Carrier-frequency-shift telegraphy

After the history of the development of carrier-frequency-shift telegraphy has been briefly outlined, methods of frequency-shift transmission are described and discussed. Methods of detection, by band-pass filter selection, or by a discriminator, are described. Practical advantages of frequency-shift operation are discussed and some factors influencing the design of carrier-frequency-shift transmitters and receivers are considered. A theoretical analysis of the threshold value of signal and the time shift produced by random noise is given in an appendix, and formulae are derived for comparing keyed-carrier and carrier-frequency-shift systems. It is shown that the discriminator method of detection gives no theoretical advantage over normal keyed-carrier operation, but that in a typical example, post-discriminator filtration gives an improvement of 14 db and band-pass filter selection an improvement of 10 db in threshold value.