Whole House Wiring Basics Part 2
"Imagination is the beginning of creation. You imagine what you desire, you will what you imagine and at last you create what you will." - George Bernard Shaw
In our last installment of Whole House Wiring Basics we explored the hottest business sector in entertainment electronics today: the implementation of multi-room and/or multi-zone media functionality in the home or office. What is multi-room? Simply put, it is a system that can deliver one source to multiple locations simultaneously. In contrast, then, a multi-zone installation is a system that can deliver more than one source simultaneously to multiple locations. Depending upon your budget, your living arrangements, and your media system's capabilities, you will likely realize a hybrid of the two. Regardless of how ambitious your A/V desires are, you'll certainly need to distribute a low-level signal to make your system perform. We hope that this article will help you understand and overcome the challenges of distributing low-level audio and video signals.
Line Level Audio Distribution
Line level audio signals are very low voltage and very delicate. Typically under 2 volts peak-to-peak, this analog waveform's soul resides in the low level details, which may be measured in millivolts or even microvolts! It should not be surprising that, with these extremely low level and delicate signals, shielding is a key attribute of proper cabling. This shielding must be carried through any distribution amplifiers used in the installation. Note: distribution amplifiers are only needed if a single output is split to several locations, or if a single output is run an exceptionally long distance (greater than 100 feet).
Line level audio cables are typically a shielded twisted-pair assembly with one conductor carrying the hot lead, one carrying the ground connection, and the outer shield tied to ground at one or both ends of the cable assembly. It's important to know that an audio cable certainly doesn't have to be a shielded twisted-pair. A coaxial cable of proper pedigree will work just fine. Impedances within distributed audio systems can vary greatly, with typical source-to-load ratios ranging from 10:1 to 1000:1. Using a 110-ohm audio interconnect is probably not worth the effort for most consumer level installations. Because of this, 75-ohm coaxial cable is suitable for line level feeds.
However, balanced line-level audio distribution within a studio environment is a different matter entirely. Low impedance 600-ohm distribution relies upon careful implementation of transmission-line principles to ensure each piece of gear receives tightly controlled signal voltages.
Typical applications of line level audio distribution within a home system are to route subwoofer signals to remotely placed active subwoofers, and to route key source signals such as the audio output of a satellite receiver to a secondary zone installation.
Let's examine some best practices for this type of common installation. First, select a quality coaxial cable with a braided shield to run inside the walls. Of course, this wire will be appropriately certified as Class 2 compliant for low voltage applications as specified by the National Electrical Code. We use a 75-ohm RG-59 with a 90% copper braid and stranded copper center for two reasons: 1) The braided shield won't be compromised by tight bends the way a spiral shield will be, and 2) the stranded center will make for a flexible, easy pull.
Read our article entitled Spiral Shields Video Cables - Bad Cable Designs for more information on this topic.
When planning the wiring run (always have a wiring strategy!), make sure that your not running this cable parallel to any AC power cables. If you must run parallel for some short distance, try to maximize the separation of the two cables. Remember, electromagnetic interference and radio frequency interference both diminish as a function of the square of the distance separating the assemblies. The potential for induced noise is 144 times lower one foot away from an AC cable than it is one inch from that same cable!
As you pull this cable you may find that you need to cross some AC wires running up and down the height of the wall. When doing this, ensure maximize separation where possible, as above. Also, route the run in such a way as to cross the power cable at a right angle to the direction of its run. This will also help to minimize induced electromagnetic interference.
At each end of the run, terminate this cable in a plastic junction box and use an appropriate wall plate to make a neat, attractive finished connection. With only a few dollars spent and a little bit of effort involved, you can run an in-wall solution to subwoofer or second zone audio distribution!
Composite Video Distribution
Composite video distribution is normally associated with closed circuit security camera equipment rack to a remote television in a master bedroom or den. This type of distribution is similar to distribution of MATC and CATV antenna signals because you'll utilize a 75-ohm RF coaxial cable. Unlike audio distribution, video distribution is susceptible to impedance mismatches.
Like the line-level audio example above, use a properly designed class 2 low voltage cables to meet the requirements of the NEC. You could use the same stranded RG-59 described above, but it is easier to terminate a video cable that uses a solid center conductor. For distribution of composite and component video, use a quad shield RG-6u with a bonded foil and braided shield. This cable design is the most impervious to RFI and EMI, yet it is relatively inexpensive.
Video signals are 1 volt peak-to-peak, but are several hundred times higher in frequency than anything an audio component would ever experience. Because of this, it is important to maintain the characteristic impedance of the system. Since video output or input is 75-ohms, the characteristic impedance of the interconnect must also be 75-ohms. The impedance of a coaxial cable is set by the geometric relationship of its center conductor to the cable's dielectric. Changing that geometric relationship can alter the impedance. This, in turn, causes internal reflections or attenuations of the signal, and thus compromises the ultimate picture quality. You must be careful not to crimp, kink or deform the coaxial cable as you place the run.
