Frequency modulation: Difference between revisions
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In [[Analog signal|analog]] frequency modulation, such as radio broadcasting of voice and music, the instantaneous [[frequency deviation]], i.e. the difference between the frequency of the carrier and its center frequency, has a functional relation to the modulating signal amplitude. | In [[Analog signal|analog]] frequency modulation, such as radio broadcasting of voice and music, the instantaneous [[frequency deviation]], i.e. the difference between the frequency of the carrier and its center frequency, has a functional relation to the modulating signal amplitude. | ||
[[Digital data]] can be encoded and transmitted | [[Digital data]] can be encoded and transmitted using a form of frequency modulation known as [[frequency-shift keying]] (FSK), in which the frequency of a carrier is switched among a discrete set of values. In its simplest form, binary FSK, two frequencies represent binary symbols 0 and 1. FSK is widely used in low to moderate data-rate applications because of its simplicity and robustness. Common uses include early computer [[Modem|modems]] (such as fax modems), telephone [[Caller ID|caller-ID]] systems, garage-door openers, remote keyless entry systems, and [[radioteletype]].<ref>{{cite book |last=Gibilisco |first=Stan |title=Teach yourself electricity and electronics |url=https://archive.org/details/teachyourselfele00gibi |url-access=registration |quote=morse-code frequency-shift-keying sent-using-fsk. |publisher=McGraw-Hill Professional |year=2002 |page=[https://archive.org/details/teachyourselfele00gibi/page/477 477] |isbn=978-0-07-137730-0}}</ref><ref>{{cite book |last=Rutledge |first=David B. |title=The Electronics of Radio |url=https://books.google.com/books?id=ZvJYLhk4N64C&q=radio-teletype+fsk&pg=RA2-PA310 |publisher=Cambridge University Press |year=1999 |page=310 |isbn=978-0-521-64645-1}}</ref> | ||
Frequency modulation is widely used for [[FM | Frequency modulation is widely used for [[FM broadcasting]]. It is also used in [[telemetry]], [[radar]], seismic prospecting, and monitoring [[newborn]]s for seizures via [[EEG]],<ref>B. Boashash, editor, ''Time-Frequency Signal Analysis and Processing – A Comprehensive Reference'', Elsevier Science, Oxford, 2003; {{ISBN|0-08-044335-4}}</ref> [[two-way radio]] systems, [[Frequency modulation synthesis|sound synthesis]], magnetic tape-recording systems and some video-transmission systems. In radio transmission, an advantage of frequency modulation is that it has a larger [[signal-to-noise ratio]] and therefore rejects [[radio frequency interference]] better than an equal power [[amplitude modulation]] (AM) signal. For this reason, most music is broadcast over FM radio. | ||
Frequency modulation and [[phase modulation]] are the two complementary principal methods of [[angle modulation]]; phase modulation is often used as an intermediate step to achieve frequency modulation. These methods contrast with [[amplitude modulation]], in which the [[amplitude]] of the carrier wave varies, while the frequency and phase remain constant. | Frequency modulation and [[phase modulation]] are the two complementary principal methods of [[angle modulation]]; phase modulation is often used as an intermediate step to achieve frequency modulation. These methods contrast with [[amplitude modulation]], in which the [[amplitude]] of the carrier wave varies, while the frequency and phase remain constant. | ||
== | ==FM Signal== | ||
According to [[Paul J. Nahin|Paul Nahin]], "To apply the [[baseband]] signal of a microphone output directly to the transmitter antenna won't work, because ...a quarter-wavelength antenna at audio frequencies is physically enormous. To have a reasonably sized antenna requires a transmitter signal at frequencies considerably higher than those of the bandwidth spectrum; that is, the baseband spectrum must be upshifted to the radio frequencies."<ref name="pn">{{cite book |last1=Nahin |first1=Paul |title=The Mathematical Radio: Inside the Magic of AM, FM, and Single-Sideband |date=2024 |publisher=Princeton University Press |location=Princeton |isbn=978-0-691-23531-8 |pages=74,210–213}}</ref> This is called [[signal modulation]]. According to [[Ron Bertrand]], "Frequency modulation is a method of modulating a carrier wave whereby the modulating audio causes the instantaneous frequency of the carrier to change. Without modulation, an FM transmitter produces a single carrier frequency."<ref name=rb>{{cite book |last1=Bertrand |first1=Ron |title=Radio Handbook |date=2022 |isbn=979-8-3625-5372-2 |pages=405,408–409}}</ref> | |||
