Editing Frequency-shift keying

{{Short description|Data communications modulation protocol}}
{{More citations needed|date=March 2009}}
{{Modulation techniques}}

[[File:fsk.svg|300px|thumb|right|An example of binary FSK]]
[[File:FSK-FMCW Principle.png|thumb]]
'''Frequency-shift keying''' ('''FSK''') is a [[frequency modulation]] scheme in which digital information is encoded on a [[carrier signal]] by periodically shifting the [[frequency]] of the carrier between several discrete frequencies.<ref>{{cite book |last1=Kennedy |first1=G. |last2=Davis |first2=B. |title=Electronic Communication Systems |edition=4th |publisher=McGraw-Hill International |year=1992 |isbn=978-0-07-112672-4 }}, p 509</ref>  The technology is used for communication systems such as [[telemetry]], weather balloon [[radiosonde]]s, [[caller ID]], [[garage door opener]]s,  and low frequency radio transmission in the [[very low frequency|VLF]] and [[extremely low frequency|ELF]] bands.  The simplest FSK is '''[[Binary numeral system|binary]] FSK''' ('''BFSK''', which is also commonly referred to as '''2FSK''' or '''2-FSK'''), in which the carrier is shifted between two discrete frequencies to transmit binary (0s and 1s) information.<ref>FSK: Signals and Demodulation (B. Watson) http://www.xn--sten-cpa.se/share/text/tektext/digital-modulation/FSK_signals_demod.pdf {{Webarchive|url=https://web.archive.org/web/20120907031840/http://www.xn--sten-cpa.se/share/text/tektext/digital-modulation/FSK_signals_demod.pdf |date=2012-09-07 }}</ref>

==Modulating and demodulating==
Reference implementations of FSK modems exist and are documented in detail.<ref>Teaching DSP through the Practical Case Study of an FSK Modem (TI) http://www.ti.com/lit/an/spra347/spra347.pdf</ref> The demodulation of a binary FSK signal can be done using the [[Goertzel algorithm]] very efficiently, even on low-power microcontrollers.<ref>FSK Modulation and Demodulation With the MSP430 Microcontroller (TI) http://www.ti.com/lit/an/slaa037/slaa037.pdf {{Webarchive|url=https://web.archive.org/web/20120406002335/http://www.ti.com/lit/an/slaa037/slaa037.pdf |date=2012-04-06 }}</ref>

==Variations==

===[[Multiple frequency-shift keying]]===

===Continuous-phase frequency-shift keying===
{{Main | continuous-phase frequency-shift keying}}

In principle FSK can be implemented by using completely independent free-running oscillators, and switching between them at the beginning of each symbol period.
In general, independent oscillators will not be at the same phase and therefore the same amplitude at the switch-over instant,
causing sudden discontinuities in the transmitted signal.

In practice, many FSK transmitters use only a single oscillator, and the process of switching to a different frequency at the beginning of each symbol period preserves the phase.
The elimination of discontinuities in the phase (and therefore elimination of sudden changes in amplitude) reduces [[sideband]] power, reducing interference with neighboring channels.

===Gaussian frequency-shift keying===
Rather than directly modulating the frequency with the digital data symbols, "instantaneously" changing the frequency at the beginning of each symbol period, '''Gaussian frequency-shift keying''' ('''GFSK''') filters the data pulses with a [[Gaussian filter]] to make the transitions smoother. This filter has the advantage of reducing [[sideband]] power, reducing interference with neighboring channels, at the cost of increasing [[intersymbol interference]]. It is used by [[Improved Layer 2 Protocol]], [[DECT]], [[Bluetooth]],<ref>[https://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1158106 ieeexplore.ieee.org], Sweeney, D.; "An introduction to bluetooth a standard for short range wireless networking" ''Proceedings. 15th Annual IEEE International ASIC/SOC Conference, Rochester, NY, US, 25-28 Sept. 2002'', pp. 474–475. 2002.</ref> [[Cypress WirelessUSB]], [[Nordic Semiconductor]],<ref>Nordic Semiconductor.  [http://www.nordicsemi.com/files/Product/data_sheet/Preliminary_Product_Specification_nRF24LU1P_v1_2.pdf nRF24LU1+ Preliminary Product Specification v1.2]{{webarchive |url=https://web.archive.org/web/20110220064408/http://www.nordicsemi.com/files/Product/data_sheet/Preliminary_Product_Specification_nRF24LU1P_v1_2.pdf |date=2011-02-20}}</ref> [[Texas Instruments]],<ref>[http://processors.wiki.ti.com/index.php/Category:LPRF LPRF] {{Webarchive|url=https://web.archive.org/web/20170104162432/http://processors.wiki.ti.com/index.php/Category:LPRF |date=2017-01-04 }}</ref> [[IEEE 802.15.4]], [[Z-Wave]] and [[Wavenis]] devices. For basic data rate [[Bluetooth]] the minimum deviation is 115&nbsp;kHz.

