AV Design Reference Manual - InfoComm

Chapter 17: Systems
Community Antenna Television (CATV) Architecture, continued
The basic analog CATV system consists of two main parts, the headend or main station and
the distribution system consisting of trunks, feeders, and drops to the subscribers. Collectively,
they provide for the acquisition, processing, and distribution of the required CATV channel
signals.
Common components found in a simple RF video distribution system typically include:
• Headend—The location of transmission equipment.
• Modulators—Convert composite or S-video signals along with corresponding audio signals
into modulated signals on a carrier channel.
• Demodulator (or tuner)—Strips the baseband signals from the carrier, producing the
composite audio and video.
• Combiners—Through multiplexing, combine the individual signals together with a carrier
signal to allow for transmission over one cable, constituting a broadband signal.
• Distribution system—Very common and useful method of distributing multiple video
signals throughout a facility; the RF video signal topology within the RF distribution system
is controlled by splitters, taps, and line amplifiers.
• Splitters—Divide signal symmetrically, sending the entire composite signal down multiple
cable paths.
• Taps—Divide the signal asymmetrically, sending part of the signal to a display.
• Line amplifiers—Boost the signal to compensate for cable attenuation.
RF parameters that the AV system designer must consider include:
• Frequency range—When designing an RF system, the maximum number of channels that
can be carried by the system must be determined. Channels are expressed in frequency
ranges of up to 450 MHz or 900 MHz.
• Signal strength—Signal strength must be managed to ensure that sufficient levels are
provided to each destination. Standards organizations provide requirements that govern
signal strength at each port and the allowable drop to the next port.
• Cabling losses—Cable attenuation of the signal can be significant and must be accurately
calculated when designing the RF distribution system. These losses affect signal strength
and vary with frequency. Equalization is used to restore the system to flat frequency
response.
• Component insertion loss—Each component in the system, including splitters and other
distribution devices, introduces an insertion loss attenuation of the signal for which the
design must also compensate.
• Bidirectional subchannels—Sometimes subchannels are used for returning a signal (e.g., a
camera feed) back to the headend. Subchannels also can be used for control signals, such
as with a media retrieval system, and for cable modem communication. The system must
incorporate bidirectional components to enable bidirectional communication.
AVDRM, 1st edition 17-124 © 2006 BICSI ® /InfoComm International ®