Incremental Frequency Keying (IFK) - an experimental modulation method invented by VK2ZTO (me:-)

(Alberto Di Bene, I2PHD, has developed a derivation of IFK called Jason - Jan 2002.    At this time (May 3, 2008) there is a link here to download Jason from Alberto's web-site.)

General Philosophy of IFK: - 

The IFK mode idea came after the AFK and FDK mode ideas.  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 IFK and FDK modes were an attempt to overcome at least the frequency accuracy hurdle.

The problem with AFK is that it requires accuracies better than 100ppm.  While most soundcards should meet this requirement, this cannot be guaranteed.  At a penalty of 3dB, FDK was an attempt to eliminate the frequency accuracy problem, leaving only the stability problem.

IFK eliminates the 3dB penalty while eliminating the accuracy problem from AFK with the penalty that the first character could be undetermined for inaccurate systems (worse than 100ppm).   Another penalty is that an error in one character causes two consecutive characters to be received incorrectly.

An advantage of IFK is that a fixed interfering tone tone can be identified and eliminated as even a string of the same character results in a different tone for each character.

IFK Transmission Mode:- When transmitting a character, the tone frequency increment is assigned to that character is calculated (say 5Hz for the space character ' ').   The incremental frequency for the current character is added/subtracted from the last tone sent (or the space tone if it is the first character in a transmission) and that frequency is sent.  Whether the incremental frequency is added or subtracted depends on the position of the last tone sent with respect to the centre frequency. The transmitting software always chooses between addition or subtraction on the basis of moving the tones back towards the centre frequency. 

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 characters incremental frequency has been initially set to 0.1Hz. A beacon mode is provided for repeating a set message if necessary.

IFK 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 the increment between the previous tone frequency and the current 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.

Absolute Frequency Keying

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

"PICCOLO

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.

OLMEC - A fun experimental mode

General Philosophy of OLMEC: - The idea for OLMEC came while reading about old South American civilisations.    The Olmecs, Mayas, etc had sophisticated writing and numbering systems.     In particular they had a dot and bar system for numbering.     I was struck by the similarity between the the dot and bar numbering system and morse code.    Having a quick look at morse code revealed that five (5) tones could be used to encode all the letters and numbers.   

To show the general idea here is a heavily compressed JPG example (part of the background picture from my OLMEC generating software).    The text OLMEC is shown with its encoding underneath.

To represent numbers requires five (5) tones.    The OLMEC software (in testing at the moment) generates the tones with a spacing of 1Hz and the basic timing is 10 second/dot.     These parameters are suitable for propagation paths with several hops on HF.   The duration of a bar is 3 dots (30 seconds).   The time between characters is one dot time (10 seconds) and the time between words is 4 dots (40 seconds).

DISCLAIMER:  I make no claims about the relative efficiency of this mode compared to other modes and point out that it is a linear mode like PSK31 and requires the same interface.    However, I make a few interesting observations after looking at OLMEC signals in poor S/N situations.

Observations of OLMEC: -   The length of each character is identical making it easier to locate the extent of an individual character (normal morse is variable length, which is obviously more efficient, but sometimes confusing visually in noise).     Because the dot time always occurs in the middle of a bar (when there is one) it is easier to sort out a dot from noise.    Basically, the regular structure of the code gives some temporal clues for the decoding of the eye-brain.

Summary:-   A new mode which can be decoded by eye and which uses the familiar (to hams) morse code is presented.    The receiving party only needs a copy of Argo or equivalent waterfall spectral display program to be able to decode it (and a morse code decoding sheet if necessary).   It is a fun code to play with and no claims are made for efficiency or suitability of the new mode (except for having a bit of fun).

Last updated Sunday, 11 May 2008