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20
The Evolution of the Analog Set-Top Terminal to a Digital
Interactive Home Communications Terminal
H. Allen Ecker and J. Graham Mobley
Scientific-Atlanta Inc.
Abstract
This paper addresses the evolution of the cable home terminal
from past to present and presents a most likely scenario for its
future evolution. Initially, a simple converter allowed subscribers
to tune more channels than the initial VHF Channel 2 through
Channel 13. Next, conditional access and addressability allowed
programmers and network operators to offer subscription-paid
premium programming. Today, advanced analog home communications
terminals (HCTs) allow downloading of software to provide
electronic program guides, virtual channels, and menu-driven
navigators. Evolution will continue from advanced analog HCTs to a
fully interactive digital HCT.
The enabling factors that allowed the cable industry to grow to
become the primary entertainment delivery system to the home are
the following:
1.
The availability of satellite delivery of content
to headends;
2.
The availability of a broadband plant allowing
many channels; and
3.
The availability of set-top terminals that have
grown in functionality to truly become HCTs with reverse path
capability.
Currently, cable systems provide broadband delivery to the home
that allows broadcasting of many channels. In the broadcast mode,
all programs pass each HCT so that each and all HCTs can receive
and display the programs. In most instances, premium programming is
scrambled before transmission so that only authorized customers can
descramble premium channels. The authorization can be handled
through individual addressing of each HCT.
To evolve to an interactive digital terminal that allows
client/server type applications, four key ingredients are
required.
The first is the requirement for a migration strategy to digital
that uses broadband hybrid fiber coaxial systems. This strategy
allows the digital signals to coexist with analog broadcast
channels from which most revenue is derived today.
The second ingredient is the development of high-density
integrated circuits to reduce the cost of the complex digital and
analog signal processing inherent in any interactive terminal.
Circuit density will increase from 0.8 micron to 0.5 micron to 0.35
micron geometries over time.
Third is the development of a multimedia operating system (OS)
and user interface (UI) that allows the user to navigate and
interact with the wide variety of content applications made
available. This OS/UI must also be programmer friendly so that many
different applications providers can develop applications that will
run on home terminals.
Finally, the system must be designed for full interoperability
at critical interfaces and must operate efficiently within the
regulatory environment. Specifically, the interactive home terminal
must contain a network interface module (NIM), which would be
provided by the network operator but could have a consumer digital
entertainment terminal (DET) that could be moved from network to
network and that has an open architecture operating system and an
open application program interface (API).
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Historical Background
Community antenna television, or CATV as it is now called, has
its roots extending as far back as the late 1940s. It is widely
accepted that the first CATV system was constructed in Mahanoy
City, Pennsylvania, by an appliance store owner who wanted to
enhance the sale of television sets by making programming available
to customers that could not get reception of off-air signals since
the community was located in a valley in the Appalachian Mountains.
A large antenna was erected on top of a utility pole atop New
Boston Mountain and was used to feed wires and repeater amplifiers.
The system was ''able to provide sufficient reception of television
programs so as to not only sell television sets but to obtain
subscribers to his cable connection"
1R. From this early attempt to provide cable service,
additional systems were constructed with the intent to provide
clear reception of locally generated signals to customers unable to
receive off-air signals. By the mid-1960s there were over 1,300
cable systems feeding approximately 1.5 million customers.
Architecturally, these early cable systems were merely
distribution systems that received and amplified off-air signals
without processing and reassigning redistribution frequency.
However, the 1970s brought new technological developments. The
launching of geostationary satellites, coupled with the allowance
by the FCC to make these satellites available for domestic
communications, brought about the growth of satellite delivery of
programming directly to cable headends. For the first time,
television programmers and programming distributors could use
satellites to broadcast their programming with complete U.S.
coverage. CATV systems could receive and distribute these signals
via cable to customers with little incremental investment.
Television broadcast stations such as WTBS in Atlanta and WGN in
Chicago made use of this technology to create superstations with
full continental U.S. coverage of their signals. In addition, FM
satellite receivers, large satellite receiving antennas, and video
processing equipment provided cable headends with the necessary
electronic equipment to receive and distribute these programs over
selected channels in the cable system.
