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MillimeterThe Surface
Aug 1, 1999 12:00 PM, June Lee and Jim Scrivner
Most homeowners live with increasingly obsolete traditional wiring - the electrical wiring and telephone wiring standard in houses. Older houses use CAT 3 telephone wiring, but most new houses have CAT 5 telephone wiring, which is capable of faster data transmission rates. When homeowners need an additional line for a fax machine, the phone company gets called in. When homeowners need to create a small LAN (local area network) so two computers can share files easily, they make a run to the local computer store for a networking kit, which includes network cards, a hub and cables. Add the cards to the computers; hook the two computers together through the hub and cables, and the network is complete.
What if the computers are in separate rooms, half a house apart, and they need to share a cable modem located in a third room? When a security system, a home theater or computers in multiple rooms are added, the fun truly starts. The cables seem to sprout at random, entangling themselves with the furniture and equipment. The homeowners eventually realize that there must be a better way to do all this wiring, and it is at this point where residential structured wiring usually makes its entrance.
Over the past several years, cable technology and standards have made huge strides in the business sector, and homeowners are beginning to demand similar technological advances at home to support their electronic needs. When homeowners want modern home theaters, telecommunications, computing networks, security and home automation, they look to contractors for residential structured wiring solutions, either for homes being built or renovations.
Structured telephone wiring has been the norm; phones are connected to wall outlets via telephone wires - simple and clean connections, but times have changed. With the home becoming increasingly dependent on other communication modes - data transmissions via the Internet or audio signals between a DVD player and a whole-house stereo audio system - homeowners want a wired house, well-planned and ready for any and all devices, and they want discreet cabling within the walls - no more bunches of cables out in the open.
Cable
Before diving into residential structured wiring systems, it is important to understand the cables that make up these wiring systems. Cable is the weakest link in any chain of electronic devices. You may not notice the difference a poor quality cable makes to a standard video signal, but you will notice it when watching an HDTV signal. An entire system can be adversely affected by poorly performing cable. To minimize signal loss for the best possible price, it is important to understand the cables available and their associated specifications.
So many different types and models of cables exist that one could easily become intimidated by simply paging through a cable catalog, but cables are really not that different. They are essentially various combinations of basic configurations. These cable types have great differences in size, capability, quality and performance within their own groups, but most cables fall into four main groups - individual conductor, twisted pair, coaxial and fiber optic. (Please see "Fiber Optics in the Home," page 32, for more information on fiber-optic cable.)
Referred to as wire, individual conductor is the most basic of cables (see Figure 1). It consists of a center conductor wrapped in a plastic orrubberized outer jacket. This cable type is generally used to distribute such lo w-frequency signals as digital communication signals and computer ID bit information. Larger variations of this cable type are used to distribute power.
Twisted-pair cable is available in two different styles. Shielded and un-shielded twisted-pair cables are found in environments ranging from telephone to computer networking to professional audio (see Figure 2 and Figure 3). Twisted-pair cable consists of two individual conductors, generally running signal and return (ground), that have been twisted together to form one pair of cables. Usually found with 100 V impedance values and in AWG#22 to AWG#24, twisted-pair cable is inexpensive and easy to handle. Because both conductors have equal exposure to extraneous noise, twisted pair cabling, by design, provides inexpensive protection from outside interference, RFI and EMI.
Most commonly seen in telecommunications and computer networking environments, unshielded twisted pair is categorized to be compatible with the many different data transfer rates associated with these types of equipment. Shielded twisted-pair cable consists of a regular twisted pair that is surrounded by a wire braid shield. This shield provides additional protection against EMI, making it useful for signal distribution of sensitive audio and computer sync information.
More complex, coaxial cable consists of two conductors (see Figure 4). The center conductor carries the signal while the outer conductor (shield) provides a return path for the current to ground and protects against outside interference. The two conductors are separated by a dielectric material (insulation) that establishes electrical characteristics and provides physical protection to the center conductor. All of these components are jacketed to make one cable.
