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Hellschreiber

One of the real pleasures of having Amateur Radio as a hobby is the amazing breadth it covers. There are so many facets, to it. One facet is the transmission modes available to experiment with. My first love in radio was Morse Code which is sent using a mode called CW (Continuous wave). This is very simply sent by keying a transmitter on and off and forming characters by using long and short pulses (dits and dahs). A development from Morse was RTTY (Radio Teletype) which uses two separate tones keyed on and off to represent characters. Following on from Morse and RTTY,¬†Hellschreiber was developed in the 1920’s. This uses a similar technique of sending two tones but rather than forming representations of characters which get translated when received, it sends graphical representations which are printed pixel by pixel on to paper or a screen. It looks very much like a Fax.

Whilst generally browsing the web for useful items, I stumbled upon Mark VandeWettering’s website and an article on getting an Arduino to send Hellschreiber. The code is amazingly small and I wondered if it would work in VAYU-NTX. Although Mark was using an ATTiny MCU, there were no specifics for that chip in the code so I changed the output pin number, compiled and uploaded it.

VAYU-NTX v1.04 FELD HELL Test

VAYU-NTX v1.04 FELD HELL Test

I was very pleased to find it worked. An interesting thing about the code is the character set glyph table held at the beginning. I wanted to hyphenate VAYU-NTX in the transmission but there was no ‘-‘ character available. This was successfully added by inserting the line:

{‘-‘, {0x0000, 0x0000, 0x0060, 0x0060, 0x0060, 0x0000, 0x0000}},

at the end of the existing glyph table. Each glyph is made up of a 7×14 pixel grid. It was a simple matter to draw a grid and fill in the cells to produce a hyphen and then break that down into hexadecimal numbers to add to the code. Adding further characters, if required, shouldn’t be very difficult.

What will be difficult, at least for me, is to get the transmission under the control of interrupts. Mark’s code relies on a delay loop which I successfully customised to produce a straight line of text. I would like to send the Hellschreiber out along with the standard telemetry string in RTTY and realistically, that will only be possible using the precise timing of interrupts. I have already experimented with Domino transmissions from VAYU-NTX and it is quite clear to me that timing is everything.

Steve Smith

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