Television

Television signals are transmitted on frequencies that are limited in range. Only persons residing within a few-dozen An HDTV (high-definition television). AP/Wide World Photos. Reproduced by permission. miles of a TV transmitter can usually receive clear and interference-free reception. Community Antenna Television systems, often referred to as CATV, or simply cable, developed to provide a few television signals for subscribers far beyond the service area of big-city transmitters. As time passed cable moved to the big cities. Cable's appeal, even to subscribers able to receive local TV signals without an outdoor antenna, is based on the tremendous variety of programs offered. Some systems provide subscribers with a choice of hundreds of channels.

Cable systems prevent viewers from watching programs they have not contracted to buy by scrambling or by placing special traps in the subscriber's service drop that remove selected channels. The special tuner box that descrambles the signals can often be programmed by a digital code sent from the cable system office, adding or subtracting channels as desired by the subscriber.

Wired cable systems generally send their programming from a central site called a head end. All TV signals are combined at the head end, then sent down one or more coaxial-cable trunk lines. Signals for various neighborhoods along the trunk split away to serve individual neighborhoods from shorter branches called spurs.

Coaxial cable, even the special type used for CATV trunk lines, is made of material that dissipates the electrochemicals passing through. Signals must be boosted in power periodically along the trunk line, usually every time the signal level has fallen by approximately 20 decibels, the equivalent of the signal having fallen to 1/100 of its original power. The line amplifiers used must be very sophisticated to handle the wide bandwidth required for many programs without degrading the pictures or adding noise. The amplifiers must adjust for changes in the coaxial cable due primarily to temperature changes. The amplifiers used are very much improved over those used by the first primitive community antenna systems, but even today trunk lines are limited in length to about a dozen miles. Not much more than about one hundred line amplifiers can be used along a trunk line before problems become unmanageable.

Cable's program offerings are entirely confined within the shielded system. The signals provided to subscribers must not interfere with over-the-air radio and television transmissions using the same frequencies. Because the cable system's offerings are confined within the shielded system, pay-per-view programs can be offered on cable.

Cable is potentially able to import TV signals from a great distance using satellite or terrestrial-microwave relays. Cable systems are required to comply with a rule called Syndex, for syndication exclusivity, where an over-the-air broadcaster can require that imported signals be blocked when the imported stations carry programs they have paid to broadcast.

The next step in CATV technology is the replacement of wire-based coaxial systems with fiberoptic service. Fiberoptics is the technology where electrical signals are converted to light signals by solid-state laser diodes. The light waves are transmitted through very-fine glass fibers so transparent that a beam of light will travel through this glass fiber for miles.

Cable's conversion to fiberoptics results in an enormous increase in system bandwidth. Virtually the entire radio and TV spectrum is duplicated in a fiberoptic system. Every radio and TV station transmitting "over the air" can be carried in a single thread of fiberglass; 500 separate television channels on a single cable system are not beyond reach. A fiberoptic CATV system can be used for two-way communication more easily than can a wire-cable plant with electronic amplifiers. Fiberoptic cable service has great potential to support interactive television services.