©2004-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.

Latest page update: April 2016

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Advances in DSP technology (hardware and software implemented) have led to the development of a number of Hellschreiber mode-variants since the 1990s. Various software packages for all major operating systems can operate in these new Hell modes (see my Hell software and PC-interfacing pages). Here is a brief summary of these modes:

  • FSK-Hell. This mode uses Frequency Shift Keying with a speed of 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, as does the Thomson Hellschreiber.
  • PSK-Hell encodes the pixels brightness in the carrier phase instead of the amplitude. Strictly speaking, it is encoded in the change of the phase (differential Phase Shift Keying): an unchanged phase at the beginning of a pixel means "white", and a reversed phase means "black". This is a narrow-bandwidth mode. 105 or 245 baud. It was conceived by Murray Greenman, ZL1BPU.
  • FM-Hell uses Minimum Shift Keying (MSK). Data rates are 105 and 245 baud; due to use of special fonts, these two 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 is a Concurrent Multi-Tone mode, that sends all rows of the text font simultaneously. Each row uses a distinct tone frequency. Hence, the transmission can be read directly on a frequency-domain display (Fast Fourier Transform (FFT) equivalent of a time domain waterfall display). Versions with 7, 9, 14, and 16 tone/rows have been developed. The principle of C/MT Hell dates back to 1937, when the 7-tone version was introduced by the LMT company of France.
  • S/MT Hell, or 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 conceived by Murray Greenman, ZL1BPU.
  • CP-16 is a 16-tone C/MT Hellschreiber mode that was developed by the Chinese Amateurs Radio Club (CRAC) and the Chinese Radio Sports Association (CRSA) in 2011/2012. As the Hellschreiber concept is based on rasterized font-images, it is an obvious choice for graphically oriented Chinese, Japanese, Arabic, etc. characters. The 16 tones are spaced at 17 Hz intervals, and the total bandwidth is 400 Hz. Transmission speed of the 16x16 pixel font is adjustable from 2 to 5 characters per sec. Transmitted text can be viewed with any standard digi-mode program that has a waterfall display. The mode was presented to the IARU Region 3 Conference in Vietnam, November 2012 (ref. 6).
  • SlowFeld is an experimental version of Feld-Hell, intended for very low-power, very narrow bandwidth DX and beacons on HF and LF. It runs 50, 100, or 200 times slower than regular Feld-Hell: 3, 1.5, or 0.75 characters/minute. It was developed in the 1990s by Lionel Sear (G3PPT). It is an adaptation of QRRS "Morse" telegraphy to Hellschreiber. A special version of SlowFeld can be used on VHF, to communicate via aircraft-scatter.

Ko Versteeg, NL9922, has this collection of screen captures of received text from various Hell modes, and an archive of Hell (and other digi/visual-mode) software on his website.

External links last checked: October 2015


I have used DL4YHF's Spectrum Lab (excellent Audio Signal Analyzer freeware) to produce the audio frequency spectrum plots for various Hellschreiber modes that are shown 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, FLdigi 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).

Sound-clips of these Hellschreiber modes are provided on the "Sounds from Hell" page.

Feld Hell spectrum

Feld-Hell (IZ8BLY software)

Feld Hell spectrum

Feld-Hell (DM780 software)

(both with raised-cosine pulse shaping)

RTTY spectrum

RTTY 45.45 Bd (DM780 software)

(shift 170 Hz)

PSK31 spectrum

PSK31 (DM780 software)

(specified bandwidth is 62.5 Hz @ -30 dB)

Hell-80 spectrum

Hell-80 (FLdigi software)

Hell-80 spectrum

Hell-80 (MULTIPSK software)

(frequency scale expanded to 200-2400 Hz)

FM hell spectrum

FM105-Hell (IZ8BLY software)

FM hell spectrum

FM245-Hell (IZ8BLY software)

FSK105 Hell spectrum


PSK105 Hell spectrum

PSK105-Hell (IZ8BLY software)

PSK245 Hell spectrum

PSK245-Hell (IZ8BLY software)

Duplo-Hell spectrum

DUPLO-Hell (IZ8BLY software)

C/MT Hell spectrum

C/MT-Hell (IZ8BLY software)

Thomson-Hell spectrum

Thomson (own recording of actual machine)

(blue: "space" tone only)

SlowFeld Hell spectrum

SlowFeld (G3PPT software)


The spectra shown below may be used to help recognize Hell-modes on the amateur radio bands.

waterfall spectrum - Feld-Hell

Feld-Hell spectrum

waterfall spectrum - Hell-72

Hell-72 "GL" spectrum

(shifted to 900 Hz)

waterfall spectrum - Hell-80

Hell-80 spectrum

(shifted to 900 Hz)

waterfall spectrum - Thomson Hellschreiber

Hell Thomson spectrum

(shifted to 900 Hz)

waterfall spectrum - FM-Hell 105

Hell FM-105 spectrum

waterfall spectrum - FM-Hell 245

Hell FM-245 spectrum

waterfall spectrum - PSK-Hell 105

Hell PSK-105 spectrum

waterfall spectrum - PSK-Hell 245

Hell PSK-245 spectrum

waterfall spectrum - Duplo hell

Duplo-Hell spectrum

waterfall spectrum - C/MT hell

C/MT Hell spectrum

Waterfall spectra of other digital modes are here.


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