A GFSK modulator differs from a simple frequency-shift keying modulator in that before the [[baseband]] waveform (with levels −1 and +1) goes into the FSK modulator, it passed through a [[Gaussian filter]] to make the transitions smoother to limit spectral width. Gaussian filtering is a standard way to reduce spectral width; it is called ''[[pulse shaping]]'' in this application.

In ordinary non-filtered FSK, at a jump from −1 to +1 or +1 to −1, the modulated waveform changes rapidly, which introduces large out-of-band spectrum. If the pulse is changed going from −1 to +1 as −1, −0.98, −0.93, ..., +0.93, +0.98, +1, and this smoother pulse is used to determine the [[carrier frequency]], the out-of-band spectrum will be reduced.<ref>{{cite web |last1=Bhagwat |first1=Pravin |title=Bluetooth: 1.Applications, Technology and Performance |url=http://faculty.kfupm.edu.sa/coe/mayez/ps-coe541/references/Chapter11.ppt |access-date=27 May 2015 |page=21 |date=10 May 2005}}</ref>

===Minimum-shift keying===
{{Main|Minimum-shift keying}}

Minimum frequency-shift keying or minimum-shift keying (MSK) is a particular spectrally efficient form of coherent FSK. In MSK, the difference between the higher and lower frequency is identical to half the bit rate. Consequently, the waveforms that represent a 0 and a 1 bit differ by exactly half a carrier period. The maximum [[frequency deviation]] is δ&nbsp;=&nbsp;0.25&nbsp;''f<sub>m</sub>'', where ''f<sub>m</sub>'' is the maximum modulating frequency. As a result, the modulation index ''m'' is 0.5.  This is the smallest FSK [[modulation index]] that can be chosen such that the waveforms for 0 and 1 are [[orthogonal]].

===Gaussian minimum-shift keying===
{{Main | Gaussian minimum-shift keying }}

A variant of MSK called Gaussian minimum-shift keying ([[GMSK]]) is used in the [[GSM]] [[mobile phone]] standard.

=== Audio frequency-shift keying ===
''Audio frequency-shift keying'' (AFSK) is a [[modulation]] technique by which [[digital data]] is represented by changes in the [[frequency]] ([[Pitch (music)|pitch]]) of an [[Sound|audio]] tone, yielding an encoded signal suitable for transmission via [[radio]] or [[telephone]].  Normally, the transmitted audio alternates between two tones: one, the "mark", represents a [[Binary numeral system|binary]] one; the other, the "space", represents a binary zero.

AFSK differs from regular frequency-shift keying in performing the modulation at [[baseband]] frequencies.  In radio applications, the AFSK-modulated signal normally is being used to modulate an [[Radio frequency|RF]] [[Carrier signal|carrier]] (using a conventional technique, such as [[Amplitude modulation|AM]] or [[Frequency modulation|FM]]) for transmission.

AFSK is not always used for high-speed data communications, since it is far less efficient in both power and bandwidth than most other modulation modes.<ref>{{Cite book |last=Ciciora |first=Walter |title=Modern cable television technology: video, voice and data communications |date=2004 |publisher=Elsevier/Morgan Kaufmann Publishers |isbn=978-1-55860-828-3 |editor-last=Ciciora |editor-first=Walter S. |location=Amsterdam ; Boston |publication-date=2004}}</ref>  In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through [[AC coupling|AC-coupled]] links, including most equipment originally designed to carry music or speech.

AFSK is used in the U.S.-based [[Emergency Alert System]] to notify stations of the type of emergency, locations affected, and the time of issue without actually hearing the text of the alert.

