Digital audio broadcasting offers the potential to give every radio in Europe the sound quality of a compact disc. To accomplish this, it requires a rugged method of transmission. The coded orthogonal frequency division multiplexing (COFDM) modulation system was developed to meet this need. This Report describes the reasons why a new modulation process was needed, and explains how the COFDM system has been optimised to meet the requirements. Note that in addition to the references embedded in the main text, further useful material can be obtained from other BBC R&D Report documents which are listed in the appendix to this report.