Some cable TV systems receive the local television stations' programming by dedicated coaxial cable, microwave link or fiber-optic line, installed between the local station and the headend. Commercial TV pre-amplifiers strengthen the weakened terrestrial TV signals for distribution, usually after re-modulation using a cable-specific analog or digital scheme. These antennas are often built into a single tower structure called a master antenna television structure.
While each terrestrial channel represents a defined frequency, one or more commercial-grade receiving television antennas are used to receive the multiple channels that the cable company wishes to distribute. Most cable TV systems also carry local over-the-air television stations for distribution. The content is then analog or digitally modulated and transmitted through the cable network (the OSP or 'OutSide Plant') to subscriber homes by means of coaxial or fiber optic cables buried underground or strung from utility poles. Encrypted to prevent unauthorized use, this content is uplinked from one or more earth stations operated by various content delivery companies. Nearly all cable TV systems carry subscription content that is relayed from a satellite in geosynchronous orbit.
