A Single-Sideband Short-Wave System for Transatlantic Telephony

T HE single-sideband suppressed-carrier method of transmission has been used to effect economies in the power capacity required, energy consumed, and space in the frequency spectrum on carrier telephone circuits for over fifteen years. On the basis of equal peak amplitudes in a transmitter a single-sideband suppressed-carrier system gives a possible theoretical improvement of 9 db in received signal-to-noise ratio over a double-sideband and carrier system. Six db of this improvement is obtained by omitting the carrier and utilizing the entire available amplitude capacity of the transmitter for the sideband. The other 3 db is obtained by reducing the band width of the receiver to only t h a t required to pass one sideband, thus reducing the noise energy at the receiver o u t p u t by one-half. In order t h a t speech may be transmitted without undue distortion over a single sideband system, it is necessary t h a t the carrier frequency at the receiver be within about ± 20 cycles of the correct value. For the transmission of music a much higher precision is required. The practical construction of a single sideband radio system at frequencies of the order of 60 kc., such as is used in the long-wave transatlantic telephone circuit, requires only a careful application of known technique to obtain the desired degree of stability of the oscillators. At the short-wave transatlantic radio telephone frequencies of from 5,000 to 20,000 kc., however, the very best crystal oscillators, such as are now used only for the very highest quality laboratory standards, would be required at both transmitter and receiver to obtain the degree of synchronization required.