Absolute Frequency Keying (Very Slow Piccolo MKI)

An Experimental Modulation Method - Absolute Frequency Keying (AFK)

Firstly, I am sure that this is also not a new idea.  If it is a new idea, you heard it first here - if not, don't be too unkind!!

NOTE:   I have discovered that AFK is NOT a new idea having being developed in 1957 and called Piccolo.   Here is an extract from IZ8BLY's MFSK page.


Work on developing a robust teleprinter system for diplomatic service use started at the Hanslope Park offices of the Diplomatic Wireless Service, part of the British Foreign and Commonwealth Office, in about 1957. Piccolo was developed first by DWS Chief Engineer Harold Robins OBE (who worked for MI6 from 1939 - 1946, and DWS from 1946 to 1971). Donald Bailey and Dennis Ralphs were development team project leaders. The system was first demonstrated at an Institute of Electrical Engineers exhibition in London in 1963, where it aroused much interest. This system was an amplitude modulated system using a carrier and with one sideband removed. The tones used were between 330 and 660 Hz, spaced at 10 Hz intervals, and operating at 10 baud."

So AFK is just a very slow version of Piccolo.     Perhaps I should refer to it as VSPMK-1 :-) Others who re-re-invented AFK (PGP-1 and PUA-43, both which followed AFK) should also give credit to the original developer.

General Philosophy of AFK: - The idea for AFK came to me around end 1998/beginning 1999 and came from looking at many LF signals (some live, but mostly pictures posted to the 'Net) using Spectrogram , a very useful FFT display especially for receiving slow CW.   In order to maximise the range of their stations many European stations use slow CW (QRSS) with dot durations sometimes over 10 seconds.   Looking at these QRSS pictures, I noticed a couple of things.

The AFK mode idea came before the FDK mode idea.  The early experiments were conducted at LF frequencies, and so it became apparent that it would be difficult to achieve the frequency accuracy/stability at those frequencies sufficiently for the AFK mode.  The FDK mode was an attempt to overcome at least the frequency accuracy hurdle.

AFK Transmission Mode:- When transmitting a character, the tone frequency assigned to that character is calculated (say 325Hz for the space character ' ').   Each burst of the tone for each character sent lasts for 60secs and is synchronised with the transmitting PC clock time.  The 'channel spacing' for each character has been initially set to 0.1Hz. A beacon mode is provided for repeating a set message if necessary.

AFK Reception Mode:- The receiver synchronises to the receiving PC clock time and acquires data for 47.6 seconds giving a record length of 524288 samples.  This is because FFT raw data should have a length which is a power of  2.   The nearest block time to one minute using 11025Hz sampling is (524288 / 11025)= 47.6 seconds.

The audio data is fed to the FFT algorithm.   The output spectrum is scanned for the maximum amplitude frequency and the character corresponding to that frequency is displayed. 

This method requires short-term drift to be less than 0.017Hz over one minute to maintain the S/N advantage and about 0.1Hz over one minute to minimise decoding errors.  At a chopping frequency of, say, 320Hz, this translates to a stability of about 50ppm and 300ppm respectively.  This should be easily done by a soundcard in a PC.

AFK can be used where the accuracy and stability of the received signal is adequate - for example in Laser Comms DX, InfraSonic and VLF communications.