nab cable television systems

1751NAB ENGINEERING HANDBOOKCopyright © 2007 Focal Press.

All rights of reproduction in any form reserved.

C H A P T E R

6.14

Cable Television SystemsPAUL HEARTY

Ryerson UniversityToronto, Ontario, Canada

INTRODUCTIONThis chapter is provided as guidance for engineeringprofessionals in terrestrial broadcasting concerningtechnology and practice in cable television.1 The intentof the chapter is to provide an overview of cable sys-tems sufficient to convey an overall understandingbut, in particular, to focus on those aspects of cablemost likely to be of operational and practical interestto the broadcaster. The information given in this chap-ter reflects cable in North America and should not beassumed to accurately portray cable elsewhere.2

Given the depth and breadth of the topic and thelimited space available, it is not possible to presentthis material in any detail. Accordingly, this chapteridentifies key standards and other sources that willallow those interested to pursue topics in greaterdepth. For a more detailed picture of digital video/audio in cable, readers might examine Ciciora et al. [1],Thomas and Edgington [2], and Ovadia [3].

CABLE IN THE UNITED STATES

At its beginnings in the late 1940s, cable was con-ceived, first as an extension to terrestrial broadcastwhich “filled in” coverage in difficult-to-reach areas,then as a community antenna television (CATV) sys-tem, which provided an alternative to the proliferationof terrestrial receive antennas. Since those early days,however, cable has developed to be a full-scale com-munications medium that offers a broad array of one-way and two-way services over vast geographicalareas.

The next stage in the development of cable began inthe late 1950s, when operators began to use CATV sys-tems to deliver “distant” television signals (signalsoriginated outside the local broadcast coverage area).This development, although seminal to the concept ofcable as we understand it today, provoked competitiveconcerns among local broadcasters and led to regula-tory actions that slowed development of the cable ser-vice concept.

The 1960s could be characterized as a period of sub-scriber growth, with little change in the cable serviceconcept. For example, the National Cable and Tele-communications Association (NCTA) estimates thatcable penetration grew from about 70 systems withabout 14,000 subscribers in 1952 to about 800 systemswith about 850,000 subscribers.3

The 1970s saw several developments that enabledmuch of the concept of cable as we see it today. First,relaxation of regulations allowed operators to enter-

1This chapter is based in part on “Carriage of Digital Video andOther Services by Cable in North America,” by Paul J. Hearty, whichappeared in Proceedings of the IEEE, vol. 94, no. 1, 2006, 148–157,©2006 IEEE. Those portions of the paper reflected here are providedwith the permission of the IEEE. Other acknowledgments are pro-vided at the end of the chapter.

2In this respect, it is important to note that, although there aremany commonalities among cable systems worldwide, there are dif-ferences in channel plans and in many aspects of technology and op-eration. In particular, there are significant differences among systemsin digital services; there, North American systems conform to Societyof Cable and Telecommunications Engineers (SCTE) standards,while systems in Europe, for example, conform to Digital VideoBroadcasting (DVB) standards.

3Source: National Cable and Telecommunications Association(NCTA), http://www.ncta.com.

SECTION 6: TELEVISION TRANSMISSION

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tain new service concepts. Second, the launch ofnational television service delivery over satellite byHome Box Office in 1972 established a new model forthe delivery of cable programming. And, third, theemergence of new program providers, such as the“Superstations,” not only added program offerings,but also cemented the concept of the cable system as aplatform for diverse program delivery.

The 1980s were a period of considerable growth inthe cable industry. Subscriptions grew from about16M at the end of the 1970s to about 53M at the end ofthe 1980s. Subscriber growth and revenue, as well asthe availability of new program sources, fueledupgrades to systems to provide greater capacity,which, in turn, allowed and stimulated the emergenceof new program sources. By the end of the 1980s, therewere 79 cable television networks, up from 28 adecade previously.

The pattern of growth in subscribers and programofferings continued through the 1990s. Upgradescontinued, not only to provide for greater capacity,but also to enable radically new service deliveryoptions: digital television (including high-definitiontelevision, HDTV), broadband (data services using acable modem), and voice (telephony). By the middleof the decade, many cable systems were transitioningto a new architecture, hybrid fiber-coax, whichallowed high bandwidth capacity at the core of thenetwork using fiber and cost-effective improvementsto network elements closer to customer premisesthrough upgrades to the existing coaxial infrastruc-ture.

The first digital offerings in the United States, com-prising both digital cable and broadband (cablemodem) services, were launched in 1996. From initiallaunch in 1996 through September 2004, digital cableachieved a penetration of 32.1% of cable subscribers,while broadband achieved a penetration of 26.9% ofcable subscribers.4

By the end of the 1990s, the number of cable sub-scribers had grown to more than 65M, and number ofnational cable programmers stood at more than 170.By early 2001, subscribers to digital television andbroadband services had grown to 12.2M and 5.5M,respectively.5

The 2000s saw further growth in digital andbroadband deployments, as well as the introductionof new services and concepts. In the early 2000s,operators began pilot testing of new services, such asinteractive TV. In 2003, operators accelerated deploy-ments of HDTV, launched Video on Demand (contentaccess with VCR-like capabilities), and introducedcable telephony service (Voice over Internet Protocol,VoIP).

As of early-to-mid 2006, there were 65.5M cablesubscribers in the United States, representing a pene-tration of 59.1% of television households. Penetrationof digital cable had reached 26.9M. That for broad-band had exceeded 27.6M, while that for cable tele-

phony had exceeded 6.6M. Of nearly 119M homespassed by cable, 96M had access to HDTV service.6

Currently, operators are exploring even moreadvanced service concepts. For example, work cur-rently is underway to add wireless extensions tocable systems, allowing cable service to extendbeyond the wired cable infrastructure to mobile, per-sonal devices.

ECONOMICS AND CONTEXT

The U.S. cable industry is expected to give a strongfinancial showing in 2006, with subscriber revenues of$69.5B and advertising revenues of $24.6B.7 Despitethis strong showing, however, the cable industry facesa number of challenges.

First, there is the ever-increasing demand for pro-gram bandwidth. Cable operators must sustain theviability of their legacy analog services, while at thesame time responding to demand for new digital andhigh-definition television services. For reference, sup-port for 80 legacy analog services in a 550 MHz plantwould leave only 13% of capacity available for digitaltelevision, HDTV, and other services, while supportfor the same would leave only 45% of capacity in an870 MHz plant.8

Second, there is competition from other multichan-nel video program distributors (MVPDs—primarilydigital satellite and xDSL sources—that have a sub-scriber base of about 28M, or approximately 30% ofthe multichannel market9). In this respect, it is impor-tant to note that other MVPDs do not have legacy ana-log service to maintain and can concentrate all of theirbandwidth resources on digital programming andother services.

Third, the competitive landscape has evolvedbeyond the unidirectional delivery of television andaudio programming. High-definition television, whichrequires approximately five times the bandwidth ofstandard-definition television, is a key competitiveoffering. Similarly, broadband Internet continues topose both opportunity and challenge; subscribers andrevenues continue to be significant, but competitivepressures have driven user bandwidth requirementsfrom their early-day limits of 1.5–3.0 Mbps to valuesas high as 8 Mbps. Additional services, such as Videoon Demand (VOD) and Voice over Internet Protocol(VoIP), round out the portfolio but add to the band-width challenge.

Recognizing the need to extract the maximum per-formance from their plant, operators expect to expend$11.1B in plant upgrades in 2006, approximately 16%of cable subscriber revenues.10

4Source: NCTA, http://www.ncta.com.5Source: NCTA, http://www.ncta.com.

6Source: NCTA, http://www.ncta.com.7Source: NCTA, http://www.ncta.com.8When technical constraints on the use of frequencies below about

54 MHz are considered, these percentages become more like 3% and39%.

9Some viewers subscribe to both cable and another multiprogramservice, so this percentage is approximate.

10Source: NCTA, http://www.ncta.com.

CHAPTER 6.14: CABLE TELEVISION SYSTEMS

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CABLE SYSTEM ARCHITECTURE

Cable systems are closed communications networksdesigned to meet particular service objectives overspecific geographical topologies. Systems vary in size,both in geographical area and in subscriber popula-tion, in bandwidth capacity, and in degree of technicalsophistication.

Accordingly, it is difficult to describe a typical plantwith any degree of detail, as it is in the details thatthey differ. However, at a generic level, the system iscomposed of four main structural elements:

• The head-end, which accommodates signal ingest and origination, as well as network control, provi-sioning, and monitoring.

• The trunk, which conveys signals from the head-end to area distribution points. Trunks typically transport signals up to 10 miles or so.

• The distribution plant, which delivers signals from the trunk to the neighborhood. Distribution plants typically transport signals about 1–2 miles.

