Residential Video Distribution – The Adventure Begins
You haven’t felt this scared since opening the doors of your security company many years ago. You’re a veteran security dealer and have just landed a residential distributed video and audio system installation. The customer has requested the works for every room of the house. The house is to be wired not only for security, but for home theater, background music, and intercom and cable video in every room.
This is a technical challenge and the beginning of a new era for you and your company. Fortunately, there are places to turn for guidance.
In addition to the valuable information in this column, electronic house (EH) equipment manufacturers are prepared to help direct you through this new experience. Organizations such as the Custom Electronic Design and Installation Association (CEDIA) and Building Industry Consulting Services Int’l (BICSI) are key resources. Both organizations have documentation, certification and training programs to help make you a savvy EH dealer.
Video Signals Are Temperamental
Building on our experiences from CCTV, we know video, and in this case video distribution, can be very unforgiving.
In CCTV, we typically deal with composite video that is distributed through switchers, matrixes and quads. In a residential video distribution system (RVDS), we are dealing with video that is routed from an input source, such as cable and satellite (CATV), and sent to a hub or distribution device (DD). Then the signal is typically distributed in a star topography and branched throughout the house, possibly using cable splitters (one to many) or reversed as cable combiners (many to one).
By splitting a video signal, you create considerable loss. A radio frequency (RF) signal has only so much power. Some basic gain/loss calculation techniques will go a long way in planning your RVDS. RF signal levels are measured and calculated in decibel millivolt or dBmV units. A dBmV is output voltage referenced to 1mVrms in the equation 20 log (Vout/1mV) where Vout is in millivolts (mV).
OK, don’t go running for your calculator. We will see in the diagram (see diagram on page 28 of November issue) how calculating in dBmV is as simple as adding and subtracting. In the diagram, an incoming signal from a cable service is to go to 12 locations in the house. Typically, a cable service should provide a 15dB signal level. Our goal is to provide an 8-15dB signal to the 12 wall plates.
The trick is to calculate backwards from the wall plates and add amplification when needed. When calculating signal attenuation (loss), a good rule of thumb is -4dB for a two-way splitter; -6.5dB for three-way; -8dB for four-way; -12dB for eight-way; and -4dB per 100 feet of RG6 coaxial cable.
In the diagram, it was decided that a +20dB inline amplifier would be placed near the input, or head-end, of the system to compensate for the overall -20dB splitter and cable loss. Notice that the downstream drops were grouped cables in order to take advantage of the splitter attenuation. It is a good idea to cap any unused downstream drops with a terminator.
It is advisable to place amplification as close to the head-end as possible. All systems will pick up some noise as the signal is distributed. Downstream amps will amplify unwanted noise as well as signal. In order to minimize noise induction, installers should be careful and keep a physical cable separation of at least one foot when running coax in parallel near 120VAC house power circuits and two feet for 240VAC circuits. Crossing of cables should be done at right angles.
Signal Testing Important, Affordable
Signal levels should be within the 8-15dB range at all outputs. Modern TV sets have what is known as a -10dBmV signal performance “knee.” Below this level, video noise (snow) will increase rapidly. On the other hand, too high a signal on video channels can cause interference on other channels in the distribution system. Testing of these signals can help identify problem areas.
A video field strength meter is a valuable piece of test equipment and should be seriously considered for RVDS installing and servicing personnel. However, the cost can run around $800, which may be too much for all service personnel and trucks to stock. A low-cost alternative to this tester is the creative use of a variable attenuator, which can be purchased at Radio Shack (Part No. 15-678) for less than $10. (Note: The latest version of the Radio Shack variable attenuator may not have dB range markings on the chassis. It is still a 20dB attenuator so you can reference the online documentation and manually place similar range markings on the chassis cover.) This technique will allow a service person to reasonably estimate signal levels.
In this test procedure, the variable attenuator is connected inline with the output in question and then directly to a TV set. The TV will be an indicator of signal level performance. Manually adjust the attenuator knob until you start to see noise (the “knee”) on the TV. Note and record the attenuator’s range marking. Add this number to -10dBmV to get the signal strength of the station. The accuracy of this method is +/- 5dB. You can add nonvariable attenuators to extend the variable attenuator’s 20dB range. If needed, then place fixed attenuators inline to reduce signal strength or add amplification.
Know, Follow Codes and Standards
Recently, I asked Dmitriy Abaimov, a registered communication distribution designer (RCDD) with Cabling-Design.com, to describe some of the biggest problems encountered when installing and designing residential A/V distribution systems.
“Lack of understanding on the architect’s part of the importance of cabling in modern living spaces,” Abaimov answered. Additionally, he stated that installations are difficult due to a lack of emphasis on system functionality. This is further compounded by the need to comply with areas such as the National Electrical Code (NEC) as we will see in a moment.
“Tech Talk” readers may want to check out the Cabling-Design Web site (www.cabling-design.com) as it has a reasonably priced e-Book titled “Residential Cabling Guide” that provides very current installation information.
Due to architectural specifications, RDVS installations often will require that video cabling and house power cabling wall plates end up in the same close proximity, such as in a multigang box. On the other hand, NEC code 800.52(A)(1)(c) is very specific in that power conductors and CATV (broadband) conductors cannot occupy the same box or conduit.
However, there is an exception to the code that will allow this if the conductors are separated by a barrier. In order to comply with this NEC code, specially designed boxes such as the Carlon (www.carlon.com) double-gang box with barrier insertions (see photo on page 30 of November issue) are available.
Organizations such as the American National Standards Institute (ANSI) and the Telecommunications Industry Association (TIA) have recently upgraded their Residential and Light Commercial Telecommunication Wiring Standards 570. The new standard is ANSI/TIA-570-B, vs. the previous 570-A. The new standard establishes a grading system for structured wiring.
Grade 1 is for the minimum requirements of basic telephone and video application support. Grade 2 is for advanced support of high-speed LANs, multimedia, Internet, etc., or “Smarthouse” configurations. You can get a peak of the standard’s specifications in a free sample chapte
r preview at Cabling-Design’s Web site.
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