Frequency Shift Telegraphy -- Radio and Wire Applications

D URING World War II, single-channel and multichannel frequencyshift radio telegraph systems proved of the utmost importance in providing the Allied Powers with a world-wide automatic printing telegraph network for handling with precision, secrecy and dispatch the unprecedented volume of traffic engendered by a war of global extent. It is expected that the next few years will witness a greatly expanded application of this method of operation by commercial telegraph companies and others interested in long distance telegraphy. Frequency Shift carrier telegraphy (FS) may be applied to any carrier telegraph circuit, but, as will appear below, it provides particularly striking advantages in H.F. radio transmission. For some other radio frequency ranges and for wire line operation the conditions are such as to limit the advantages of the FS method. The main advantages of the FS over the AM method are a greater ability to accept rapid level changes, which results in better stability and lower distortion, and an improvement in signal-to-noise ratio, which permits a reduction in carrier amplitude. It is therefore of particular importance where automatic printing is desired over H.F. radio circuits. When it is necessary to transmit through very high noise levels, low speed AM signaling with aural reception of an audio beat note remains the superior method. FS is a form of frequency modulation in which signaling is accomplished by shifting a constant amplitude carrier between two frequencies representing respectively the marking and spacing conditions of the telegraph code.