• The drop, which delivers signals from the distribu-tion plant to customer premises. Drops typically transport signals 100–300 feet.

Originally, all three stages in the signal delivery chainwere mediated by coaxial cable—rigid cable in trunkand distribution and flexible cable in the drop. How-ever, recent years have seen increasing use of fiber intrunk and distribution. Modern hybrid fiber/coaxialsystems deliver not only improvements in capacity,but also increased quality due to a reduction in thenumber of amplifiers needed for service delivery anddue to its inherent indifference to ingress from RadioFrequency transmissions.

PLANT AND SERVICE TOPOLOGY

A large cable network may consist of a Master Head-end, a number of Regional Head-ends, a number ofLocal Head-ends (often called hubs), and a large num-ber of District/Neighborhood nodes. The interchangeof content and other data between Master andRegional Head-ends may be by satellite or fiber (tele-communications link). That between Regional Head-ends and Local Head-ends typically is by fiber. Thatbetween Local Head-ends and District/Neighborhoodnodes almost always is by fiber. The final link, thatbetween the District/Neighborhood node and cus-tomer premises, typically is by coaxial cable.

Bandwidth

Plant bandwidths typically are in the range of 550–870MHz, although systems with higher and lower band-widths exist.

Frequency and Channel Plans

Frequency utilization typically is 8-40 MHz for signalsoriginating in subscriber terminals and 50–870 MHz,or higher, for signals directed toward subscribers.

For historical reasons, the television channel plan isbased on the 6 MHz channel bandwidths establishedby the FCC for terrestrial VHF/VSB broadcast. Histor-ically, three channel assignment plans have been used:

• The standard plan, in which carriers are spaced at 6 MHz increments, beginning at 55.25 MHz.

• The incrementally related carriers (IRC) plan. Here, carriers are phase locked and are located at multi-ples of 6 MHz beginning at 55.25 MHz.

• The harmonically related carriers (HRC) plan. Here, carriers are phase locked and are located at multi-ples of 6 MHz beginning at 54 MHz.

In the analog domain, noise is the limiting factor fortelevision service quality in systems carrying smallnumbers of channels (say, fewer than 30). However, asthe number of channels increases, the limiting factorbecomes distortions introduced by cascaded amplifi-ers in the distribution plant. The IRC and, in particu-lar, the HRC assignment plans were developed tominimize the visible impact of these distortions.

Due to the existence of legacy analog television ser-vices, digital television, radio/audio, and broadband(cable modem) services use the existing 6 MHz assign-ment plan. Analog FM radio/audio is carried between88 MHz (90 MHz for IRC) and 108 MHz with 200 kHzcarrier spacing.

Service Topology

Currently, available bandwidth typically is partitionedby frequency among five types of services:

• Analog television and audio services (legacy);

• Digital television and audio services;

• Video on Demand (VOD) services;

• Cable modem (broadband) services; and

• Voice over IP (VoIP) services.

In the near future, it is to be expected that the digitalbandwidth will be shared between conventional(broadcast) services and switched (scheduled unicastor multicast) services. In the less immediate future,additional services, such as those supporting mobiledevices, also will occupy spectrum. At some point,when digital-capable devices become ubiquitous, leg-acy analog services may be terminated.

Standards

The primary body for standardization in North Amer-ican cable television is the Society of Cable Telecom-munications Engineers (SCTE), an American NationalStandards Institute (ANSI) accredited StandardsDevelopment Organization.11 All standards developed


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by SCTE are available at http://www.scte.org. Inaddition to the ANSI-SCTE standards, many elementsof cable technology are recognized in InternationalTelecommunications Union standards.

ANALOG TELEVISION AND AUDIO SERVICES

The first analog cable services were launched in thelate 1940s, and analog services support more than halfof cable subscribers today, nearly 60 years later. Thetechnology, of course, has developed considerablyduring that period, providing better quality, greatercapacity, and more robust and reliable service.

Operative subcommittees for standards in analogcable plants in North America include the SCTE’sInterface Practices and In-home Cabling and HybridManagement Sub-layer Subcommittees.

Analog Network Architecture

An analog cable network, although in some respectssimpler than its digital companion described later,nevertheless is a complex assembly of many compo-nents. An exhaustive description of these componentsand their interrelation would be beyond the scope ofthis chapter. Moreover, because analog cable plantsvary widely, such a detailed description could giverise to potentially misleading impressions of specificplant architectures.

At a conceptual level, however, the key elements ofthe analog video/audio delivery architecture are asgiven in Figure 6.14-1. As the figure shows, a varietyof analog source signals are made available to thecable plant. Some of these are received off-air frombroadcasters in analog NTSC and are simply passedon unencrypted, at delivery frequency, through Het-erodyne Processors. Signals from other sources aredemodulated and decoded (if needed) by Receiver/Decoders, and modulated in AM-VSB in accordance

with the NTSC standard by NTSC Modulators, andplaced on-frequency for delivery.

After they have been modulated and placed on-fre-quency for delivery, the signals comprising the analogcable service are directed to a Combiner or set of Com-biners, which interface to the delivery plant.

Most signals originated from non-broadcast sourcesare encrypted for secure delivery. In plants that pro-vide addressable Access Control, an Authorization Con-troller is used to deliver authorizations either in-bandor through an out-of-band channel. In plants that donot use addressable control, scrambling is applied toprovide authorized access for recognized customerpremises equipment.

Services and Components

Television

Television services consist of standard-definitionvideo (525 lines, with a 4.2 MHz bandwidth), withassociated audio on an FM subcarrier 4.5 MHz abovethe video carrier that contains monaural (L+R) audiowith a bandwidth of 15 kHz, a stereo pilot at 15.734kHz, and a subcarrier at 31.468 kHz for L–R differenceaudio for stereo services. The signal may also containa 10 kHz audio bandwidth Secondary Audio Program(SAP) channel on a subcarrier at 78.7 kHz and closedcaptions and/or extended data services on line 21 ofthe video vertical interval per CEA-608-C [4].

Additional data, such as SID/AMOL (for whichstandards are under development in SCTE and CEA)and Content Advisory information, may be conveyedin the Vertical Blanking Interval (lines 21 and 22).Additional data services, such as teletext, which mayor may not be related to the program, also may bepresent.

Audio/Radio

Cable systems may carry analog radio services in theFM band (88–108 MHz). Main audio channel band-width is 15 kHz for monaural services (L+R). A stereopilot is placed at 19 kHz with a subcarrier at 38 kHzfor L–R difference audio for stereo services.

Program Guide

To assist viewers in accessing programming, operatorstypically devote an analog channel to delivery of anoninteractive program guide. The guide presents, ingrid format, channel offerings as a function of time.The time window is fixed at any given time, and thevideo scrolls down through the channels at a fixedpace.

Service Delivery Paradigms

Linear Programming

In analog cable, all program delivery is linear, in that itis delivered according to a fixed program schedule.Access to such programming is through basic and

11Standards from other bodies, such as the Consumer ElectronicsAssociation, also inform certain elements of cable practice.

FIGURE 6.14-1 Conceptual representation of keyarchitectural elements in an analog cable network.

HOME PREMISES SET-TOP- BOX

HOME PREMISES SET-TOP-BOX

HOME PREMISES SET-TOP BOXES

ANALOG

SOURCES

RECEIVER/DECODER

SCRAMBLER

NTSC MODULATOR

HETERODYNEPROCESSOR

AUTHORIZATIONCONTROLLER

In-Band Path

Out-of-Band Path

CO

MB

INE

R

Note: all paths to and from Set-top Boxes are through the coaxial cable.


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tiered subscription, as well as Pay Per View (PPV);access is secured by a variety of security mechanisms(see following sections).

Near Video on Demand

Pay Per View (PPV) content can be staggered in timeto allow greater choice in program access times. How-ever, such offerings still are linear, in that they aredelivered according to a fixed delivery schedule. Inanalog cable, staggered delivery is very costly in termsof bandwidth utilization.

Technical and Service Issues

In the analog domain, the challenge is to provide themaximum number of television (and audio) serviceswhile maintaining quality and reliability of service.Unlike digital services, which maintain their initialquality until error protection is overtaxed, analog ser-vices exhibit impairments reflecting imperfections inthe plant and interference from other signals.

