An Introduction to Basic CATV

Historic Perspective

From An Overview of Cable Television in the United States: by Dr. Walter Ciciora
"Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary and often sole purpose was the transportation of a variety of entertainment television signals tosubscribers. Thus, they needed to be one-way transmission paths from a central location, called a headend, to each subscriber’s home, delivering essentially the same signals to each subscriber. The signals are intended for use with the consumer-electronics equipment that subscribers already own.  This equipment is built to operate on the current U.S. television technical standard called NTSC after the organization that created it in 1941, the National Television Systems Committee. This black-and-white television standard was modified in 1953 to provide compatible color information to color television receivers, and again in 1984 to add compatible stereo sound.

"The original purpose for cable television was to deliver broadcast signals in areas where they were not received in an acceptable manner with an antenna. These systems were called community antenna television, or CATV. In 1948, Ed Parson of Astoria, Oregon, built the first CATV system consisting of twin-lead transmission wire strung from housetop to housetop.  In 1950, Bob Tarlton built a system in Lansford, Pennsylvania, using coaxial cable on utility poles under a franchise from the city."

"In most CATV systems, off-air signals were not available or were very weak because of the terrain or the distance of the receiver from television transmitters.  In some areas, such as New York City, multiple signal reflections and shadows cast by buildings made reception difficult.  In both of these environments, a hard-wire method of delivery of signals to subscribers was welcomed.  The first operators of these systems were retail TV receiver dealers who sought to expand the market for the sale of their products by also providing the signals that the products required.  By the late 1960s, nearly all of the areas of the U.S. that could benefit from a community antenna had been served.  Growth in the cable industry all but stopped.

"In the mid 1970s, an embryonic technology breathed new life into cable television.  This technology was satellite delivery of signals to cable systems, which added more channels than were available from terrestrial broadcasters.  While satellites and earth stations were very expensive investments, these programming pioneers understood that the costs could be spread over many cable operators who, in turn, serve many subscribers."

Network Topology -- Distribution Plant

There are five major parts to a traditional coaxial cable system: 1) the headend, 2), the trunk cable, 3) the distribution (or feeder) cable in the neighborhood, 4) the drop cable to the home and in-house wiring, and 5) the terminal equipment (consumer electronics).
           

Two Key CATV Components

Hermetic Outdoor Taps

Hermetic Inline Amplifiers

 
CATV System Topologies
 

Channel Carriage Capacity

TV Broadcast Frequency Plan:

CATV Systems:


Power Considerations

Signals in cable systems are measured in dB relative to 1 mV(millivolt) across 75 ohms. This measure is called dBmV. Expressed in dBmV, the minimum room-temperature noise in a perfect cable system is -59.17 dBmV.

Starting at the home, the objective is to deliver at least 0 dBmV, but no more than 10 dBmV to the terminal on the television receiver. Lower numbers produce snowy pictures and higher numbers overload the television receiver’s tuner, resulting in cross modulation of the channels.  If a converter or a descrambler is used, its noise figure must be taken into account. There are two reasons for staying toward the low side of the signal range: cost and the minimization of interference in the event of a signal leak caused by a faulty connector, damaged piece of cable, or defects or inadequate shielding in the television receiver.  Low signal levels may cause poor pictures for the subscriber who insists on unauthorized splitting in the home to serve multiple receivers.  Working our way back up the plant, we need a signal level of 10 to 15 dBmV at the tap to compensate for losses in the drop cable.  The design objectives of the distribution part of the cable system involve an adequate level of power not only to support the attenuation characteristics of the cable, but to allow energy to be diverted to subscribers’ premises.  Energy diverted to the subscriber is lost from the distribution cable.  This loss is called flat loss because it is independent of frequency.  Loss in the cable itself is a square-root function of frequency (Cable Properties) and is therefore contrasted to flat loss.  Because of flat losses, relatively high power levels are required in the distribution part of the plant, typically 48 dBmV at the input to the distribution plant. These levels force the amplifiers in the distribution part of the plant to reach into regions of their transfer characteristics that are slightly non-linear. As a result, only one or two amplifiers, called line extenders, can be cascaded in the distribution part of the plant. These amplifiers are spaced 300 to 900 feet apart depending on the number of taps required by the density of homes.  Because the distribution part of the plant is operated at higher power levels, non-linear effects become important. The television signal has three principal carriers, the video carrier, the audio carrier, and the color subcarrier. These concentrations of energy in the frequency domain give rise to a wide range of “beats” when passed through non-linearities. To minimize these effects, the audio carrier is attenuated about 15 dB below the video carrier.

In the days when cable systems only carried the 12 VHF channels, second-order distortions created spectrum products that fell out of the frequency band of interest. As channels were added to fill the spectrum from 54 MHz to as much as 750 MHz (1 GHz in a couple of systems), second-order effects were minimized through the use of balanced, push-pull output circuits in amplifiers. The third-order component of the transfer characteristic dominates in many of these designs. Figure 1.8 demonstrates the triple-beat phenomena. The total effect of all thecarriers beating against each other gives rise to an interference called composite triple beat (CTB). CTB is measured with a standard procedure involving 35-channel carriers. In a 35-channel cable system, about 10,000 beat products are created. Channel 11 suffers the most with 350 of these products falling in its video passband. Third-order distortions increase nearly 6 dB for each doubling of the number of amplifiers in cascade. A 1-dB reduction in amplifier output level will generally reduce CTB by 2 dB.

The Advent of FO CATV Trunks:

Again from An Overview of Cable Television in the United States: by Dr. Walter Ciciora
"The late 1980s and early 1990s brought a high level of interest in and excitement about the 'Information Superhighway,' also called the National Information Infrastructure (NII).  A variety of drivers are energetically pushing the NII.   Early efforts came from the telephone industry seeking to justify upgrading heavily depreciated
plant.   Since the twisted-pair plant did a fine job of delivering plain old telephone service (POTS), another reason for replacing it was needed. That reason needed to be a bandwidth hog.  Digitized voice did not fill the bill.
It could be handled over the existing twisted-copperpairs.  Only computer data and digitized video held the promise of providing a justification for declaring twisted-copper pair obsolete.  So the telephone systems began a quest for permission to add fiber and digital technology to the rate base.  Congressmen caught the bug and decided that a vast government-sponsored program was needed to build the Information Superhigh-way.  The NII was born.

"……The last few years have brought exciting trends employing new technologies. Fiber is now being installed to upgrade older systems and as part of rebuilds and new builds. The old trunk system of long cascades of amplifiers is now considered obsolete. Work on new amplifier technologies will allow a realization of cable’s inherent bandwidth, which exceeds 1 GHz.  A bandwidth of 1 GHz contains 160 slots of 6 MHz. These can be allocated to NTSC, HDTV simulcast, and to new services.  The most exciting potential lies with utilizing video-compres-sion technology to squeeze four or more NTSC-like quality signals in a 6-MHz slot."
 
 
 



 

References:

Connecting Homes to the Internet: An Engineering Cost Model of Cable vs. ISDN by Sharon Eisner Gillet
An Overview of Cable Television in the United States: by Dr. Walter Ciciora. Updated 1995

The Cable Connection: The Role of Cable Television in the National Information Infrastructure

The Cable Modem Info Center:

Cable Modem Primer

Cable Modem Tutorial

 
 
 
 

This page was prepared and is maintained by R. Victor Jones
Comments to: jones@deas.harvard.edu.

Last updated May 10, 2004