:<math>\begin{align} | The FM signal produced by a [[sinusoidal]] carrier of frequency ω<sub>c</sub>, modulated by an audio tone of frequency ω<sub>a</sub> with amplitude A, can be written as:<ref name=pn/> | ||
:<math>\begin{align} | |||
e(t) &= \sin \left(\omega_c t + kA \sin\left(\omega_at\right)\right) \\ | |||
\end{align}</math> | \end{align}</math> | ||
We need the [[Instantaneous phase and frequency| instantaneous frequency]], which describes a frequency varying above and below the carrier frequency at the audio tone frequency, which we derive by using [[John Renshaw Carson|Carson's]] time derivative method:<ref name=pn/> | |||
:<math>\begin{align} | |||
\frac{d}{dt}\left(\omega_c t + kA \sin\left(\omega_at\right)\right) &= \omega_c + kA \omega_a\cos\left(\omega_at\right) \\ | |||
\end{align}</math> | |||
The amplitude factor kAω<sub>a</sub> defines the maximum [[Frequency deviation]] around ω<sub>c</sub>. Dividing by ω<sub>a</sub>, gives us the modulation index kA,<ref name=pn/> which "is the ratio of the amount of frequency deviation to the audio modulating frequency."<ref name=rb/> | |||
While most of the energy of the signal is contained within ''f<sub>c</sub>'' ± ''f''<sub>Δ</sub>, it can be shown by [[Fourier analysis]] that a wider range of frequencies is required to precisely represent an FM signal. The [[frequency spectrum]] of an actual FM signal has components extending infinitely, although their amplitude decreases and higher-order components are often neglected in practical design problems.<ref name=TGTSCS05/> | While most of the energy of the signal is contained within ''f<sub>c</sub>'' ± ''f''<sub>Δ</sub>, it can be shown by [[Fourier analysis]] that a wider range of frequencies is required to precisely represent an FM signal. The [[frequency spectrum]] of an actual FM signal has components extending infinitely, although their amplitude decreases and higher-order components are often neglected in practical design problems.<ref name=TGTSCS05/> | ||
==Sinusoidal baseband signal== | |||
Mathematically, a baseband modulating signal may be approximated by a [[Sine wave|sinusoid]]al [[continuous wave]] signal with a frequency ''f<sub>m</sub>''. This method is also named as single-tone modulation. The integral of such a signal <math>x_m(t) = cos(2\pi f_m t)</math> is: | Mathematically, a baseband modulating signal may be approximated by a [[Sine wave|sinusoid]]al [[continuous wave]] signal with a frequency ''f<sub>m</sub>''. This method is also named as single-tone modulation{{Dubious|date=April 2026|reason=No, an approximation for mathematical understanding is not a ''Method'', thus this is wrong}}. The integral of such a signal <math>x_m(t) = \cos(2\pi f_m t)</math> is: | ||
:<math>\int_0^t x_m(\tau)d\tau = \frac{\sin\left(2\pi f_m t\right)}{2\pi f_m}\,</math> | :<math>\int_0^t x_m(\tau)d\tau = \frac{\sin\left(2\pi f_m t\right)}{2\pi f_m}\,</math> | ||
In this case, the expression for y(t) above simplifies to: | In this case, the expression for y(t) above simplifies to:<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=0-02-341860-5 |pages=143–144}}</ref> | ||
:<math>y(t) = A_c \cos\left(2\pi f_c t + \frac{f_\Delta}{f_m} \sin\left(2\pi f_m t\right)\right)\,</math> | :<math>y(t) = A_c \cos\left(2\pi f_c t + \frac{f_\Delta}{f_m} \sin\left(2\pi f_m t\right)\right)\,</math> | ||
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where the amplitude <math>A_m\,</math> of the modulating [[sine wave|sinusoid]] is represented in the peak deviation <math>f_\Delta = K_f A_m</math> (see [[frequency deviation]]). | where the amplitude <math>A_m\,</math> of the modulating [[sine wave|sinusoid]] is represented in the peak deviation <math>f_\Delta = K_f A_m</math> (see [[frequency deviation]]). | ||
The [[harmonic]] distribution of a [[sine wave]] carrier modulated by such a [[sinusoidal]] signal can be represented with [[Bessel function]]s; this provides the basis for a mathematical understanding of frequency modulation in the frequency domain. | The [[harmonic]] distribution of a [[sine wave]] carrier modulated by such a [[sinusoidal]] signal can be represented with [[Bessel function]]s; this provides the basis for a mathematical understanding of frequency modulation in the frequency domain.<ref name=jg/> | ||
