10.1 Introduction

Distribution system design is, ideally, the process of specifying the most economical network that will provide the required bidirectional bandwidth to the required number of terminal points and still meet the performance goals. The performance goals, in turn, may be derived from internal quality standards, governmental regulations, the perceived requirements for reliable signal transportation, or all of the above.

Typical system specifications will include noise, distortion, response variation, delivered signal levels (and their stability), and hum. Reliability, another important consideration, is discussed in Chapter 17, and the effects of imperfect impedance matching among components are discussed in Chapter 13. The distribution system performance must also be considered as part of a chain that includes the headend channel processing and terminal equipment effects of signals. These are considered in separate chapters as well. Finally, the coaxial distribution system may well be cascaded with a signal transportation link such as microwave or, more frequently, a fiber-optic circuit. These, also, are treated in different chapters.

The final leg in the broadband distribution system is the drop wiring leading from tap ports to and throughout homes. By tradition, broadband networks are designed to meet certain goals from headend output to tap port, with an additional allowance made for the effects of in-home wiring. Modern drop systems, however, may vary widely in their complexity and quality, and in fact, may constitute minidistribution networks by themselves. The final section of this chapter deals with drops.

We begin by discussing basic principles and applying them to the hard cable portion of the plant.

10.2 Carrier-to-Noise Ratio

Noise is defined by the IEEE as unwanted disturbances superposed upon a useful signal that tend to obscure its information content. ¹ In many communications systems, that is a good working definition. In cable television, however, the term noise is generally applied to thermal noise, which is treated independently from disturbances caused, for instance, by intermodulation products, ingress, or other causes. One of the reasons for treating thermal noise differently from other disturbances is that the subjective effects of noise and various other types of interference are different for analog video signals. Another is that, unlike many communications systems, the sources and mechanisms by which interfering signals are generated are well understood, and a detailed independent analysis of each has proved useful. Therefore, in this book, unless otherwise stated, noise will be used to refer exclusively to thermal noise.

In general, thermal noise is uncorrelated, that is, unpredictable and nonrepetitive. Noise generated in one device or at one...