Editing Frequency-shift keying (section)

==Applications==
{{Listen
|filename=AFSK 1200 baud.ogg
|title=1200&nbsp;baud AFSK signal
|description=Listen to an example of a 1200&nbsp;baud AFSK-modulated signal.
|format=[[Ogg]]
}}
In 1910, [[Reginald Fessenden]] invented a two-tone method of transmitting Morse code. Dots and dashes were replaced with different tones of equal length.<ref>{{Harvnb|Morse|1925|p=44}}; Morse cites British patent 2,617/11.</ref>  The intent was to minimize transmission time.

Some early Continuous Wave (CW) transmitters employed an [[arc converter]] that could not be conveniently [[On-off keying|keyed]]. Instead of turning the arc on and off, the key slightly changed the transmitter frequency in a technique known as the ''compensation-wave method''.<ref>{{Harvnb|Bureau of Standards|1922|pp= 415–416}}</ref> The compensation-wave was not used at the receiver. [[Spark-gap transmitter|Spark transmitters]] used for this method consumed a lot of bandwidth and caused interference, so it was discouraged by 1921.<ref>{{Harvnb|Little|1921|p=125}}</ref>

Most early telephone-line [[modem]]s used audio frequency-shift keying (AFSK) to send and receive data at rates up to about 1200&nbsp;bits per second. The [[Bell 103 modem|Bell 103]] and [[Bell 202 modem|Bell 202]] modems used this technique.<ref>{{Harvnb|Kennedy|Davis|1992|pp=549–550}}</ref> Even today, North American [[caller ID]] uses 1200&nbsp;baud AFSK in the form of the [[Bell 202]] standard. Some early [[microcomputer]]s used a specific form of AFSK modulation, the [[Kansas City standard]], to store data on [[audio cassette]]s.<ref>{{Cite web |last=Peschke |first=Manfred |last2=Pesche |first2=Virgina |date=2016-12-24 |title=Kansas City Standard |url=http://www.swtpc.com/ |url-status=dead |archive-url=https://web.archive.org/web/20161224113109/http://www.swtpc.com/mholley/AC30/KansasCityStandard.htm |archive-date=2016-12-24 |access-date=2023-01-09 |website=swtpc.com}}</ref> 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.{{citation needed|date=August 2012}} It is used at higher [[bitrate]]s for [[Weathercopy]] used on [[Weatheradio]] by [[NOAA]] in the U.S.

The [[CHU (callsign)|CHU]] [[shortwave radio]] station in [[Ottawa|Ottawa, Ontario]], Canada broadcasts an exclusive digital time signal encoded using AFSK modulation.{{citation needed|date=August 2012}}