The Story of Hellschreiber
Hellschreiber - an
antique analog mode that still delivers the DX.
Hellschreiber was the one of the earliest technologies for sending information over radio, predated only by Morse Code. Yet, today modified Hellschreiber is considered one of the most effective modes for sending text in weak signal conditions, with Morse itself still the recognized leader. But why would a primitive technology like Hellschreiber beat out modern error correcting modes like PSK-31 and MFSK 16 and 8? Essentially, for the same reason that Morse itself still claims the lead, because the human brain does the detection and error correction.
Hellschreiber can be considered a form
of fax. However, instead of building an image by scanning across the page,Hellschreiber
prints small sections of a line of text, from the top of the text to the
bottom. You'll see a sample screen image to the right. It is 
essentially
sending an image, which is one of the reasons for its effectiveness.
Hellschreiber's primitiveness is part of its robustness. The original Feld-Hellschreiber (meaning for military field use, as coined by the Nazis) did not require that the transmitter or receiver be synchronized at all. Two identical lines of text are received, so that even with major timing errors the text remains visible. Unlike, MFSK-16 (which takes quite some time to re-synchronize, when synch is lost) or PSK-31, the receiving software does not require synchronization. The image may fade but the printing continues on the receiving end.
The other factor is the brain and eye's ability to recognize shapes, especially text shapes, against a background. While noise may really reduce the clarity of the text image, the eye is unparalleled in picking out the shape from the noise and distortion.
To give you some history on the mode, Dr. Hell patented it under the name Hellschreiber in 1929. He started a company to build and sell the machines, and it was used not only on radio circuits, but also over telegraph circuits. By the time World War II rolled along, the Germans had their own patented and efficient method of remaining in contact with their troops in Feld-Hellschreiber. At the time of the fall of the German army to the Russians, one FH circuit still remained linking Hitler to the front. Commonly, the messages were sent in battle conditions with makeshift antennas, such as barbed wire just laying on the ground. As can be imagined, the British and Americans had built many Hellschreiber receivers. Dr. Hell is still alive today, living in Bavaria, and perhaps surprised that his invention has risen again.
Modern Hellschreiber has been improved in a number of ways. Modulation techniques have been changed. The initial Hellschreiber was an amplitude-modulated signal. Modern variations have been phase-modulated, frequency modulated, and finally, mininum-frequency-shift modulated. The latter has proven to be best where the propagation is difficult, although each has its strong points depending on the conditions encountered.
Hellschreiber does have some advantages beyond immunity to noise. It is also less sensitive to adjacent channel interference, which is becoming more common now that the sound card modes have risen in popularity. It is almost as narrow banded as PSK, and certainly more so than MFSK.
Hellschreiber does not have the popularity of PSK or MFSK, so it can be a bit more difficult to find someone to have a QSO. It is my experience from just a short time monitoring, that the standard old-tech Feld version of the system is still most used by Hams. However, the receive quality of this mode is improved over the original, since IZ8BLY's software uses DSP techniques to improve on the original.
Here's a screen image of his software. An interesting feature of the QSO show here is that the parties switched from Feld to PSK modulated Hellschreiber towards the bottom of the screen.
There are some inconveniences with Hellschreiber compared to the other modern sound card modes. Backspacing to correct an error will only work if you've typed sufficiently far ahead in the transmit buffer, otherwise the error will print on the receive screen. I would expect that Hellschreiber would remain a weak signal mode, with other modes being favored when conditions are better.
If you are interested in playing with Hellschreiber, the location of the IZ8BLY download site is http://iz8bly.sysonline.it/ . Also, there is an excellent site that covers all the history of the mode, frequencies, etc. and it is at http://www.qsl.net/zl1bpu/ .
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