In the A-V industry, these coaxial cables generally have 75 V impedance and are used in video applications and for applications involving test equipment, RF distribution and audio equipment. Because of the complex design, coaxial cable is more expensive, but it provides enhanced reliability. The frequency and resolution of a signal and the distance of the cable are important determining factors in deciding which grade of coaxial cable should be used because some provide lower signal loss characteristics than others. The crimp style of cable termination (connectorization) used with these cables makes them easy to terminate in field applications, and they offer consistent, reliable connections.
Cable anatomy
Many people view cable simply as what is used to connect two pieces of equipment. Not much attention is given to what is inside, even though the components and the materials used are absolutely critical in establishing a given cable's performance in an application. These components determine the electrical and physical properties of the cable. For this section we will use the example of a coaxial cable because it contains all of the following components - conductor, insulation, shields and jacket. In one form or another, most cables use these various components in their construction.
The center conductor carries the signal from one point to another. Center conductors are made of highly conductive materials that are capable of carrying electrical current. The most commonly used material is copper because of its overall conductive properties, availability, cost and ease of use. Other materials - aluminum, silver, gold - are available and used in specific applications. Conductors have either a solid or stranded center.
A solid-center conductor is made of a single wire. The diameter or AWG size may vary, but solid-conductor cables are generally easier to manufacture and are therefore less expensive. These conductors are formable but not all that flexible. A stranded center conductor consists of multiple small gauge wires that are twisted together to form a larger single conductor. These cables are flexible and easily handled. Generally speaking, however, performance will be slightly less in comparison to a solid conductor of the same size and material.
Cable insulation, also called the dielectric, serves an important dual role. It is the material that separates the center conductor from the outer shield. This serves to protect the center conductor, but more importantly, it establishes electrical characteristics - impedance and capacitance - that greatly affect cable performance. Some of the more common insulation materials include polyethylene for general purpose cable and fluoropolymer for plenum-rated cables.
Shields also serve an important dual role. A shield works as a second conductor, which acts as a return path for the signal current to ground and also protects the signal from outside interference. Because there are different types of interference that cables may encounter (see Figure 5 and Figure 6), there are different shielding methods - foil, braid, and combination foil/braid.
Foil shields protect signals from picking up high-frequency interference (RFI or radio frequency interference) from telephone, microwave, television station and other transmissions. The foil shield is a foil-sided tape that is wrapped completely around the cable's insulation. A thin drain wire usually accompanies a foil shield, enabling connection to the shell of its respective connector. To form a braid shield that encompasses the cable's insulation, many fine strands of conductive material, usually tinned copper, are woven together. This method keeps out unwanted EMI (electromagnetic interference), a low-frequency, induced magnetic field caused by such high-current machinery as air-conditioning units or electric motors.
The outer jacket physically protects all the inner components from weather, chemicals, liquids, sunlight and other factors. Because of the diverse locations through which cables run, there are classifications to which a cable must conform in order to be installed in specific environments. These standards are governed by the NEC (National Electric Code) and have certain UL (Underwriters Laboratories) certifications.
Jacket material primarily determines a cable's overall flexibility. For example, a plenum-rated cable will be stiffer and therefore more difficult to maneuver and terminate. Depending on the system's certification needs, you would normally want to use the most flexible cable available.
Performance and specifications
Cable performance in a particular system is largely determined by the cable's physical construction but is also influenced by length, frequency, interference, crosstalk, temperature and cable specifications.
As cable length increases, performance decreases. As the signal travels through the cable, it is affected by the cable's materials. The more material through which the signal travels, the weaker it becomes. Cable performance is also affected by signal frequency. As the signal frequency increases, the detrimental effects that capacitance and conductor materials have on the signal increase.
Outside interference can distort the signal if the cable is not properly shielded. Interference can be caused by many factors, especially RFI and EMI. Crosstalk is a type of interference, but it is not induced from the outside. It is created internally by signals being run together on adjacent, improperly shielded wires. When crosstalk occurs, data transmitted through one line bleeds over and adds itself to the signal of the adjacent cable.
