L.M.T. laboratories 7-frequency radio printer

Deals with a printer of the type in which each character is analysed into a number of elementary lines which are transmitted as a series of dashes and spaces of varying length, and in which at the receiving end the original character is built up. Such printers are well suited to radio circuits since interference cannot change a letter into another totally dissimilar, the only effect being to print extra elements or to suppress simple elements of the transmitted lines. In the printer described the characters are analysed into seven horizontal lines, each such line being represented by one definite audio frequency. The lines are transmitted simultaneously instead of in sequence and the radio carrier is therefore modulated simultaneously by all the audio frequencies corresponding to the line. The equipment operates on the start-stop principle. There is therefore no need to maintain synchronism between the transmitting and receiving mechanisms and the receiver may be left unattended. The "marking" or printing condition may correspond to the existence or absence of the seven frequencies. The latter condition has been chosen, as the effect of static is then not to print unwanted components of letters but to suppress points in the characters, which in the majority of cases remain perfectly legible. The apparatus and the electrical circuits are fully described by means of line drawings and photographs and examples of the printed record received in Paris from Algiers at times of heavy disturbance show that the received text is easily read under these conditions.