F.M. systems of exceptional bandwidth

Until the advent of television tape recorders, the only frequency-modulated systems in use were ones in which the frequency spectra of the modulating and modulated signals were well separated. In television tape recorders these bands are not merely adjacent but actually overlap. The analysis of such systems of `exceptional bandwidth¿ emphasises three points: (a) The instantaneous frequency and amplitude of a waveform cannot be uniquely defined; the approximations involved become serious in such systems. (b) It follows that one cannot synthetise a distortionless f.m. system. It is therefore important to distinguish the implicit distortion from that which may be avoided. (c) These systems are very sensitive to amplitude distortion and, especially, to second-order components. Finally, the paper discusses the practical design of modulators and demodulators.