Field Guide · protocol

Also known as: Hellschreiber, Feld-Hell, Hell

Hellschreiber (German for “Hell writer,” after inventor Rudolf Hell) is a text mode that transmits characters not as data bits to be decoded, but as a facsimile of the printed glyph — each letter is scanned as a grid of dots and sent column by column, so the receiver simply paints the dots and the operator reads the words by eye.1 Because no character-recognition step can fail, it is one of the classic fuzzy modes: noise sprinkles speckle across the page but the human eye still resolves the letters, much as it copes with weak RTTY or faint Morse.

glyph as dot grid scan columns carrier keyed on/off per pixel →
Each glyph is scanned into a dot grid; the carrier is keyed on for dark pixels and off for light ones, so the receiver reprints the letter shape rather than decoding a code.

Overview

The classic variant, Feld-Hell, sends 245 characters per minute using a 7-pixel-high font scanned in columns. A “dark” pixel keys the carrier on; a “light” pixel keys it off — plain on-off keying. At the receiver each received pulse blackens the corresponding cell of a scrolling raster, and successive columns build up the letters. There is deliberately no forward error correction and no synchronisation to lock: the display just scrolls, and the reader’s brain does the pattern recognition. To counter timing drift, most software prints two stacked copies of the text so at least one row is always readable.

Variants

Modern sound-card software has spawned a family of Hell modes on the same visual principle:

  • Feld-Hell — the original on-off-keyed mode, ~35 baud, ~75 Hz wide.
  • PSK-Hell / Hell-PSK — replaces keying with phase-shift signalling for a narrower, cleaner signal.
  • FM-Hell (MT-Hell) — shifts dot rows to different tones (multi-tone), improving sensitivity and tolerance to selective fading.
  • Slow-Hell — very low speeds for extreme weak-signal work.

History

Rudolf Hell patented the Hellschreiber in 1929 as a rugged, low-cost teleprinter for landline and radio press circuits; it saw wide use through the mid-20th century, including military and news traffic, precisely because a simple electromechanical machine could send and print it without complex decoders.1 Amateurs revived it in the sound-card era, where “fuzzy mode” ruggedness on HF is its main appeal.

Deployment

Today Hellschreiber is an amateur curiosity and weak-signal mode, run on HF phone/data sub-bands via Fldigi and similar programs. It has no commercial deployment but retains a loyal following for QRP and marginal-path text contacts.

Decoding it with GopherTrunk

GopherTrunk does not decode Hellschreiber. It is a visual facsimile mode read by eye, with no framed data output to recover, so it sits well outside GopherTrunk’s trunking and data-decoding scope. Fldigi is the standard tool. The underlying on-off keying is a primitive GopherTrunk understands, but the Hell raster display is not something GopherTrunk produces.

Sources

  1. Hellschreiber — Wikipedia, for the facsimile dot-scan principle, Feld-Hell timing, mode variants, and Rudolf Hell’s invention. ↩ ↩2

See also