Hellschreiber ✕
hell.odd.place
About Controls How to use Sharing Signal Stack About
Hellschreiber doesn't send a code for each letter. It sends a picture of the letter. Every character sits on a grid 7 pixels wide and 14 tall; the transmitter scans it column by column, top to bottom, switching a 900 Hz tone on for black pixels and off for white, 245 pixels a second. The receiver paints those pixels onto a paper tape as they arrive. There is no decoder and no character recognition: your eye reads the tape.
That sounds primitive, but it's clever. Noise smears letters without destroying them, and a character can be a third obliterated and still be legible, because human vision is good at filling in. Each row is printed twice, at the top and bottom of the tape, so when the sender's and receiver's clocks drift apart one copy is always cleanly inside the readable band.
History
Rudolf Hell patented the mechanical Hellschreiber in 1929. The German army carried it through the Second World War because it needed no skilled operator: anyone literate could read the tape. Afterwards it lived on in amateur radio as Feld-Hell, and programs like Fldigi keep its visual-decoding spirit: the machine does the listening, the person does the reading.
What each control does
Tape The paper tape. Drag it to read back; it follows the printing again when you scroll near the end. TX and RX light while sending or receiving; POS is the column being printed and RATE the speed in baud.
Transmit and Stop Play the message from your speaker and print it onto the tape as it plays.
Export WAV and Export Tape Save the sound (with any channel noise) as a WAV file, or the printed tape as a PNG picture. Clear Tape starts a fresh tape.
Carrier Freq The tone's pitch, 400 to 1500 Hz. Feld-Hell traditionally uses 900 Hz.
Speed Slow Hell is half speed, Feld-Hell the 245 baud standard, and Fast Hell two or four times faster. Faster prints narrower letters.
Volume and Tape Color How loud the transmission plays, and the color of the print: amber or green phosphor, paper white, or cyan.
Channel Noise Adds hiss to what you send, measured against the tone. Push it past 75% and watch the letters stay readable: that resilience is the whole idea.
Rate Trim and Auto Corrects a recording made by a sender whose clock ran slightly fast or slow, which shows up as letters leaning on the tape. Auto measures the lean in a loaded file and straightens it.
Cipher Encrypts the pictures themselves. Each letter's 98 pixels are mixed with a keystream made from your key (SHA-256 and AES-CTR). It still sounds like Hellschreiber, but without the key the tape prints even noise. Both ends need the same key. The key is never saved or sent anywhere.
Receive from file Drop in a WAV, MP3, M4A, OGG or FLAC recording and it prints onto the tape.
Listen, Auto-Tune and Spectrum Prints live from your microphone. Auto-Tune snaps the carrier to the loudest tone between 400 and 1500 Hz; the spectrum shows 0 to 2 kHz with the signal-to-noise estimate.
How to use it
Send: type a message and press Transmit . The tape prints as the tones play.
Receive on another device: open hell.odd.place there, press Listen , and hold it near the sender's speaker. If the letters don't print, press Auto-Tune while the sender is playing.
Read through noise: raise Channel Noise and transmit again. The tape gets grainy, but you can still read it.
Send a secret: turn on Cipher, enter a key, and transmit. Only someone with the same key sees letters.
Sharing a message
Press Share (top right), check the message, and choose whether the share page shows the words or keeps them secret. Each share gets its own page and picture for Facebook and other link previews. When a friend opens it, the Hellschreiber loads your carrier, speed, color and noise, and Receive plays it and prints it onto their tape.
Encrypted shares: if Cipher is on with a key when you share, only the scrambled pixels are uploaded; the words and the key never leave your device. Tell your friend the key some other way. Without it their tape prints noise, exactly what an eavesdropper would see.
A secret, unencrypted share just keeps the words off the share page and preview. Your friend's browser still receives the pixels, so it's a surprise, not a lock.
The Signal Stack
Three machines for sending words without a voice, oldest first. Each one solves the same problem, getting letters through a noisy channel, in the way its era could.
I Morse Trainer Dots and dashes, from the 1840s
II Hellschreiber Letters printed by sound, 1929
III Odyssey Modem Tones and error correction, today
Discover more Oddities at odd.place Generative art, games, science toys and signal machines from the same workshop.Made by Jason Buchheim. Produced by Odyssey Expeditions , marine biology adventure travel since 1995. Everything runs in your browser; your microphone and camera never leave your device.