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| Analog modulation |
| Digital modulation |
| Hierarchical modulation |
| Spread spectrum |
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Frequency-shift keying (FSK) is afrequency modulation scheme in which digital information is encoded on acarrier signal by periodically shifting thefrequency of the carrier between several discrete frequencies.[1] The technology is used for communication systems such astelemetry, weather balloonradiosondes,caller ID,garage door openers, and low frequency radio transmission in theVLF andELF bands. The simplest FSK isbinary FSK (BFSK, which is also commonly referred to as2FSK or2-FSK), in which the carrier is shifted between two discrete frequencies to transmit binary (0s and 1s) information.[2]
Reference implementations of FSK modems exist and are documented in detail.[3] The demodulation of a binary FSK signal can be done using theGoertzel algorithm very efficiently, even on low-power microcontrollers.[4]
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) reducessideband power, reducing interference with neighboring channels.
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 aGaussian filter to make the transitions smoother. This filter has the advantage of reducingsideband power, reducing interference with neighboring channels, at the cost of increasingintersymbol interference. It is used byImproved Layer 2 Protocol,DECT,Bluetooth,[5]Cypress WirelessUSB,Nordic Semiconductor,[6]Texas Instruments,[7]IEEE 802.15.4,Z-Wave andWavenis devices. For basic data rateBluetooth the minimum deviation is 115 kHz.
A GFSK modulator differs from a simple frequency-shift keying modulator in that before thebaseband waveform (with levels −1 and +1) goes into the FSK modulator, it passed through aGaussian filter to make the transitions smoother to limit spectral width. Gaussian filtering is a standard way to reduce spectral width; it is calledpulse 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 thecarrier frequency, the out-of-band spectrum will be reduced.[8]
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 maximumfrequency deviation is δ = 0.25 fm, wherefm is the maximum modulating frequency. As a result, the modulation indexm is 0.5. This is the smallest FSKmodulation index that can be chosen such that the waveforms for 0 and 1 areorthogonal.
A variant of MSK called Gaussian minimum-shift keying (GMSK) is used in theGSMmobile phone standard.
Audio frequency-shift keying (AFSK) is amodulation technique by whichdigital data is represented by changes in thefrequency (pitch) of anaudio tone, yielding an encoded signal suitable for transmission viaradio ortelephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents abinary one; the other, the "space", represents a binary zero.
AFSK differs from regular frequency-shift keying in performing the modulation atbaseband frequencies. In radio applications, the AFSK-modulated signal normally is being used to modulate anRFcarrier (using a conventional technique, such asAM orFM) 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.[9] In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass throughAC-coupled links, including most equipment originally designed to carry music or speech.
AFSK is used in the U.S.-basedEmergency 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.
Phase 1 radios in theProject 25 system use 4-level frequency-shift keying (4FSK).[10][11]
In 1910,Reginald Fessenden invented a two-tone method of transmitting Morse code. Dots and dashes were replaced with different tones of equal length.[12] The intent was to minimize transmission time.
Some early Continuous Wave (CW) transmitters employed anarc converter that could not be convenientlykeyed. Instead of turning the arc on and off, the key slightly changed the transmitter frequency in a technique known as thecompensation-wave method.[13] The compensation-wave was not used at the receiver.Spark transmitters used for this method consumed a lot of bandwidth and caused interference, so it was discouraged by 1921.[14]
Most early telephone-linemodems used audio frequency-shift keying (AFSK) to send and receive data at rates up to about 1200 bits per second. TheBell 103 andBell 202 modems used this technique.[15] Even today, North Americancaller ID uses 1200 baud AFSK in the form of theBell 202 standard. Some earlymicrocomputers used a specific form of AFSK modulation, theKansas City standard, to store data onaudio cassettes.[16] AFSK is still widely used inamateur 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] It is used at higherbitrates forWeathercopy used onWeatheradio byNOAA in the U.S.
TheCHUshortwave radio station inOttawa, Ontario, Canada broadcasts an exclusive digital time signal encoded using AFSK modulation.[citation needed]
Frequency-shift keying (FSK) is commonly used over telephone lines forcaller ID (displaying callers' numbers) andremote metering applications. There are several variations of this technology.
In some countries ofEurope, theEuropean Telecommunications Standards Institute (ETSI) standards 200 778-1 and -2 – replacing 300 778-1 & -2 – allow 3 physical transport layers (Telcordia Technologies (formerly Bellcore),British Telecom (BT) andCable Communications Association (CCA)), combined with 2 data formatsMultiple Data Message Format (MDMF) &Single Data Message Format (SDMF), plus theDual-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".
TheTelcordia Technologies (formerly Bellcore) standard is used in theUnited States, Canada (but see below),Australia,China,Hong Kong andSingapore. It sends the data after the first ring tone and uses the 1200bits per secondBell 202 tone modulation. The data may be sent in SDMF – which includes the date, time and number – or in MDMF, which adds a NAME field.
British Telecom (BT) in theUnited Kingdom developed their own standard, which wakes up the display with a line reversal, then sends the data asCCITT v.23 modem tones in a format similar to MDMF. It is used by BT, wireless networks like the lateIonica, and some cable companies. Details are to be found in BTSupplier Information Notes (SINs)227Archived 2014-07-26 at theWayback Machine(link broken 28/7/21) and242Archived 2014-07-26 at theWayback Machine(link broken 28/7/21); another useful document isDesigning Caller Identification Delivery Using XR-2211 for BTArchived 2016-03-06 at theWayback Machine from theEXAR website.
TheCable Communications Association (CCA) of the United Kingdom developed their own standard which sends the information after a short first ring, as eitherBell 202 orV.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 areNortel DMS-100; some areSystem X;System Y; andNokia 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, soNorth American or European equipment is more likely to detect it.