TV Reception: Getting the Most out of your Cable
Lucas Valley Cable, being charged with the maintenance of the Lucas Valley Homeowner's Association's television distribution system, is committed to the delivery of the highest quality signals possible. Our responsibility ends, however, with the delivery of a signal to your home. This article is intended to give you do-it-yourself information on how to realize the full potential of the signal that we provide. If you would rather not deal with the details presented here, you may contact us and have a technician audit your home distribution, correct any problems, and give you a quote for any work that you might want done to improve your service.
Homes in our subdivision are among the earliest to receive television signals by underground cable. The original system was capable of distributing channels 2 through 13, and used components suitable for that service.
Over the past several years the public portion of the distribution system has been rebuilt to the much higher standards required for the extended service that we now enjoy. There are still some homes, however, with older equipment, and it is probably necessary to bring these up to date before a satisfactory signal can be presented on all channels. Also, the original cables and fittings may have degraded severely since your system was installed.
In order to understand the material in this article, you will need some familiarity with how television signals are measured. While these signals may be measured in volts or watts, the industry has standardized on "decibels over one millivolt," or dBmV. That sounds like an unnecessary complication but, in fact, dBmV is far easier to deal with because all the necessary calculations are done with simple addition and subtraction.
A decibel is a relational term--a kind of logarithmic way of saying "nine-tenths of the power." 6 dB equates to a power change of 4 times, or a voltage or current change of 2 times. When modified by "mV," it becomes an absolute measurement by expressing a comparison against a 1 millivolt point of reference. When this is done, it may have either a positive sign (greater than 1 mV) or a negative sign (less than 1 mV).
The television signal that is passed to your home system originates in a device called a "tap." Taps are located in green boxes that are distributed throughout the community, and usually serve two or more homes per box. The signal at the tap is routed to your home through a "drop cable." It is then either routed to your television set, or it passes through one or more "splitters" in order to serve several televisions and/or FM receivers.
To deliver quality signals to every television set in a home, it is necessary to "engineer" the TV distribution components inside your home properly and use parts that are appropriately rated. The signal at the tap is much stronger than the level needed at a typical television set in order to overcome the normal signal losses that are associated with the coaxial cable, splitters, and other devices that participate in the distribution of the signal in your home. The success of that distribution system depends on how well it budgets the excess signal level among these lossy components so that each receiver is presented with a signal of adequate strength. Let's look at these components by the numbers:
The cable system has been designed to give each tap about the same signal level at Channel 70. The signals below or above Channel 70 can be somewhat stronger or weaker, depending on the distance between the tap and the amplifier that supplies it. All taps will deliver about 19 dBmV at Channel 70. If your tap is located at an amplifier, Channel 2 will be about 5 dB weaker than Channel 70. On the other hand, if you tap is at the maximum distance from an amplifier, your signal at Channel 2 will be stronger than Channel 70 by about the same 5 dB. This is the result of the fact that signal loss on the "trunk" cable between between amplifiers is greater at the higher channels. This is compensated for by deliberately "tilting" each amplifier to favor the higher channels, so taps at an amplifier tend to be overcompensated and those at a distance tend to be under compensated. Each amplifiers is also individually "equalized" to overcome the losses in the cables that feeds it to ensure that the level and tilt are precise at each amplifier's output. In our system, the worse-case number of amplifiers between the head end and any given tap is seven.
|Type||Loss per 10 ft.|
|RG-59 Cable||1.5 dB||.242 in.|
|RG-6 Cable||.7 dB||.332 in.|
|RG-11 Cable||.4 dB||.412 in.|
The signal loss in your drop cable depends mainly on its length, but also on the kind of coaxial cable that is used and its condition. When our homes were built, fairly lossy cable was used, designated RG-59. Intended for the old 2-13 channel plan, it's loss at channel 70 rises to about 1.5 dB every 10-feet, so if your drop cable is 50-feet long you can count on about a 7 dB loss on channel 70. Drop cables gradually degrade as they are exposed to moisture, so drop cable loss tends to increase over time. A technician can inspect the cable and measure its actual loss. When drop cables are replaced, the technician usually uses RG-6 cable, which is slightly thicker but which has about half the loss at Channel 70 (.7 dB every 10-feet). If your drop cable is very long, it may be necessary to substitute RG-11, which has a loss of only .4 dB every 10-feet.
If you have LVC's broadband Internet service, your drop cable will terminate in a small "TV Service Box" that separates your cable modem signals from your cable connection. Your cable modem and your first TV splitter will be routed to this box, which reduces your TV signal by 2 - 4 dB. The components inside the Service Box are owned by Lucas Valley Cable and you should not attempt to break the seal on the box. If you feel that you need to make changes that might require opening the box, please call our Support number for help.