How the Analog Cable Set Top
Evolved
For the first time, there were more channels available over the
CATV system than the normal Channel 2–13 VHF television could
tune. This availability of greater channel capacity led to the
development of the cable tuner or "set-top converter." These early
units, developed in the late 1970s, were capable of tuning up to 36
channels. They did little more than convert the tuned cable channel
to a standard Channel 3 or Channel 4 output frequency so that a
standard VHF television set could receive the cable signals. In
effect, these units were the cable analog of the UHF converter
prevalent in the late 1950s. Recognizing the vast market made
available by the satellite and cable distribution system in place,
content providers such as Home Box Office, Cinemax, Showtime, The
Movie Channel, and The Disney Channel as well as ESPN, USA Network,
and others sought to use the existing infrastructure to provide
"cable only" delivery of premium and "niche" programming content to
a growing audience hungry for content variety.
This programming was meant to service only those who purchased
it and therefore required means for selectively enabling only those
viewers who paid for these special services. To fulfill the
requirement, frequency traps were installed in cable drops to
nonsubscribing customers. This technique had several drawbacks:
1.
Traps were used to prevent reception, proving to
be a costly proposition since the expense was distributed over
nonsubcribing customers.
2.
Customers who changed their viewing tastes
required a service call by the cable operator to either remove or
add traps.
3.
If the cable operator added a new premium service,
new traps were needed for each subscriber not desiring the
new service.
Scrambling and addressability removed these road blocks and made
premium programming a viable business. In the 1980s, scramblers
began replacing frequency trap technology. Premium programming
content was scrambled before being redistributed over the cable
system. Newer set-top converters were provided that
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contained addressable descramblers. These units had unique
preloaded serial numbers so that individual converters could be
addressed remotely from the cable headend and either enabled or
disabled for descrambling of particular premium programming content
or tiers of programs.
More Channels and Better Signal
Quality Using Analog Fiber
In an attempt to increase channel capacity, improve penetration,
and provide better quality of service, cable systems in the 1990s
substituted analog fiber technology for the coaxial RF trunk and
distribution systems. Analog fiber provided a number of
advantages:
1.
It reduced the number of amplifiers needed to span
large distances, thereby increasing reliability and signal
quality.
2.
It allowed cable systems to penetrate deeper into
outlying areas without a sacrifice in signal quality.
3.
It reduced the powering requirements.
4.
Analog fiber was selected instead of digital fiber
because it made conversion from fiber back to analog coaxial
distribution inexpensive and simple.
The addition of analog fiber also changed the architecture of a
typical cable distribution plant. In the process of providing
better signal quality deeper into the distribution system, the
topology slowly converter from a "tree and branch" layout, in which
feeder lines split from a main trunk, to a "star" topology where
clusters of subscribers were connected to a fiber node that was fed
directly from the cable headend. This architecture, known as fiber
to the serving area (FSA) or hybrid fiber coax (HFC), reduced the
number of subscribers being fed from a single node and increased
the number of nodes fed directly from the cable headend. This
evolution has enhanced a cable system's ability to convert from a
broadcast topology, where bandwidth is assigned to a program, to an
interactive topology, where bandwidth is assigned specifically to
connect a content provider and a single customer.
Hybrid fiber coax also provides more bandwidth for reverse path
signaling. In the Time Warner Full Service Network in Orlando,
reverse path amplifiers are used to connect HCTs to the fiber node
in the spectrum between 750 MHz and 1 GHz. Reverse path time
division multiple access (TDMA) signals are converted from RF to
analog, fiber at the nodes and piped back to the headend over
fiber. Other systems operate in the same way except that the band
from 5 to 30 MHz is used between the fiber nodes and the home HCT.
The "star" topology of HFC provides more reverse path bandwidth per
home because of the lower number of homes connected to a node.
Cable Becomes a System
Cable in the 1990s has evolved into a complete communications
system that provides broadband delivery to the home of many
channels, both basic unscrambled as well as premium scrambled
channels. Through the use of computer-controlled system managers
coupled with a conditional access system, individual set-top
terminals can be quickly and efficiently electronically addressed
to allow for the descrambling of particular premium channels.
