Other Hell Modes
S/MT HELL
Sequential tone Multi-Tone Hellschreiber is a frequency domain technique, and as such requires FFT
demodulation. It represents something of a half-way house between Feld-Hell and C/MT-Hell. On the
one hand, like Feld-Hell, it is a sequential transmission technique, yet like C/MT-Hell, it is a
frequency domain technique. Unlike Feld-Hell and C/MT-Hell, S/MT-Hell is believed to be a modern
(1998) invention.
S/MT-Hell characters are sent using a modified raster scan method, where the timing of the pixels is
unimportant, so long as the sequence is maintained and the speed kept relatively constant, both to
preserve character shape. There is no synchronism, and no accurate timing of the scanning process is
necessary. In addition, within limits, the non-printing pixels can be omitted, ie. not sent at all!
The advantages of S/MT-Hell are speed, immunity to timing and time domain interference, simplicity
of signal generation and ease of transmission. In addition the transmitted power is concentrated in a
single dot, like Feld-Hell, so performs well with low power.
C/MT HELL
Concurrent tone Multi-Tone Hellschreiber is a frequency domain technique, and as such requires FFT
demodulation. It was first described in 1937[6].
C/MT-Hell characters are sent using a parallel row scan method, where the timing of the pixels is
unimportant, so long as the pixels of the same column occur together and the speed kept relatively
constant, to preserve character aspect ratio. The shape is not otherwise affected by pixel timing. There
is no synchronism, and no accurate timing of the scanning process or receiving process is necessary.
The advantages of C/MT-Hell are high resolution, immunity to timing and time domain interference,
and font flexibility. The transmitting and receiving equipment must be very linear to handle multiple
tones concurrently.
DUPLO HELL
This is a dual-tone mode invented in 1999 by Nino IZ8BLY. The font and format are identical to Feld-Hell, except that two columns are transmitted at the same time, using two on-off keyed tones.These tones (980 Hz and 1225/1470 Hz for 245/490 Hz shift) are obviously sometimes sent at the same time. The lower tone is used for the left column, and the upper for the right. Rather than operate at double throughput, the transmission runs at the same throughput as Feld-Hell (2.5 characters/sec), with the dot elements doubled in length (61.25 baud).
This mode is remarkably immune to noise, due to the higher integration time for each dot element. The wider shift is more noise immune, but more sensitive to selective fading, which causes a "venetian blind" effect. The mode is little used