The level of the signal delivered to the subscriber’stelevision (after any intervening decoding/descram-bling devices) has to fall within a constrained range.Measured in dB relative to 1 mV (0 dBmV) across 75ohms, signals should be in the range of 0–3 dBmV.Delivered signal levels below this range presentsnowy pictures, while those above this range (forexample, at 10 dBmV) may begin to overload televi-sion receiver tuners, introducing cross-modulation,which presents as horizontal and vertical synchroniz-ing bars moving through the picture.12

The operator has to deal with two classes of impair-ments in television pictures: noncoherent impair-ments, such as noise, which present as “snow” whenvisible; and coherent impairments, which present aspattern when visible. The primary source of noncoher-ent impairment is noise; sources of coherent impair-ments include cross-modulation of video signals,cross-modulation of carriers within signal, signalreflections resulting from discontinuities in imped-ance along the transmission line (also called microre-flections, these are similar in aspect to multipath interrestrial broadcast), and ingress of RF signals origi-nating outside the cable system. Of the cross-modula-tion impairments, which arise primarily fromnonlinear amplifiers, the primary concerns are com-posite second-order (CSO), distortion from second-order carrier inter-modulation products (which pre-sents as a herringbone pattern), and composite triplebeat (CTB), distortion from third-order carrier inter-modulation products (which presents as noise at lowlevels and as streaks at high levels).

Analog cable services are implicitly noise limited.Noise will become apparent in pictures as the carrier-to-noise ratio (CNR) approaches 43–44 dB and objec-tionable as CNR approaches 40–41 dB; a good designtarget is 48–50 dB.13

As the number of channels in a cable system isincreased, inter-modulation products become increas-ingly determinative of service quality, as the occupiedfrequency range increases to the point at which higherorder products fall within occupied spectrum. For sys-tems with 12 channels or fewer (appropriately placed),inter-modulation is not an issue. As the number ofchannels is increased to 30 or so, cross-modulationmay become the limiting factor. For systems with 60 ormore channels, CSO and CTB become limiting factors;a good design target for each of CSO and CTB is –53dB relative to carrier.14

Ingest

In the early days, when cable content consistedentirely of local broadcast content, ingest primarilywas accomplished by direct, over-the-air reception ofthe broadcast signal. Later, as more distant signalswere added, radio and microwave relays wereemployed. With the advent of analog satellite distribu-tion in the 1970s, cable operators began to acquire con-tent from satellite, initially from C-Band (4 GHz) and,later, from a mix of C-Band and Ku-Band (12 GHz)sources. In the 1990s, cable operators increasinglybegan to acquire program content, including localbroadcast content, by telecommunication links, suchas DS3 and fiber. By the mid-1990s, the analog satellitefeeds began to be replaced by feeds using digital com-pression and transmission.

Terrestrial Broadcast Sources

Analog Broadcast Sources

Originally, cable operators captured the terrestrialbroadcast signal off-air, subjected the incoming signalto heterodyne processing to render it to intermediatefrequency, and then up-converted the signal to the fre-quency used for transmission through the cable plant.However, capture of analog broadcast signals off-air isa less than desirable approach in that the signal trans-mitted through the cable plant will include, not onlyany impairments that might originate in the plant, butalso any impairments introduced in the terrestrialbroadcast path.

At present, most local broadcaster signals arereceived by fiber or satellite. In the case of fiber, ana-log-to-fiber equipment is installed at the broadcastfacility for the analog feed and receive equipment isinstalled in the cable head-end. In the case of satellite,the local broadcast signals are up-linked by the broad-caster or its service provider; the down-linked signal iscaptured by satellite receive equipment in the head-end.

In principle, the entire signal is passed through theplant, although certain non-program-related data ser-vices may be removed, depending on particular oper-ator-broadcaster business arrangements.

12Source: Ciciora, 9th ed. NAB Engineering Handbook, Chapter 6.13.13Source: Ciciora, 9th ed.

14Source: Ciciora, 9th ed. CSO and CTB are measured by a stan-dardized procedure with 35 channels.


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Digital Broadcast Sources

Currently, broadcasters provide both analog and digi-tal signals so, with rare exceptions, there is no need toingest the digital signal for distribution in analogcable.

Non-Broadcast Sources

Broadcaster-originated programming is, of course, onlypart of the content delivered by the cable operator.Most of the content delivered by the cable operatorderives from non-broadcast sources. For the most part,this content is delivered by satellite transmission,although some content is delivered by fiber-optic link.

For signals delivered by fiber-optic link, the incom-ing signal must be demodulated, converted to elec-tronic form, and rendered to baseband analog. Forsignals delivered by digital satellite, two modulationschemes are currently in use: Quaternary Phase-ShiftKeying (QPSK), the more common; and 8-state Phase-Shift Keying (8-PSK). In both cases, the signal must bedemodulated, decrypted, and decoded to analog base-band. Originally, the device used for such processingwas an Integrated Receiver Decoder (IRD); now, otherhead-end devices can provide the same functionality,but with a denser configuration.

Regulations and Requirements for Carriage

Operators are subject to a number of technical andother regulatory requirements, as well as a number ofvoluntary standards established by industry consen-sus. Some of these requirements are specific to broad-caster-originated signals, whereas others relate tocompatibility with consumer electronics equipment.

The cable operator is subject to the FCC’s Code ofFederal Regulations (CFR) 47, Section 76 (basic techni-cal) [5]. An excellent summary of regulatory technicalrequirements established by the FCC is given inCiciora et al. [1, page 621]. The operator also operatesin cognizance of CFR 47, Section 15.118 (Cable Ready)[6] and is bound by parts of the TelecommunicationsAct of 1996, notably Section 304 (Competitive Avail-ability of Navigation Devices) [7].

For all signals, the cable operator is required to passthrough closed caption data per CEA-608-C [4], whenpresent. With regard to broadcast-originated signals,the operator is additionally required to pass throughSecond Audio Program (SAP), Content Advisory infor-mation, and other program-related data, if present.

Delivery

If they are not already so modulated, the analog sourcesfor services are modulated in AM-VSB in accordancewith NTSC standard and up-converted to the channelfrequencies assigned by the cable operator.

Security

In analog cable, there are two general approaches tosecured program delivery: trapping and scrambling.

In trapping, a filter is applied to the drop to thepremises. Trapping can be done in one of two ways.

• In negative trapping, the filter removes signals to which the household is not entitled. This works well for homes that are not subscribed to cable ser-vice. However, homes that are subscribed for cable services may experience reduced quality for ser-vices delivered on frequencies adjacent to those of trapped services due to limited selectivity of the fil-ters, particularly at higher frequencies.

• In positive trapping, an interfering (or jamming) carrier is introduced to the protected signal in the cable plant, and the trapping filter is used to remove the interfering carrier at the drop to an authorized viewing household. This method, par-ticularly with modern filter technology, offers a fair level of security, but the filter may reduce the reso-lution of the protected video.

Both negative and positive trapping can be defeated,the former by physical removal of the trap and the lat-ter by introduction of a “pirate” filter.

In scrambling, the protected signal is perturbedsuch that it is not usable without the application of arestorative (descrambling) process. There are twoscrambling methods in use:

• In RF Synchronization Suppression, various meth-ods are used to prevent proper synchronization to the protected signal. In the most common methods, horizontal or vertical synchronizing pulses in the protected content may be attenuated (gated or pulsed suppression), or the video carrier may be modulated with a sine wave to prevent synchroni-zation (sine-wave suppression). Authorized descramblers restore the signal to synchronizable form by restoring the synchronizing pulses to proper level or by modulating the signal with the inverse to the sine wave used in scrambling.

• In Baseband Scrambling, protected content is sub-jected to pseudorandom synchronization suppres-sion, video inversion, or both. In some cases, encrypted digital audio is introduced into the hor-izontal and vertical synchronizing intervals of the protected video. Authorized descramblers per-form the inverse process to the initial scrambling operation.

Both scrambling approaches offer a higher level ofsecurity than trapping. However, inasmuch as theyphysically perturb the protected signal, special effortmust be made to minimize impacts on the quality ofthe protected video.

Trapping typically is done outside the customerpremises. Descrambling typically is done inside cus-tomer premises with a set-top converter but can alsobe done with specialized equipment outside customerpremises. Security devices may be addressable (con-trolled by a data signal from the cable head-end deliv-ered in the VBI or by separate carrier) ornonaddressable (programmed prior to arrival at cus-tomer premises).


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DIGITAL TELEVISION AND AUDIO SERVICES

The North American cable industry began the transi-tion to digital cable delivery in the early 1990s, withthe development, and subsequent approval by theInternational Telecommunications Union (ITU) in1995, of its specification for 64-QAM and 256-QAMmodulation formats (ITU Recommendation J.83,Annex B) [8]. The initial draft specification providedfor 64-QAM, with a payload of 26.97 Mbps; a laterdevelopment added 256-QAM, with a payload of 38.8Mbps.

The operative subcommittee for standards in digitalcable television in North America is the SCTE’s DigitalVideo Subcommittee.

Digital Network Architecture

A digital cable network is a complex assembly ofmany interrelated elements, and an exhaustivedescription of these elements would be beyond thescope of this chapter. At a conceptual level, however,the key elements of the digital video/audio deliveryarchitecture are as given in Figure 6.14-2. It should benoted, however, that the actual embodiments of theseelements vary. For example, in some implementations,encryption, modulation, and up-conversion may becarried out in the same device.