==Modulation index== | |||
As in other modulation systems, the modulation index indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the [[carrier frequency]]: | As in other modulation systems, the modulation index indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the [[carrier frequency]]: | ||
:<math>h = \frac{\Delta{}f}{f_m} = \frac{f_\Delta \left|x_m(t)\right|}{f_m}</math> | :<math>h = \frac{\Delta{}f}{f_m} = \frac{f_\Delta \left|x_m(t)\right|}{f_m}</math> | ||
where <math>f_m\,</math> is the highest frequency component present in the modulating signal ''x''<sub>''m''</sub>(''t''), and <math>\Delta{}f\,</math> is the peak frequency-deviation{{snd}}i.e. the maximum deviation of the ''[[instantaneous phase#Instantaneous frequency|instantaneous frequency]]'' from the carrier frequency. For a sine wave modulation, the modulation index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave. | where <math>f_m\,</math> is the highest frequency component present in the modulating signal ''x''<sub>''m''</sub>(''t''), and <math>\Delta{}f\,</math> is the peak frequency-deviation{{snd}}i.e. the maximum deviation of the ''[[instantaneous phase#Instantaneous frequency|instantaneous frequency]]'' from the carrier frequency. For a sine wave modulation, the modulation index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave. | ||
{{anchor|narrowband_FM_anchor}}If <math>h \ll 1</math>, the modulation is called '''narrowband FM''' (NFM), and its bandwidth is approximately <math>2f_m\,</math>. Sometimes modulation index <math>h < 0.3</math> is considered NFM and other modulation indices are considered wideband FM (WFM or FM). | {{anchor|narrowband_FM_anchor}}If <math>h \ll 1</math>, the modulation is called '''narrowband FM''' (NFM), and its bandwidth is approximately <math>2f_m\,</math>. Sometimes modulation index <math>h < 0.3</math> is considered NFM and other modulation indices are considered wideband FM (WFM or FM).<ref name=jg/> | ||
For digital modulation systems, for example, binary frequency shift keying (BFSK), where a binary signal modulates the carrier, the modulation index is given by: | For digital modulation systems, for example, binary frequency shift keying (BFSK), where a binary signal modulates the carrier, the modulation index is given by: | ||
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{{anchor|narrowband FM}} | {{anchor|narrowband FM}} | ||
Frequency modulation can be classified as narrowband if the change in the carrier frequency is about the same as the signal frequency, or as wideband if the change in the carrier frequency is much higher (modulation index > 1) than the signal frequency.<ref>Lathi, B. P. (1968). ''Communication Systems'', pp. 214–17. New York: John Wiley and Sons, {{ISBN|0-471-51832-8}}.</ref> For example, narrowband FM (NFM) is used for [[two-way radio]] systems such as [[Family Radio Service]], in which the carrier is allowed to deviate only 2.5 kHz above and below the center frequency with speech signals of no more than 3.5 kHz bandwidth. Wideband FM is used for [[FM broadcasting]], in which music and speech are transmitted with up to 75 kHz deviation from the center frequency and carry audio with up to a 20 kHz bandwidth and subcarriers up to 92 kHz. | Frequency modulation can be classified as narrowband if the change in the carrier frequency is about the same as the signal frequency, or as wideband if the change in the carrier frequency is much higher (modulation index > 1) than the signal frequency.<ref>Lathi, B. P. (1968). ''Communication Systems'', pp. 214–17. New York: John Wiley and Sons, {{ISBN|0-471-51832-8}}.</ref><ref name="sh">{{cite book |last1=Haykin |first1=Simon |last2=Moher |first2=Michael |title=Communication Systems |date=2010 |publisher=John Wiley & Sons |isbn=978-81-265-2151-7 |pages=110–116}}</ref> For example, narrowband FM (NFM) is used for [[two-way radio]] systems such as [[Family Radio Service]], in which the carrier is allowed to deviate only 2.5 kHz above and below the center frequency with speech signals of no more than 3.5 kHz bandwidth. Wideband FM is used for [[FM broadcasting]], in which music and speech are transmitted with up to 75 kHz deviation from the center frequency and carry audio with up to a 20 kHz bandwidth and subcarriers up to 92 kHz. | ||
