Telegraph codes and code convertors

The paper discusses those aspects of the problem of interconnecting telegraph systems which are due to the variety of codes in use. The common and more important codes are reviewed on the basis of their traffic-carrying capacity and undetectable-error vulnerability. Methods of working and the characteristic forms of distortion on both submarine cable and radio circuits, in so far as they contribute to the error rate, are discussed. Methods of detecting and correcting errors are reviewed, and a 7-unit-5-unit electronic convertor and a 5-unit-7-unit mechanical convertor, which incorporate some of these features, are described. It is concluded that some forms of 7-unit code give adequate protection against undetectable errors. The final choice is shown to depend on a number of factors. The most suitable codes are those in which each symbol consists of an arrangement of three marks and four spaces.