Editing Frequency-shift keying (section)

==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>