==Bessel functions== | |||
[[File:Waterfall FM.jpg|thumb|Frequency spectrum and [[waterfall plot]] of a 146.52{{nbsp}}MHz 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.]] | [[File:Waterfall FM.jpg|thumb|Frequency spectrum and [[waterfall plot]] of a 146.52{{nbsp}}MHz 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 his 1922 FM paper, Carson pointed out an infinite number of side frequencies are generated when a carrier frequency is modulated by a signal frequency, the amplitudes expressed as Bessel functions. The separation is determined by the frequency of the modulating signal, and the amplitude dependent upon the modulation index. A table of Bessel functions of the first kind is used to determine the side frequency amplitudes.<ref name=rb/><ref name=pn/>{{rp|214}} | |||
For the case of a carrier modulated by a single sine wave, the resulting frequency spectrum can be calculated using [[Bessel function]]s of the first kind, as a function of the [[sideband]] number and the modulation index. The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals. For particular values of the modulation index, the carrier amplitude becomes zero and all the signal power is in the sidebands.<ref name=TGTSCS05>T.G. Thomas, S. C. Sekhar ''Communication Theory'', Tata-McGraw Hill 2005, {{ISBN|0-07-059091-5}} p. 136</ref> | For the case of a carrier modulated by a single sine wave, the resulting frequency spectrum can be calculated using [[Bessel function]]s of the first kind, as a function of the [[sideband]] number and the modulation index. The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals. For particular values of the modulation index, the carrier amplitude becomes zero and all the signal power is in the sidebands.<ref name=TGTSCS05>T.G. Thomas, S. C. Sekhar ''Communication Theory'', Tata-McGraw Hill 2005, {{ISBN|0-07-059091-5}} p. 136</ref> | ||
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|} | |} | ||
==Carson's rule== | |||
{{Main|Carson bandwidth rule}} | {{Main|Carson bandwidth rule}} | ||
A [[rule of thumb]], ''Carson's rule'' states that | A [[rule of thumb]], ''Carson's rule'' states that the frequency-modulated signal lies within a [[bandwidth (signal processing)|bandwidth]] <math> B_T\, </math> of:<ref name=jg/>{{rp|146}} | ||
:<math>B_T = 2\left(\Delta f + f_m\right) = 2f_m(h + 1)</math> | :<math>B_T = 2\left(\Delta f + f_m\right) = 2f_m(h + 1)</math> | ||
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===Modulation=== | ===Modulation=== | ||
FM signals can be generated using either direct or indirect frequency modulation: | FM signals can be generated using either direct or indirect frequency modulation: | ||
* Direct FM modulation can be achieved by directly feeding the | * Direct FM modulation can be achieved by directly feeding the modulating audio voltage into a [[voltage-controlled oscillator]].<ref name=rb/> | ||
* For indirect FM modulation, the message signal is integrated to generate a [[phase modulation|phase-modulated signal]]. This is used to modulate a [[crystal oscillator|crystal-controlled oscillator]], and the result is passed through a [[frequency multiplier]] to produce an FM signal. In this modulation, narrowband FM is generated leading to wideband FM later and hence the modulation is known as indirect FM modulation.<ref>Haykin, Simon [Ed]. (2001). ''Communication Systems'', 4th ed.</ref> | * For indirect FM modulation, the message signal is integrated to generate a [[phase modulation|phase-modulated signal]]. This is used to modulate a [[crystal oscillator|crystal-controlled oscillator]], and the result is passed through a [[frequency multiplier]] to produce an FM signal. In this modulation, narrowband FM is generated leading to wideband FM later and hence the modulation is known as indirect FM modulation.<ref>Haykin, Simon [Ed]. (2001). ''Communication Systems'', 4th ed.</ref> | ||
===Demodulation=== | ===Demodulation=== | ||
{{see also|Detector (radio)#Frequency and phase modulation detectors}} | {{see also|Detector (radio)#Frequency and phase modulation detectors}} | ||
[[File: | [[File:Frequency-modulation.svg|thumb|FM modulation]] | ||
