The Hellschreiber , actually called type field teletype , is a teleprinter invented by Rudolf Hell , which was used in the middle of the 20th century on transmission channels that were particularly susceptible to interference. The principle was patented in 1929 and used both with radio transmission and via cable. It was of particular importance in the transmission of press radio messages until the 1980s. Today it is still partly used by radio amateurs .
In more modern systems, each character was broken down into a grid of 7 lines and 7 columns, i.e. 49 pixels. Through the raster transmission, the Hellschreiber can transmit any character that can be reproduced on the raster, which is why the process has also been used successfully in Asia.
In the following, the first four columns of the letter W are transferred, column by column from bottom to top and then from left to right.
The transmitter contains a cam disc for each character, which rotates one revolution when the button is pressed. Their cams generate current pulses of different lengths depending on their length.
In the receiving device, a tape of paper is pressed by the armature of an electromagnet against a writing screw rotating at 60 revolutions per second in the rhythm of the received impulses . One column of a character is written per revolution of the writing screw, so that approximately 8½ characters can be transmitted per second. Translated into today's conditions, the data transfer rate of this system was 420 bit / s. The bandwidth of telex connections was sufficient for transmissions with the Hellschreiber.
Decoding and error correction
If the sender and receiver were not synchronized, the only consequence was that the letters were printed at an angle, but were still legible. Furthermore, the susceptibility of the receiving device to failure was reduced by the fact that the writing snail wrote the letters twice under each other.
Furthermore, there was no technical error correction or decoding in the true sense of the word with this analog transmission method - the character recognition is done by the reader. For a long time, this achieved recognition rates that were only achievable with other technical means much later through highly developed channel coding .
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The Hellschreiber patents can be found on Wikimedia Commons.
- Feld Hell Club A club of radio amateurs who practice bright writing.
- Very detailed pages about Hellschreiber by Frank Dörenberg (eng)