Because of heat-related conditions, all of the materials used in a cable are susceptible to performance variance. This can result in difficult troubleshooting sessions. Cables are typically installed in such areas as walls, ceilings and equipment racks. Because these areas are not always properly ventilated, they tend to be exposed to higher heat conditions. Therefore, it is important to select cables that will perform as consistently as possible in these environments.
Cable specifications are the most important information that manufacturers supply, so you should pay careful attention to these numbers before you choose which cable to purchase. Many different specifications exist, the most critical of which are impedance, resistance, capacitance, and attenuation.
Impedance establishes the baseline for the flow of the signal. This flow must be maintained throughout the entire system for proper transfer of power. If there are impedance mismatches, the possibility of reflections occurring is high. The signal that is reintroduced to the original signal is called a reflection. Reflections are caused by impedance mismatch and improperly terminated connectors.
Resistance is defined as "the property of a substance that impedes current and results in the dissipation of power in the form of heat." It is the characteristic that blocks the flow of electrons, hence affecting the voltage (amplitude) of a signal. In cable, resistance is expressed as ohms/1,000 ft (305 m). The resistance of a material is determined by material type, dimensions and temperature.
Capacitance is defined as "the property of an electric system that determines how much electrical charge will be stored in the dielectric for a given potential difference between the conductors." In other words, capacitance is the ability of a component to charge, hold a charge and discharge. The unit of measurement for capacitance is farad (F). In cable, capacitance is expressed as pF/ft (picofarads per foot). A coaxial cable is really nothing more than a large capacitor.
Capacitance affects the rise and fall times of a signal. When a cable charges, the electron flow first starts as a trickle and then gradually builds to the full pressure. When a cable discharges, the electron flow slows and then stops. For example, the supposedly black-to-white transition of a video signal may actually be displayed as a black-to-gray-to-white transition instead. Attenuation is the most important performance specification, the cable equivalent to bandwidth. Measured in dB (decibels), attenuation, or insertion loss, is a measurement of the amount of loss that will occur from one point to another. For cable, it is referenced at a fixed length, normally 100 ft (30.5 m), and is rated at different frequencies. For example, -2.2dB/100 ft @ 100 MHz would be a typical way of indicating the specification. At 200 MHz, the same 100 ft cable would have a different attenuation value. With this specification, a system designer can get an excellent idea of how much loss will be incurred in a system because the designer will know the signal rates and cable lengths used. Remember that attenuation value changes as cable length changes.
Cable connectors
We begin to appreciate what important roles these cable components play in maintaining the signal integrity of a system. If any component is damaged or not connected properly, many problems can occur, and system performance could be significantly diminished. The type of connectors used will make a difference with such high-quality signals as HDTV signals. Compared to RCA connectors, BNC connectors are sturdier and enable a more secure connection. Additionally, most BNC connectors have 75 V impedance, which is appropriate for video use. Impedance mismatches in video systems can result in signal reflections, which, in turn, can cause signal attenuation and for video signals, loss of picture detail and ghost images.
Signal enhancement
Various pieces of equipment offer features that enhance signals and/or enable the use of longer cable runs. We will describe two features and one device that specifically enhance video signals, but they do have counterparts in such other fields as audio.
Level control increases the voltage of an entire signal so it can travel longer distances and withstand resistance in the system. Compensating for loss due to resistance, level is analogous to a brightness control - brightness increases as a function of voltage.
Peaking control sharpens the edges of a picture's vertical lines. Compensating for signal loss due to capacitance, peaking increases a signal's voltage only at the high frequency range.
A ground loop isolator eliminates hum bars and sync wiggles caused by improper equipment grounding. A hum bar appears as a horizontal bar or line moving vertically through the picture. Sync wiggles are represented as continuous movement or picture squiggling. Both are also referred to as ground loops. A ground loop in the power or video signal occurs when the various system components are receiving power from different grounds, or the ground potential between two pieces of equipment is not identical. The different AC potentials between the two grounds cause the loops to appear. One way to correct this problem is to have all the system components use power from the same ground. In most cases, this is not an option. The other solution is to use a ground loop isolator.