To aid in program selection, advanced analog set-top terminals
today feature downloaded fully integrated program guides and a
system navigator that informs subscribers of the vast programming
available. Also, using the 150 kbps average bandwidth available in
the vertical blanking interval of a standard video channel, these
units incorporate "virtual channels" that can be used to send
sports updates, financial services, news, and other types of
digital services. These units also can be equipped with a standard
read-only-memory (ROM)-based operating system or with an
addressably renewable operating system (AROS) that allows the
operator to download new software to the HCT via the system manager
to provide new user interfaces, unique electronic program guide
on-screen formats, bitmapped graphics, and multilingual on-screen
displays.
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Also, reverse path signaling has made impulse pay-per-view
(IPPV) and near-video-on-demand (NVOD) a practicality. With an
integrated IPPV module, subscribers can view events at their
leisure. Also, NVOD capability offers subscribers advantages
similar to those of VCR tape rental by defining events with
staggered start times on multiple channels. Advanced HCTs have the
capability to emulate a VCR's pause, fast-forward, and rewind
features by making use of the staggered simulcast nature of NVOD
channels and the built-in intelligent software resident in the HCT
to keep track of the appropriate channel needed to view a desired
program segment.
Digital Communications with Analog TV
Signals: An Added Dimension
While current cable systems use analog signals for video and
audio, advancements in digital technology now allow cable systems
to add a digital video layer to increase channel capacity with
little or no increased distribution bandwidth.
Two technological breakthroughs in digital processing are
clearing the way for digital video and audio program content. The
first is the adoption of standards for digitizing, compressing, and
decompressing video programming. In 1992, the Moving Picture
Experts Group (MPEG) of the ISO set out to develop a set of
standards for digitizing and compressing an analog video signal.
This international standards group laid the groundwork for
standardizing the algorithms, syntax, and transport format
necessary to allow interoperability among different suppliers of
video compression and decompression systems. The attitude at the
outset was that if digital television was to flourish, equipment
built by different vendors must be compatible and interchangeable.
The international standards adopted by the MPEG committee allow
freedom to be creative in the encoding process within the
parameters of the defined "encoding language" while maintaining
compatibility with standard MPEG decoders. The analogy in the
computer programming world is to say that software programmers can
approach a programming problem in many different ways; however, if
their program is is written in C, for example, any C compiler can
compile and execute the program.
The second development was a means for delivering the digital
signals to the customer. Schemes for modulating and demodulating an
RF carrier using either quadrature amplitude modulation (QAM) or
vestigial sideband modulation (VSB) have been developed. These
approaches are compatible with standard analog cable systems and
can deliver data information rates up to 36 Mbps in a single 6-MHz
channel. The combination of digital compression of video and audio
and digital transmission over cable can increase the number of
video services in a single 6-MHz channel by a factor of
approximately 5 to 10 depending on programming content and picture
resolution.
Not only does digital processing increase capacity, but it also
allows greater flexibility in the types of services that can be
provided. No longer will there be the restriction that the
information provided be a video signal. Digital computer games,
digital music, and other multimedia applications will all be likely
candidates for the new broadband digital cable.
The cable set-top terminal needed to fully utilize these new
services will evolve to a fully interactive home communications
terminal (HCT) that allows client/server functionality.
Migration Strategy for Broadband Cable
Systems of the Future
So how do we evolve from the cable system of today that
broadcasts analog video services to the home to the fully
interactive digital system of the future? This evolution will most
likely occur in four phases.
The first phase in the migration would employ advanced analog
home communications terminals (HCTs) that are compatible with
existing hybrid fiber coax distribution plants. These terminals,
available today, not only can tune up to 750 MHz but also have both
forward- and reverse-path digital communications capability for
downloading digital applications, providing on-screen display of
program guides and hand-held remote navigators for ordering
pay-per-view and NVOD.
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The second phase most probably will use digital broadcast HCTs
that can not only tune and display the standard analog video but
also receive and decode digital video services. These digital
services would be overlaid on the HFC cable plant using unused
spectrum above the standard analog video channels. This strategy
would allow the digital services to coexist with analog broadcast
channels from which most of today's revenue is derived. System
upgrades and even "new builds" will need to provide today's analog
channels to existing set-top terminals and customers who are
content in viewing basic and premium scrambled broadcast services.
