Hellschreiber GL 72 (1950) | ||||||
The "Hellschreiber" is a teletype device, which transmits characters as a matrix of black and white picture elements (pixels), as opposed to normal teletype writers, which transmit a 5-bit-code for each character. Thus, the Hellschreiber transmits an "image" of the character, the image being made up of a matrix of 7 x 7 pixels. This method makes the transmission much less sensitive to noise on the transmission channel. The first Hellschreiber models were developed by Hell already since 1929. Just be-cause of the high noise immunity these devices were also used by military forces. For that purpose a special very compact model was developed, named "Feld-Hellschreiber". |
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The Hellschreiber GL 72 can send and receive text messages, up to
360 characters per minute.
When sending text, the text is typed in on a keyboard just as with a
normal teletype writer.
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For each keystroke a corresponding notched disk is started to rotate (one rotation). The position of the notches on the circumference of the disk corresponds to the pixel pattern of the character matrix. The disk is divided into seven sectors corresponding to the columns in the matrix, and each sector has notches corresponding to the black pixels along each column. For each disk there is a contact which senses the notches. Each time a notch passes the contact the device sends out a tone pulse to the receiver. In this way the character is transmitted pixel by pixel. |
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With this method a disturbance on the transmission channel can only distort a few pixels; however, the character is generally still readable. If in a normal teletype writer transmission the 5-bit-code is distorted, a completely different character is received.
When the Hellschreiber operates as a receiver, it writes the received pixels on a paper strip.
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The helix has two turns with respect to the width of the paper, i.e. there are two touching points between the helix and the blade and the characters are printed twice, one below the other. This has the effect that the text is still readable, even if the transmitter and the receiver are not exactly synchronized with respect to their motor speeds. In such a case the text lines are printed slightly slanted, but at least one of the two characters is recorded completely on the paper strip.
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(German; 1,4 Mbyte)
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(German; 38 pages; 4,5 Mbyte)
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