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9DISTRIBUTION SYSTEMThe process of transporting signals trom theheadend to the subscriber is commonly referredto as "downstream" communication; the traditional cable system,is capable of communicationin the downstream direction only. The distributionsystem for one-way communication consists of acoaxial cable and the electronics necessary totransport signals from the headend to the subscriber. The basic element is a coaxial cable; thecable consists of an inner conductor, generally,-!aFigure 1. Control Center for a Cable Television System Headend.sion signal received by the system (see Figure 2).The antenna is sited to provide the best availablesignal reception. Each received television signalpasses through a processing amplifier, whichseparates the sound and picture components, andadjusts the signal levels to prevent a strong signalfrom one channel from overpowering another onan adjacent channel (as often happens in broadcast television): Thus, all 12 VHF channels can beused. The amplifier then reassembles the pictureand sound signals for transmission th roughout thedistribution system.UHF signals are broadcast at levels above therange of frequencies which can be transmittedmade of copper or copper-clad aluminum surrounded by an insulator (such as extruded foampolystyrene or extruded foam polyethylene)which is in turn enclosed by an outer conductor,typically aluminum (see Figure 3). Coaxial cablesused by the cable television industry range indiameter from one-quarter to one inch.plastic sheathing\1outer conductorvia cable electronics. In a cable system which alsocarries UHF television signals, the headendincludes electronic equipment to convertUHF signals to unused VHF channel frequencies withinthe cable system spectrum.plasm foiminner conductorFigure 3. Coaxial Cable.The distribution network, or "plant," is dividedinto two portions, trunk lines and feeder lines.Trunk lines, which are constructed of one-half tothree-quarter inch coaxial cable, are the majorarteries of the cable system. They transport thesignals from the headend to the extremities ofthe cable system. Feeder lines, which are connected to the trunk lines, carry signals past eachhome. In a typical distribution plant, there is twoto five times as much feeder cable as trunk cable.Although individual trunk lines are longer thanfeeder lines, there are a far greater number offeeder lines.Both the trunk line and feeder lines employ elec-tronic amplifiers which are installed at specificintervals to boost the cable signal, as shown inFigure 4. Feeder lines are connected to the trunkFigure 2. Antennas and Tower for a Cable Television SystemHeadend.lines by an electronic device called a bridgeramplifier. This amplifier draws signal power from

10cable and amplifiers introduce random electronicimpulses into the television signal as it travelsthrough the distribution system. These impulsesare called "noise," and appear on the televisionscreen as snow. Since each amplifier boosts boththe signal and the noise, there is a limit to thetotal number of amplifiers which can be attachedto a length of cable. Current design permits signalsto be processed by 18 to 25 trunk amplifiers ina row ("cascade") before the amount of snowbecomes objectionable.The distance between amplifiers depends uponthe degree to which each amplifier boosts thesignal. The amount of boost is limited becauseat high signal output, the amplifier produces otherkinds of signal degradation. These degradations,called intermodulation and cross-modulation dis-tortion, appear on the television set as movingbars or stripes.Trunk cable amplifiers are operated at lowpower levels to minimize amplifier-produceddegradation. In practice, the distance betweentrunk amplifiers is about 2,000 feel. Thus, in combination, the amplifier cascade limit and the maxFigure 4. Cable and Amplifier.the trunk for further distribution, and protects thetrunk lines from electronic disturbance in thefeeder lines.Subscribers' television sets are connected to thefeeder lines with. a subscriber "drop" line. Thesubscriber drop consists of a tap at the feeder,the drop cable (typically one-quarter inch diameter cable), and a room wall plate connector. Ashort length of coaxial cable runs from the wallplate and is connected to the antenna terminalsof the television set via a matching transformer.An understanding of the electronics in the distribution system is helpful in making decisionsabout system layout and capacity.imum distance between amplifiers defines themaximum distance for trunk cable to be 7 to 10miles.Feeder cable amplifiers (sometimes called lineextenders) feed signals to the subscriber. Theyare operated at higher power levels than trunkamplifiers in order to permit more subscribers tobe fed from a single amplifier. Since the feederamplifiers are operated at higher power levels thanthe trunk amplifiers, the number of cascadedfeeder amplifiers is normally limited by (1:stortionconsiderations to three. The distance betweenfeeder amplifiers is less than between trunkamplifiers, because the large number of subscribers drain signal from the feeder cable. In prac-Two problems in the electronic operation ofcable system distribution limit the size of the geographical area a single cable system can serve. Theyi»are termed attenuation and distortion.Coaxial cable attenuates, or weakens, a signalit carries. As a television signal, flows throughcable, the signal strength, or power of the television signal, diminishes; a typical coaxial cabletrunk line has its signal reduced to 1/100 of itsoriginal strength for approximately every 2,000III)) I4'4feet of cable. The reduction of signal strengthbecomes perceptible to the viewer as a 'snowypicture. To counteract this attenuation, amplifiersare connected to the coaxial cable to boost signalstrength.Unfortunately, the amplifiers themselvescause the second problemdistortion. BothFigure 5. Cable Distribution Network for a Cable TelevisionSystem.

