples are [[Physics:Amplitude modulation|amplitude modulation]] (AM) in which the [[Company:Amplitude|amplitude]] (strength) of the carrier wave is varied by the modulation signal, and [[Physics:Frequency modulation|frequency modulation]] (FM) in which the [[Physics:Frequency|frequency]] of the carrier wave is varied by the modulation signal. These were the earliest types of modulation , and are used to transmit an [[Engineering:Audio signal|audio signal]] representing sound in AM and FM [[Engineering:Radio broadcasting|radio broadcasting]]. More recent systems use '''digital modulation''', which impresses a digital signal consisting of a sequence of [[Binary digit|binary digit]]s (bits), a [[Bitstream|bitstream]], on the carrier, by means of mapping bits to elements from a discrete alphabet to be transmitted. This alphabet can consist of a set of real or [[Complex number|complex number]]s, or sequences, like oscillations of different frequencies, so-called [[Engineering:Frequency-shift keying|frequency-shift keying]] (FSK) modulation. A more complicated digital modulation method that employs multiple carriers, [[Orthogonal frequency-division multiplexing|orthogonal frequency-division multiplexing]] (OFDM), is used in WiFi networks, digital radio stations and digital cable television transmission.

==Analog modulation methods{{anchor|Analog}}==
[[File:Amfm3-en-de.gif|thumb|right|200px|A low-frequency message signal (top) may be carried by an AM or FM radio wave.]]
[[File:Waterfall AM.jpg|thumb|[[Waterfall plot]] of a 146.52&nbsp;MHz radio carrier, with amplitude modulation by a 1,000&nbsp;Hz sinusoid. Two strong sidebands at + and - 1&nbsp;kHz from the carrier frequency are shown.]]
[[File:Waterfall FM.jpg|thumb| A carrier, frequency modulated by a 1,000&nbsp;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.]]

In [[Engineering:Analog signal|analog]] modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques include:<ref name="jg" />{{rp|105-168}}<ref name="hb" />{{rp|125,181-184}}
* [[Physics:Amplitude modulation|Amplitude modulation]] (AM) (here the amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal)
** Double-sideband modulation (DSB)
*** Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band)
*** [[Physics:Double-sideband suppressed-carrier transmission|Double-sideband suppressed-carrier transmission]] (DSB-SC)
*** [[Physics:Double-sideband reduced-carrier transmission|Double-sideband reduced-carrier transmission]] (DSB-RC)
** [[Physics:Single-sideband modulation|Single-sideband modulation]] (SSB, or SSB-AM)
*** Single-sideband modulation with carrier (SSB-WC)
*** Single-sideband modulation suppressed carrier modulation (SSB-SC)
** Vestigial-sideband modulation (VSB, or VSB-AM)
** [[Quadrature amplitude modulation]] (QAM)
* [[Physics:Angle modulation|Angle modulation]], which is approximately [[Constant envelope|constant envelope]]
** [[Physics:Frequency modulation|Frequency modulation]] (FM) (here the frequency of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal)
** [[Physics:Phase modulation|Phase modulation]] (PM) (here the phase shift of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal)
** Transpositional Modulation (TM), in which the waveform inflection is modified, resulting in a signal where each quarter cycle is transposed in the modulation process. TM is a pseudo-analog modulation (AM). Where an AM carrier also carries a phase variable phase f(ǿ). TM is f(AM,ǿ)
{{Clear}}

==Digital modulation methods==
<!-- This section is linked from [[Phase-shift keying]] -->

In [[Digital data|digital]] modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the ''modulation alphabet'').

[[File:baud.svg|thumb|right|200px|Schematic of 4 baud, 8 bit/s data link containing arbitrarily chosen values]]

<blockquote>'''A simple example:''' A telephone line is designed for transferring audible sounds, for example, tones, and not digital bits (zeros and ones). Computers may, however, communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the [[Symbol rate|symbol rate]] is 1000 symbols/second, or 1000 [[Baud|baud]]. Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the [[Bit rate|bit rate]] is twice the symbol rate, i.e. 2000 bits per second.</blockquote>

According to one definition of digital signal,<ref>{{Cite web|title=Modulation Methods {{!}} Electronics Basics {{!}} ROHM|url=https://www.rohm.com/electronics-basics/wireless/modulation-methods|website=www.rohm.com|access-date=2020-05-15}}</ref> the modulated signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as [[Analog transmission|analog transmission]].{{cn|date=September 2024}}

===Fundamental digital modulation methods===
The most fundamental digital modulation techniques are based on [[Engineering:Keying (telecommunications)|keying]]:<ref name="fx">{{cite book |last1=Xiong |first1=Fuqin |title=Digital Modulation Techniques |date=2000 |publisher=Artech House |location=Norwood |isbn=9780890069707 |page=8-9,13}}</ref>
* [[Phase-shift keying|PSK (phase-shift keying)]]: a finite number of phases are used.
* [[Engineering:Frequency-shift keying|FSK (frequency-shift keying)]]: a finite number of frequencies are used.
* [[Amplitude-shift keying|ASK (amplitude-shift keying)]]: a finite number of amplitudes are used.
* [[Quadrature amplitude modulation|QAM (quadrature amplitude modulation)]]: a finite number of at least two phases and at least two amplitudes are used.

