Editing Signal modulation (section)

{{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)}}
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{{Technical|date=February 2017}}
{{More citations needed|date=June 2008}}
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{{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]] in [[electronics]] and [[telecommunication]] for the purpose of transmitting information.

The process encodes information in form of the modulation or message signal onto a [[carrier signal]] to be transmitted.<ref>{{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 [[audio signal]] representing [[sound]] from a [[microphone]], a [[video signal]] representing moving images from a [[video camera]], or a [[digital signal]] representing a sequence of binary digits, a [[bitstream]] from a computer. 

This carrier wave usually has a much higher [[frequency]] than the message signal does. This is because it is impractical to transmit signals with low frequencies. Generally, receiving a [[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.

Another purpose of modulation is to transmit multiple [[Communication channel|channels]] of information through a single [[transmission medium|communication medium]], using [[frequency-division multiplexing]] (FDM). For example, in [[cable television]] (which uses FDM), many carrier signals, each modulated with a different [[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 [[demodulation|demodulated]] to extract the information bearing modulation signal.

A '''modulator''' is a device or [[electric circuit|circuit]] that performs modulation. A '''demodulator''' (sometimes ''[[detector (radio)|detector]]'') is a circuit that performs [[demodulation]], the inverse of modulation. A [[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]]'', while the higher frequency band occupied by the modulated carrier is called the ''[[passband]]''.{{Citation needed|date=March 2023}}

In '''analog modulation''', an [[analog signal|analog]] modulation signal is "impressed" on the carrier. 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. These were the earliest types of modulation{{Citation needed|date=March 2023}}, and are used to transmit an [[audio signal]] representing sound in AM and FM [[radio broadcasting]]. More recent systems use '''digital modulation''', which impresses a [[digital signal]] consisting of a sequence of [[binary digit]]s (bits), a [[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]]s, or sequences, like oscillations of different frequencies, so-called [[frequency-shift keying]] (FSK) modulation. A more complicated digital modulation method that employs multiple carriers, [[orthogonal frequency-division multiplexing]] (OFDM), is used in [[WiFi]] networks, [[digital radio]] stations and digital cable television transmission.