Many FM detector circuits exist. A common method for recovering the information signal is through a [[Foster–Seeley discriminator]] or [[ratio detector]]. A [[phase-locked loop]] can be used as an FM demodulator. ''Slope detection'' demodulates an FM signal by using a tuned circuit which has its resonant frequency slightly offset from the carrier. | Many FM detector circuits exist. A common method for recovering the information signal is through a [[Foster–Seeley discriminator]] or [[ratio detector]]. A [[phase-locked loop]] can be used as an FM demodulator.<ref name=rb/>{{rp|415-419}} ''Slope detection'' demodulates an FM signal by using a tuned circuit which has its resonant frequency slightly offset from the carrier. The input FM wave of constant amplitude and instantaneous frequency, is converted to an FM wave with instantaneous frequency and instantaneous amplitude, which is then sent to an envelop detector.<ref name=pn/>{{rp|224-228}} AM receivers may detect some FM transmissions by this means, although it does not provide an efficient means of [[detector (radio)|detection]] for FM broadcasts. | ||
In [[software-defined radio]] implementations, the demodulation may be carried out by using the [[Hilbert transform]] (implemented as a filter) to recover the instantaneous phase, and thereafter differentiating this phase (using another filter) to recover the instantaneous frequency. Alternatively, a complex mixer followed by a bandpass filter may be used to translate the signal to baseband, and then proceeding as before. For sampled signals, phase detection, and therefore frequency modulation detection, can be approximated by taking the IQ (complex) sample and multiplying it with the complex conjugate of the previous IQ sample, <math>x[n]\cdot \overline{x[n-1]}</math>.<ref>{{Cite book |last=Shima |first=James Michael |url=https://books.google.com/books?id=Aq7uygAACAAJ |title=FM Demodulation Using a Digital Radio and Digital Signal Processing |date=1995 |publisher=University of Florida |language=en}}</ref> If the demodulated signal is sampled at or above Nyquist, this allows for recovery of near-instantaneous phase changes. | In [[software-defined radio]] implementations, the demodulation may be carried out by using the [[Hilbert transform]] (implemented as a filter) to recover the instantaneous phase, and thereafter differentiating this phase (using another filter) to recover the instantaneous frequency. Alternatively, a complex mixer followed by a bandpass filter may be used to translate the signal to baseband, and then proceeding as before. For sampled signals, phase detection, and therefore frequency modulation detection, can be approximated by taking the IQ (complex) sample and multiplying it with the complex conjugate of the previous IQ sample, <math>x[n]\cdot \overline{x[n-1]}</math>.<ref>{{Cite book |last=Shima |first=James Michael |url=https://books.google.com/books?id=Aq7uygAACAAJ |title=FM Demodulation Using a Digital Radio and Digital Signal Processing |date=1995 |publisher=University of Florida |language=en}}</ref> If the demodulated signal is sampled at or above Nyquist, this allows for recovery of near-instantaneous phase changes. | ||
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==Applications== | ==Applications== | ||
=== Doppler effect=== | === Doppler effect=== | ||
In 1968, Schnitzler noted certain bats lower the [[animal echolocation]] emission frequency by 13 to 16 kHz, compensating for [[Doppler shift]]s caused by the bat’s own movement. [[Doppler shift compensation]], dynamic frequency modulation, ensures that the returning echo frequency is optimally adjusted for the bat's auditory fovea.<ref>{{cite journal |last1=Schnitzler |first1=HU |last2=Denzinger |first2=A |title=Auditory fovea and Doppler shift compensation: adaptations for flutter detection in echolocating bats using CF-FM signals |url=https://link.springer.com/article/10.1007/s00359-010-0569-6 |journal=Journal of Comparative Physiology A |publisher=Journal Comparative Physiology A 197 |pages=541–559 |access-date=28 October 2025 |date=2011 |volume=197 |issue=5 |doi=10.1007/s00359-010-0569-6 |pmid=20857119 |bibcode=2011JCmPA.197..541S |url-access=subscription }}</ref><ref>{{cite journal |last1=Schnitzler |first1=Hans-Ulrich |title=Die Ultraschall-Ortungslaute der Hufeisen-Fledermäuse (Chiroptera-Rhinolophidae) in verschiedenen Orientierungssituationen |url=//doi.org/10.1007/BF00303062 |journal=Zeitschrift für Vergleichende Physiologie |access-date=28 October 2025 |pages=376–408 |date=1968 |volume=57 |issue=4 |doi=10.1007/BF00303062 |bibcode=1968JCmPA..57..376S }}</ref> | |||
