SlowfeldXPAS...An Experimental Slow Speed Variant of the Hellschreiber System Modified For Use in Communication via Aircraft reflections on VHF.

Acknowledgements

This is an ongoing development taking place within that part of the radio amateur fraternity involved with the use of digital signal processing to further communication modes. Specifically, much support has come from those interested in "fuzzy" modes such as the various forms of Hellschreiber, and the focus for this interest centres on Murray Greenman’s site:

http://www.qsl.net/zl1bpu/ And specifically:

http://www.qsl.net/zl1bpu/FUZZY/Feld.htm/

Many enthusiasts have helped with testing and encouragement including:

Paul PA0OCD, Ko Versteeg NL9222, Bernard Spencer G3SMW and Ray Scrivens G3LNM.

The program PathSim by Moe Wheatley, AE4JY has been indispensable during the development of this and other programs.

Program overview

FeldHell is a fuzzy mode which means that the transmitter uses no coding and the receiver does not decide when or if data is present. It is in fact a form of FAX; the character is in the form of a font of 7 vertical pixels and up to 7 horizontal pixels and these are scanned through from bottom to top and left to right. The really clever part is that on receive, the detected signal is plotted twice, one above the other with the lower version plotted one pixel to the right. This means that whatever the timing between receiver and transmitter, one readable line of text is always visible. In the example shown below, a test recording has been stopped and started in a random fashion, but legibility is maintained.

The technique is very tolerant of errors in the speed of transmission and reception. In the example below, the reception speed is 4 percent different to that of transmission but again, legibility is maintained.

Some years ago, the author developed a very slow version of Feldhell called SlowFeld. This was aimed at the sort of applications where QRSS CW is used. In SlowFeld, the speed of transmission is slowed right down from the normal 245 baud speed to some 2 baud or less and the signal detection is performed by plotting the bins of sequential Fast Fourier Transforms (FFT's). This makes for great sensitivity but very slow speed at 3 characters/minute. The problem of tuning to a single pixel on a FFT waterfall is overcome by plotting a number of FeldHell lines side by side and a drifting signal would move out of one line into another. The problem of synchronizing a received pixel with the start of a FFT is overcome by clocking the data through the FFT's in stages. Lowest S/N is better than -30dB.

SlowFeldXPAS is a modification of the program SlowFeldXP which itself is a complete re-write of the original SlowFeld program using Borland C++ Builder V6 under WindowsXP. The original Slowfeld program was written some six years ago using C++ Builder V1 under Windows95 and had become useless due to it hanging up when trying to run under modern Windows systems. The re-write has allowed for a number of improvements that make the program more useful and easy to use. The modifications in SlowFeldXPAS allow the program to track signals reflected by aircraft. These signals are typically very weak and can swing wildly in frequency due to the Doppler effect. Additionally there may be several reflections seen simultaneously arising from different aircraft. This makes the use of aircraft reflection as a medium for VHF communication extremely difficult, but the glorious simplicity of the Hellschreiber principle with its total lack of a need for synchronization allows its use here.

For reception, the spectrum between 700 and 1506Hz is shown on a waterfall display. The actual reception window (some 100Hz) wide is positioned on the display by a left-click of the mouse and is denoted by an azure centre marker with two red outer markers. In contrast with the original SlowFeld concept, the strongest bin within this reception window is plotted on a single Hellschreiber line on the main dislay below. If there is more than one reflection in the passband then the strongest one is selected. The contents of this scroll up when the line is full. If the signal drifts in frequency to the point where it threatens to move out of the reception window, manual repositioning of the reception window via the mouse will be required. The transmit frequency is set to the centre of the reception window.

There are three transmission speeds: 3, 6 and 12 characters/minute with FFT bin widths of approximately 1, 2 and 4Hz respectively. If an operating procedure related to that used for meteor scatter (MS) is deployed, the 3 characters/minute speed used with 15 minute periods of transmission/reception should allow the transfer of information considered accepatable by MS enthusiasts. The use of the 12 character/minute speed may require better equipment and a higher quality of reflection but much greater information interchange may be possible.

