We will see that such fine graphics are needed to emulate a HELL printer. Hell printing is a sort of facsimile or picture printing transmission system. The letters are made up of vertical lines which are drawn on paper tape by means of a hammer tapping on the inky threads of a quick threaded screw. The lines are arranged from left to right by moving the paper tape, which is transported to the left. The pictures of the letters are transmitted in the form of on-off modulation of a tone or carrier, which is converted in the receiving apparatus to recognizable figures or letters.
Interference can distort tbe signal.
The receiving apparatus does not have to make a decision in real time, as is the case with a Murray coded teleprinter. Receiving and transmitting are a-synchronous. The received text is printed twice on the paper, one above the other, which is necessary if the text is always to be legible. If the transmitter and receiver do not run at exactly the same speed, the text is printed at an angle on the paper. It climbs up or down, but is always legible thanks to the double row of letters.