Frequency-shift keying
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| Topics in Modulation techniques |
| Analog modulation |
| Digital modulation |
Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.
Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency.
Continuous phase forms of FSK exist in which there is no phase discontinuity in the modulated signal. The example shown at right is of such a form.
Other names for FSK are frequency-shift modulation and frequency-shift signaling.
Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent frequency-shift keying. In MSK the difference between the higher and lower frequency is identical to half the bit rate. As a result, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard.
FSK is commonly used in Caller ID and remote metering applications: see FSK standards for use in Caller ID and remote metering for more details.
Mark frequency is the frequency used to represent binary 1 and the space frequency is used to represent binary 0.
Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented as changes in the frequency (pitch) of an 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 one; the other, the "space", represents a binary zero.
AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies. In radio applications, the AFSK-modulated signal is normally used to modulate an RF carrier (using a conventional technique, such as AM FM or ACSSB(R)(LM Mode(R)) for transmission.
AFSK is not generally used for high-speed data communications, as it is less efficient than other modulation modes. In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through AC-coupled links, including most equipment originally designed to carry music or speech.
Listen to an example of a 1200 baud AFSK-modulated signal.
[edit] Applications
Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. The common Bell 103 modem used this technique, for example. Some early microcomputers used a specific form of AFSK modulation, the Kansas City standard, to store data on audio cassettes. 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. It is used at higher bitrates for Weathercopy used on Weatheradio by NOAA in the U.S., and more extensively by Environment Canada.
The CHU shortwave radio station in Ottawa, Canada broadcasts a Exclusive digital time signal encoded using AFSK modulation.
[edit] See also
- Amplitude-shift keying
- Multiple frequency-shift keying (MFSK)
- Orthogonal frequency division multiplexing
- Phase-shift keying
- Dual-tone multi-frequency (DTMF) is another encoding technique which represents data using pairs of audio frequencies.