Referring to the figure, the Authorization Controllerdeals with all aspects of subscriber authorization,whether for individual programs or for tiers (pack-ages) of programs. Alone, or in conjunction with aseparate key server, it is responsible for the generationand secure handling of authorization keys, which arepropagated through the network to the customer’sset-top box. The Authorization Controller also inter-faces with the operator’s business system to confirmsubscription entitlements and to report program andevent purchases.

The MPEG-2 Multiplexers accept compressedMPEG-2 single-program transport streams and assem-ble them into MPEG-2 multiple-program transportstreams. They also insert the transport infrastructureneeded to facilitate program access and assembly atthe set-top box. Note: In practice, Multiplexer topol-ogy is varied. The Multiplexer may be interfaced to anassembly of analog-to-digital converters and MPEG-2encoders that process a group of analog programsources intended to constitute the line-up for a givencable channel. Alternatively, the Multiplexer maytranscode (and pass through) an entire incoming digi-tal multiplex (as delivered by satellite, for example)that is intended to constitute the line-up for a channel.And, in still other cases, the Multiplexer may process anumber of in-bound single-program and/or multiple-program digital sources, selectively dropping individ-ual sources to assemble a multiple-program transportstream for a channel.15

The Encryption Engines interface with the Authori-zation Controller and the MPEG-2 Multiplexers toencrypt outbound content using the appropriate keys.The Quadrature Amplitude Modulation (QAM) Modula-tors apply Forward Error Correction and QAM modu-lation to the outbound MPEG-2 transport streams,while the Up-converters shift the QAM-modulatedIntermediate Frequency (IF) signals to the frequenciesused for transmission to the home.

The Out-of-Band Modulators deliver System Infor-mation, authorization data (for example, decryptionkeys, etc.), messages, and application data to the set-top through a separate channel that is processed by asecond demodulator in the set-top. The Return-PathDemodulators collect messages transmitted to the net-work by the set-tops using a modulator/up-converterintegrated in the set-top. Messages issued to the net-work using this path include Instant (or Impulse) PayPer View (IPPV) and VOD requests, program pur-chase/credit status reports, service access requests,and status and health information.

Services and Components

Television

Digital cable television services consist of ANSI/SCTE43 2004 [9] video, one or more ATSC A/53C Annex B(Dolby™ AC-3) [10] audio program streams, and closedcaptioning per CEA-608-C [4] (ANSI/SCTE 20 2004[11]) and per CEA-708-B [12] (ATSC A/53C [10]), asappropriate, as well as other data, as identified inANSI/SCTE 43 2004 [9], ANSI/SCTE 54 2004 [13],ANSI/SCTE 19 2006 [14], and ANSI/SCTE 27 2003 [15].

The basic video compression in digital cable con-forms to ISO/IEC 13818-2 Main Profile [16]. Specifi-cally, it is Main Profile with constraints, spanning therange from Main Level through High 1440 to HighLevel. The video coding also incorporates compatibleextensions to support captions and other data ser-vices. The video specification is given in ANSI/SCTE43 2004 [9].

The video formats used in digital cable also arespecified in ANSI/SCTE 43 2004 [9]. For convenience,they are shown in Table 6.14-1.

15The most sophisticated Multiplexer of this sort can adjust the bitrates of the individual single-program transport streams.

FIGURE 6.14-2 Conceptual representation of keyarchitectural elements in a digital cable network.

Path

Path

Path HOME PREMISES

SET-TOP-BOX

QAM MODULATOR & UP-CONVERTER

RETURN PATH DEMODULATOR

OUT-OF-BAND MODULATOR


HOME PREMISES SET-TOP-BOXES

ENCRYPTION ENGINE

MPEG-2 MULTIPLEXER

AUTHORIZATION CONTROLLER

Out-of-Band

In-Band

Return

Note: all paths to and from Set-top Boxes are through the coaxial cable.


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ANSI/SCTE 43 2004 [9] supports a number ofextensions for delivery of program-related servicesthrough video_user_data. Specifically, it supportsdelivery of advanced DTV captions (encoded perCEA-708-B [12], with transport per ATSC A/53C [10]),NTSC closed captions (encoded per CEA-608-C [4],with transport according to ANSI/SCTE 20 2004 [11],ATSC A/53C [10], or both), and other NTSC VerticalBlanking Interval data (encoded per ANSI/SCTE 202004 [11] and ANSI/SCTE 21 2001 [17]). In addition, itsupports delivery of Bar Data and Active FormatDescriptor data per ATSC A/53C [10].

Although ANSI/SCTE 43 2004 [9] is very similar toATSC A/53C [10], there are a few differences:

1. In compression format constraints, digital cable allows more formats within the Main-Profile/Main-Level to Main-Profile/High-Level envelope. In addition to those formats supported by ATSC (shaded cells in the table), digital cable supports other 1080-line and 480-line formats. The additional formats provide options for different horizontal res-olutions; these reflect prior practice in the digital cable environment, mostly to allow for more band-width-efficient transmission.

2. In sequence_header constraints, digital cable allows a bit rate of 26.97 Mbps for 64-QAM, while ATSC

allows a value of 19.39 Mbps for 8-VSB. This differ-ence reflects differences in payload between 64-QAM and 8-VSB. Values for 256-QAM and 16-VSB are identical at 38.8 Mbps.

3. In sequence_display_extension constraints, digital cable requires encoding of 480-line formats in accordance with SMPTE 170M [18] colorimetry, unless otherwise indicated; for such formats, ATSC acknowledges, but does not require, SMPTE 170M [18]. This difference reflects prior practice in digital cable.

4. In transport of NTSC captions (CEA 608-C [4]), digi-tal cable allows carriage in accordance with ANSI/SCTE 20 2004 [11], A/53C [10], or both, while ATSC requires carriage in A/53C [10] only. This difference reflects a balance between prior practice in digital cable and the need to ensure harmonization between digital cable and digital terrestrial broadcast.

5. In transport of other “NTSC” Vertical Blanking Interval (VBI) data, digital cable employs ANSI/SCTE 20 2004 [11] and ANSI/SCTE 21 2001 [17]. This difference reflects prior practice in digital cable.

The basic audio compression in digital cable is asgiven in ATSC A/53C, Annex B (Dolby™ AC-3) [10].

TABLE 6.14-1Compression Format Constraints

Vertical_size_value Horizontal_size_value aspect_ratio_information frame_rate_code Progressive_sequence

1080 1920 1, 3 1, 2, 4, 5 1

4, 5 0

1440 3 1, 2, 4, 5 1

4, 5 0

720 1280 1, 3 1, 2, 4, 5, 7, 8 1

480 720 2, 3 1, 2, 4, 5, 7, 8 1

4, 5 0

704 2, 3 1, 2, 4, 5, 7, 8 1

4, 5 0

640 1, 2 1, 2, 4, 5, 7, 8 1

4, 5 0

544 2 1 1

4 0

528 2 1 1

4 0

352 2 1 1

4 0Legend:

aspect_ratio_information: 1 = square pixels; 2 = 4:3 aspect ratio; 3 = 16:9 aspect ratio.frame_rate_code: 1 = 23.976 Hz; 2 = 24 Hz; 4 = 29.97 Hz; 5 = 30 Hz; 7 = 59.94 Hz; 8 = 60 Hz.Progressive_sequence: 0 = interlaced; 1 = progressive.


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Audio can be offered in monaural, stereo, or 5.1-chan-nel surround. Monaural audio, which rarely is used,typically would be offered at 64 or 128 kbps. Stereoaudio, which currently is most common, typicallywould be offered at 160 or 192 kbps. The 5.1-channelaudio typically is offered at 384 kbps, although 448kbps is not precluded.

Audio service configurations and constraints arethe same as those given in ATSC A/53C, Annex B [10].

Audio/Radio

Audio services conform to ATSC A/53C, Annex B(Dolby™ AC-3) [10] and may be associated with otherdata, as with television services.

Electronic Program Guide

In the multichannel universe of digital cable, the Elec-tronic Program Guide is fundamental to providing theviewer with a satisfactory experience. Currentlydeployed set-tops host proprietary guide applications,with “look and feel” typically customized to the cableoperator’s needs. However, the North American cableindustry is considering the need for standardized pro-tocols for guide applications.

Service Delivery Paradigms

Linear Programming

At launch, all programming on digital cable in NorthAmerica was linear, that is, available according to apre-established program schedule. Access to such pro-gramming was through basic and tiered delivery orthrough Call-Ahead or Instant (Impulse) Pay Per View(CAPPV or IPPV). The former Pay Per View optioninvolves a telephone call to the cable operator’s sup-port center to arrange authorization for a future pro-gram offering, whereas the latter involves directaccess through the set-top box, which is provisionedwith an amount of purchase credit and which periodi-cally reports purchases and seeks refreshment of thecredit amount. However, both PPV options are linearprogramming, in that the program availability sched-ule is fixed by the operator.