===Magnetic tape storage=== | ===Magnetic tape storage=== | ||
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{{Main|FM broadcasting}} | {{Main|FM broadcasting}} | ||
[[File:FM Broadcast Transmitter High Power.jpg|thumb|An American FM radio transmitter at [[WEDG]] in Buffalo, New York]] | [[File:FM Broadcast Transmitter High Power.jpg|thumb|An American FM radio transmitter at [[WEDG]] in Buffalo, New York]] | ||
[[Edwin Howard Armstrong]] (1890–1954) was an American electrical engineer who invented wideband frequency modulation (FM) radio.<ref>{{Cite book | [[Edwin Howard Armstrong]] (1890–1954) was an American electrical engineer who invented wideband frequency modulation (FM) radio.<ref>{{Cite book | ||
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|publisher = Artech House | |publisher = Artech House | ||
|year = 2001 | |year = 2001 | ||
|isbn = 978- | |isbn = 978-1-58053-284-6 | ||
|page = [https://archive.org/details/principlesofmode0000noll/page/104 104] | |page = [https://archive.org/details/principlesofmode0000noll/page/104 104] | ||
|url = https://archive.org/details/principlesofmode0000noll | |url = https://archive.org/details/principlesofmode0000noll | ||
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|date= May 1936 | |date= May 1936 | ||
|doi = 10.1109/JRPROC.1936.227383 | |doi = 10.1109/JRPROC.1936.227383 | ||
|s2cid = 43628076 | |bibcode = 1936PIRE...24..689A | ||
}}</ref> | |s2cid = 43628076 | ||
}}</ref> The first experimental station, [[W2XMN]], went on the air in 1937.<ref name="nab">{{cite book |last1=Welton |first1=Jeff |editor1-last=Cavell |editor1-first=Garrison |title=VHF (FM) Radio Transmitters in National Association of Broadcasters Engineering Handbook, 11th Edition |date=2018 |publisher=Routledge |location=New York |isbn=978-1-138-93051-3 |page=1331}}</ref> | |||
As the name implies, wideband FM (WFM) requires a wider [[signal bandwidth]] than [[amplitude modulation]] by an equivalent modulating signal; this also makes the signal more robust against [[Noise (radio)|noise]] and [[Interference (communication)|interference]]. Frequency modulation is also more robust against signal-amplitude-fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, [[high fidelity]] [[radio]] transmission, hence the term "[[FM radio]]" (although for many years the [[BBC]] called it "VHF radio" because commercial FM broadcasting uses part of the [[VHF]] band{{snd}}the [[FM broadcast band]]). FM [[receiver (radio)|receivers]] employ a special [[Detector (radio)|detector]] for FM signals and exhibit a phenomenon known as the ''[[capture effect]]'', in which the [[Tuner (radio)|tuner]] "captures" the stronger of two stations on the same frequency while rejecting the other (compare this with a similar situation on an AM receiver, where both stations can be heard simultaneously). [[Frequency drift]] or a lack of [[selectivity (radio)|selectivity]] may cause one station to be overtaken by another on an [[adjacent channel]]. Frequency [[drift (telecommunication)|drift]] was a problem in early (or inexpensive) receivers; inadequate selectivity may affect any tuner. | As the name implies, wideband FM (WFM) requires a wider [[signal bandwidth]] than [[amplitude modulation]] by an equivalent modulating signal; this also makes the signal more robust against [[Noise (radio)|noise]] and [[Interference (communication)|interference]]. Frequency modulation is also more robust against signal-amplitude-fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, [[high fidelity]] [[radio]] transmission, hence the term "[[FM radio]]" (although for many years the [[BBC]] called it "VHF radio" because commercial FM broadcasting uses part of the [[VHF]] band{{snd}}the [[FM broadcast band]]). FM [[receiver (radio)|receivers]] employ a special [[Detector (radio)|detector]] for FM signals and exhibit a phenomenon known as the ''[[capture effect]]'', in which the [[Tuner (radio)|tuner]] "captures" the stronger of two stations on the same frequency while rejecting the other (compare this with a similar situation on an AM receiver, where both stations can be heard simultaneously). [[Frequency drift]] or a lack of [[selectivity (radio)|selectivity]] may cause one station to be overtaken by another on an [[adjacent channel]]. Frequency [[drift (telecommunication)|drift]] was a problem in early (or inexpensive) receivers; inadequate selectivity may affect any tuner. | ||
