Asymmetric-Side-Band Broadcasting

An economy in the frequency-channel width occupied by stations sending telephony signals can be made by removing one sideband from the transmitted spectrum. This operation however produces a distortion of the received signals which can only be minimized by an intensification of the carrier component compared with the side-band component. If this intensification is performed at the transmitter the carrier power must be enormously increased and the scheme would be impracticably uneconomic; it is furthermore impossible in broadcast technology to make the necessary alterations to intensify the carrier component in existing receivers all at once because they are publicly owned and extremely numerous. The distortion produced in the absence of carrier-wave intensification at either transmitter or receiver is mainly directly proportional to modulation. The modulation demanded in the transmission of ordinary speech and music is much less at the higher frequencies of modulation than that taking place in the lower middle frequencies. In order, therefore, to approach the ideal of the carrier- and single-side-band system, circuits have been devised to produce what is called an asymmetric side-band transmission in which only the outer parts of the side band are cut away.