# Difference between FFSK and FSK modulation schemes

Can somebody explain to me what is the difference between FFSK (Fast Frequency Shift Keying) and FSK (Frequency Shift Keying) modulation schemes?

• Never heard of FFSK! Seems to be interesting, could you tell us (ideally with a link) where you've encountered that? Often, things get different names in different contexts :) Aug 20, 2021 at 7:39
• Me neither. I first heard of that at my job. I am involved in a project where one of my task is to implement different waveform generators and FFSK is one of the required waveform to be implemented. First thing was to google the FFSK term but sadly, there are lack of concrete information about this scheme. One paper to which I have no access but there is a little information about FFSK you can find e.g. here; infona.pl/resource/bwmeta1.element.ieee-art-000006830823 Aug 20, 2021 at 8:10
• If someone at works ask you create 'X" they should provide a clear definition what X actually is. If they want it, they should be able to define it. Aug 20, 2021 at 11:40
• I think you might be right. I will ask for further details what the author meant by this scheme. Thank you Aug 20, 2021 at 12:30

Fast FSK is just another term for minimum-shift keying (MSK). In the modern literature, the term fast FSK has been almost completely replaced by MSK.

There is a very good tutorial paper on MSK by S. Pasupathy:

Subbarayan Pasupathy, Minimum Shift Keying: A Spectrally Efficient Modulation, IEEE Communications Magazine, 1979.

The following is a quote from that paper:

Since the frequency spacing is only half as much as the conventional $$1/T$$ spacing used in noncoherent detection of FSK signals, MSK is also referred to as Fast FSK [...]

You should also be able to find a section on MSK in any good text on digital communications.

• +1 for reminding everyone about Pasupathy's tutorial paper. It is a gem. Aug 21, 2021 at 15:33