This White Paper originally appeared as an internal BBC R&D Report in 1997, before the start of regular digital television transmissions. The aim of the work described in the document was to discover the best way of quantifying typical impairments introduced by the transmission system. It was hoped that the individual impairments could be added in a convenient way to provide a measure of total impairment. Fortunately, this proved to be true. Although both the theory and practice of COFDM systems have moved on considerably since those early days, the author believes that the original work is still of interest. In particular, the concepts of equivalent noise floor (ENF) and equivalent noise degradation (END) may not be widely known outside organisations such as the Digital Television Group. Also, the reader may find it comforting that these abstract concepts have direct application to measurements in the real world.