Frequency shift keying
Frequency shift keying (FSK) is a digital modulation technique in which the frequency of a carrier is modulated to convey digital information. The image to the right conveys the concept using the simplest form of FSK known as binary frequency shift keying (BFSK). In that scheme, one frequency is chosen to represent a zero and another to represent a one. There are more complex forms of FSK that use more frequencies, but few are actively used today.
A specialized form of FSK is called minimum frequency shift keying (MFSK), also known as minimum shift keying (MSK). In this form, the difference between the two frequencies is equal to half the signaling rate (which is also the bit rate in BPSK). This means that the waveforms used to represent ones and zeroes are offset by half the cycle of a single sine wave of the carrier, making the two signals orthogonal using the smallest modulation index possible for this encoding scheme. A small modulation index means a small channel. Gaussian minimum shift keying (GMSK) is a variation found in the GSM cellular system.
There is a version of FSK known as audio frequency shift keying (AFSK). The primary difference between AFSK and FSK is that the former is a baseband signal whereas the latter modulates a carrier at a specific frequency. AFSK is used on North America for caller ID on an analog POTS line. AFSK was also used in the early, low-speed modems, like the Bell 103. Today it is also used in the U.S. Emergency Alert System.
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q2/ask-fsk.mp3%7Cdownload</mp3> | Amplitude shift keying (ASK) and frequency shift keying (FSK)|