TV Cable Distribution Systems

Once the TV cable reaches your house it's up to you to decide how to use it. Most homeowners connect it directly to the set in the family room, or split the signals with a two-way splitter to supply two different TV sets. Each time the signals are split they get weaker, and at some point the video quality will become degraded and require amplification - this page describes the theory and practice of TV cable distribution.

The signals provided in the cable cover a range of frequencies from 54-88 MHz (VHF/low channels 2 to 6), 88-108 MHz (FM radio), 174-216 MHz (VHF/high channels 7 to 13), to 470-806 MHz (UHF channels 14 to 69). Because cable doesn't carry actual UHF frequencies very efficiently (100 feet of RG-59 loses 80-90 percent of UHF), the UHF channels are converted by your cablevision company to a set of lower frequencies. This is why you need a converter box, or a "cable-ready" TV set.

There are numerous sources of interference that will add noise (static) to the cable. To keep the quality of the picture and sound high, you need to keep the signal/noise ratio as large as possible. The cable shield and good clean connections help keep noise low, and amplification of the signal as soon as possible (before noise creeps in and while the signal/noise ratio is high) will be needed if distances are large, or if the signal will be split and sent to several places.

Whenever the signal is split, it becomes half as strong. It isn't like the three-way outlet of an extension cord where all the appliances receive the same voltage, as they would if connected directly to the wall. It's more like a farmer irrigating a crop by dividing a stream of water, every time it is split in two there is only half as much water.

Relative signal strength is measured in decibels. Alexander Graham Bell worked with sound. When a sound has 10 times as much energy as another sound, it is one bel stronger. One hundred times = two bels, one thousand times = three bels, etc. These would be ten, twenty and thirty decibels, respectively. A two-way splitter changes the signal by -3.5 db (negative because it is a loss of signal), a four-way splitter causes -8 db, and an eight-way splitter -12 db. Long runs of cable will also lose a few db, more for longer lengths, and more for higher frequencies. RG59 cable loses a little more than the heavier RG6. To keep the signal from becoming weak compared to the noise, it should be amplified before it is split. A typical amplifier for home use may amplify 10 to 20 db, and it may have a built-in four- or eight-way splitter.

Connections from the splitter to wall outlets in your home are made with RG-59 coaxial cable. Putting the F-fittings on the ends of the cable is not difficult, but if you don't want to do this, just buy lengths of cable with the fittings already attached, and coil up any excess cable or stuff it into a wall cavity. The excess length may have a slight loss, but since it has been amplified anyway it won't make any noticeable difference.

Unused outlets (outlets which are not connected to TV sets) used to require terminating resistors to prevent reflection of signals. This is something you might try if you find poor reception on only one or two channels using an older amplifier. The resistors are designed to plug directly into the unused outlets. Today you can find amplifiers that don't require terminators.

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