A hybrid fiber coax system transmits the digital information on
analog RF carriers designed to be frequency multiplexed into a
normal broadband channel. The bandwidth required is compatible with
a normal analog video channel so that the digital signals can be
assigned a vacant space in the spectrum or, because of enhanced
ruggedness, can be placed at the upper end of the spectrum outside
the guaranteed performance range of a distribution plant.
The third phase in the migration would provide the
analog/digital HCTs with full interactive capability. This phase
would require an incremental upgrade in the HCT to provide more
memory and enhanced reverse path signaling and full interactivity.
However, the major upgrades would evolve over time as the plant
architecture migrates from a "tree and branch" to a "star"
configuration and more cable headends become interconnected. This
change would be coupled with the growth of content-provider digital
applications stored on file servers, market demand for specialized
applications, and the success of true ''video dial tone" as
compared with NVOD.
The final phase, illustrated in Figure 1, is a fully integrated
broadband network that provides full connectivity to the home for
POTS services, broadband digital modems for personal computers
(PCs), analog and digital video services, digital applications, and
utility monitoring. Either a fully integrated HCT or a split
between the functionality of an HCT and a customer interface unit
(CIU) could be used. This last phase could happen as slowly or as
rapidly as needed depending on market demand for interactive
services.
The phases outlined make sense because they allow for the
coexistence of analog only, digital broadcast, and full interactive
services and HCTs on the same system and do not require flash cuts
to implement, nor do they sacrifice analog video revenues. Thus,
the revenues track with the cost of upgrades.
Cost Drivers for the Fully Interactive
Home Communications Terminal
This migration will be paced by key cost drivers in the HCT, the
distribution plant, and the headend. HCT cost appears to be the
controlling factor because each digital user must have a digital
HCT. Also, developing an economic model that allows enough
incremental revenues to support needed upgrades will be key.
The cost of the fully interactive digital HCT depends on the
availability of a cost-effective set of application-specific
integrated circuits (ASICs) for digital and analog signal
processing and digital memory inherent in the interactive digital
terminal. Currently most if not all the necessary ASICs have been
designed to perform the critical processing functions. These
functions include digital demodulation of 64/256 QAM digital
signals, adaptive delay equalization of the digital signals, error
correction, program demultiplexing and conditional access, and
finally MPEG video and audio decompression. Also, microprocessors
and graphics accelerators are available for a platform for the
terminal operating system (OS) and user interface (UI). Several
trial systems such as the Time Warner Full Service Network in
Orlando, Florida, and the US West fully interactive trial in Omaha,
Nebraska, are using HCTs that include these ASICs. In 1995 other
systems using these chips will be deployed by BellSouth, Ameritech,
Southern New England Telephone, and PACTEL.
However, these chips are currently designed using today's
0.8-micron technology, and the number needed to build a fully
interactive HCT is about seven to nine chips containing about
975,000 gates and 1,500 pinouts. To reduce cost, the goal is to
design the required functionality into one or two chips.
Projections are that by mid-1996 the number of ASICs could be
reduced to four using 0.5-micron technology, and by 1998 the goal
of two chips could be realized using 0.35-micron technology.
Reducing the number of chips would dramatically reduce the number
of pinouts, thereby decreasing cost and increasing liability.
The second key cost driver is the cost per subscriber for the
headend and distribution plant. In an analog broadcast or digital
broadcast scenario, including NVOD, the cost per subscriber for the
headend and distribution plant can be normalized over the number of
subscribers. Therefore, increasing penetration reduces operating
cost
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Figure 1
A fully integrated broadband network providing full connectivity to
the home.
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per subscriber. However, in a fully interactive network,
interactive server/client bandwidth must grow proportionally with
the number of subscribers, so that plant and headend cost increases
with penetration to meet the anticipated heavier demand for
simultaneous interactive sessions between client and server. An
acceptable value for subscriber costs for the fully interactive
system is heavily dependent on the economic model for incremental
revenues that might be expected from the additional services
available to a customer. To put it another way: Just how much is a
customer willing to pay and for what types of services? To answer
this question, a number of fully interactive trials are being
conducted for consumer acceptance as well as technical
evaluation.
