Single-Side-Band Telephony Applied to the Radio Link between the Netherlands and the Netherlands East Indies

An historical introduction reviews the initial long-wave telegraph circuit with the East Indies in 1923, the first short-wave telegraph circuit in 1925, and the inauguration of short-wave telephony in 1928, this being accomplished by means of the usual double-side-band and carrier system. The principal limitations attending this method of operation are discussed; viz., the phenomena of fading, susceptibility to frequency and phase modulation, and the fact that it does not lend itself to multiplexing, because of the presence of the high-powered carrier. The features which characterize the present system are, first, that of single-side-band as applied to high-frequency transmission, and, second, that of multiplexing, whereby a plurality of channels are enabled to be transmitted by means of a single power amplifier and directive antenna system. In the transmitter the group of channels which is to be transmitted, representing one telegraph and two telephone messages, is assembled at low power and low carrier frequency and is then stepped up in power and in frequency in a succession of stages. The spectrum transmitted comprises (1) the two telephone channels located in frequency as single bands on opposite sides of the suppressed carrier; (2) the telegraph channel superimposed thereon as two tone frequencies, one for spacing and one for marking, and transmitted on a double-side-band basis to obtain the advantage of frequency diversity; and (3) the pilot channel transmitted at a frequency 5000 cycles removed from the carrier position.