Over the years, an increasing need for information transmitting media has resulted in a series of developments aimed at providing a more efficient utilization of available information bearing channels. Thus, once the basic problems involved in the generation of clean radio signals were solved in the early part of this century, development then turned toward more efficient techniques for separating the desired signal from a group of signals sharing the same general part of the spectrum.
In contrast to the first problem of generating a harmonically pure continuous wave signal or a harmonically pure carrier signal with sidebands extending from the carrier frequency within very well defined frequency limits, the problem of selecting desired frequency components of electrical signals has yet to be the object of an optimum solution. Generally, such techniques involve the use of conventional tuned circuits whose effectiveness is enhanced by reducing the carrier frequency by heterodyning techniques, regenerative feedback, or a combination of these techniques. In the case of heterodyning, reduction of the carrier frequency results in the possibility of designing a filter with a narrower passband with components having the same "Q" or quality factor. The quality factor is, a measure of the relationship between a manufactured electrical component, such as an inductor or capacitor, to an ideal component having no spurious resistive component. The quality factor of an electrical component may thus be expressed in terms of the ratio of reactance to resistance which, in turn, is proportional to the ratio of the frequency of a tuned circuit incorporating the component to the width of that circuit's passband. Thus, reducing the frequency of a radio signal to a lower frequency results in a passband which, in absolute terms, is fewer hertz in width for tuned components of a given Q.
Given these theoretical considerations, three basic techniques evolved for improving the quality factor of tuned circuit components. These involve decreasing the resistance of circuit components, increasing the reactive component of manufactured electrical parts and electronically removing resistance from the tuned circuit. The first of these approaches, namely, decreasing series resistances was relatively simple to overcome. This involved using relatively thick wire of high conductivity in winding inductors and using thick highly conductive plates in capacitors. Naturally, however, the physical size of components limited the extent to which this avenue of approach would yield useful results. The reactive component of an electrical element could, likewise, be increased without changing its series resistive components by, such techniques as incorporating a magnetic core in an inductor or using a material having a high dielectric constant, such a Mylar (a plastic film manufactured by the E. I. DuPont Company of Wilmington, Del.) instead of air. The third technique, the use of regenerative feedback, essentially comprises balancing the positive resistance inherent in all real electrical parts with a negative resistance generated by an electronic circuit. In principle, this technique, also known as Q-multiplication, as well as the others all have limits beyond which they cease to be useful.
As the principles involved in applying the above techniques were being refined, attention again focused on the generated signal. In particular, crowded conditions in the spectrum were dramatically alleviated through the use of single-sideband supressed carrier systems. In single-side band transmission, a conventional amplitude-modulated signal which comprises a carrier and two sidebands on opposite sides of the carrier is modified by the elimination of the carrier and one of the sidebands. This results in no loss of information, inasmuch as the second sideband contains a mirror image of the information in the other sideband and the carrier contains no information and may be generated at the receiving point. Presently, single-sideband transmission is very extensively used in areas where high fidelity is not required because of the efficiency with which it utilizes available band space.
Nevertheless, the readability of single-sideband signals is often impaired by interference from adjacent channels. In this respect, it is not unusual for relatively strong signals having frequencies higher and lower than that of the desired signal to be heard at the same time as the desired signal. While it is possible to reduce the interference from adjacent channels by increasing the selectivity of the receiver's tuned circuits, this also results in attenuating the high and low frequency components of the desired single sideband signal, thus resulting in distorted and often unintelligable signal reception.