lice, feeder cable extends no more than 4,000 feetfrom the trunk, and often less. Figure 5 illustratea typical trunk and feeder network.SUBSCRIBER TERMINAL"Subscriber terminal" is a term more appropriate to the description of advanced rather thanMHz-S06UHF tekvision(channels 14 to 70) contemporary systems. Here, in the case of tradi-600tional CATV systems, the term refers simply tothe subscriber's television receiver; in an advanced cable system it would consist of a television receiver in addition to other facilities, andwould he capable of two-way communication andother services.The standard television receiver is capable ofreceiving two bands of television broadcast frequencies: VHF and UHF. Unfortunately, as Figure6 illustrates, present cable electronics design frequencies are not broad enough to encompass UHFtelevision frequencies. System operators deal withthis problem partially, by converting UHF signalsat the headend to unused VHF channels. But whilethe operators can deliver as many as 35 channelsto subscribers' homes, the standard television receiver can receive no more than 12 of thosechannels.'500400Aeronautical and mobile. communications300VHF television(channels 7 to 13)Aeronautical and mobilecommunicationsbroadcast signals. These circuits are not necessaryFM radioVHF television(channels 2 to 6)all 35 channels from the cable. Research anddevelopment is taking place, but as yet there isIIIII200174The standard receiver is equipped with electronic circuits that enable it to tune and receivefor cable reception and may introduce unwantedbroadcast signals that will interfere with cable,systerns. This problem can be reduced if the cableoperator is permitted to modify the-Inside of thetelevision receiver or connect the cable directlyto the tuner.A more practical solution is the developmentof a special cable receiver, designed to receive)16Mobile communicationsApproxin ate rangeof fret uencieson cable10888100II543011011111113Figure 6. Television Frequency Spectrumsmall to support a television station, or too farfrom cities that did have stations. Traditional CATVwas essentially a retransmission service; but asthe cable industry flourished, traditional systemsno such cable receiveron the market. In the mean-provided the economic base for the furthertime, other arrangements have been devised todevelopment of cable television technology.Traditional cable television systems developedincrease capacity. These arrangements will be discussed later in more detail.B. Modern Cable TelevisionTraditional cable television was developed toprovide television service to communities too'Although cable electronics permit transmission of 40 television channels simultaneously, at present no manufacturer hasdesigned a system that delivers more than 35 channels to subscriber's': Thus, although the electronic capacity of cable is40 ch;?,nnels, subicriber service for the time being will probahlybe iunited to 35 channels.in areas where broadcast television is poor. Today,systems are built in areas where broadcast televi-sion service is available. Consequently, the provision of standard television signals will not beenough, and cable operators in urban environments will have to provide new and more diverseservices. The result will be a more technically complex cable system.1. Headends are becoming more sophisticated,with the addition of new features:Electronic importation of distant television stations