===Multilevel frequency-shift keying===
Phase 1 radios in the [[Project 25]] system use 4-level frequency-shift keying (4FSK).<ref>
Essam Atalla et al.
[http://www.utdallas.edu/~kamran/SDR.pdf "A Practical Step Forward Toward Software-Defined Radio Transmitters"].
p. 1.
</ref><ref>
Steve Ford.
[https://books.google.com/books?id=mdXqWt8On88C "ARRL's VHF Digital Handbook"].
2008.
p. 6-2.
</ref>

==Applications==
{{Listen
|filename=AFSK 1200 baud.ogg
|title=1200&nbsp;baud AFSK signal
|description=Listen to an example of a 1200&nbsp;baud AFSK-modulated signal.
|format=[[Ogg]]
}}
In 1910, [[Reginald Fessenden]] invented a two-tone method of transmitting Morse code. Dots and dashes were replaced with different tones of equal length.<ref>{{Harvnb|Morse|1925|p=44}}; Morse cites British patent 2,617/11.</ref>  The intent was to minimize transmission time.

Some early Continuous Wave (CW) transmitters employed an [[arc converter]] that could not be conveniently [[On-off keying|keyed]]. Instead of turning the arc on and off, the key slightly changed the transmitter frequency in a technique known as the ''compensation-wave method''.<ref>{{Harvnb|Bureau of Standards|1922|pp= 415–416}}</ref> The compensation-wave was not used at the receiver. [[Spark-gap transmitter|Spark transmitters]] used for this method consumed a lot of bandwidth and caused interference, so it was discouraged by 1921.<ref>{{Harvnb|Little|1921|p=125}}</ref>

Most early telephone-line [[modem]]s used audio frequency-shift keying (AFSK) to send and receive data at rates up to about 1200&nbsp;bits per second. The [[Bell 103 modem|Bell 103]] and [[Bell 202 modem|Bell 202]] modems used this technique.<ref>{{Harvnb|Kennedy|Davis|1992|pp=549–550}}</ref> Even today, North American [[caller ID]] uses 1200&nbsp;baud AFSK in the form of the [[Bell 202]] standard. Some early [[microcomputer]]s used a specific form of AFSK modulation, the [[Kansas City standard]], to store data on [[audio cassette]]s.<ref>{{Cite web |last=Peschke |first=Manfred |last2=Pesche |first2=Virgina |date=2016-12-24 |title=Kansas City Standard |url=http://www.swtpc.com/ |url-status=dead |archive-url=https://web.archive.org/web/20161224113109/http://www.swtpc.com/mholley/AC30/KansasCityStandard.htm |archive-date=2016-12-24 |access-date=2023-01-09 |website=swtpc.com}}</ref> AFSK is still widely used in [[amateur radio]], as it allows data transmission through unmodified voiceband equipment.

AFSK is also used in the United States' [[Emergency Alert System]] to transmit warning information.{{citation needed|date=August 2012}} It is used at higher [[bitrate]]s for [[Weathercopy]] used on [[Weatheradio]] by [[NOAA]] in the U.S.

The [[CHU (callsign)|CHU]] [[shortwave radio]] station in [[Ottawa|Ottawa, Ontario]], Canada broadcasts an exclusive digital time signal encoded using AFSK modulation.{{citation needed|date=August 2012}}

==Caller ID and remote metering standards==
Frequency-shift keying (FSK) is commonly used over telephone lines for [[caller ID]] (displaying callers' numbers) and [[remote metering]] applications. There are several variations of this technology.

===European Telecommunications Standards Institute===
In some countries of [[Europe]], the [[European Telecommunications Standards Institute]] (ETSI) standards 200 778-1 and -2 &ndash; replacing 300 778-1 & -2 &ndash; allow 3 physical transport layers ([[Telcordia Technologies]] (formerly Bellcore), [[British Telecom]] (BT) and [[Cable Communications Association]] (CCA)), combined with 2 data formats [[Caller ID|Multiple Data Message Format]] (MDMF) & [[Caller ID|Single Data Message Format]] (SDMF), plus the [[Dual-tone multi-frequency]] (DTMF) system and a no-ring mode for meter-reading and the like. It's more of a recognition that the different types exist than an attempt to define a single "standard".

===Telcordia Technologies===
The [[Telcordia Technologies]] (formerly Bellcore) standard is used in the [[United States]], Canada (but see below), [[Australia]], [[China]], [[Hong Kong]] and [[Singapore]]. It sends the data after the first ring tone and uses the 1200 [[Bit rate|bits per second]] [[Bell 202]] tone modulation. The data may be sent in SDMF &ndash; which includes the date, time and number &ndash; or in MDMF, which adds a NAME field.