Drop cables (or the TV Box if you have our Internet service) usually connect to a splitter so that it can feed more than one receiver. Splitters come in many varieties. The basic splitter divides the incoming signal two ways, which corresponds to a signal division loss of 3 dB, plus some additional loss for a total of 3.5 dB. Splitters with more than two outputs are often comprised of 2-way splitters in combination. A 3-way splitter, in other words, will often have one output with 3.5 dB loss and two outputs with a 7 dB loss that emerge from a second 2-way splitter in the same enclosure. A four-way split ter will have about a 7 dB loss on each of its four of its outputs. Note that all splitters must be rated to at least 750 MHz, a specification not likely to be met by the original splitters installed in your home. Therefore, many homes can benefit greatly by doing nothing more than replacing old splitters with new ones. Note: It is often necessary to have a hierarchy of splitters when a home has several receivers. One may have a splitter in a distant part of the home that serves two receivers in adjacent rooms. When designing such a configuration, remember to feed that distant splitter with a stronger level to compensate. Also, if you have a VCR, it may have an internal splitter or you may have installed a splitter to enable you to watch one channel while recording another.
The length and type of cable in your home participates in signal loss exactly the same as your drop cable. Original wiring was RG-59, which was fine for Channels 2-13. We now recommend RG-6, which is far more appropriate for the extended services in place in the majority of Lucas Valley homes.
Many homeowners found it necessary to install home amplifiers years ago to compensate for the weak signals delivered to their homes before the community system was rebuilt. Home amplifiers have negative loss, or "gain", which may be used to offset splitter and wire losses. They may have a "tilt" control to compensate for the increased loss of wiring at the higher numbered channels. These amplifiers, however helpful they may have been when they were installed, are probably no longer necessary. Many are not capable of amplifying signals higher than Channel 13, resulting in a signal loss rather than the gain that is expected. But worse, they usually create serious problems because they are so often over-driven by the signal levels that now appear at each tap. This results in the production of "intermodulation distortion," meaning that the stronger channels tend to modulate the weaker ones. This distortion manifests itself on television receivers in a manner that can be easily confused with a weak signal. Where a home is experiencing severe signal problems, we often trace the result to a house amplifier that is no longer needed. If you think that you need a house amplifier, read its specifications carefully before installing it to make sure that it has the correct gain, that it extends to 750 MHz, and that it will not be over-driven by the signal coming from the tap.
|Summary of Losses|
|RG-59 Cable||1.5||dB per 10 ft.|
|RG-6 Cable||.7||dB per 10 ft.|
|RG-11 Cable||.4||dB per 10 ft.|
The information presented above, when combined with the actual wire lengths in your home, are sufficient to determine the signal level that is delivered to each receiver in your home. Begin by drawing a map of your home, and the entry point of the drop cable, the approximate length of the drop cable. Then sketch the location of each splitter and the length of wire to each receiver. Determine the loss of each cable from the information above, plus the loss of the splitter(s), and simply do the arithmetic. When designing your distribution system, you should end up with about 0 dBmV at each television receiver (3 dBmV if you have a VCR that has an internal splitter).
The example below shows how a 19 dBmV level at the tap (at the top of the drawing) gradually diminishes as the signal passes through the wiring in a typical home. The signal level at each point is shown in red. (All levels are measured at Channel 70.)
The first splitter is a 3-way, with two 7.0 outputs and one 3.5 output. The level at each of the 7.0 outputs therefore measures 7.2 dBmV. The first level at TV Receiver 1 is therefore 0.2 dBmV because of the 7 dB loss in 40 feet of RG 59, however Receiver 2 has a level of 3.0 dBmV over a longer cable because that cable is RG 6.
The signal first must pass through the drop cable (2.8 dB loss) and the TV Box (3 dB loss), so it arrives at the first splitter at a level of 19 dBmV - 2.8 dB - 3.0 dB = 14.2 dBmV.
The remaining receivers, located at the other end of the house, are supplied by a remote splitter at the end of a 40-foot run of RG 6.
It is very important that you use good components and technique when rebuilding or modifying the television distribution in your home. The cable must be properly prepared at each point of connection and the connector must be carefully installed and crimped to ensure that the shield connection is reliable and secure. Poor fittings are the cause of many problems. In addition to poor reception, they can result in turning your distribution system into an antenna that radiates the cable signal. Such radiation can interfere with licensed radio services and is not permitted under FCC rules. We periodically "walk the neighborhood" with a calibrated receiver looking for this kind of problem, and may be forced to decrease your signal level at your tap if we find radiation levels in excess of legal limits radiating from your home. If this happens, you will be notified so that a you may arrange for a service call to correct the problem.