Modern (software) Hellschreiber modes

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©2007-2010 F. Dörenberg. All rights reserved worldwide. No part of this publication may be used without permission from the author. It has taken considerable effort to create these pages. If you "borrow" content from them, at least reference the source.

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Advances in DSP technology (hardware and software implemented) has led to the development of a number of Hellschreiber variants since the 1990s. Various PC (and Mac) software packages can operate in these new Hell modes (see my Hell software and interfacing page). A brief summary of these modes is listed below.

	FSK-Hell. This mode operates at 105 or 245 baud. The average duty cycle is about 80%. Actually this is not a new Hellschreiber mode at all (not even for amateur radio). It was actually already tested by the end of World War II (ref. 1), and the Hell-80 also uses FSK.
	PSK Hell encodes the pixels brightness in the carrier phase instead of the amplitude. Strictly speaking, it's encoded in the change of the phase (differential phase shift keying): an unchanged phase in the beginning of a pixel means white, and a reversed phase means black. PSK Hell is a narrow bandwidth mode. 105 or 245 baud. It was invented by Murray, ZL1BPU.
	FM Hell uses minimum shift keying (MSK). Data rates are 105 and 245 baud; due to use of different fonts, these modes have the same character transmission rate.
	Duplo Hell is a two-tone mode in which columns are transmitted in pairs, each at a distinct frequency (980 Hz and 1225/1470 Hz).
	C/MT Hell (Concurrent Multi-Tone) sends all rows simultaneously; each row uses a distinct tone frequency. Hence, the transmission can be read on a frequency-domain display (Fast Fourier Transform (FFT) equivalent of a time domain waterfall display). The principle of C/MT dates back to 1937, when it was introduced by the LMT company of France. It was adapted by Peter, G3PLX.
	S/MT Hell (Sequential Multi-Tone) is like C/MT Hell, but the rows are transmitted sequentially rather than simultaneously. One, five, or seven distinct frequencies are used for the rows. It was invented by Murray, ZL1BPU.
	Slowfeld is an experimental Hell-mode with a very small bandwidth (2 Hz), and requires very stable and precise tuning (20 Hz). The receiving process applies FFT techniques. It is to Hellschreibers what QRSS is to CW. This mode was invented by Lionel, G3PPT.

	Ref. 1: "Hellschreiber Modes - Technical Specifications", by Murray Greenman, ZL1BPU (used with permission)
	Ref. 2: "Hell formats", by Murray Greenman, ZL1BPU (used with permission)
	Ref. 3: "Erfahrungen mit MtHell" and "MtHell - Ein Fernschreibverfahren für Soundkarten", Manfred Salzwedel (DK4ZC), cq-DL, 1998, nr.10, p. 811
	Ref. 4: "Digital Modes", Murray Greenman, ZL1BPU, Break-In, Nov/Dec 2010, pp. 26-28
	Ref. 5: pp. 294,295 in "The fading machine, and its use for the investigation of the effects of frequency-selective fading", W.J. Bray, H.G. Lillicrap, F.C. Owen, pp. 283-297 in "Journal of the IEE, Part IIIa: Radiocommunication", Vol. 94, Issue 12, March-April 1947

Ko Versteeg, NL9922, has an extensive collection of screen captures of received text from various Hell modes on his website.

I have used DL4YHF's Spectrum Lab (excellent Audio Signal Analyzer freeware) to produce the audio frequency spectra for various Hellschreiber modes. See figures below. This allows to get a feel for the minimum required bandwidths. The analyzer was configured for a 16348-point FFT (the Spectrum Lab setup file that I used is here). The spectra were generated for the transmission of random text.

Note: the real Feld-Hell machines apply on-off-keying (OOK) of the internally generated 900 Hz signal or of a CW transmitter; the IZ8BLY, DM780, FLdgi and MULTIPSK software applies raised-cosine pulse shaping. In absence of transmitter non-linearities and over-driving the modulator, the latter minimizes the signal bandwidth to something close to the theoretical Nyquist minimum of 2x the signaling rate (i.e., 2x122.5 = 230 Hz for Feld-Hell).

Note: the analyzed audio signals were "PC-internal". The signal path did not include a soundcard, nor a transmitter-receiver or "propagation".

Note: the plot for Hell-80 with FLdigi software has a Iarger frequency scale (200-2400 Hz vs 600-1400 Hz for all others).

Note: The spectra for RTTY and PSK31 digi-mode were added as a reference; PSK31 specified bandwidth is 62.5 Hz @ -30 dB.

Note: I still have to add the spectrum for Hell-72 "GL".

- (double) click on any of the spectrum plots below to get the full-size picture -

Feld-Hell (IZ8BLY software) Feld-Hell (DM780 software)
- both with raised-cosine pulse shaping -