Chapter 3: The Basics of In-House Cable TV Distribution Networks

The in-house TV distribution network carries the channel content from the headend to the receivers.  The physical network may be coaxial (coax) wiring, CAT 6 wiring (computer networking), or even wireless.  The TV channels can be transmitted as modulated radio frequency (RF) signals or as data over a computer network.  RF networks are very similar to terrestrial broadcast TV signals.  Terrestrial broadcast TV signals are distributed wirelessly from a transmitter mounted high on a transmission tower, typically hundreds of feet high.  The signals are transmitted through the air and are capable of reaching millions of antenna equipped consumer TVs.   

RF cable TV networks work on the same principle except the signals are transmitted in wires, not through the air.  This allows the cable company to utilize additional frequencies beyond those licensed for broadcast.  With more frequencies you can transmit more channels.  The most common wiring used for RF TV distribution is coax.  It is also possible to carry these signals via CAT 6 wiring, but this requires special equipment called a balun which is typically reserved for retrofitting RF distribution where coax wires are not available. 

When a computer network is used to transmit the in-house TV channels it is often referred to as IPTV.  IPTV networks can carry the same content as coax RF networks however unlike MATV RF networks IPTV networks require special receiving devices to display the channels on the TV.   WiFI is typically not used to distribute this type of IPTV because it requires a large amount of multicast one way (UDP) traffic which does not easily recover from data loss which is common to WiFi networks.

The most important component of the in-house cable TV system is the distribution network.  These must be properly designed to preserve signal integrity for each connected device.  The in-house cable TV system consists of some or all of the following.

• Amplifiers – A powered device that increases the strength of the signals that are carrying the TV channels.  
• Splitters – A passive device that splits a coax wire into two or more wires without harmful signal degradation.  These are rated by the frequency range they support.  Modern cabled TV networks require support for up to 1 GHz.
• Combiners – A passive device that combines signals from multiple sources onto a single coax wire.  Sources must coordinate their use of carrier frequencies so as not to conflict when combined.  Combiners are used for inserting a private TV channel into an existing cable network.  Combiners are simply splitters installed backwards.
• Filters – A passive device that blocks some signals and passes others through.  These fall into three categories, high pass, low pass, and notch filters – referring to the carrier frequencies that are passed through and not blocked.  Notch filters are often used to block a single carrier frequency so that it can be replaced using a combiner by a signal from another source.  
• Coax Wiring – A single conductor wire surrounded by shielding to protect the integrity of the TV signals.  Coax wiring comes in a variety of sizes and shielding capabilities, the most commonly used for in-house cable TV is 75 Ohm RG-6.  
• Terminators – Caps placed on open coax ports so that they don’t allow signals in the air to bleed into the coax cable TV network.
• Attenuators – A passive, in line, device used to reduce the strength of TV signals.  These are sometimes used to balance the strength of two different signal sources before they are combined.

The goal of a good coax cable distribution network is to provide equal signal strength to all endpoints on the coax network.  Each additional foot of cable and each additional cable split reduces the strength.  If one leg of the network has significantly more splitters than another amplification is needed to balance that leg of that network.  To prevent this, many designs use a homerun design.  In a home run design, all wires run from a distribution closet to the room where it will be used.  This design makes it easy to count the number of splitters in the network, and hence manage even signal distribution.

Homerun designs are often impractical or impossible due to long distance in the runs or a great variety of in the lengths of each run.  In such a case, a similar result is possible by simply managing the splitter count and cable lengths.