Reliable EDTV/HDTV Transmission in Low-Quality Analog Channels

Abstract

The presence of random noise, interference, echoes, and imperfect frequency response in terrestrial broadcasting channels, and of pervasive low-level reflections in cable systems, degrades the quality of most television reception. These effects tend to reduce the difference between the quality of NTSC and that of various advanced television systems, as actually delivered to the home via such channels. In this article, several methods are presented to deal with these effects, and thus to preserve the improved quality made possible by currently proposed EDTV and HDTV systems. Automatic channel equalization is used for first-order correction of echoes and frequency distortion. A scrambling method is described that transforms all remaining degradations to additive random noise, which, for a given noise power, is of minimum visibility. Random noise, whether additive or produced by scrambling of other defects, is suppressed by adaptive modulation of the various signal components. If the channel carrier-to-noise ratio (CNR) is high enough, some digital data can be transmitted in addition to the analog signal by a method called “data under.” These methods are applied to a receiver-compatible system that utilizes a very low power 3-MHz augmentation channel as well as to a bandwidth-efficient 6-MHz system that is not receiver compatible. One version of the latter system can be operated at such low CNR that it may be possible to utilize the taboo channels.