Near Video on Demand

Soon after the launch of digital cable in North Amer-ica, operators recognized viewer demand for morechoice in program delivery times and introduced NearVideo on Demand (NVOD). In NVOD, PPV offeringsare staggered in time to allow more frequent and con-venient access times from the viewer’s perspective,but this is done at a cost in system bandwidth. Suchofferings also are linear, in that offerings are by pre-established schedule.

Video on Demand

Initial launches of Video on Demand (VOD) began in2000. In VOD, the digital cable portion of the networkis subdivided into service groups comprising a group

of nodes, each of which serves a number of homes.Delivery of VOD programming to a service group isconditional upon a viewer request, either for access orfor adaptation of delivery in accordance with VCR-like control requests such as Pause, Fast Forward,Rewind, and so on. VOD represents the first instanceof the delivery of nonlinear programming services.

Essentially, VOD involves a high-speed interactiveapplication enacted through a “thin client” applicationhosted by the set-top box, a “thick” application hostedby the digital cable network, and a set of server andprocessing resources made available through the net-work. A schematic of a VOD infrastructure is pro-vided in Figure 6.14-3.

The VOD architecture makes use of the existingdownstream and upstream out-of-band communica-tion resources of the digital cable plant. The viewer’srequest, interpreted by the client application, isdirected to the cable plant through the QPSK returnchannel. This request is processed by the NetworkVOD Controller, which accesses programmingresources from the VOD Servers and directs this con-tent to the Multiplexer/Encryptor/Modulator/Up-con-verter assemblies that serve the viewer’s node.Authorization data, as well as content access informa-tion, is directed to the viewer’s set-top through theout-of-band QPSK downstream channel. The VODapplication in the viewer’s set-top processes this infor-mation and accesses the “new” program content sup-plied for the viewer.

The VOD architecture scales well with demand.As more VOD-active subscribers come on line, ser-vice groups are subdivided (potentially down to thelevel of a single node serving 500 or fewer homes),and additional Multiplexer/Encryptor/Modulator/Up-converter assemblies are introduced to supportthe newly created service groups. Quality of Service ismaintained by ensuring that nodes are sized to ensurethat contention for VOD resources among VOD-active

FIGURE 6.14-3 Conceptual representation of incre-mental architectural elements necessary to supportVOD in a digital cable network (basic digital infra-structure that already is in place, but is leveraged byVOD, is shown in gray).

Path

Path HOME PREMISES

SET-TOP-BOX

NETWORK VOD CONTROLLER

QAM MODULATOR UP-CONVERTER

RETURN PATH DEMODULATOR

OUT-OF-BAND MODULATOR


HOME PREMISES SET-TOP-BOXES

SESSION-LEVEL ENCRYPTOR

VOD MPEG-2 MULTIPLEXER

VOD FILE SERVERS

Out-of-Band

In-Band

Return

Path


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viewers cannot exceed a given service threshold (suchas 4% contention) at peak VOD viewing times.

At this point, cable networks in North Americahave nearly complete VOD coverage, defined in termsof the availability of VOD programming to subscriberswith digital set-top boxes. The limiting factor in VODpenetration is the number of subscribers with suitablyequipped set-tops. At present, business models forVOD range from free access (either as an introductoryoffering or as an advertising-supported service)through subscription to Pay Per View.

Digital Simulcast

Some operators have begun service delivery in what iscalled digital simulcast. In digital simulcast, analogservices are encoded in MPEG-2 and replicated in digi-tal service tiers. This is a strategic move to facilitate thetransition to all-digital service and eventual reclama-tion of spectrum dedicated to analog services. Thisallows the deployment of lower cost set-tops but, moreimportantly, allows the operator to offer higher qualitypictures and to enhance competition in the Multichan-nel Video Program Delivery (MVPD) environment.

Switched Program Delivery

Building on its success in launching VOD, the NorthAmerican digital cable industry recently has begunexperimentation with switched program delivery.This future potential offering could leverage theinfrastructure developed for VOD but use this infra-structure to deliver programming that previouslywould have been available network-wide as basicsubscription or subscription-tier programming. Thekey element of this approach is that programmingwould be delivered linearly (by schedule) but wouldbe provided to a given service group only whenrequested by one or more viewers in that group. Itshould be noted, however, that this flexibility can beachieved only at the expense of increased infrastruc-ture with more Multiplexer/Encryptor/Modulator/Up-converter assemblies.

Interactive Programming

Electronic Program Guide and VOD, of course, areinteractive services. The cable industry has developedspecifications for common middleware to support otherinteractive applications, and set-tops that support thesespecifications are being deployed worldwide. The keyspecifications are given in Open Cable ApplicationsPlatform (OCAP) and are reflected in SCTE CableApplications Platform (CAP) standards and in ITUstandards J.200 [19], J.201 [20], and J.202 [21].

Technical and Service Issues

As noted previously, analog services reveal impair-ments arising from imperfections in plant and fromingress by external RF signals. Digitally transmittedservices, however, retain their initial picture and

sound quality up to the point at which their error pro-tection and concealment are overtaxed.

That said, however, noise, cross-modulation, inter-modulation, ingress, and the like do challenge the dig-ital signal. If the strength of such sources is too great,visible and audible impairments will become apparentand, in cases at the limit, the signal may be lostentirely. Moreover, because the material, and in partic-ular the video, is highly compressed, the visible oraudible impact of uncorrected transmission errors canbe very substantial.

Service targets for digital cable are given in ANSI/SCTE 40 2004 [22] and are incorporated in the FCC’sCode of Federal Regulations CFR 47, section 76.640 [5].

Ingest

Terrestrial Broadcast Sources

Currently, most broadcasters generate two signals: aconventional analog NTSC (standard-definition) sig-nal and a digital 8-VSB signal, which may host a digi-tal high-definition television program or a multiplexof multiple digital standard-definition programs.

Analog Broadcast Sources

Ingest of the analog NTSC signal is as described in thesection on analog cable services. However, when thisis the only source available for the digital cable ser-vice, the operator is obliged to convert the analogbaseband signal to digital baseband and executeMPEG-2 and AC-3 compression encoding. The ensu-ing MPEG-2 single-program multiplex then is insertedinto an MPEG-2 Multiplexer.

Digital Broadcast Sources

Ingest of the digital signal typically is by fiber,although over-the-air and satellite receptions are notuncommon.

Interestingly, because off-air transmission is errorprotected (from digital channel coding), it is now pos-sible to take the signal off-air at the same quality it hadat source, provided the received signal is reliablyabove the 8-VSB threshold. In the instance of off-airreception, an 8-VSB receiver delivers the MPEG-2transport in DVB-ASI [23] or SMPTE-310M [24], whichis inserted into an MPEG-2 Multiplexer. The multi-plexing device combines individual program streamsand processes PSIP data to eliminate potential con-flicts and to aggregate data; see CFR47, Section76.640(b)(1)(iv) [5]. Typically, two DTV signals arecombined into a single 256-QAM channel, althoughother aggregation arrangements are possible.

At the time of writing, analog standard-definitionand digital high-definition signals are available to theoperator. In rare cases, digital standard-definition sig-nals also are available.

Non-Broadcast Sources

Receipt of signals from non-broadcast sources is asdescribed in the section on analog cable service.


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However, instead of being decoded to analog andinserted into the plant on frequency in NTSC, thedigital signals are directed to the appropriateresources for transmission through the plant. In theparticular case of digital satellite reception (for exam-ple, Head in the Sky, HITS), two scenarios exist. In somecases, the entire incoming transport multiplex isQAM-modulated, up-converted, and passed throughthe plant. In others, the incoming programs are multi-plexed together with other nonbroadcast or locallyoriginated programs for greater bandwidth efficiencyin transmission.

Regulations and Requirements

In all cases, the operator is obliged to pass throughCEA-608-C [4] and CEA-708-B [12] captions, as well asContent Advisory data, if present, in program streamsdelivered through the cable system.

In the case of broadcaster-originated content, thereare no specific requirements for carriage of content.

The operator also is bound by FCC CFR 47, Section76.640 (Support for Unidirectional Cable product) [5].

Delivery

Preparation for Delivery

Forming a transport multiplex for delivery throughthe plant is a complex exercise involving the selectionand combination of programs (video and/or audioand associated data). In general, there are three possi-ble operating scenarios.

In the first, an entire group of services may simplybe passed through. This typically occurs with incom-ing satellite signals, for example, HITS feeds, whichhave been assembled for maximum bandwidth effi-ciency through the cable plant.