Current projections for 1995–96 revenues that might be
expected with various services using advanced analog HCTs, digital
broadcast mode HCTs, and digital interactive HCTs are shown in
Table 1.
TABLE 1 Projected Revenues from Services over
Various HCTs, 1995–96 (billions of dollars)
Advanced Analog HCT
Digital Broadcast HCT
Digital Interactive HCT
Broadcast video
28
28
28
Pay per view/NVOD
8
14
16
Video on demand
—
—
4
Other services
4
8
28
TOTAL
40
50
76
SOURCE: Paul Kagan, 1994, Projections for 1995/96
Cable Revenues.
These projections indicate that a fully interactive HCT could
provide almost a 100 percent increase in monthly revenues over an
advanced analog unit. However, a large portion of the projected
increase is in the vague category defined as "other services" that
include home shopping and other services whose revenue potential is
not proven.
Currently, a reasonable compromise is to provide advanced analog
HCTs with NVOD capability for the near term, but add additional
capacity using digital broadcast, and offer digital interactive
services on a customer demand basis with digital interactive
HCTs.
Requirement for Interoperability and
Industry Standards
The promise of emerging digital interactive services cannot
reach critical mass without the elements of interoperability,
portability, and open standards at critical interface
points in the system. Because of the complexity of the systems
involved, these elements are best resolved through industry groups
representing major providers of equipment as opposed to government
standards that could overlook critical issues.
The following are one set of definitions for the above
terms:
•
Interoperability is the ability to easily
substitute or mix similar products from different vendors. Examples
are television sets, VCRs, fax machines, and almost any electronic
product sold at retail.
•
Equipment portability is defined as
consumers' ability to move their owned equipment across a town,
state, or country and still be able to use the equipment as before
in a new location. Examples are TVs, VCRs, radios, cellular
telephones, direct-TV, and almost any public telecommunications
equipment. Currently, CATV set-top converters do not fit this
category.
•
Open standards are standards that provide
complete descriptions of interfaces at critical points in the
system with reasonable, nondiscriminatory license fees if any
intellectual property is involved. Examples are NTSC, PAL, DOS,
MPEG, telecommunications interface protocols, and many others.
Interoperability, consumer equipment portability, and open
standards are critical to the success of a fully interactive system
for at least three reasons.
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First, the risk is lowered for all participants. Equipment
manufacturers are able to invest in the development of products
that are guaranteed to be compatible with all networks and the
suppliers of content on those networks.
Second, it assures the lowest cost hardware and content.
Competition between equipment manufacturers to produce in large
volume and therefore achieve economies of scale is enhanced because
these products can serve the entire market base as opposed to many
smaller proprietary networks. Also, content providers will be
inclined to participate in content and applications development
because these applications will be compatible with all consumer
terminals.
Third, it will jump start the creation of many new interactive
applications by removing the chicken-and-egg phenomenon. Once
standards are agreed upon, content providers can begin working on
applications software knowing that network architectures and
consumer terminals will be available in the near future. Likewise,
once attractive applications are available, consumers can acquire
home terminals and subscribe to the network.
A major step toward the goals of achieving interoperability,
portability, and industry standards has been accomplished by the
MPEG committee in the adoption of standards for compressing,
transporting, and decompressing video signals. As a result, chip
manufacturers have developed a set of decompression chips that are
functionally interchangeable, and equipment manufacturers now have
a video standard to work from. Because of MPEG's success, a digital
audiovisual council called DAVIC was formed in April 1994 to help
resolve remaining compatibility issues. This committee currently
has over 120 member companies worldwide. The charter for DAVIC is
to assist in the success of the emerging digital video/audio
applications and services by providing timely agreed-upon
specifications for interfaces and protocols. DAVIC is working on a
timetable that called for a strawman baseline document in December
1994, experiments and tests to occur during the spring and summer
of 1995, and publication of final recommendations by December
1995.
While the goals of DAVIC are ambitious to say the least, much of
the intent could be realized by recognizing that two critical
interface points exist in the fully interactive network.
