Incremental frequency keying

Incremental frequency keying, also known as IFK or IFK+, is a modified type of MFSK modulation where the data to be transmitted is represented by the difference in frequency between the previously received tone and the currently received tone.{{cite book|author1=Roland Proesch|author2=Aikaterini Daskalaki-Proesch|title=Technical Handbook for Radio Monitoring VHF/UHF: Edition 2013|url=https://books.google.com/books?id=B7iuCQAAQBAJ&pg=PA56|date=26 May 2015|publisher=BoD – Books on Demand|isbn=978-3-7322-4147-7|pages=56–}}{{cite book|author=Steve Ford|title=ARRL's HF Digital Handbook|url=https://books.google.com/books?id=STpiwLFYSHoC&pg=SA10-PA11|year=2007|publisher=American Radio Relay League|isbn=978-0-87259-103-5|pages=10–}}

This modulation produces a signal which is much more tolerant of receiver mis-tunings and frequency drift than MFSK modulation. Additionally, IFK modulation is more resistant to multipath interference and intersymbol interference caused by multipath propagation than traditional MFSK.{{cite book|author=Roland Proesch|title=Technical Handbook for Radio Monitoring HF: Edition 2015|url=https://books.google.com/books?id=4reuCQAAQBAJ&pg=PA71|date=26 May 2015|publisher=BoD – Books on Demand|isbn=978-3-7322-4142-2|pages=71–}}{{Cite web|url=http://www.arrl.org/domino|title = Domino}} This combination of features makes IFK modulation well suited for high frequency communications.

This modulation is used in the amateur radio data-modes DominioEX and THOR.

{{Digital modes}}