In the second, the transport payload simply isassembled, without further processing, from single-program transport streams (SPTS) drawn from multi-ple sources. This is likely to be inefficient in that it isunlikely that the sum of all payload components willreliably fully occupy the transport capacity of thetransmission channel. This inefficiency is worsenedwhen constituent signals are encoded in constant bitrate (CBR), as is the case with a 19.39 Mbps broadcastHDTV source. Because the bit rate actually required ata given point in time varies quite widely (as a functionof individual field or frame requirements), the outputfrom CBR encoding tends to include null or zero-valuedata packets and, sometimes, a considerable quantityof such packets.

In the third, the transport payload is groomed froma group of single-program transport streams. This isthe most efficient scenario in terms of system band-width and other resources such as multiplexers, mod-ulators, and up-converters.

At its simplest, grooming supports the assembly ofmultiplexes with minimal null-packet payload. Thiskind of grooming has absolutely no impact on qualityand demonstrates that throughput bit rate should notbe a point for discussion or contention.

In its more complex form, sometimes called Statisti-cal Multiplexing (StatMUX) or rate-shaping, groomingcan achieve enhancements to efficiency yet, if donewith due care, will have little or no impact on quality.New developments in Statistical Multiplexing, whichemploy a different approach from that taken in previ-ous generations of StatMUX products, offer the prom-ise of very significant improvements in efficiency withno loss in quality whatsoever.

Transport

The syntax and semantics of the transport for digitalcable conform to ISO/IEC 13818-1 [25]. More specifi-cally, transport for digital cable is a compatible subsetof ISO/IEC 13818-1 [25]. Constraints and extensions toISO/IEC 13818-1 are as specified in ANSI/SCTE 542004 [13].

The key elements of the digital video transport arethe Program Association Table (PAT), the ConditionalAccess Table (CAT), and the Transport Stream Pro-gram Map Tables (PMTs). The PAT and CAT are iden-tified according to Packet Identifiers (PIDs) 0x00 and0x01; the PIDs for the PMTs corresponding to individ-ual programs are identified in the PAT.

Programs compliant with SCTE or ATSC standardsmay be identified with MPEG-2 Registration Descrip-tors of 0x5343 5445 (“SCTE” in ASCII) or 0x4741 3934(“GA94”). These descriptors may be provided as partof the program_map_section describing the programin its PMT.

Under the standard, MPEG-2 programs are con-strained to carry no more than one MPEG-2 videostream, which is identified by a stream_type code of0x02 or 0x80. If the program contains audio compo-nents, at least one such is to be a complete main audioservice, as specified in A/53C [10]. Audio programscomplying with A/53C [10] are identified by astream_type code of 0x81. Transport stream packetsidentified by a particular PMT_PID value are con-strained to carry only one program definition, asdescribed in the PMT.

The standard imposes a number of restrictions onthe timing and total bandwidths of the PAT, CAT, andPMT sections. Additional constraints are imposed onthe use of adaptation headers and on Packetized Ele-mentary Stream packet headers and extensions.

The standard also provides for delivery of Emer-gency Alert Messaging per ANSI-J-STD-042-2002, ajoint SCTE-CEA standard [26]. Such messages are car-ried in transport packets identified with PID 0x1FFB.

In comparison with ATSC A/53C, digital cabletransport imposes fewer restrictions on ISO/IEC13818-1 [25]. The more notable of these are:

1. Digital cable does not constrain PES packets always to begin with a video access unit (alignment_type value of 0x02) aligned with the packet header.

2. Digital cable does not constrain PES packets to con-tain only one video frame.

3. Digital cable permits use of MPEG-2 still pictures.


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Both ATSC A/53C [10] and ANSI/SCTE 54 2004 [13]permit specification of private data services. However,ANSI/SCTE 54 2004 [13] permits not only the ATSCservices, but also additional SCTE-specific services.

Because throughput rates differ between 64-QAMand 8-VSB, the transport encoder bit rates differbetween the SCTE and ATSC standards. Those for 256-QAM and 16-VSB are essentially identical.

System/Service InformationOut-of-Band

The primary method in digital cable for the delivery ofSystem/Service Information is through the Out-of-Band Channel (Downstream). System/Service Infor-mation (SI) delivered out-of-band is as described inANSI/SCTE 65 2002 [27]. This specification presentssix profiles for potential use in SI. They range frombaseline, which corresponds to current practice in dig-ital cable, through a series of intermediate profiles thatadd features and/or elements of ATSC A/65B (PSIP)[28], to a further profile that consists solely of PSIP.The SI tables are formatted in accordance with thedata structures defined for Program Specific Informa-tion in ISO/IEC 13818-1 [25].

The key tables in the Baseline Profile are the Net-work Information Table (PID 0xC2, which comprisesthe Carrier Definition Sub-table and the ModulationMode Sub-table), the optional Network Text Table(PID 0xC3, which comprises the Source [Program Pro-vider] Name Sub-table), and the Short-form VirtualChannel Table (0xC4, which comprises the VirtualChannel Map, the Defined Channels Map, and,optionally, the Inverse Channel Map).

Descriptions of other profiles are given in ANSI/SCTE 65 2002, Annex A [27].

In-Band

ANSI/SCTE 54 2004 [13] recognizes the potentialpresence in the digital cable network of devices thatare unable to process out-of-band SI data (that is, dig-ital cable-ready television receivers or set-top boxesnot equipped with CableCARD™16). Accordingly, itallows for provision of in-band SI data in PID0x1FFB, in accordance with ATSC A/65B [28]. Specif-ically, it provides for the inclusion of, and the repeti-tion rates for, the PSIP Master Guide Table, SystemTime Table, and either a Cable Virtual Channel Tableor a Terrestrial Virtual Channel Table. If the PMT oran Event Information Table in the Master GuideTable includes a content_advisory_descriptor refer-ring to a Rating_Region_Table other than the UnitedStates or its possessions, the standard also requiresinclusion of the Rating Region Table. Details are avail-able in ANSI/SCTE 54 2004 [13].

TransmissionIn-Band Modulation

A conceptual diagram of the in-band Forward ErrorCorrection (FEC) encoding and decoding process, aswell as the modulation and demodulation processes,is given in Figure 6.14-4.

Forward Error Correction uses a concatenatedcoding approach for high performance with modestcomplexity. The outer stage involves a (128, 122)Reed-Solomon Block Encoding. This allows correc-tion of up to three symbols per Reed-Solomon block.Inner coding involves fixed (I=128; J=1) or variable(I=128,64,32,16,8; J=1,2,3,4,8,16) convolutional inter-leaving. This allows the operator to explicitly trade offthe level of burst-error protection against decodinglatency. Inner coding continues with randomization tofacilitate demodulator synchronization and then withtrellis coding to further mitigate random channelerrors. The trellis coding uses rate 1/2 convolutionalencoding with a 4/5 puncture rate.

Modulation in digital cable may be in either 64-QAM or 256-QAM, as specified in ANSI/SCTE 072006 [29] (also, ITU Recommendation J.83, Annex B[8]), although many systems currently employ onlythe 256-QAM mode. FEC framing in 64-QAM consistsof 80 Reed-Solomon blocks, followed by a 42-bit synctrailer. That for 256-QAM consists of 80 Reed-Solomonblocks, followed by a 40-bit sync trailer.

The principal parameters of the downstream modu-lation approach are given in Table 6.14-2.

As was indicated previously, the 64-QAM modewas standardized and development begun before theFCC Advisory Committee process had reached con-clusion. The 256-QAM mode was added in 1995.

Out-of-Band Signaling and Modulation

There are two types of out-of-band signaling: that fromthe network to the set-top and that from the set-top tothe network. In both cases, modulation is QuaternaryPhase-Shift Keying (QPSK), which provides highrobustness with good payload. For each case, there are

16A CableCARD™ is a PCMCIA card that incorporates the securi-ty and other network-specific elements of the digital cable network.

FIGURE 6.14-4 Block diagram for coding and trans-mission in in-band channel.

REED-SOLOMON ENCODER

INTERLEAVER RANDOMIZER

CHANNEL

REED-SOLOMON DECODER

DERANDOMIZER DEINTERLEAVER

DOWNCONVERT/ DEMODULATOR

MODULATOR/ UPCONVERTER

FEC ENCODING

FEC DECODING

TRELLIS ENCODER

TRELLIS DECODER


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two protocols: that in systems provided by Motorolaand that in systems provided by Scientific-Atlanta.These are given in ANSI/SCTE 55-1 2002 (Motorola)[30] and ANSI/SCTE 55-2 2002 [31] (Scientific-Atlanta).

