Picture transmission by submarine cable

The direct transmission of pictorial matter over transatlantic submarine cable has become practical through the development of some special types of networks and amplifiers. These are required because of the rapid increase of the circuit attenuation with frequency, and in order to override certain characteristic electrical interference to which the cable is subject. A summary is presented of the fundamental theory which governs facsimile and telegraph transmission, and the methods are shown by which this is applied to meet the conditions of cable picture working. An efficient system for transmission of facsimile matter from London to New York by submarine cable was made available to the public in April 1939. This provides an important advantage over radio sending of pictorial subjects, in that the cable system is not subject to unpredictable variations in the transmitting medium. But while the path from sender to receiver is inherently constant with time, the cable possesses characteristics which provide some unusual problems of transmission. The measures which were adopted to insure an undistorted likeness of the original and to transmit the intelligence at a maximum rate within the limited frequency spectrum available through the cable, appear to be sufficiently unique to justify a description at some length. This new system should not be confused with the “Bartlane” system of picture sending which was applied to cables for several years. In the Bartlane system the picture was analyzed into small unit squares, and a code character was set up corresponding to each unit. The characters were sent by means of the equipment ordinarily serving for telegraph transmission, and were decoded, usually automatically, at the receiving end. That system was inherently slow and made inefficient use of the available cable capacity as compared with the arrangement described herein.