A system that receives and processes television signals from various sources and retransmits these signals through cables to subscribers' homes. The sources of the signals include broadcast transmissions, satellite-delivered programming, and local television studio productions. The facility that receives, processes, and retransmits the signals is called a headend.
Unlike broadcast television signals, which travel through free space, cable signals travel through coaxial cable or optical fiber, with different programs or channels traveling at different frequencies (much the same as frequency-division multiplex). In effect, the coaxial cable or optical fiber acts as a self-contained, closed, noninterfering frequency spectrum, created inside the cable by the reuse of the spectrum already in use for other purposes.
Cable television is made possible by the technology of coaxial and optical-fiber cable and is subject to the principles of transmission-line theory. The primary disadvantage of coaxial-cable distribution systems is their relatively high loss or attenuation regarding television signals at the frequencies normally used in cable television systems (50–550 MHz, extending up to 1 GHz in newer systems). Amplifiers are required to overcome this signal loss, and the farther the subscriber is from the cable headend the more amplifiers are needed. Noise and intermodulation distortions created by many cascaded amplifiers limit the practical length of any coaxial cable network. See also Coaxial cable.
Optical fiber does not have the same high attenuation or loss characteristics as coaxial cable, and optical-fiber networks can therefore be built without amplifiers. Most cable systems that use optical fibers do so in a hybrid fashion. Optical fiber is connected from the headend to some localized node or terminating location. The subscriber is then connected to the optical-fiber node by short distances of coaxial cable. See also Optical communications; Optical fibers.
The system design or architecture is known as a tree-and-branch design. The tree-and-branch architecture is the most efficient way to transmit a package of multiple channels of programming from a headend to all subscribers. See also Closed-circuit television.