Emerging technologies
Various methods of communications are entering the home. You will encounter more and more home-owners asking you to add HDTV, Ethernet or other technologies to their homes. Below are several technologies with which you should acquaint (or reacquaint) yourself. Research their pros and cons thoroughly before you decide which residential structured wiring solutions to implement.
- HDTV: Component video is a higher-quality video format than composite video. For standard TV signals, all the picture information is crammed into one composite video signal and carried on one wire. For HDTV, in order to maintain picture quality, you need to run component video lines for the three HDTV signals (Y, R-Y, B-Y), which are carried on three wires.
- Serial data communications: Control systems are convenient, user-friendly remote control products that integrate the control functions of different types of environmental, video, audio, lighting and/or security devices. Many third-party control systems, such as AMX or Crestron, use RS-232, a serial interfacing standard. Present in various presentation and communication systems, RS-232 is used by computers and other devices for control communications.
- Ethernet: Ethernet is the most popular LAN type. The most common Ether-net networks are used by computers to share data and peripheral devices. Most Ethernet networks use ordinary telephone wire (twisted pair), but some do use coaxial cable.
- CAT 5 cable: Most PC networks use CAT 5 cabling, which uses four twisted pairs of wires (see Figure 7). Each wire consists of a single strand (solid) or multiple strands (flex). CAT 5's maximum transmission rate is 100 Mbps (megabits per second). No need for multiplexing - one CAT 5 cable can carry up to four different signals (audio, video, power and control). CAT 5 does not carry high-resolution video signals well, but there are alternative cables available that bundle together twisted pair, coaxial, and single wire conductors.
- DSL: Digital Subscriber Line (DSL) is a modem-based technology that enables concurrent high-speed Internet access and voice calls. The DSL modem and splitter (if required) are connected to the existing phone line and keep the voice signals separate from high-speed data.
- Cable modem: A cable modem enables a PC to be connected to a local cable TV coax line for a faster data transmission rate. The cable line is divided up between the (1) television and (2) the cable modem and PC.
- ISDN: This digital telephone service is called Integrated Services Digital Network (ISDN) and works over existing telephone wiring for a faster data transmission rate. The most common ISDN service uses an adapter to digitize the telephone line between a home and the local telephone network.
- Structured wiring networks: Some companies now offer structured wiring network packages to residential contractors. All the necessary wiring (coax, twisted pair or fiber optic) as well as distribution hubs or panels are included in each package.
Factors of purchase
Before making a decision to purchase cable, address these three main factors - applicability, performance and price.
What is the application? To choose a cable, you must first know how it will be used. Consider the source devices and destination devices of the signals the cable will carry. Are the cables you are considering appropriate for this application? After you have narrowed down the field of cables, you must then decide which cable is easier to work with. Is it flexible and installer friendly? Ease of use translates into reduced installation time - time saved for yourself and your client. Also ask yourself if there are any specific fire code requirements. There are plenum-rated cables, CL2-rated cables and even unrated cables. Make sure your cables comply with all the relevant codes.
What types of sources will be distributed? There are different signal types of varying frequencies - computer, audio, video, control and voice. Each has individual needs that may require specific cable considerations for optimal signal performance. What are the distances of the cable runs? If cable runs are long, consider a low-loss cable. Also consider the possibility of a future upgrade. In case of future changes, it is a good idea to have a reliable cable infrastructure in place - no need for a costly, inconvenient retrofit later on.
Cable costs can have a significant impact on the overall price of a system. Cabling is often one of the last items addressed in the design stage. Resorting to low-quality cables in order to meet budgetary constraints will never improve the signal quality.
Cables are available in many sizes and shapes. Manufacturers of high-quality cables - Belden, Extron, Mogami or Monster - specialize in various markets. A word of advice, when shopping around for cables, look beyond all of the adjectives in the marketing materials to the numbers that really matter - the specifications. Only through examining a given cable's technical specifications can you make an informed decision about which one to select for a given application.