Editorial Note on Cable Impedance
Some of our articles are recommended for further elaboration on cable impedance and mismatch issues at video frequencies, such as Component Video Cables - The Definitive Guide and Transmission Line Effects in Video Cables
For this example, assume you are distributing the output of a satellite receiver to several monitors in your home. You have a single output and it needs to go to four separate inputs. This will require a distribution amplifier such as the one shown. A good distribution amplifier offers at least 10MHz of bandwidth, unity gain, and excellent isolation between ports.
Place the distribution amplifier as closely to the source component as possible to avoid degradation before distribution. The signal you end up with at each location can only be as good as the signal you start out with. Because the distribution amplifier will require a source of power, your installation must have access to AC. Also, you should select a location that has stable temperature, humidity and clean environment. Remember, a distribution amplifier is a delicate component and it should be properly protected.
Run two runs of your copper RG59 to carry audio from the distribution amplifier to each remote zone. We'll also run a single RG6u Quad Shield coax for video from the distribution amplifier to each remote zone. Compression style RCA connectors will make secure, quality terminations for the wire runs. As with the audio runs detailed above, make sure to avoid running your cables parallel to AC power cables. Where you have to cross over AC cables, do so at a right angle.
Connection of the output from the source component to the input of the distribution amplifier can use a custom built cable, or pre-terminated interconnect assembly. Feel free to mix and match, performance of the system won't be affected. Conversely you could use wall-plate terminations and interconnect cables from the wall plates to the actual components if that makes for a more attractive and flexible installation. Remember that you should design your system to have the minimal amount of breaks in the cabling path, guaranteeing maximum performance and system integrity. Every connector is a possible source of error!
Using a distribution amplifier and quality coaxial cable as described above will allow installation of runs up to 150 feet or more. Additionally, you can "daisy chain" two or more amplifiers if you need to distribute a single signal to more than four locations - a sports bar would be a good example of this requirement.
S-Video sends an analog video signal on two 75-ohm coaxial cables. One conductor carries a luminance signal, which is a black and white wide-bandwidth television signal. The other conductor carries a chrominance (color) signal. In a way, S-Video is a form of component video because we are sending "components" of the complete video signal along discrete signal paths.
You distribute S-Video signals for much the same reasons you distribute composite video signals. For source materials that originate in a digital domain, such as DVD, S-VHS tapes, DBS satellite transmissions and ATSC off-air transmissions; you can realize a significant performance advantage by selecting S-Video over composite video.
It should come as no surprise that there are S-Video distribution amplifiers and that they are used similarly to composite distribution amplifiers. A single input to four output configuration is particularly popular.
Now you could try to purchase S-video cable of the right length for each custom application, but this may be expensive and/or impractical. Plus, Class 2 rated bulk S-Video cable is expensive and hard to find. There is an easier way. S-Video cables consist of two parallel 75-ohm coaxial cables. You can distribute S-Video using the very same Quad Shield RG-6u coaxial cable as used for composite video cables. The only difference is that you'll need two runs to handle the separate luminance and chrominance signals. To connect the two separate coaxial cables to the mini-Din S-Video connector on the component or distribution amplifier, use an S-Video separator cable.
An S-Video Separator is a simple cable that takes the connections of an S-Video mini-DIN connector and "breaks" them out to dual coaxial cables. You can easily connect any S-Video source to a dual run of RG-6u using this adapter cable without having to solder small pins in a very tight connector!
Component Video Distribution
By now you shouldn't be surprised that component video can also be easily distributed using Quad Shield RG-6u and appropriate amplification devices. As with S-Video and Composite video, you must avoid introducing noise into the system. Do this by keeping runs of coaxial cable as far as possible away from the power cable. Where the coax will cross the power cable, it should do so at a right angle. And like Composite and S-Video, it is very important to maintain the characteristic impedance of the cable assembly by ensuring that the cable is not kinked, crimped, bent or otherwise deformed.
Component video distribution amplifiers often have provisions for left and right audio channels and S/PDIF digital audio. Most HDTV and DVD sources feature multi-channel digital audio soundtracks. If you are distribute the signal from an HDTV DBS Satellite receiver to multiple rooms or multiple locations, for instance, you'd certainly want to be able to send that impressive digital audio soundtrack along with the HDTV image.
Digital Audio Distribution
Treat digital audio distribution exactly like composite video distribution. That is, it can be admirably handled by a Quad Shield RG-6u, has a bandwidth somewhat less than that of composite video, operates in a 75-ohm characteristic impedance environment, and delivers about 1 volt peak-to-peak. To distribute progressive-scan extended definition video from a multi-DVD changer to systems located in various rooms of the house, you should simply add a fourth RG-6u for the S/PDIF signal!
In our next article - Digital Video Distribution…
We would like to thank Cables to Go for allowing us to reprint this informative article.