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Frequency Difference Keying (Wanjina)

An Experimental Modulation Method - Frequency Difference Keying (FDK)

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

Secondly, please don't confuse FDK with DFCW or VFSKCW. That mode was one that I abandoned in favour of FDK a long time ago and which later was brought to practical fruition by Rik Strobbe. It is simply a way of encoding the dots and dashs of CW in a more efficient way. FDK is completely different to that mode.

General Philosophy of FDK: -

The idea for FDK 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.

Firstly, at poor S/Ns or in the presence of heavy QRN, because the decoding requires some fuzzy logic (deciding if the carrier is there or not), the full benefit of using narrowband reception is not realised. To receive a character using 10 second dot durations you would need in the order of a minute to receive the character. To make sure that the dots and dashes are not blurred, the narrowest FFT BW would be about 0.3Hz. However, if you only needed to detect the presence of a carrier (not dots and dashes) in that minute then you could use an FFT BW of around 0.017Hz (1/60). You can readily detect the presence of carriers as traces on a FFT spectrum display at S/Ns well below the S/N levels needed to visually decode QRSS.

Secondly, for the decoding of the signals a visual display is necessary, ie, an operator needs to be watching the display as no more than about a dozen characters could be displayed at any one time. For slow data rates which might take tens of minutes to pass relevant data, this can be a problem.

These are the reasons for investigating other narrowband methods for LF.

Note: The character duration for FDK has been initially chosen to be one minute per character.

Why Frequency Difference Keying: -

Obviously different characters cannot be encoded by the the presence of a carrier alone. Some unique characteristic needs to be able to be indentified to enable decoding of the characters at the other end.

Some possibilities are:-

FDK Transmission Mode:-

When transmitting a character, the difference frequency assigned to that character is calculated (say 5Hz for the space character ' '). Two tones are generated with that frequency difference (5Hz) spaced equally around the centre frequency (nominally 1000Hz), ie, (1000 - 2.5) = 997.5Hz and (1000+2.5)=1002.5Hz - giving a difference of the required 5Hz. Each burst of the tones 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.

FDK 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.

Two modes of analysis of this data are available:-

Some Notes on the Linear Mode:- Observing the butterfly display of the linear FDK reception mode brings to mind some possibilities:-

  1. Even with errors rates of 50% or more it is easy to see the symmetrical spacing of the two tones. Either by manual or automatic means it looks possible to identify the centre frequency and thereby adjusting the centre and the range of the scan for the maximums. This would improve the error rate by eliminating from the scan tones outside the character tone difference range.

  2. Following on from above - after indentifying the centre frequency, the error rate could further reduced by doing a SmartScan algorithm. Starting from the strongest tones find the strongest pair which are symmetrically placed around the centre frequency. Kind of like the slow tuning lock found in PSK31.

Test Results:- Ok, so that's the theory, what about the practice? As luck would have it (in this case good luck) there was a very high level of QRN when the first live tests were done.

The pictures below are low-resolution to reduce their size (the FDK screenshots were taken in 1024x768 mode), but hopefully you 'get the picture' :-)

A Spectrogram of the audio with the FDK signal just visible.

Note that while the signal is discernible, trying to use QRSS under these conditions would be well-nigh impossible.

A screenshot of the FDK software in Linear Mode using the same signal as the Spectrogram above.

The butterfly shape can be clearly seen. The center frequency could be easily determined. The error rate is around 6%.

Here is a screenshot of the FDK display in Square Law Mode at a S/N where PSK31 has well-and-truly dropped out.

Here the display only shows the difference frequency as appropriate for the square-law mode. 0% error rate.