Spread side-band system

One of the consequences of selective fading with waves below 100 m. is the enhanced production of spurious audio-frequencies when the highly modulated carrier fades appreciably in relation to the side-bands. The new system eliminates this effect by transposing the speech-band, 250-2500 hertz, to another band, say, 2750-5000 hertz, by balanced modulation, familiar in telephone carrier systems, before modulation in the radio transmitter. After demodulation in the radio receiver, all the spurious frequencies, sum-and-difference speech frequencies, are outside the specified band, 2750-5000 hertz, and therefore can be removed by filters before the required speech is transposed back to its proper band, 250-2500 hertz. The spread side-band system is particularly advantageous when used with secrecy systems, because intermodulation within the latter normally introduces inharmonic frequencies in the reproduced speech. In short-wave systems, diurnal and general rapid fading is compensated by automatic gain-controls in the radio receiver; a change of 40 db is held automatically within plusmn2 db.