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".


 Kulturdenkmal SH en

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.
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.
   hellschreiber GL72 nockenscheibe

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.
hellschreiber GL72 schreibmechanismus
The writing mechanism in the receiver has a
rotating spindle with a raised helix, which is
inked. The received tone pulses actuate a blade
by an electromagnetic relay, which moves the
blade up such that the paper tape is pressed
against the inked spindle.
This writes a black pixel on the paper for each
tone pulse, while the paper tape is transported
slowly along. The touching point between the
rotating spindle and the blade sweeps across
the width of the paper, corresponding to the
column direction of the pixel matrix.
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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