===British Telecom===
[[British Telecom]] (BT) in the [[United Kingdom]] developed their own standard, which wakes up the display with a line reversal, then sends the data as [[ITU-T|CCITT]] v.23 modem tones in a format similar to MDMF. It is used by BT, wireless networks like the late [[Ionica (company)|Ionica]], and some cable companies. Details are to be found in BT [[Supplier Information Note]]s (SINs) [http://www.sinet.bt.com/sinet/SINs/pdf/227v3p6.pdf 227] {{Webarchive|url=https://web.archive.org/web/20140726125549/http://www.sinet.bt.com/sinet/SINs/pdf/227v3p6.pdf |date=2014-07-26 }}(link broken 28/7/21) and [http://www.sinet.bt.com/sinet/SINs/pdf/242v2p4.pdf 242] {{Webarchive|url=https://web.archive.org/web/20140726130003/http://www.sinet.bt.com/sinet/SINs/pdf/242v2p4.pdf |date=2014-07-26 }}(link broken 28/7/21); another useful document is [http://www.exar.com/common/content/document.ashx?id=248 Designing Caller Identification Delivery Using XR-2211 for BT] {{Webarchive|url=https://web.archive.org/web/20160306003503/http://www.exar.com/common/content/document.ashx?id=248 |date=2016-03-06 }} from the [[EXAR]] website.

===Cable Communications Association===
The [[Cable Communications Association]] (CCA) of the United Kingdom developed their own standard which sends the information after a short first ring, as either [[Bell 202 modem|Bell 202]] or [[ITU V.23|V.23]] tones. They developed a new standard rather than change some "street boxes" (multiplexors) which couldn't cope with the BT standard. The UK cable industry use a variety of switches: most are [[Nortel]] DMS-100; some are [[System X (telephony)|System X]]; [[System Y]]; and [[Nokia]] DX220. Note that some of these use the BT standard instead of the CCA one. The data format is similar to the BT one, but the transport layer is more like Telcordia Technologies, so [[North America]]n or European equipment is more likely to detect it.

==See also==
* [[Amplitude-shift keying]] (ASK)
* [[Continuous-phase frequency-shift keying]] (CPFSK)
* [[Dual-tone multi-frequency]] (DTMF), another encoding technique representing data by pairs of audio frequencies
* [[Frequency-change signaling]]
* [[Multiple frequency-shift keying]] (MFSK)
* [[Orthogonal frequency-division multiplexing]] (OFDM)
* [[Phase-shift keying]] (PSK)
* [[Federal Standard 1037C]]
* [[MIL-STD-188]]
* [[Spread frequency-shift keying]] (S-FSK)

==References==
{{reflist|30em}}

* {{Citation |author=Bureau of Standards |author-link=NIST |title=The Principles Underlying Radio Communication |edition=Second |year=1922 |id=Radio Communications Pamphlet No. 40 |publisher=U.S. Army Signal Corps |url=https://books.google.com/books?id=TsTZCjhSG2EC |isbn=9781440078590 }}. Revised to April 24, 1921.
* {{Citation |first=D. G. |last=Little |journal=Electric Journal |title=Continuous Wave Radio Communication |date=April 1921 |volume=18 |pages=124&ndash;129 |url=https://archive.org/details/electricjournal18elecuoft }}
* {{Citation |last=Morse |first=A. H. |title=Radio: Beam and Broadcast |year=1925 |location=London |publisher=Ernest Benn Limited |url=https://archive.org/details/radiobeamandbroa029214mbp }}

==External links==
*[https://ieeexplore.ieee.org/document/1312602 dFSK: Distributed Frequency Shift Keying Modulation in Dense Sensor Networks]
* [http://dl.acm.org/citation.cfm?id=2663508 M Nasseri, J Kim, M Alam - Proceedings of the 17th Communications & Networking, 2014, Unified metric calculation of sampling-based turbo-coded noncoherent MFSK for mobile channel]
* [http://dl.acm.org/citation.cfm?id=2499616 J Kim, P Raorane, M Nasseri, M Alam - Proceedings of the 46th Annual Simulation Symposium, 2013, Performance analysis of sampling-based turbo coded NCQFSK for image data transmission]

[[Category:Amateur radio]]
[[Category:Caller ID]]
[[Category:Quantized radio modulation modes]]