Television, Cable and Satellite

In the early days of television experiments, transmission of signal was by wire connections and transmissions over phone lines. Radio communications came later. For example, the demonstration in April 1927 by Bell Telephone Labs between Washington, D.C. and New York City was transmitted by both wire and radio, and John Logie Baird also transmitted television between London and Glasgow using conventional telephone lines in 1927.

All prewar British television outside broadcasts used post office telephone lines to get the video signal back to Alexandra Palace prior to radio broadcast. However, consumers received their signal only by radio, and television distribution by cable did not start until the 1950s, and even then in only a limited way.

During World War II, television broadcasts halted in Europe, but they carried on in the U.S., although in a reduced sense. After the war, television broadcasts in England restarted and by 1950 there were early attempts in London to use existing radio relay cables to distribute television. Radio relay was used in England, Germany, and Russia to distribute radio from a master antenna via cable to blocks of houses. By 1953, cable systems were being designed and installed elsewhere in England, still on the radio relay principle.

While there is dispute over who built the first U.S. system, the National Cable Telecommunications Association (NCTA) has given credit to Ed Parsons, who set up a local cable system in Astoria, Oregon. Seattle radio station KRSC had announced it would build television station KRSC-TV, now KING-TV. Parsons’ wife had seen television in 1947 at a convention of the National Association of Broadcasters (NAB), and remarked that she would like to have television at home.

When KRSC-TV went on the air November 25, 1948, Parsons had an antenna and booster installed on the roof of the eight-story John Jacob Astor Hotel, connected with twin lead to his penthouse apartment a short distance down Commercial Street. By New Year’s Day 1949 he had run a feed to Cliff Poole’s music store across the street from the hotel, using coaxial cable, and later to the whole town. Early cable transmission was by ordinary cable, but coaxial cable began to be used from 1941 in the U.S. and the 1950s in the U.K.

Cable TV developed as a medium for distributing or relaying broadcast signals to outlying communities which could not get good reception of broadcast signals due to being out of range of the transmitter. Early cable operators such as those in Pennsylvania in 1949 were mostly local entrepreneurs operating community antenna television or CATV, which consisted of a tall antenna with a repeater station and amplifier, connected by wire to a few homes. From about 1953, operators began to build microwave relays to bring in distant television signals.

By 1961 there were 700 CATV systems in the U.S., though the industry soon coalesced into a few large operators called MSOs, or multiple system operators, as entrepreneurs began to consolidate small networks into larger operations. The industry continued to be primarily a master antenna service operating outside of major metropolitan areas until 1975, when Home Box Office (HBO) inaugurated satellite distribution of its pay movie service.

The inaugural broadcast occurred on 1 October 1975 (Philippine time), with the transmission of the Ali-Frazier heavyweight prize fight from the Philippine Islands (the ‘‘Thrilla from Manila’’) to cable television systems in Florida and Mississippi. HBO then began to sell its satellite-delivered movie service to cable operators, who in turn sold it to subscribers. Shortly after, a number of cable networks, both pay and advertiser-supported, began to appear. With these new sources of programming not available off the air, cable television began to penetrate larger cities.

Communications satellites receive television signals from a ground station, amplify them, and relay them back to earth. Satellite distribution of television began in 1962. On 11 July 1962 satellite dishes at the Radome in Pleumeur-Bodou, France and British Telecom’s Goonhilly Earth Station, in Cornwall, U.K. received the first transatlantic transmission of a television signal from a twin station in Andover, Maine, in the U.S. via the TELSTAR satellite. TELSTAR had a low, elliptic orbit and was only usable for three or four 40- minute periods in each 24 hours. It delivered satellite television during its seven months in orbit.

The satellites used today for cable and broadcast program distribution are in geosynchronous orbit, so that they appear to be stationary in space, affording the use of relatively low-cost fixed receiving antennas. The early antennas were 10 meters in diameter, but antennas smaller than 3 meters can be used today.

The original downlink (satellite-to-ground) was the so-called C band, 3.7 to 4.2 gigahertz. Analog frequency modulation was used, and remains in declining use today. Some newer satellites use downlinks in the Ku band, about 11.7 to 12.2 gighertz. Most newer satellite links use digital transmission of MPEG-2 (Motion Picture Experts Group) compressed video. This permits more programs to be transmitted in the same bandwidth, and permits use of smaller earth station antennas.

Cable television systems start at ‘‘headends,’’ where signals are brought together from a number of sources. Headends supply signals directly to subscribers located close by, and supply signals to more distant subscribers using fiber-optic cable. Figure 3 illustrates a modern cable television system distributing signals to a large metropolitan area.

