An extended analysis of echo distortion in the F.M. transmission of frequency-division multiplex

When a frequency-division-multiplex telephony signal is subjected to some non-linear process, intermodulation distortion is generated, being usually audible as an unintelligible noise. In f.m. systems an important source of such non-linear distortion is to be found in small echoes of the main r.f. signal. Albersheim and Schafer have investigated theoretically and experimentally the distortion due to a single echo, but the analysis applied only to limited ranges of the available parameters, and the agreement with experiment was not always good. It now appears that, by a combination of analytical and semi-empirical methods, Albersheim and Schafer´s results can be extended to enable prediction of distortion levels for most cases of practical interest (in the absence of pre-emphasis). Most of the discrepancy between theory and the measurements of Albersheim and Schafer can also be accounted for, the formulae being in good agreement with measurements carried out by the present authors.