With the advent of Open Cable Applications Plat-form (OCAP) middleware, operators recognized theneed for a more flexible, higher capacity mechanismfor out-of-band signaling to support interactive andother high-demand applications. Leveraging therobust and mature data infrastructure of Data overCable Service Interface Specification (DOCSIS), whichalready was in place, the operators developed theDOCSIS Signaling Gateway (DSG). DSG uses theDOCSIS channel on an on-demand basis to supportfaster downstream and upstream data transfer andsmoother running interactive applications. DSG canefficiently support many applications, including soft-ware download. It also allows for simplification in thedesign of set-top terminal devices by elimination of theout-of-band downstream tuner and upstream modula-tor. For the foreseeable future, in recognition of thedeployed base of set-tops and Consumer Electronicsdevices, DSG will be used in parallel with the conven-tional signaling methods described in forthcoming sec-tions of this chapter. Specifications for DSG are givenin ANSI/SCTE 106 2005 [32] and ITU J.128 [33].

Downstream

At the Physical Layer, both downstream encodingapproaches involve Reed-Solomon Block Encoding,followed by Convolutional Interleaving. The principalparameters of the two downstream modulationapproaches are given in Table 6.14-3.

At the Data Link Layer, both downstreamapproaches employ roughly similar architectures. Atsource, Network-related control messages are format-ted as Media Access Control (MAC) messages, com-plete with MAC headers and trailers (including Cyclic

Recovery Clock, CRC). These are encapsulated inMPEG-2 transport stream packets and delivered asMPEG-2 private streams. On receipt, the set-topaccesses the MPEG-2 private stream, de-encapsulatesto retrieve the MAC packets, and performs error-cor-rection to recover the MAC packet payload. The pay-load then is processed by the terminal.

Return Path (Upstream)

At the Physical Layer, both upstream encodingapproaches involve Reed-Solomon Block Encoding,followed by Convolutional Interleaving. The principalparameters of the two upstream modulationapproaches are given in Table 6.14-4.

At the terminal, both upstream approaches employroughly similar architectures. The higher-level proto-cols hand off a Service Data Unit to the Link Layer,which forms a Protocol Data Unit (PDU), completewith upstream link header and trailer. The PDU isparsed into MAC-packet-payload-sized data fields,which are encapsulated into MAC packets, completewith headers and trailers (FEC). These, then, are trans-mitted upstream to the head-end, using one or anotherform of contention-based traffic managementschemes.

Security

Digital cable systems in North America employ veryrobust security systems. Encryption is done usingData Encryption Standard (DES) Cipher Block Chain-ing, as described in ANSI/SCTE 52 2002 [34]. TheAccess Control systems, which manage the generationand secure distribution of decryption keys, as well asgeneral authorization, provisioning, and service con-figuration, are proprietary systems supplied by theNorth American cable industry’s primary OEM devel-opers, Motorola and Scientific-Atlanta.

TABLE 6.14-2Principal Parameters of Downstream In-Band Modulation

Parameter 64-QAM 256-QAM

Modulation 64-QAM, rotationally invariant 256-QAM, rotationally invariant

Symbol Size 3 bits for I and 3 bits for Q 4 bits for I and 4 bits for Q

Transmission Band 54–860 MHz 54–860 MHz

Channel Spacing 6 MHz 6 MHz

Symbol Rate 5.056941 Msps +/– 5 ppm 5.360537 Msps +/– 5 ppm

Information Bit Rate* 26.97035 Mbps +/– 5 ppm 38.81070 Mbps +/– 5 ppm

Frequency Response (Nyquist pulse shaping filters)

Square Root Raised Cosine (band-edge roll-off ≈ 0.18)

Square Root Raised Cosine (band-edge roll-off ≈ 0.12)

FEC Framing 42-bit sync trailer following 60 Reed-Solomon blocks

40-bit sync trailer following 88 Reed-Solomon blocks

QAM Constellation Mapping 6 bits per symbol 8 bits per symbol*The information bit rate is the effective payload once forward error correction overhead is applied.


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Embedded Security

Digital cable set-top boxes supplied by North Ameri-can cable operators are manufactured primarily byMotorola and Scientific-Atlanta, although other manu-facturers are entitled to build under license. Themajority of set-tops deployed to date incorporatesembedded security; that is, the security element,which accomplishes decryption and other Access Con-trol functions, is entirely internal to the set-top, ratherthan partially externalized, as is the case with “SmartCard” approaches, such as the CableCARD™described in the following section. This embeddedsecurity approach allows high levels of physical pro-tection for the secure element and eliminates opportu-

nities for wrongful interception of secure messagessuch as Control Words.

CableCARD™

Responding to regulatory and business requirementsfor the availability of digital cable-ready devices atretail and portability of devices between different net-works, the North American cable industry has collab-orated with the Consumer Electronics industry andother interested parties to develop specifications forCableCARD™. Originally called the Point of Deploy-ment (POD) module, the CableCARD™ is a PCMCIAcard that incorporates the security and other network-specific elements of North American digital cable

TABLE 6.14-3Principal Parameters of Out-of-Band Downstream Modulation Approaches

Parameter Motorola

Scientific-Atlanta

Grade A Grade B

Modulation Differentially encoded Quaternary Phase-Shift Keying (QPSK)

Transmission Rate 2.048 Mbps 1.544 Mbps 3.088 Mbps

Channel Spacing 1.8 MHz 1.0 MHz 2.0 MHz

Frequency Band 70–130 MHz

Carrier Center Frequency 75.25 MHz ± 0.01%* 70–130 MHz in 250 kHz steps

Frequency Response(Nyquist pulse shaping filters at receiver)

Raised Cosine(band-edge roll-off = 0.5)

Square Root Raised Cosine (band-edge roll-off = 0.3)

R-S Forward Error Correction (96,94) (59,53)

Convolutional Interleaving (96,8) (55,5)*72.75 MHz and 104.2 MHz are optionally supported.

TABLE 6.14-4Principal Parameters of Out-of-Band Upstream Modulation Approaches

Parameter Motorola

Scientific-Atlanta

Grade A Grade B Grade C

Modulation Differentially encoded Quaternary Phase-Shift Keying (QPSK)

Transmission Rate 256 kbps ± 50 ppm* 256 kbps 1.544 Mbps 3.088 Mbps

Channel Spacing 192 kHz 200 kHz 1.0 MHz 2.0 MHz

Frequency Band 8–40.16 MHz 8–26.5 MHz

Carrier Center Frequency 8–40.16 MHz (in 8 kHz steps)

8–26.5 MHz in 50 kHz steps

Frequency Response (Nyquist pulse shaping filters at receiver)

Square Root Raised Cosine (band-edge

roll-off = 0.5)

Square Root Raised Cosine (band-edge roll-off =0.3)

R-S Forward Error Correction (62,54) (59,53)

Convolutional Interleaving (96,8) (53,6)* Higher rates are available with Extended Practice.


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systems, allowing appropriately manufactured Con-sumer Electronics devices to support one-way lineardigital cable services and allowing suitably equippedConsumer Electronics devices and set-tops to bemoved across system boundaries.

The basic specification for the Host-CableCARD™interface is given in ANSI/SCTE 28 2004 [35]. A sup-plementary specification for Host-CableCARD™Copy Protection is given in ANSI/SCTE 41 2004 [36].A specification for a one-way (receive-only) digitalcable receiver is given in ANSI/SCTE 105 2005 [37],while discussions concerning a specification for two-way capability are in process.

Copy Protection

The North American cable community recognizes thatproviders of high-value content have the right to beassured that their content, when transmitted throughthe digital cable network, is protected against unau-thorized copying. To that end, the content community,in collaboration with the Consumer Electronics indus-try and other interested parties, has developed a stan-dard for Copy Protection.

Copy Protection technology provides methods forauthentication and revocation of devices, for securelybinding devices, for generating copy protection keys,and for rescrambling high-value content locally toprotect against unauthorized copying. A full descrip-tion of such systems would be beyond the scope ofthis chapter. However, at the heart of the system is anagreement between the content provider and the pro-gram distributor concerning copying permissions forprogram content. Information concerning these per-missions is delivered securely to a device, which usesthis information to enforce restrictions on copying.Sample permissions are shown in Table 6.14-5.

Similar capabilities are available to set-tops withembedded security.

New Security Initiatives

Recently, operators have begun exploring new Condi-tional Access technologies to replace the Cable-CARD™. Under current FCC rules for separablesecurity, operators must use CableCARD™ technol-ogy in leased-out set-top boxes after July 1, 2007. TheDownloadable Conditional Access System (DCAS)

was created to replace the older CableCARD™ tech-nology, which is expensive and cumbersome to use,with downloadable Conditional Access technology.DCAS uses a secure microprocessor in the subscriber’sterminal device to host the downloaded ConditionalAccess system software. DCAS allows operators to usemultiple Conditional Access systems within or acrosscable systems and to change CA systems as necessaryor appropriate. DCAS, which has been successfullydemonstrated to the FCC and across the United States,accomplishes separability objectives, while allowingconsiderable savings in device design and license fees.PolyCipher is the name of the official licensing author-ity and keeper of the DCAS specifications.

