HamRadio Tech

DIGITAL MODES: the new generation of HF HAM Radio !


What is so good about Feld Hell?

The oldest keyboard text communications protocol is now one of the newest ham radio modes! The hellschreiber method was used over landline for many years before its adaption to radio waves. This simple method has one major advantage over all other HF keyboard modes... The advantage comes from the fact that it is NOT a true digital mode in the way that modes like RTTY, PSK31 and Pactor are. With Hell modes the text characters are not decoded by the computer program. For this reason, you will hear hellschreiber described as a "fuzzy mode".
Hell text is "painted" to the screen as the Hell tone is being received through the sound card. To receive a Feld Hell signal, the computer program sets up the sound card to use DSP techniques for detecting the signal and places the output on a short vertically scanning line that is moving horizontally across the screen. This action produces a single line of text in a marque fashion that gets wrapped around to eventually create a full screen of text. Really simple! The computer never has to decide (decode) the characters and so no missed characters or wrong characters will ever be sent to the screen! To send Hell characters, the computer program uses the sound card to produce the audio signal to modulate the transmitter in the same way that other sound card modes are transmitted. For Feld Hell, it is an on/off single tone the same as used for MCW. Each tone burst is conditioned by the DSP to have the correct shape that will not cause harmonics and "clicks" to be produced by the radio's modulator. A properly adjusted Feld Hell input level will produce a bandwidth of no more than 75 Hz. Over driven levels will appear as wider signals on the waterfall display.
With Hell modes, the only time that a received character appears wrong on the screen is when it is typed wrong at the other end. The only time when a character or word is not readable on the screen is when radio propagation is so bad that not even DSP techniques can pull it out of the mud! This occurs at signal levels considerably lower than those tolerated by all other modes. The weakest link in decoding Hell characters will be the operators ability distinguish letters under noisy conditions. Another advantage of Feld Hell mode over most others is the ability to print readable text under conditions of polar phase shift. A severely phase shifted signal will print a shifted character, but it can still be read on the screen. Each Hell character is sent on a double line, so that if one character is shifted off the marque, the other will still be visible. Another nice feature of Feld Hell is that the RF power output cycle is less than 50% during TX, saving energy. One nice feature that feld hell has in common with PSK31 is a very narrow bandwidth used by the signal (about the same as for CW). The end result is the most reliable keyboard communications achievable on HF ham radio. So why isn't everyone using Hellschreiber mode?

How does Feld Hell compare with PSK31?

- The hardware setup and cabling is exactly the same.
- Good freeware and shareware software is available for both.
- Tuning in a hellschreiber signal is easier and faster.
- Hellschreiber is a little slower for sending text.
- No cut-n-paste from the receive screen with hellschreiber.
- You can chose the text font for sending with hellschreiber.
- Weak signals have better readabilty with hellschreiber.

PSK31 is a true digital mode and for some people that makes it a "sexier" mode than Hellschreiber. Hell modes are more closely related to CW than to other digital mode. With CW you use your ears and for Hell modes your eyes. Having both fuzzy and digital modes available on the computer desk top has been a good strategy for me. The Hellschreiber frequencies are next to the PSK31 frequencies on most ham bands, so they are easy to mix. As more PSK31 ops discover how nicely the two operating modes compliment each other, we will see more Hellschreiber activity.

What about the other Hellschreiber modes?

The very popular Hellschreiber freeware program from IZ8BLY and the Shareware program from UT2UZ have other experimental Hell modes included. Of these, there are two modes besides the traditional Feld Hell mode that hams have found to be useful and are currently using. PSK HELL and FM HELL modes both come in a 105 baud and 245 baud rate. With these modes, the ON/OFF keying needed to paint the Hell characters across the marque are sent by shifting the phase of two tones (PSK Hell) or by shifting the frequency of one tone (FM Hell). The faster baud rate for both modes does not increase text speed, but it does increase resolution, producing a sharper font. Both modes have a 100% duty cycle during transmit, but the additional power will produce a less noisy background, making the text easier to read. With either of these modes, it is fun to see the font flow across the screen like a video, as apposed to computer type set. When your eyes get tired of reading computer text, try Hell modes!

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