3.2 Services general sense, it has connotations of a limited relay service. The term 'Cable TV' is superseding it for modern systems (those in Until recently the cable TV service package has been straightfor- the industry often just contract this to 'Cable'). ward multi-channel TV and, probably, FM radio. National regula- MATV. Mast Antenna TV; a local system for a hotel or appartment tions may define a 'must carry' set of channels, probably those block. Where a satellite dish supplements the off-air reception available locally off air and the radio. The operator would then the term SMATV is used. normally for marketing reasons wish to segment the channels into Trunk (primary) Customer To head Hub site (secondary) links Switch point Figure 57.20 Switched star cable TV network Cable distribution systems 57/19 tiers, a low cost basic tier incorporating the must carry set, and a number of premium tiers culminating in the recent film channel. 1. Audience interactive; voting, home shopping, home banking, This market need for varying the channel selection delivered to different customers, SO called 'conditional access', has important 2. Information services; data to the home or as text on a screen. technical implications; the pursuit of a cost effective and safe (from 3. Low speed data. programme theft) means of doing this has been a major preoccupa- 4. Telephony. tion of the cable TV industry. Generally the video format to the customer has conformed to the 57.3.3 Network types standard TV receiver input, namely vestigial sideband amplitude modulation (VSB-AM) of the national standard format (NTSC, 57.3.3.1 H.f. multipair PAL, SECAM) in a frequency division multiplex (FDM) as illus- trated in Figure 57.21 Because of the large channel capacity of cable TV systems there are three major differences from receiving In this old type of system individual channels were modulated at off air signals. Firstly the frequency range covered is larger, high frequency (h.f. 3MHz to 30MHz region) onto individual meaning conventional TV 