New Technologies

The cable industry continues to explore ways ofimproving existing services and of leveraging the digi-tal plant to offer new or expanded services. In video/audio services, operators are considering advancedvideo and audio compression systems (AVC/MPEG-4Part 10, SMPTE VC-1, Dolby™ E-AC-3, and MPEG-4AAC).

BROADBAND (CABLE MODEM) AND IP TELEPHONY SERVICES

Coverage of broadband and IP telephone serviceswould be well beyond the scope of this chapter. How-ever, it is important to note that these are key servicesin the cable operator’s portfolio and represent, notonly increasing revenues to the operator, but alsoincreasing challenges for bandwidth. Both serviceshave exhibited remarkable uptake by subscribers andare making increasing demands for bandwidth to sup-port Quality of Service objectives. Broadband, in par-ticular, shows this trend. At launch, the Broadbanddownstream path was limited to a maximum of 3Mbps, with actual user allocations often much lower.However, user demand and competition from xDSLhave put pressure on operators to raise downstreamlimits, such that many operators now allow down-stream rates of 6–8 Mbps.

The operative subcommittee for standards inBroadband and IP Telephony services in North Amer-ica is the SCTE’s Data Standards Subcommittee.

OTHER SERVICES

The industry is seriously investigating wireless serviceextension. In this offering, the cable plant would be“extended” using wireless transmission to supportaccess to “cable” services with mobile devices.

The industry also is considering CableHome™, aninitiative to leverage the two-way capabilities of thedigital cable plant to offer convenient support for themanagement of communications, appliances, andsecurity in the home. Further details are available athttp://www.cablelabs.com/.

TABLE 6.14-5Sample Digital Copy Permission Values

Encryption ModeIndicator Value Digital Copy Permission

00 Copying not restricted

01 No further copying is permitted

10 One generation copy is permitted

11 Copying is prohibited


1766

It is not as yet clear what impacts such new servicesmight have on analog and digital cable television ser-vices.

CONCLUSION

This chapter provides a brief overview of architec-tures, technologies, and services in cable in NorthAmerica. Given the depth and breadth of the topic andthe limited space available, it was not possible topresent this material in any detail. Instead, the chapteridentifies key standards and other sources that willallow those interested to pursue topics in greaterdepth.

ACKNOWLEDGMENTS

The author would like to acknowledge with gratitudethe contributions of Bill Warga and Andy Scott, whogave freely of their time to improve the quality andaccuracy of this chapter. The author also would like toacknowledge the NAB Engineering Handbook, 9th edi-tion, predecessor to this chapter, an excellent overviewof cable by Dr. Walter Ciciora. Dr. Ciciora’s chapterhelped me ascertain what would be of interest to thisaudience and provided many important insights.

References[1] Ciciora, W., Farmer, J., Large, D., and Adams, M. Modern cable

television technology (2nd ed.). Morgan Kaufmann, 2004.[2] Thomas, J., and Edgington, F. Digital basics for cable TV sys-

tems. Prentice Hall, 1998. [3] Ovadia, S. Broadband cable TV access networks: From technol-

ogies to applications. Prentice Hall, 2001.[4] Consumer Electronics Association, CEA-608-C, “Line 21 Data

Services,” 2005.[5] FCC Code of Federal Regulations (CFR) 47, Section 76, “Multi-

channel video and cable television service.”[6] FCC Code of Federal Regulations (CFR) 47, Section 15, Part

118, “Cable ready consumer electronics equipment.”[7] The Telecommunications Act of 1996, 47 U.S.C., Section 304.[8] International Telecommunications Union (ITU), Recommenda-

tion J.83, “Digital multi-programme systems for television,sound and data services for cable distribution.”

[9] Society of Cable Telecommunications Engineers, ANSI/SCTE43 2004, “Digital video systems characteristics for cable televi-sion,” 2004. Available: http://www.scte.org/standards.

[10] Advanced Television Systems Committee, ATSC A/53C, “Digi-tal television standard,” 2004. Available: http://www.atsc.org/standards.html.

[11] Society of Cable Telecommunications Engineers, ANSI/SCTE20 2004, “Method for carriage of closed captions and non-real-time sampled video,” 2004. Available: http://www.scte.org/standards.

[12] Consumer Electronics Association, CEA-708-B, “Digital televi-sion (DTV) closed captioning,” 1999.

[13] Society of Cable Telecommunications Engineers, ANSI/SCTE54 2004, “Digital video service multiplex and transport systemfor digital cable,” 2004. Available: http://www.scte.org/stan-dards.

[14] Society of Cable Telecommunications Engineers, ANSI/SCTE19 2006, “Methods for isochronous data services transport,”2006. Available: http://www.scte.org/standards.

[15] Society of Cable Telecommunications Engineers, ANSI/SCTE27 2003, “Subtitling methods for broadcast and cable,” 2002.Available: http://www.scte.org/standards.

[16] [MPEG-2] International Standards Organization/InternationalElectrotechnical Commission, Standard ISO/IEC 13818-2,“Information technology—generic coding of moving picturesand associated audio—Part 2: Video coding,” 1994.

[17] Society of Cable Telecommunications Engineers, ANSI/SCTE21 2001, “Standard for carriage of NTSC VBI data in cable digi-tal transport streams,” 2001. Available: http://www.scte.org/standards.

[18] Society of Motion Picture and Television Engineers, SMPTE170M-2004, “Television—Composite analog video signal—NTSC for studio applications,” 2004.

[19] International Telecommunications Union (ITU), Recommenda-tion J.200, “Worldwide common core—Application environ-ment for digital interactive television services.”

[20] International Telecommunications Union (ITU), Recommenda-tion J.201, “Harmonization of declarative content format forinteractive television applications.”

[21] International Telecommunications Union (ITU), Recommenda-tion J.202, “Harmonization of procedural content formats forinteractive TV applications.”

[22] Society of Cable Telecommunications Engineers, ANSI/SCTE40 2004, “Digital cable network interface standard,” 2004.Available: http://www.scte.org/standards.

[23] European Telecommunications Standards Institute, EN 50083-9, “Cabled distribution systems for television, sound and inter-active multimedia signals; Part 9: Interfaces for CATV/SMATVheadends and similar professional equipment for DVB/MPEG-2 transport streams,” (DVB Blue Book, A010), Annex B,Asynchronous Serial Interface.

[24] Society of Motion Picture and Television Engineers, SMPTE310M-2004, “Television—Synchronous serial interface forMPEG-2 digital transport streams,” 2004.

[25] [MPEG-2] International Standards Organization/InternationalElectrotechnical Commission, Standard ISO/IEC 13818-1,“Information technology—generic coding of moving picturesand associated audio—Part 1: Systems,” 1994.

[26] American National Standards Institute, ANSI-J-STD-042-2002,“Emergency alert message for cable,” 2002.

[27] Society of Cable Telecommunications Engineers, ANSI/SCTE65 2002, “Service information delivered out-of-band for digitalcable television,” 2002. Available: http://www.scte.org/stan-dards.

[28] Advanced Television Systems Committee, ATSC A/65B, “Pro-gram and system information protocol for terrestrial broadcastand cable, 2003. Available: http://www.atsc.org/stan-dards.html.

[29] Society of Cable Telecommunications Engineers, ANSI/SCTE07 2006, “Digital video transmission standard for television,”2006. Available: http://www.scte.org/standards.

[30] Society of Cable Telecommunications Engineers, ANSI/SCTE55-1 2002, “Digital broadband delivery system part 1: ModeA,” 2002. Available: http://www.scte.org/standards.

[31] Society of Cable Telecommunications Engineers, ANSI/SCTE55-2 2002, “Digital broadband delivery system part 2: Mode B,”2002. Available: http://www.scte.org/standards.

[32] Society of Cable Telecommunications Engineers, ANSI/SCTE106 2005, “DOCSIS Set-Top Gateway (DSG) specification,”2005. Available: http://www.scte.org/standards.

[33] International Telecommunications Union (ITU), Recommenda-tion J.128, “Set top Gateway specification for transmission sys-tems for interactive cable television services.”

[34] Society of Cable Telecommunications Engineers, ANSI/SCTE52 2002, “Data encryption standard cipher block chainingpacket encryption,” 2002. Available: http://www.scte.org/standards.

[35] Society of Cable Telecommunications Engineers, ANSI/SCTE28 2004, “Host-POD interface standard,” 2004. Available:http://www.scte.org/standards.

[36] Society of Cable Telecommunications Engineers, ANSI/SCTE41 2004, “POD copy protection system,” 2004. Available: http://www.scte.org/standards.

[37] Society of Cable Telecommunications Engineers, ANSI/SCTE105 2005, “Uni-directional receiving device standard for digitalcable,” 2005. Available: http//www.scte.org/standards.

nab cable television systems

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