Multi-Tone Hell
Traditional Feld-Hell transmits the dots of each character at specific times, because it operates in the time domain. Each dot is sent at the same audio or radio frequency, at a different but precise point in time. This makes Feld-Hell a time domain mode.MT-Hell or Multi-Tone Hell transmissions are broadly similar in concept to Feld-Hell, but each row of dots or pixels in the character is at a different frequency, with quite different (much relaxed) timing, and thus text is transmitted in the frequency domain. The timing of the transmitted dots has more to do with the column in which the dot resides, rather than the position withing the column. Since columns are always sequential, there is no notion of synchronism in MT-Hell. Reception of MT-Hell requires a conversion technique called the Fast Fourier Transform to convert the signal back into a human readable form. Although the MT-Hell concept is simple, and not by any means new (it was described in 1937!), MT-Hell relies on high performance digital computers and the high speed repetitive Fast Fourier Transform calculations. Don't be frightened off by that though, because all the hard work has been done for you by the developers.
By transmitting each row of character dots at a different frequency, the characters can be read using only the frequency domain. Since almost all noise and interference is based in the time/amplitude domain, frequency domain modes provide significant rejection of noise and interference, as we know from comparing the quality of AM and FM broadcast reception. The data rate of MT-Hell is not fixed, because there is no problem of synchronism. The only timing constraints are those that ensure that the parts of the transmitted characters come out in the right order, and don't look wobbly. By slowing down the transmitting rate, the receiving software is able to integrate (or average) the received signal over many samples. When this is done, the signal information tends to add, while the noise information tends to subtract, providing further noise rejection. In other words when you send slower, copy is better.
There are two main variants of MT-Hell, based on whether the transmitter can send dots from different rows in each column at the same time or not. These variants sound different, and have different properties, but may be received fairly interchangeably using waterfall plot (spectrogram) software -
C/MT-Hell or Concurrent MT-Hell uses many tones (from 7 to 16 tones), which are often transmitted at the same time.
S/MT-Hell or Sequential MT-Hell uses only a few tones, typically five or seven, but never more than one at a time.You can generally easily distinguish Feld-Hell and its derivatives, Sequential MT-HELL, and Concurrent MT-HELL:
The text has different characteristics too:
- Feld-Hell does not appear to have much of a musical sound - just a brrt - brrt noise (machine gun)
- Sequential MT-Hell transmissions sound like "tweedle - tweedle" (bird chirps)
- Concurrent MT-Hell signals sound like "eeee - awww" (door squeaking)
- Feld-Hell text is usually very sharp and may lean slightly to the right, and is always double printed
- Sequential MT-HELL text leans strongly to the right, and may have a dotty (pixellated) appearance
- Concurrent tone MT-HELL has very detailed characters, and is always upright - unless the user has chosen to use an italic font!.
Double printed Feld-Hell Sequential MT-HELL leans to the right Concurrent MT-HELL is always upright
C/MT-Hell has the following potential advantages:Sensitivity is not as high as other modes unless slow speeds are used. The transmitter must operate very linearly. G3PLX has a DSP system using 16 tones and Windows fonts! G3PPT has a seven tone sound card version, and an excellent 9 tone transmit only version. IZ8BLY has a 14 tone Windows font option.
- Speed, since several dots can be sent at the same time
- Better readability, since more dots provide better resolution
- Better looking text since the characters need not slope and many fonts can be used
S/MT-Hell also has its advantages:
Reception tends to become blurry as speed is increased. G3PPT has developed a seven tone upper-case only Sound Blaster sound card version. ZL1BPU has developed software for direct FSK of CW transmitters or audio VCOs, and a version for the Hamcomm interface,but the most successful and widely used is the version that transmits via the PC speaker. IZ8BLY has demonstrated excellent WINDOWS™ S/MT-Hell, but it is not in the current release.
- Weak signal performance, since all the transmitter power is applied to a single dot
- High efficiency, since the transmitter need not be linear, Class C transmitters can be used
- Very simple transmitter requirements - easily adapted to QRP and LF transmitters
- Simple signal generation with a PC - even via the PC speaker or a Hamcomm interface
ZL1BPU in QSO with EA2BAJ on 20m - world record DX for S/MT-Hell
(ZL1BPU software in double-column mode).
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