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 Hellschreiber 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 rows are on different frequencies and 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.
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, slanted, or too fat or too skinny. 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 does not, providing further noise rejection. In other words when you send slower, copy is better.
Fred OH/DK4ZC in Finland, copied in ZL on 20m (G3PLX C/MT-Hell DSP software)
There are three main variants of MT-Hell, based on how many dots the transmitter can send at the same time. 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 transmits more than one at a time.
- Chirped Hell can send one or a few tones at the same time, and 'scans' the columns several times faster than the column rate.
The text sent by these modes has different characteristics. Let's compare them with Feld-Hell:
- Feld-Hell text is usually very sharp and may lean slightly to the right, and is always double printed
- Concurrent tone MT-HELL has very detailed characters, and is always upright - unless the user has chosen to use an italic font!.
- Sequential MT-HELL text leans strongly to the right, and may have a dotty (pixellated) appearance
- Chirped Hell tends to be blurry, often striped, can be leaning or upright, depending on the settings. It can be used to send very complex pictures.
Double printed Feld-Hell Concurrent MT-HELL is always upright Sequential MT-HELL leans to the right Chirped Hell can be blurry or striped
Sensitivity is not as high as other modes unless slow speeds are used. The transmitter must operate very linearly, and has problems when vertical lines are transmitted, since all tones are sent at once. 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 all dots in a column are 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
Reception tends to become blurry as speed is increased. Text speed can be high because white space duration can be reduced or eliminated. G3PPT has developed a seven tone upper-case only Sound Blaster sound card version. Direct FSK of CW transmitters is very popular for QRP use. The ZL1BPU LF Exciter operates S/MT-Hell. Double-column mode (see example below) gives improved readability, more upright text, and is easy to generate.
- 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 DDS VFOs, QRP, MEPT and LF transmitters
- Simple signal generation with a PC or a micro, or even via the PC speaker or a Hamcomm interface
Chirped Hell advantages:
Most operators use DF6NM's flexible Chirppix 'compiler' to convert bit map images to .WAV files, which are then transmitted by a media player. Chirped Hell is not intended as a QSO mode. It is popular for experimental propagation transmissions.
- Flexibility - if you can draw a picture as a bit map, you can send it!
- Peak power issues of C/MT-Hell are eased (but TX must still be linear)
- Wide variety of bandwidth and speed options