In QAM, an in-phase signal (or I, with one example being a cosine waveform) and a quadrature phase signal (or Q, with an example being a sine wave) are amplitude modulated with a finite number of amplitudes and then summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK.

In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary [[Bit|bit]]s. 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, frequency or amplitude.

If the alphabet consists of <math>M = 2^N </math> alternative symbols, each symbol represents a message consisting of ''N'' bits. If the [[Symbol rate|symbol rate]] (also known as the baud rate) is <math>f_{S}</math> symbols/second (or [[Baud|baud]]), the data rate is <math>N f_{S}</math> bit/second.

For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate.

In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on a [[Constellation diagram|constellation diagram]], showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis for each symbol.

===Modulator and detector principles of operation===
PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex-valued signal ''I''+''jQ'' (where ''j'' is the [[Imaginary unit|imaginary unit]]). The resulting so-called equivalent lowpass signal or equivalent baseband signal is a complex-valued representation of the real-valued modulated physical signal (the so-called passband signal or RF signal).

These are the general steps used by the modulator to transmit data:
# Group the incoming data bits into codewords, one for each symbol that will be transmitted.
# Map the codewords to attributes, for example, amplitudes of the I and Q signals (the equivalent low-pass signal), or frequency or phase values.
# Adapt [[Pulse shaping|pulse shaping]] or some other filtering to limit the bandwidth and form the spectrum of the equivalent low-pass signal, typically using digital signal processing.
# Perform digital to analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using [[Digital signal processing|digital signal processing]], DSP).
# Generate a high-frequency sine carrier waveform, and perhaps also a cosine quadrature component. Carry out the modulation, for example, by multiplying the sine and cosine waveforms with the I and Q signals, resulting in the equivalent low-pass signal being frequency shifted to the modulated passband signal or RF signal. Sometimes this is achieved using DSP technology, for example, [[Direct digital synthesis|direct digital synthesis]] using a waveform table, instead of analog signal processing. In that case, the above DAC step should be done after this step.
# Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum.

At the receiver side, the demodulator typically performs:
# Bandpass filtering.
# [[Physics:Automatic gain control|Automatic gain control]], AGC (to compensate for [[Physics:Attenuation|attenuation]], for example fading).
# Frequency shifting of the RF signal to the equivalent baseband I and Q signals, or to an intermediate frequency (IF) signal, by multiplying the RF signal with a local oscillator sine wave and cosine wave frequency (see the [[Engineering:Superheterodyne receiver|superheterodyne receiver]] principle).
# Sampling and analog-to-digital conversion (ADC) (sometimes before or instead of the above point, for example by means of [[Undersampling|undersampling]]).
# Equalization filtering, for example, a [[Matched filter|matched filter]], compensation for multipath propagation, time spreading, phase distortion and frequency selective fading, to avoid [[Engineering:Intersymbol interference|intersymbol interference]] and symbol distortion.
# Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal.
# Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values.
# Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups).
# Parallel-to-serial conversion of the codewords into a bit stream.
# Pass 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 has 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.

Asynchronous methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier signal. In this case, modulation symbols (rather than bits, characters, or data packets) are [[Asynchronous communication|asynchronously]] transferred. The opposite is synchronous modulation.