12Live studio programming exclusively for thecable systemAutomated programming of news and weatherMultiple headend interconnection for largesystems.2. Distribution systems are beginning to havemore capacityas many as 35 channels deliveredto subscribers.3. Subscriber terminals will include devices thatbypass weakne,ses of the teleion receiver.HEADENDAs in the traditional cable system, the headendin a modern system includes the antennae, andthe electronic control center for amplifying broadcast signals received and converting UHF signalsto VHF frequencies before transmitting themthrough the distribution system. Additionalmunications Commission now reouires that allsystems with more than 3,500 subscribers mustoperate to a significant extent as a local outletby origination cablecasting.'Studio facilities for cablecasting vary in cost,ranging from as low as several thousand dollarsfor facilities bUilt around a half-inch black andwhite video tape recorder, to a full color professional studio that may cost S1 million or more.Automated ProgrammingMany system headends now include automatedorigination providing specialized services otherthan conventional telecasts. Automated equipment packages provide time, weather, stock tickerand news (AP or UP) wire services, many withalphanumeric message capability. At least onemanufacturer markets a computer controlled vid-facilities are now required to produce the neweocassette player, which permits the systemoperator to play pre-recorded three fourth inchservices.videotapes automatically.Electronic Importation ofDistant Television SignalsCable systems are required to carry certain television stations which place a signal over the system's service area. Cable operators offer additional program services by importing televisionsignals from distant markets. Since the signals arereceived near the distant market and transportedto the local headend by microwave radio systems,one additional feature of a modern headend isthe equipment necessary to receive and modulatemicrowave signals.Microwave transmission is a system of broadcasting from one point to another 20 to 35 milesdistant. To import a television station, a transmitter near the station radiates the signal in a specificrathertoward the distant receiverdirectionthan uniformly in all directions as is done withradio and television broadcasting. Microwave stations must be cascaded when stations from greatdistances are imported. For example, importinga television station from a market 200 miles awaymight require "hops" between three to five microwave stations. Microwave service may be pro-vided by the telephone company, or byspecialized common carriers, or it may be ownedby the cable system operator himself.Live Studio ProgrammingSystem operators frequently operate !ocalstudios to program a channel. The Federal Com-Multiple Headend InterconnectionHeadends are now becoming increasingly com-plex because of the need for interconnection of.systems. Interconnection is necessary when ast: large that it must be served bycommunit-,several headends and distribution systems (called"hubs"): It may also develop when several contiguous cable systems in neighboring communities interconnect to share local programming.Interconnection can be achieved by cable (including a new one inch cable with low signal losscharacteristics, called "supertrunk"), or by microwave. This microwave is not the one-pointto-another kind used to import distant televisionsignals, but a new service called Local DistributionService (LDS). These microwave systems have thecapability of transporting a large number of televi-sion pictures (from 8 to 38) from one site to anumber of receiving locations, without visibledeu.Idation of signals (see Figure 7). Supertrunkis usually favored over LDS where weather conditions do not allow reliable LDS operation or wheretwo-way capability is required, such as in a systemwith a number of local origination studios thatneed to be interconnected. However, in urban'In United States v. Midwest Video Corp., 406 U.S. 649 (1972),the Supreme Court upheld the FCC's authority to impose theorigination requirement. In the course of that litigation, however, the commission stayed the effect of the rule. Upon affirmation by the Supreme Court, the commission did not moveto vacate the stay. There is some question, though, whethersuch action was necessary and the commission has not statedwhether the rule is in effect.

