FSK-Hell, of recent invention, uses differential Minimum Shift Keying (2-MSK) with a raised cosine envelope to minimize bandwidth. The technique is similar to PSK with one tone suppressed. These modes are much more sensitive than all other Hell modes, and have much better resistance to multi-path and Doppler. Of all Hell modes, only FSK-Hell 245 can correctly use a regular computer font. The FSK-Hell 105 version uses a special restricted resolution font to achieve very narrow bandwidth. Great for DX!
The Hell 80 mode is a Hell original from the 1970s. It is a rival to RTTY, with similar speed and bandwidth.
All Hellschreiber modes are based on character scanning, reproducing characters in a similar way to a dot-matrix printer. This technique uses a digital transmission, yet allows the received result to be interpreted by eye, a similar concept to the reception of Morse by ear. The character is scanned upwards, then left to right. There are typically 14 pixels (transmitted dot elements) per column (although single pixels are never transmitted) and up to seven columns per character including inter-character space.
These remarkably simple modes are easy to use, easy to tune, and although not especially sensitive, are entirely suited to HF since they use no sync and the eye can discern the text even in high levels of noise.
Mode | Symbol Rate | Typing Speed1 | Duty Cycle2 | Modulation | Bandwidth3 | ITU Designation4 |
Feld-Hell5 | 122.5 baud | ~ 2.5 cps (25 wpm) | ~ 22% | OOK ASK | 350 Hz | 350HA1B |
Slow Hell | 14 baud | ~ 0.28 cps (2.8 wpm) | ~ 22% | OOK ASK | 40 Hz | 40H0A1B |
Feld-Hell X5 | 612.5 baud | ~ 12.5 cps (125 wpm) | ~ 22% | OOK ASK | 1750 Hz | 1K75A1B |
Feld-Hell X9 | 1102.5 baud | ~ 22.5 cps (225 wpm) | ~ 22% | OOK ASK | 3150 Hz | 3K15A1B |
FSK-Hell 245 | 245 baud | ~ 2.5 cps (25 wpm) | ~ 80% | 2-MSK6 | 490 Hz | 490HF1B |
FSK-Hell 105 | 105 baud | ~ 2.5 cps (25 wpm) | ~ 80% | 2-MSK6 | 210 Hz | 210HF1B |
Hell 80 | 245 baud | ~ 5.0 cps (50 wpm) | 100% | 2-FSK (480Hz) | 800 Hz | 800HF1B |