Hellschreiber
| Hellschreiber | |
|---|---|
| Also called | Hell, Feld Hell |
| Invented by | Rudolf Hell, Germany |
| Invented | Late 1920s |
| Mode type | Text sent as a picture of each letter, read by eye |
| On the ham bands | |
| Main variant | Feld Hell, on-off keyed |
| Speed | About 2.5 characters per second for Feld Hell |
| Other variants | FSK Hell, Slow Hell and others |
| Software | fldigi and other multimode programs |
Hellschreiber is a method of sending text in which each letter is transmitted as a small picture rather than as a code. The receiving station draws the dots on a screen or a strip of paper, and the operator reads the letters by eye. It was invented in Germany in the late 1920s by Rudolf Hell, and the name means "Hell's writer". Long obsolete in commercial service, it survives on the amateur bands, where its simple, forgiving nature and its look of an old teleprinter tape have kept a dedicated following.
How it works
In most text modes, such as RTTY or PSK31, each character is sent as a code, and the receiving equipment must decode that code correctly or print the wrong character. Hellschreiber works more like a fax machine. Each character is divided into a small grid of dots, and the transmitter scans the grid one column at a time, sending a burst of signal for each black dot and nothing for each white one. The receiver paints whatever it hears, in step, so the shape of the letter is rebuilt on the page.
Because the receiver never has to decide which character was sent, there is nothing to decode wrongly. Noise appears as scattered specks, and a fading signal produces faint or broken letters, but the human eye is remarkably good at reading shapes through that kind of damage. A signal that would turn RTTY into gibberish can still be readable as Hell.
The receiver prints every column twice, one copy above the other. If the two stations' timing differs slightly, the text slowly slants up or down, but one complete copy of each line is always visible, so the operator can keep reading without any synchronization.
History
Rudolf Hell developed the system as a simple, rugged way to send text without the complex mechanisms of a teleprinter. The version known as Feld Hell, short for Feldfernschreiber (field teleprinter), was used by the German army during the Second World War, where its tolerance of poor radio conditions and its simple equipment were valuable in the field. After the war, Hellschreiber machines were used for years by news agencies to distribute press reports by wire and radio.
Amateurs revived the mode once personal computers with sound cards could generate and display it in software, with no mechanical printer needed. Today it is a niche mode, valued by operators interested in radio history and by those who enjoy a mode that rewards the operator's own skill.
Variants
- Feld Hell is the classic version and the most common on the ham bands. The transmitter simply keys a carrier on and off, much like CW but far faster, sending about 2.5 characters per second.
- FSK Hell shifts between two tones instead of keying on and off, which can be more robust on some paths.
- Slow Hell sends more slowly in a smaller bandwidth, for very weak signals, and faster, wider versions exist for strong ones.
The common multimode programs, such as fldigi, support most of these. A Feld Hell signal is a few hundred hertz wide, and because the transmission is made of pulses, transmit levels must be set carefully to keep it clean.
Operating
Hellschreiber needs only an HF transceiver, a computer and a digital mode interface. Activity is usually found in the data portions of the HF bands, often at announced times. The Feld Hell Club is an active international group that issues member numbers, organizes regular on-air sprints and offers awards, and it is the best place to find out when and where others are operating.
Because the operator reads the screen directly, a little practice helps. Adjusting the receive width and the display speed makes letters easier to read, and most programs let the operator choose the font that is transmitted. Unusual characters and simple symbols can be sent, since the receiver draws exactly what it is sent.