Hellschreiber

The Hellschreiber, actually called type field telegraph, invented by Rudolf Hell is a teletype machine that has been used mid-20th century on a particularly vulnerable transmission paths. The principle was patented in 1929 and used both with radio transmission or via cable. He had particular importance in the transmission of radio messages to the press in the 1980s. Today he is still partially used by radio amateurs.

Transfer principle

In more modern systems, each character is in a pattern of 7 lines and 7 columns so decomposed 49 pixels. By scanning transmission of Hellschreiber can transfer any character that can be mapped on the grid, which is why the method has been used successfully in the Asian region.

For each character of the transmitting device includes a cam which performs a rotation in pressure on the encoder key. Here, their cams produce different long current pulses depending on the length.

The receiving apparatus, a paper strip is pressed in the rhythm of the pulses received by the armature of an electromagnet to a 60 revolutions per second write rotary screw. A column of a character is written per revolution of the writing screw so that a second approximately 8 ½ characters can be transmitted. Translated into today's standards, the data transfer rate of this system was 420 bit / s The range of telex connections handed out for so that communications with the Hellschreiber.

Decoding and error correction

Walked transmitter and receiver are not synchronized, this was only the consequence that the letters were printed at an angle, but were still readable. Furthermore, the susceptibility of the receiver was reduced by the fact that the writing snail letters twice wrote to each other.

There was also in this analog transmission no technical error correction decoding or in the real sense - the character recognition is performed by the reader. This achieved a long time Recognition rates that were much later with other technical means by sophisticated channel coding achievable.

Sound Samples

Patent

The patent for Hellschreiber, No. DE540849, was valid in Germany from 3 April 1929 and 17 September in 1930 was followed by an additional (No. DE541935 ) to do so. The PDFs of patents can be found on the German Patent and Trademark Office under Search / beginners.