Abstract :
The major factor which hampers the progress of broadcasting is the lack of a sufficient number of channels in which contain the increasing number of transmissions. In the present state of the art few, except local, stations can be received without interference from other broadcasting stations on frequency-contiguous channels unless the upper frequencies of modulation of the wanted station are severely attenuated and unless, therefore, reproduction is lacking in intelligibility and naturalness. This inter-station interference can be eradicated or minimized, and quality improved as regards ?top? reproduction, if all or part of one sideband of the transmitted spectrum is cut away. The cutting-away operation on the side-band, however, produces harmonic distortion in the received signals. It is the object of the asymmetric-sideband system of transmission to cut away part of one sideband without introducing audible harmonic distortion. The paper gives a quantitative analysis which shows how asymmetry, either of phase or of magnitude, between two sideband components, created by any given modulation component, is related to harmonic distortion; and it is shown that this distortion is proportional, over a large range of values, to depth of modulation. The nature of the sound spectrum is such that the intensities in the upper register are much less than those in the lower middle register. Since the degree of asymmetry between counterpart sideband components is a measure of the attenuation of the cut-away sideband, this attenuation can be increased as the modulation frequency is made greater. This is tantamount to saying that, as the sideband frequency becomes more and more removed from the carrier frequency, it may be more and more attenuated while the distortion can be made to remain constant and small. Thus in asymmetric technique the outer parts of one sideband may be removed without the introduction of audible distortion. Curves are derived, based upon the evaluation of e- xpressions given in the quantitative analysis, which show the required, attenuation-constant of filters which cut away part of one sideband but produce no more than a constant distortion. It is proved that phase asymmetry between counterpart sidebands may give rise to more serious distortions than are created by magnitude asymmetry acting alone, and a network is described which keeps phase asymmetry to a minimum and yet gives a close approximation to the required attenuation. Accounts are given of practical tests wherein the quality of reproduction obtainable from an asymmetric transmitter is compared with that given by orthodox modulation. While it is true that a highly trained ear may, on rare occasions, be able to detect some slight differences between the two types of reproduction, when they are directly compared, the general public would be quite incapable of so doing, even if they possessed receiving apparatus so free from distortion itself as to give them an opportunity to do so. It is pointed out that it is better to face the occasional minute deteriorations in quality given by the asymmetric system than to suffer from the continuous hissing and splitting sounds brought about by existing inter-station interference. In the latter part of the paper an analysis is given which shows that the asymmetric system has three possible applications to space broadcasting as it is practised to-day, namely (1) It could be used to allow carrier difference frequencies of the order of 11?12 kc. without introducing any sideband overlap interference. (2) It could be applied to existing conditions, where carrier difference frequencies are of the order of 9 kc, and would reduce inter-station interference to the order of one-tenth its present value. (3) If existing interferences are considered to be tolerable, then carrier difference frequencies can be reduced to 6 kc. without increasing interference above its existing values; this implies that 1? times the number of stations wo