13environments where underground construction isnecessary, microwave may be the least expensiveone-way interconnection system. In some circumstances, a combination microwave and cable sys-tem for two-way interconnection may be lessexpensive than an all-cable system.If interconnection of cable systems is desired(either presently or in the future), the technicalstandards for the distrih,n system must bemore strict than those for.ind alone" system.Whether microwave or caith., is used to interconnect hubs, some additional distortionsbothmore noise (snow) and more in termodulation aridcross-modulation (moving bars)will occur. Asa result, to obtain the same quality signals as thosecarried by a single hub system, the number oftrunk and feeder amplifiers cascaded in a multiplehub system must be smaller.Traditional systems use a one-way "tree" layoutthat is, allthat can be likened to a party linesubscribers share the same facilities and receiveFigure 7. Microwave Interconnection of C.:ble Television Systems.the same programming (see Figure 8). A "fullyswitched" system such as the nationwide telephone network permits two-way private line com-munications by connecting each subscriberdirectly to a vast network of switching centers andalternate signal routes (see Figure 9). Intercon-nection, if properly designed, helps offset theparty line character of cable system.Interconnection permits headends to act asswitching centers. Interconnection switchingfacilities allow one hub to receive programs ofparticular interest to its subscribers, while anotherhub does not. Or, switching facilities within a hubmay also permit subscribers connected to aspecifi, trunk to receive special programming (forexample, a locally cablecast high school baseballFigure 8. Cable Television Systems Use a Tree-Shaped Network.game) while other trunks do not. Such limitedswitching capabilities are not nearly as sophisticated as the fully switched telephone system,but they do increase the flexibility of downstreamone-way service and in the future may increasethe efficiency of two-way services.Although switched cable systems exist, they areriot widespread in the United States. Switchingcenters are costly, and urban communities wouldrequire a large number of centers. Furthermore,the cost of laying the large number of cablesrequired might be prohibitive, especially if thecables were placed underground.DISTRIBUTION SYSTEMf",The modern distribution system consists of thecoaxialsame elements as the traditional onecable, amplifiers, and so on. However, with theFigure 9. Telephone Systems Use a Switched Network.

14increase in the number of services provided,several problems arise. As mentioned earkr, astandard television receiver is capable of receivingonly 12 of the35 channels cable can carry. Further,a strong local broadcast signal will interfere witha cable signal carried on the same channel whenthe two signals meet in the television receiver,making that channel unusable. Thus, distributionsystems have to be modified to overcome theselimitations.Two approaches have been developed to solvethese problems. The first, and generally the mostexpensive, is to bring additional cables to eachhome and provide a switch allowing the subscriberto switch from cable to cable, receiving up to 12channels on each one. This approach uses only12 of the 35 channels in each distribution cablebut it does provide a means of receiving morethan the 12 channels on the television set. Further,this approach led the cable industry to developprototype advanced two-way systems based onthe presumption of a dual cable distributionsystem.The second approach does not involve the dis-tribution system, but rather the subscriber terminal.SUBSCRIBER TERMINALMore recently, system operators have increasedcapacity by installing a converter instead of extracables. Depending on the model, the converterpermits the subscriber to tune any of 26 to 35cable channels. The converter translates theincoming signal selected by the subscriber to aVHF frequencythe best suited to reception bythe television set. Thus, with a converter, a subscriber may receive from 26 to 35 channels froma single cable. Greater channel capacity can beachieved by using both multiple cable and converters.The converter rests on top of the televisionreceiver and takes the place of the receiver's tuning mechanism (see Figure 10). Current versionscost about 23 to 35 each.Figure 10. A Switched Converter.C. Broadband CommunicationsCable's FutureModern cable television has evolved from aretransmission system providing basic receptionto an expanded medium for delivering one-wayservices to subscribers. With the addition ofimported signals, au tomated origination, and localprogramming, cable television has surpassed thecapabilities of broadcast television and is well onthe way to becoming the "television of abundance" envisioned by the Sloan Commission.'Cable technology promises two-way communications for subscribers, and the potential of broad-band communication networks in the future.THE EVOLUTION OFCABLE TECHNOLOGY:TWO-WAY COMMUNICATIONCable's most glamorous aspect is its "bidirectional," or two-way, capability. For the mostpart, this capability exists now only in prototype.Little is known about the future market for twoway communications; system operatoi ,, therefore, are likely to be cautious when consideringinvestment in two-way equipment.Two fundamental kinds of two-way systems areenvisioned for cable communications: institutional two-way systems for a few users, and subscriber systems for all subscribers. Institutionaltwo-way systems may involve two-way data communications or two-way video communications.Most of the electronics for these systems arealready on the market.Subscriber systems involve three prospectiv

However, cable TV systems do use coaxial cable as a medium for delivering television signals, with . Control Center for a Cable Television System Headend. sion signal received by the system (see Figure 2). The antenna is sited to provide the best available signal reception. Each received television signal