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===Hearing assistive technology=== | ===Hearing assistive technology=== | ||
Frequency modulated systems are a widespread and commercially available [[assistive technology]] that make speech more understandable by improving the signal-to-noise ratio in the user's ear. They are also called ''auditory trainers'', a term which refers to any sound amplification system not classified as a [[hearing aid]]. They intensify signal levels from the source by 15 to 20 decibels.<ref>{{cite tech report |author=ASHA Ad Hoc Committee on FM Systems |date=2002 |orig-date=Original March 1994 |edition=Revised |title=Guidelines for Fitting and Monitoring FM Systems |institution=[[American Speech–Language–Hearing Association]] |url=https://www.asha.org/policy/gl2002-00010/ |doi=10.1044/policy.GL2002-00010}}</ref> FM systems are used by hearing-impaired people as well as children whose listening is affected by disorders such as [[auditory processing disorder]] or [[ADHD]].<ref>{{Cite journal |last1=Schafer |first1=Erin C. |last2=Bryant |first2=Danielle |last3=Sanders |first3=Katie |last4=Baldus |first4=Nicole |last5=Algier |first5=Katherine |last6=Lewis |first6=Audrey |last7=Traber |first7=Jordan |last8=Layden |first8=Paige |last9=Amin |first9=Aneeqa |date=June 1, 2014 |title=Fitting and Verification of Frequency Modulation on Children with Normal Hearing |journal=Journal of the American Academy of Audiology |volume=25 |issue=6 |pages=529–540 |doi=10.3766/jaaa.25.6.3 |issn=1050-0545 |pmid=25313543 |id={{EBSCOhost|107832936}} | Frequency modulated systems are a widespread and commercially available [[assistive technology]] that make speech more understandable by improving the signal-to-noise ratio in the user's ear. They are also called ''auditory trainers'', a term which refers to any sound amplification system not classified as a [[hearing aid]]. They intensify signal levels from the source by 15 to 20 decibels.<ref>{{cite tech report |author=ASHA Ad Hoc Committee on FM Systems |date=2002 |orig-date=Original March 1994 |edition=Revised |title=Guidelines for Fitting and Monitoring FM Systems |institution=[[American Speech–Language–Hearing Association]] |url=https://www.asha.org/policy/gl2002-00010/ |doi=10.1044/policy.GL2002-00010}}</ref> FM systems are used by hearing-impaired people as well as children whose listening is affected by disorders such as [[auditory processing disorder]] or [[ADHD]].<ref>{{Cite journal |last1=Schafer |first1=Erin C. |last2=Bryant |first2=Danielle |last3=Sanders |first3=Katie |last4=Baldus |first4=Nicole |last5=Algier |first5=Katherine |last6=Lewis |first6=Audrey |last7=Traber |first7=Jordan |last8=Layden |first8=Paige |last9=Amin |first9=Aneeqa |date=June 1, 2014 |title=Fitting and Verification of Frequency Modulation on Children with Normal Hearing |journal=Journal of the American Academy of Audiology |volume=25 |issue=6 |pages=529–540 |doi=10.3766/jaaa.25.6.3 |issn=1050-0545 |pmid=25313543 |id={{EBSCOhost|107832936}} }}</ref> For people with [[sensorineural hearing loss]], FM systems result in better speech perception than hearing aids. They can be coupled with behind-the-ear hearing aids to allow the user to alternate the setting.<ref>{{Cite journal |last1=Lewis |first1=M. Samantha |last2=Crandall |first2=Carl C. |last3=Valente |first3=Michael |last4=Enrietto Horn |first4=Jane |date=2004 |title=Speech perception in noise: directional microphones versus frequency modulation (FM) systems |url=https://digitalcommons.wustl.edu/audio_hapubs/5 |journal=Journal of the American Academy of Audiology |volume=15 |issue=6 |pages=426–439 |doi=10.3766/jaaa.15.6.4 |pmid=15341224 |doi-access=free|url-access=subscription }}</ref> FM systems are more convenient and cost-effective than alternatives such as [[cochlear implants]], but many users use FM systems infrequently due to their conspicuousness and need for recharging.<ref>{{Cite journal |last1=McArdle |first1=Rachel |last2=Abrams |first2=Harvey B. |last3=Hnath Chisholm |first3=Theresa |date=2005 |title=When Hearing Aids Go Bad: An FM Success Story |journal=Journal of the American Academy of Audiology |volume=16 |issue=10 |pages=809–821 |doi=10.3766/jaaa.16.10.5 |pmid=16515133 |id={{EBSCOhost|106441304}} }}</ref> | ||
==See also== | ==See also== | ||