Figure 3. Traditional cable television program distribution

The headend supplies signals to a number of ‘‘hubs.’’ Radiating from each hub are a number of fiber-optic cables, each connecting to one or more ‘‘nodes.’’ The nodes convert signals from optical to electronic form, where they are distributed via coaxial cable (coax) and radio-frequency amplifiers, to individual homes. A portion of the signal is removed to send to a group of homes, at a ‘‘tap.’’ The tap is a passive (nonpowered) device that draws a predefined portion of the signal from the cable and sends it to one or more homes. Homes are attached to the tap by way of ‘‘drop’’ cable, smaller, flexible coaxial cables.

Signal strength is reduced as the signals travel through the coaxial cable. The signal strength reduction is due to two phenomena:

(1) every time some of the signal is taken out of the cable at a tap, the remaining signal going downstream is weakened by the signal removed to supply the homes attached to that tap; and

(2) the coaxial cable itself causes signal loss due to resistance in the conductors and losses in the dielectric insulator between the conductors. Because of these losses, amplifiers must be used periodically to increase signal strength.

Traditional cable television program distribution is downstream only (see Figure 3). However, many modern services require two-way communications. These services include cable modem and telephone service, as well as interactive video. Bidirectional amplifiers facilitate this two-way communications by amplifying signals flowing in both directions. In order to separate the upstream and downstream signals, it is necessary to transmit them in different frequency bands.

North American practice is to use the band from 54 megahertz to as high as 870 megahertz for downstream transmissions, and the frequency band from 5 to 42 megahertz for upstream transmission. Other parts of the world use slightly different frequency plans. Each amplifier uses ‘‘diplex filters’’ to separate these bands at both the input and output of the amplifier, then uses separate amplification circuits for the two frequency bands. Separate fiber strands usually carry different direction signals between the hub and the optical node.

Each amplifier adds distortion and noise to the signal, so the number of amplifiers used in any one signal path (called a cascade) must be limited. Also, since the amplifiers tend to be somewhat trouble- prone due to their complexity, reliability suffers as the cascade is lengthened. In early systems, amplifier cascades of 20 were common, and cascades of 50 amplifiers were sometimes used. Today, with the introduction of fiber optics, the number of amplifiers in a cascade has been reduced to typically six, with a strong trend toward fewer amplifiers.

Fiber-optic cables are being brought deeper into the cable plant to facilitate this reduction in the number of amplifiers in cascade. Long distances can be achieved with fiber-optic transmission (Figure 4). This figure plots the loss per kilometer for two sizes of coax, and also for fiber-optic cable. The loss on the vertical axis is measured in decibels (dB), with lower numbers (less loss) being better. This lower loss of fiber-optic cable permits long distances between the hub and the node, with no amplification being needed in most cases. Optical amplification is practical and is used where needed.

Figure 4. Signal loss per kilometer measured in decibels in fiber-optic cable and two sizes of coaxial cable. Lower numbers indicate less loss and therefore permit long distances between hub and node with no amplification needed in most cases

The systems described so far are called hybrid fiber-coax (HFC) systems. The latest trend is toward either fiber-to-the-curb (FTTC) or fiber- to-the-home (FTTH) systems, neither of which use RF amplifiers. In FTTC systems, the fiber is brought to a location that serves between four and sixteen homes, and broadcast services are distributed over coax from there. Telephone service is distributed from the end of the fiber to homes, using conventional telephone cable. Data is distributed on data cables, or in some cases may use the same cable as the phone service uses.

FTTH systems bring a fiber cable to the side of the home, where it terminates in a conversion device that supplies video, voice, and data signals for use in that one home. FTTH systems can be divided into passive systems, which have no active (amplification and signal processing) devices at all between the hub and the home (PONs, or passive optical networks); and active systems, which have one active device.

The active systems can achieve much longer reach between the hub and the home, and can use lower cost optical components. These systems are beginning to be deployed at the time of writing, in competition with conventional HFC plants. These new architectures offer superior reliability, higher quality signals, and much higher data rates than are practical with HFC networks.

Signal distribution employs ‘‘frequency division multiplexing,’’ or FDM, in which each signal is assigned a frequency band at which it is transmitted. The frequency band is called a channel. The signal is modulated, or impressed on, a carrier frequency in that assigned channel. For analog transmissions, one program is assigned to each channel, whereas for digital transmissions, several programs may be transmitted simultaneously on one channel. Simultaneous transmission of multiple programs is done using ‘‘time division multiplexing,” or TDM.

In TDM, a portion of one program is transmitted, followed by a portion of another and so on. The transmission is fast enough that all programs appear to the user to be transmitted simultaneously. At the television, when the viewer selects a channel, he or she is tuning the television to that carrier frequency, and the television is recovering, or demodulating, the channel.

In HFC networks, data and telephone traffic are also transmitted using a similar combination of FDM and TDM. In FTTC and FTTH systems, television programs may be transmitted using FDM as on HFC networks, but data and telephone traffic normally use base-band TDM, in which they are not modulated onto carriers, but rather are time division multiplexed and used to turn optical transmitters on and off at a fast rate. Video programs may also be transmitted this way, although it remains more common to use FDM to transmit video programs.