Editing Frequency-shift keying (section)

=== 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.