Program Installation

Unzip the file "slowfeldxpas.zip" into a suitable folder (directory). This folder can be placed anywhere but is probably best to create a new one for it at, for instance, C:\slowfeldxpas\. The help file "slowfeldxpas.html" needs to be in the same directory. The program consists of a single .exe file, slowfeldxpas.exe, which may be run from the Windows Start/Run menu or a short cut can be set up in the normal way. There are four additional files bundled in to the zip file: borlndmm.dll - cc3260.dll - rtl60.bpl - vcl60.bpl. These should reside in the same directory as the main program and all or some of these may be needed to enable the program to run under earlier versions of Windows.

To remove this program, just delete all the files in the folder where slowfeldxpas.exe resides.

Hardware Installation

Connect a screened audio cable from the audio output of your SSB receiver to the line-input jack of your soundcard. In some cases it will be necessary to use the microphone input but be careful of any AGC or boost facilities associated with this input. Either left or right input (or both) will work. If available, it is best to use a dedicated auxiliary audio output on the receiver that does not vary in level with speaker volume.

Connect a screened audio cable from the soundcard audio output to the microphone input of a suitable SSB rig. Some rigs provide an accessory input that may be more suitable.

Operation

On running the program, there will be a pause and then the waterfall display should appear and show a moving representation of the audio spectrum. Adjust the sensitivity of the waterfall display to suit conditions using the TrackBar provided. If the lines across the display are shown as red then the input is overloaded and the soundcard audio-control window should be called up and the input level adjusted accordingly. By the same means the audio output level may be set to suit the transmitter.

Run set-up from the dropdown menu to enter your callsign for the CW ident. You can also enter a COM Port setting to allow TX switching, which switches DTR and RTS when going to transmit.

Note that the function buttons 'toggle'; for instance the 'TX ON' button becomes the 'TX OFF' button when in TX mode.

A "tune up" single tone is provided when in transmit, and this runs continuously until it is cancelled.

A 'Beaconing' facility is given which allows text to be entered in the Edit Window provided and this is then sent repeatedly while in transmit.

The soundcard audio control window may have "advanced" tone control setting facilities, if so set these for minimum bass/treble.

There are three transmission speeds. Slowfeld is a rather slow method as you have to laboriously plot through each 'pixel' of a character. Stations with good facilities and an easy path may be able to use 12 char/min. In other circumstances the slowest speed might have to be used. Whatever the speed, it is hoped that the information passed will greatly exceed that accepted for MS work for instance.

There is a 'Hell Plot Threshold' control which allows direct subtraction from the detected signal. Use the 'Hell Plot Threshold' control with care, it can be easily overdone. Note that the 'Hell Plot Threshold' works only on the plotted hellschreiber line and not on the waterfall display.

Note that because the soundcard input and output functions are separated, it is possible to externally connect the centre connection of the soundcard input and line output with a screened lead, and record the program output to a .WAV file. Any simple recording program such as the Windows generic one may be used. Please use 11025Hz sampling frequency 16 bit mono. In the same way, pre-recorded .WAV files may be played into the program, but they MUST be 11025Hz 16 bit mono also or converted to this format. Used in conjunction with Moe Wheatley’s PathSim program this is a very useful investigative tool, but bear in mind that PathSim works with 8000Hz 16 bit Mono files and a conversion must be made using Goldwave or similar. Also, it is possible to record signals off-air and play these throught the program.

Deliberate communication via aircraft reflections on VHF is relatively unknown although it must have been a factor in short SSB QSO's in the past. No protocol exists but it would seem reasonable to treat it in the same way as meteor scatter and have the transmitting station send a short message repeatedly for ten or fifteen minutes using the 'beaconing facility' and then hand over to the other station to do likewise. Aircraft fly high but not that high and ranges up to three hundred miles or so may be anticipated. Stations in poor locations will be restricted to aircraft visible at a higher angle and ranges achieved may be less but it is hoped that such stations will be able to do much better than is normally the case.

Caveat....

I am neither a professional computer programmer nor a radio engineer. This is for fun...

73

Lionel Sear G3PPT

March 2006.