===List of common digital modulation techniques===
The most common digital modulation techniques are:
* [[Phase-shift keying]] (PSK)
** Binary PSK (BPSK), using M=2 symbols
** Quadrature PSK (QPSK), using M=4 symbols
** 8PSK, using M=8 symbols
** 16PSK, using M=16 symbols
** Differential PSK (DPSK)
** Differential QPSK (DQPSK)
** Offset QPSK (OQPSK)
** π/4–QPSK
* [[Engineering:Frequency-shift keying|Frequency-shift keying]] (FSK)
** Audio frequency-shift keying (AFSK)
** Multi-frequency shift keying ([[M-ary transmission|M-ary]] FSK or MFSK)
** [[Engineering:Dual-tone multi-frequency signaling|Dual-tone multi-frequency signaling]] (DTMF)
* [[Amplitude-shift keying]] (ASK)
* [[Engineering:On-off keying|On-off keying]] (OOK), the most common ASK form
** M-ary vestigial sideband modulation, for example [[Physics:8VSB|8VSB]]
* [[Quadrature amplitude modulation]] (QAM), a combination of PSK and ASK
* [[Physics:Continuous phase modulation|Continuous phase modulation]] (CPM) methods
** [[Engineering:Minimum-shift keying|Minimum-shift keying]] (MSK)
** Gaussian minimum-shift keying (GMSK)
** Continuous-phase frequency-shift keying (CPFSK)
* [[Orthogonal frequency-division multiplexing]] (OFDM) modulation
** Discrete multitone (DMT), including adaptive modulation and bit-loading
* [[Wavelet modulation]]
* [[Trellis coded modulation]] (TCM), also known as [[Trellis modulation]]
* [[Spread spectrum]] techniques
** [[Direct-sequence spread spectrum]] (DSSS)
** [[Chirp spread spectrum]] (CSS) according to IEEE 802.15.4a CSS uses pseudo-stochastic coding
** [[Frequency-hopping spread spectrum]] (FHSS) applies a special scheme for channel release

[[Engineering:Minimum-shift keying|MSK]] and GMSK are particular cases of continuous phase modulation. 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 signaling).

OFDM is based on the idea of [[Frequency-division multiplexing|frequency-division multiplexing]] (FDM), but the multiplexed streams are all parts of a single original stream. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The modulated subcarriers are summed to form an OFDM signal. This dividing and recombining help with handling channel impairments. OFDM is considered as a modulation technique rather than a multiplex technique since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user [[Channel access method|channel access method]] in the [[Orthogonal frequency-division multiple access|orthogonal frequency-division multiple access]] (OFDMA) and [[Engineering:Multi-carrier code-division multiple access|multi-carrier code-division multiple access]] (MC-CDMA) schemes, allowing several users to share the same physical medium by giving different sub-carriers or spreading codes to different users.

Of the two kinds of [[Engineering:RF power amplifier|RF power amplifier]], switching amplifiers (Class D amplifiers) cost less and use less battery power than [[Engineering:Linear amplifier|linear amplifier]]s of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and CDMA, but not with QAM and OFDM. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often the QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers.

===Automatic digital modulation recognition (ADMR)===
Automatic digital modulation recognition in intelligent communication systems is one of the most important issues in [[Software-defined radio|software-defined radio]] and [[Physics:Cognitive radio|cognitive radio]]. According to incremental expanse of intelligent receivers, automatic modulation recognition becomes a challenging topic in telecommunication systems and computer engineering. Such systems have many civil and military applications. Moreover, blind recognition of modulation type is an important problem in commercial systems, especially in [[Software-defined radio|software-defined radio]]. Usually, in such systems, there are some extra information for system configuration, but considering blind approaches in intelligent receivers, we can reduce information overload and increase transmission performance. Obviously, with no knowledge of the transmitted data and many unknown parameters at the receiver, such as the signal power, carrier frequency and phase offsets, timing information, etc., blind identification of the modulation is made fairly difficult. This becomes even more challenging in real-world scenarios with multipath fading, frequency-selective and time-varying channels.<ref>
{{cite journal
 | title = Survey of automatic modulation classification techniques: classical approaches and new trends
 | author = Dobre, Octavia A., Ali Abdi, Yeheskel Bar-Ness, and Wei Su. Communications, IET 1, no. 2 (2007): 137–156.
 | journal= IET Communications
 | volume = 1
 | issue = 2
 | year = 2007
 | pages = 137–156
 | url = http://web.njit.edu/~abdi/IEE_COM0176_WithFigures.pdf
 | doi = 10.1049/iet-com:20050176
 }}</ref>

There are two main approaches to automatic modulation recognition. The first approach uses likelihood-based methods to assign an input signal to a proper class. Another recent approach is based on feature extraction.

===Digital baseband modulation===
Digital baseband modulation changes the characteristics of a baseband signal, i.e., one without a carrier at a higher frequency.

This can be used as an equivalent signal to be later [[Frequency mixer|frequency-converted]] to a carrier frequency, or for direct communication in baseband. The latter methods both involve relatively simple [[Line code|line code]]s, as often used in local buses, and complicated baseband signalling schemes such as used in DSL.

