Hellschreiber and Feld Hell on the Ham Bands: How It Works

Of all the digital modes on the amateur bands, Hellschreiber is the one that most people find strange at first sight. There is no decoder that turns the signal into clean letters. Instead, the text appears on the screen as a line of small pixels, much like a dot-matrix printout, and you read it with your eyes. It sounds like a step backwards, and yet the mode is a favourite of experimenters, because it can be copied through noise that defeats many other modes and because its history links a 1920s mechanical invention with the sound card in a modern computer. This guide explains how Hellschreiber works, why a human reader makes it robust, which variants you will meet and how to try it with free software.

A Teleprinter From the 1920s

According to the Wikipedia article on the Hellschreiber, the device, also called Feldhellschreiber or Typenbildfeldfernschreiber, is a facsimile-based teleprinter invented by Rudolf Hell. It was developed in the late 1920s, came into use in the 1930s and served mostly press services on landlines. It was also used by the German military during World War II, and after the war it became common among newswire services and stayed in use well into the 1980s. The same article states that it was much simpler and more robust than the typewriter-based teleprinters of its time, with far fewer moving parts, and that it can provide intelligible communication over very poor quality radio or cable links where voice or other data would be unintelligible. Today the mode lives on among radio amateurs who use computers and sound cards. The resulting mode is called Hellschreiber, Feld-Hell, or simply Hell.

How the Signal Is Built

The idea is to send pictures of letters instead of codes. The same article explains that each line of text is sent as a series of vertical columns, and each column is broken down into pixels, normally on a 7 by 7 grid for a character. The data of a line is sent as a series of on-off signals, normally at a rate of 112.5 baud for the original mode. The receiver draws the pixels in the same order, one column after another, so the letters appear on the screen as images. Because no information about which letter was sent is needed by the receiver, the mode is independent of any character set: anything that can be drawn on a grid of seven pixels in height can be transmitted.

The original machines printed every column twice, one below the other. This compensated for small timing errors and gave the text a slant, as the article describes: the received text may look like two identical lines, one below the other, or a line of text in the middle with chopped-off lines above and below, but at least one whole letter can be read at all times. Software versions copy this idea. In a program such as fldigi, you see the pixel text scroll across a window, and a small timing error simply skews the letters instead of making them unreadable.

Why a Human Reader Helps

Modern digital modes such as FT8 use error-correcting codes and a decoder that either produces correct text or none at all. Hellschreiber does the opposite. It has no error correction and no decoder, but your eye and brain are very good at recognising a shape from fragments, even when many of the pixels are missing or extra pixels from noise are present. The software versions improve this further by showing the signal in shades of grey instead of black and white, as the Wikipedia article says: depicting the received signal as shades of grey makes it much easier to read weak signals. A weak signal then looks like a faint ghost of the letters, which you can still read, while a decoder with a hard threshold would see nothing.

This also explains the limits. Hellschreiber is slow, since each character takes the time of seven columns, and the typical speed is only a few words per minute. It is therefore a mode for experiments and for friendly, short contacts. It is not a good mode for contests or for fast exchanges of data.

The Variants You Will Meet

All of these variants are descended from the Hellschreiber principle and many were created by radio amateurs in the 1990s. If you start with only one, start with the classic Feld Hell or with FSK Hell-105, because they are the ones that other operators are most likely to use, and every serious program supports them.

What You Need to Try It

You do not need special hardware. A transceiver with an SSB mode, a computer with a sound card and a program that supports the mode are enough. The software that most amateurs use is fldigi, a free multimode program described on Wikipedia and in our fldigi setup guide. The setup of the audio path is the same as for any sound card mode. If you have not yet connected the radio to the computer, our guide to the virtual audio cable setup and the article on sound card isolation and ground loops cover the connections.

Step by Step: First Reception

1. Start fldigi and select the Hell family of modes from the operating mode menu, then choose Feld Hell or FSK Hell-105.
2. Tune the radio in USB to a frequency where activity is reported, or listen on a quiet part of a digital segment on 40 or 20 metres. Check the current band plan and the operator reports to find where the mode is used, since the activity moves.
3. Watch the receive window. The text appears as a pixel strip. If the letters look like a smear, adjust the tuning slightly: the pixels line up only when the audio frequency is correct.
4. Adjust the audio level so that the signal is clear without clipping. The received text should have a good contrast and not look like a solid block.
5. Be patient. The text scrolls slowly, and you read it as it goes by. Use the two-line display to read the letters in the way the old machines did.

