coaxial systems can carry 50 or more channels Penetration. The number of homes connected as a percentage of the per cable, and fibre optics is starting to find a foothold. homes passed. Cable TV remains very variable in terms of its impact in different Churn. The proportion of customers ceasing and taking up the countries. From the 1970s it began to increase rapidly in North service. America and certain European countries, such that by the end of the Spin. The changing of customers between levels of services (tiers 1980s some very high penetrations of the service (up to 90%) had of channels). been achieved. However in other places it remains in its infancy, or Head end. The source point for service on a cable TV network. This at least confined to certain special areas. Its spread has been in- is where the video channels, radio, and any other service are fluenced by broadcast TV quality, local terrain, prosperity, commer- formulated into a multiplex to be launched out on the network. cial pressures, and government regulations. The last of these Typically it might be fed itself by links from remote aerial sites became increasingly important when additional services were intro- and a studio (where the cable operator may generate tape based duced, all the more now that carriage of non-TV services like programming and process other channels). telephony and data is under consideration in some countries. Hub site. Further site(s) after the head end where sufficient pro- cessing occurs to warrant internal housing of equipment. 57.3.1 Terminology Super-trunk distribution. Divisions of the external cable network as it gets progressively closer to the customer (with the implica- tion of different equipment and practices being cost effective). It is useful to establish at the outset some of the general terms used about cable distribution systems. The technical terms will be ex- Figures 57.19 and 57.20 illustrate somes of these terms. plained later as they arise. CATV. Community Antenna TV; although still widely used in a 57.