==Pulse modulation methods==
Pulse modulation schemes aim at transferring a narrowband analog signal over an analog baseband channel as a two-level signal by modulating a [[Pulse wave|pulse wave]]. Some pulse modulation schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e., as a [[Quantization (signal processing)|quantized]] discrete-time signal) with a fixed bit rate, which can be transferred over an underlying digital transmission system, for example, some [[Line code|line code]]. These are not modulation schemes in the conventional sense since they are not channel coding schemes, but should be considered as source coding schemes, and in some cases, analog-to-digital conversion techniques.<ref name="jg" />{{rp|233-259}}<ref name="hb">{{cite book |last1=Black |first1=Harold |title=Modulation Theory |date=1953 |publisher=D. Van Nostrand Company, Inc. |location=New York |page=237}}</ref>

;Analog-over-analog methods
* [[Engineering:Pulse-amplitude modulation|Pulse-amplitude modulation]] (PAM)
* [[Pulse-width modulation]] (PWM) and pulse-depth modulation (PDM)
* [[Engineering:Pulse-frequency modulation|Pulse-frequency modulation]] (PFM)
* [[Engineering:Pulse-position modulation|Pulse-position modulation]] (PPM)

;Analog-over-digital methods
* [[Pulse-code modulation]] (PCM)
** Differential PCM (DPCM)
*** Adaptive DPCM (ADPCM)
* [[Delta modulation]] (DM or Δ-modulation)
** [[Delta-sigma modulation]] (ΣΔ)
** [[Continuously variable slope delta modulation]] (CVSDM), also called adaptive delta modulation (ADM)
* [[Pulse-density modulation]] (PDM)

==Miscellaneous modulation techniques==
* The use of [[Engineering:On-off keying|on-off keying]] to transmit [[Engineering:Morse code|Morse code]] at radio frequencies is known as [[Continuous wave|continuous wave]] (CW) operation.
* Adaptive modulation
* [[Engineering:Space modulation|Space modulation]] is a method whereby signals are modulated within airspace such as that used in [[Engineering:Instrument landing system|instrument landing system]]s.
* The [[Physics:Microwave auditory effect|microwave auditory effect]] has been pulse modulated with audio waveforms to evoke understandable spoken numbers.<ref>{{cite book |last= Lin|first= James C.|date= August 20, 2021|title= Auditory Effects of Microwave Radiation|location= Chicago|publisher= Springer|page= 326|isbn= 978-3030645434}}</ref><ref>{{cite magazine |last= Justesen|first= Don|date= March 1, 1975|title= Microwaves and Behavior|url= http://www.mitchelleffect.com/1973_voice_to_skull.pdf|magazine= American Psychologist|location= Washington, D.C.|publisher= American Psychological Association|archive-url= https://web.archive.org/web/20160910133313/http://www.mitchelleffect.com/1973_voice_to_skull.pdf|access-date= October 5, 2021|archive-date= 2016-09-10}}</ref><ref>{{cite magazine |last= Justesen|first= Don|date= March 1, 1975|title= Microwaves and Behavior|magazine= American Psychologist|volume= 30|issue= 3|pages= 391–401|location= Washington, D.C.|publisher= American Psychological Association|doi= 10.1037/0003-066x.30.3.391|pmid= 1137231}}</ref>

==See also==

{{div col|colwidth=20em}}
* [[Channel access method]]s
* Channel coding
* [[Codec]]
* Demodulation
* [[Electrical resonance]]
* [[Heterodyne]]
* [[Line code]]
* [[Modem]]
* [[Medicine:Neuromodulation|Neuromodulation]]
* [[Engineering:RF modulator|RF modulator]]
* [[Physics:Ring modulation|Ring modulation]]
* [[Physics:Types of radio emissions|Types of radio emissions]]

{{div col end}}

==References==
{{Reflist}}

==Further reading==
*{{cite journal
|url=http://www.analog.com/library/analogDialogue/archives/47-06/multipliers_modulators.pdf
|title=Multipliers vs. Modulators
|first1=James|last1=Bryant
|last2=Analog Devices
|journal=Analog Dialogue
|volume=47
|issue=2
|page=3
|year=2013
}}

==External links==
* [http://webdemo.inue.uni-stuttgart.de/webdemos/02_lectures/communication_3/soft_demapping Interactive presentation of soft-demapping for AWGN-channel in a web-demo] Institute of Telecommunications, University of Stuttgart
* [http://ittrap.com/modemmodulation-and-demodulation/ Modem (Modulation and Demodulation)]
* [https://www.ac.uma.es/~guille/codsim2.0/ CodSim 2.0: Open source Virtual Laboratory for Digital Data Communications Model] Department of Computer Architecture, University of Malaga. Simulates Digital line encodings and Digital Modulations. Written in HTML for any web browser.

{{Audio broadcasting}}
{{Digital_modes|state=collapsed}}
{{Telecommunications}}

[[Category:Frequency mixers]]
[[Category:Radio technology]]
[[Category:Television technology]]
[[Category:Physical layer protocols]]
[[Category:Radio modulation modes| ]]
[[Category:Telecommunication theory]]
[[Category:Television terminology]]

{{Sourceattribution|Signal modulation}}