Step by Step: Transmitting

Hellschreiber is a constant-envelope or keyed mode, and the usual rules for sound card modes apply. Keep the audio level low enough to avoid driving the transmitter into compression, use no more power than you need and listen before you call. With keyed on-off modes the abrupt keying makes sidebands, so keep the audio drive modest and follow your radio’s recommendations. Type a short message, such as a CQ with your call sign repeated, and give the other station time to read it: send your call twice and slowly. Hellschreiber contacts are short, friendly and often experimental, and operators like to compare how the signal looks on each side.

• Use a clean signal. A pixel signal that is distorted looks worse on the receiving screen, so reduce the drive if the letters look fuzzy in a monitor.
• Send your call sign repeatedly in a CQ, since the receiver may need several lines to tune the audio.
• Keep the message brief. Exchange the call signs and a signal report, and perhaps a name and location.
• Use a waterfall check. If your program shows the transmitted spectrum, verify that the signal is narrow and has no unwanted tones.

Reading the Screen: Practical Tips

The first minutes with Hell are the hardest, because your brain is not used to the pixel font. A few habits help. Make the receive window as wide as the program allows, so that a whole line is visible, and keep the zoom at a size where each letter is about as tall as a normal text character. If the letters slope, the audio frequency or the timing is slightly off, and a small change in the tuning straightens them; because each column is printed twice, you can also read the letters from whichever of the two rows is cleaner. A signal that jumps between two positions on the screen suggests that the transmitting station is drifting, or that your own receiver has a slow drift. In the multitone variants, the signal is a pattern of tones on the waterfall, and the letters are visible as shapes in the display itself, so you read them from the waterfall and do not need a separate text window.

It also helps to know that noise looks different in this mode from other modes. A burst of static makes a bright vertical stripe across the letters, and fading makes the pixels lighter or darker over a few seconds. Your eye learns to ignore both and to follow the repeated shapes. After an evening of watching signals you will find that you can read callsigns that would be a few faint dots on the first night. That learning curve is a large part of the pleasure of the mode.

Where Hell Fits Among the Digital Modes

Compared with the main modes, Hell is a niche. The overview in our article on digital modes beyond FT8 shows where it sits, and for a mode that is also designed for weak signals but with a decoder, the Olivia and Contestia guide is the natural comparison. Those modes decode text with forward error correction and give exact characters at very low signal levels, which Hell cannot. The reason to use Hell is different: it is simple, it has a long history, it looks interesting and it makes you look at the signal and think about what the pixels mean. For many amateurs it is one of the most pleasing ways to spend an evening with a computer and a radio.

A Mode for Experimenters

Hellschreiber has a place in amateur radio mostly because it invites experiments. You can design your own font and send symbols that no other mode can carry, as the Wikipedia article points out when it says that the mode is independent of character sets. You can compare the mode with others on the same signal, send it through very narrow filters or watch how it behaves in fading. Slowfeld pushes the idea to the extreme, using a very narrow bandwidth, so it belongs to the same family of slow experiments as QRSS, the very slow Morse code that hams use to span long distances with tiny power. These activities appeal to the part of the hobby that is about learning and curiosity, and they cost nothing beyond the time you spend. If you enjoy them, you will probably find them more rewarding than the quick exchanges of the busier modes.

The Bottom Line

Hellschreiber sends text as columns of pixels, normally on a 7 by 7 grid at 112.5 baud for the original mode, and the receiver draws the same pixels on the screen. There is no decoder: you read it by eye, which makes it very tolerant of noise, especially in software versions that show shades of grey. Variants such as FSK Hell, PSK Hell, Duplo Hell, C/MT Hell and Slowfeld extend the idea. To try the mode, use fldigi and a sound card interface, choose Feld Hell or FSK Hell-105, tune carefully and keep the drive level modest. It is slow and quirky, but it is a mode with a story, and it is great fun.

Frequently Asked Questions

What is Hellschreiber?

It is a facsimile-based teleprinter system invented by Rudolf Hell in which text is sent as columns of pixels. Radio amateurs use it as a digital mode called Hellschreiber, Feld Hell or simply Hell.

How do I read Hellschreiber?

You read it by eye. The software draws the received pixels as a line of text on the screen, and no decoder turns it into characters.

Why is Hell good for weak signals?

The human eye can recognise letters from incomplete pixel patterns, and software shows grey levels, so a signal that a decoder would reject can still be readable.

What software can I use for Hell?

Fldigi is the most common program and supports Feld Hell and several variants. Any other multimode program with Hell modes works too.

Is Hellschreiber fast?

No. Each character is drawn as seven columns, so the speed is only a few words per minute. It is suited to short, experimental contacts.

What is the difference between Feld Hell and FSK Hell?

Feld Hell keys a single tone on and off. FSK Hell uses frequency shift keying with controlled phase, which keeps the amplitude constant and makes better use of the transmitter.