The first interface where standards are important is the
connection between broadband integrated gateways at a cable headend
and the data networks used to achieve full connectivity between
cable systems and between cable systems and telephone companies. An
interface standard must be adopted that allows SONET, FDDI,
Ethernet, ATM, and other protocols to provide inputs to the access
part of the network easily. This approach would provide the
connectivity necessary to access remote content providers either
over satellite, or from remote video file servers on the
metropolitan network or connection through a trunk from other
areas. Interactive applications providers with the capability to
communicate over the Internet or get applications resident outside
the local cable network can also be sources of content.
The second critical interface connects the broadband network to
the home. If interoperability and portability are to be realized
along with network integrity and flexibility, the home terminal
must split into a part that must be provided by the network
operator and a part that is not necessarily a part of the network
and could evolve to becoming consumer electronics sold at
retail.
A first part of the terminal that could be called the network
interface module (NIM) must be provided by the network operator for
several reasons:
•
The technical characteristics of networks will
evolve over time. This evolution will include network-unique
modulation of digital streams that will start out using 64 QAM but
migrate to 256 QAM or even VSB modulation, upgrades in
network-specific signaling techniques for both forward and reverse
path, and a reallocation of bandwidth used to transmit the digital
channels.
•
The HCT will need to interface with other types of
delivery media including 28 GHz wireless, 2 GHz wireless, or even a
satellite network.
•
Networks will have different security and
conditional access systems.
•
Software needed to run the HCT and communicate
with network control will be network specific.
The NIM would be provided by the network provider as a standard
sized plug-in module with standardized I/O on the customer side and
specialized interface to the network. It would contain the tuner,
demodulators, conditional access system, and reverse path signaling
common to a specific network and would
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allow the many different network architectures, signaling
structures, and connection management systems, many of which are
already in place today, to communicate with a part of the HCT that
one day might be purchased at retail. To add further flexibility
and network integrity, the addressability and conditional access
system (AC&E) could be installed on a "smart card" that plugs
into the NIM.
This HCT concept can be explained by Figure 2, which shows the
major functional blocks contained in an interactive digital HCT.
These include the following:
•
A network interface module (NIM) as described
above to provide connectivity, conditional access, and control
between the network and the HCT;
•
A platform for executing multimedia applications,
including an operating system (OS) and a published application
program interface (API) and a user interface (UI) to process
downloaded applications;
•
A digital entertainment terminal (DET) to receive
and process both MPEG compressed digital video and audio, as well
as analog video for presentation to a video monitor or TV set (it
would also contain all the necessary home entertainment interfaces
for TVs, stereos, PCs, and other consumer electronics); and
•
Software that is network-specific to interface
with the DET and the NIM for constructing reverse path messages and
interpreting addressable instructions sent downstream to the
terminal.
Figure 2
Proposed two-part broadband digital home communications terminal
architecture to resolve interoperability
and retail sales issues.
Concluding Remarks
The following statements are supported by the previous
sections:
•
Today's advanced analog HCTs include digital
communications that provide controlled access, downloadable digital
applications (e.g., electronic program guides and virtual
channels), and a significant revenue-producing service with analog
video and NVOD.
•
Digital interactive HCTs have processing power
equivalent to PCs and the additional ability to communicate over
broadband networks and provide multimedia applications.
•
Digital HCTs are already available for trials in
digital networks. Their wide-scale deployment will be made possible
by using highly integrated silicon to reduce cost of the HCT.
•
For an economic transition from analog to digital
that does not preclude revenue-producing analog video, digital HCTs
will also incorporate analog as well as digital capability.
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•
Standards will allow the market to develop more
rapidly and will help drive the cost of the fully interactive HCT
down faster.
•
NIMS and DETS allow networks to be deployed that
provide interoperability and portability for the network provider
and the customer without inhibiting technology and service
growth.
•
HFC provides the plant configuration that allows a
smooth transition to the future and can be the platform for a fully
interactive network that includes video, audio, telephony, and data
without sacrificing analog services and associated revenues.
Reference
[1] Goodale, James C., 1989, "All About
Cable," Law Journal Seminars-Press, Chapter 1, p. 6.
Representative terms from entire chapter:
analog video
