BBC R&D

Abstract

The introduction of new transmitter designs in the LF, MF and HF broadcast bands has not only improved efficiency directly, but permitted use of modulation-dependent control system which can themselves further reduce operating costs without loss of coverage.

This report describes the design and development of one such modulation-controlled system known as Amplitude Modulation Companding (AMC). In this method both carrier and sideband levels are progressively reduced as the modulation level is increased this effect being compensated in the receiver by the normal action of Ihe automatic gain control circuit. Extensive trials of a fully developed system for 50 kW MF transmitters have achieved input power savings of between 20% and 40% depending on programme content. This achieved with imperceptible loss of coverage and further savings may be practicable.