Coaxial Cable
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ANALOG ELECTRICAL and DIGITAL VIDEO FORMATS and CONNECTORS Analog Electrical Formats/Connectors Component Video Component video is a type of video information that is transmitted or stored as two or more separate signals (as opposed to composite video, such as NTSC or PAL, which is a single signal). Most component video systems are variations of the red, green and blue signals that make up a television image. The simplest type, RGB, consists of the three discrete red, green and blue signals sent down three wires. This type is commonly used in Europe through SCART connectors. Outside Europe, it is generally used for computer monitors, but rarely for TV-type applications. Another type consists of R-Y, B-Y and Y, delivered the same way. This is the signal type that is usually meant when people talk of component video today. Y is the luminance channel, B-Y (also called U or Cb) is the blue component minus the luminance information, and R-Y (also called V or Cr) is the red component minus the luminance information. Variants of this format include YUV, YCbCr, YPbPr and YIQ. In component systems, the synchronization pulses can either be transmitted in one or usually two separate wires, or embedded in the blanking period of one or all of the components. In computing, the common standard is for two extra wires to carry the horizontal and vertical components, whereas in video applications it is more usual to embed the sync signal in the green or Y component. The former is known as sync-on-green. Component digital video signals are sometimes referred to as 4:2:2, meaning that for every 4 bits that are dedicated to the Y component, 2 bits each are dedicated to the U & V components on both even (second 2) and odd lines (third 2) of the image. The luminance or Y channel carries most of the image detail and is, therefore, assigned more bits. Another common method, 4:2:0, is used on DVDs. In this case, only the even lines have color information; for the odd lines it is approximated by interpolation. This signal is often converted to 4:2:2 inside the player before it is sent out to other devices. Composite Video Composite video is the format of an analog television signal before it is modulated onto an RF carrier. It is usually in a standard format such as NTSC, PAL or SECAM. It is a composite of three source signals called Y, U and V (together referred to as YUV) with sync pulses. Y represents the brightness or luminance of the picture and includes synchronizing pulses, so that by itself it could be displayed as a monochrome picture. U and V between them carry the colour information. They are first mixed with two orthogonal phases of a colour carrier signal to form a signal called the chrominance. Y and UV are then added together. Since Y is a baseband signal and UV has been mixed with a carrier, this addition is equivalent to frequency-division multiplexing. Composite video can easily be directed to any broadcast channel simply by modulating the proper RF carrier frequency with it. Most home video equipment records a signal in composite format: VCRs and laserdiscs both work this way, and then give the user the option of outputting the raw signal, or mixing it with RF to appear on a selected TV channel. In the United States, the composite video signal is typically connected using an RCA jack, normally yellow (often accompanied with red and white for right and left audio channels, respectively). In Europe, this is sometimes replaced by a coax or SCART connector. BNC connectors are used for commercial variations of video media. Some devices that connect to a TV, such as videogame consoles (and the ubiquitous home computers of the 1980s), naturally output a composite signal. This may then be converted to RF with an external box known as an RF modulator that generates the proper carrier (often for channel 3 or 4 in North America). The RF modulator is preferably left outside the console so the RF doesn't interfere with the components inside the machine. VCRs and similar devices already have to deal with RF signals in their tuners, so the modulator is located inside the box. Also, most home computers usually employed an internal RF modulator. Yellow is the default color for composite video cables. The process of modulating RF with the original video signal, and then demodulating the original signal again in the TV, introduces several losses into the signal. RF is also "noisy" because of all of the video and radio signals already being broadcast, so this conversion also typically adds noise or interference to the signal as well. For these reasons, it's typically best to use composite connections over RF connections if possible. Almost all modern video equipment has composite connectors, so this typically isn't a problem. However, just as the modulation and demodulation of RF loses quality, the mixing of the various signals into the original composite signal does the same. This has led to a proliferation of systems such as S-Video and component video to separate out one or more of the mixed signals. Composite video is often designated by the CVBS acronym, meaning either "Color, Video, Blank and Sync" or "Composite Video Baseband Signal" or "Composite Video Burst Signal" or "Composite Video with Burst and Sync". Coaxial cable Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer. It is used as a high-frequency transmission line to carry a high- frequency or broadband signal. Sometimes DC power (called bias) is added to the signal to supply the equipment at the other end, as in direct broadcast satellite receivers. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields. Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, both usually of thin copper wire. The inner insulator, also called the dielectric, has a significant effect on the cable's properties, such as its characteristic impedance and its attenuation. The dielectric may be solid or perforated with air spaces. Connections to the ends of coaxial cables are usually made with RF connectors (usually F Connectors), though other connectors are possible (BNC, etc). Signal propagation Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, twisted or otherwise shaped without changing their characteristic impedance. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line. Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The transmission of energy in the line occurs totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore be bent and twisted (subject to limits) without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them. The inner conductor can be made of braid and the outer conductor can be made of corrugated tube for greater flexibility, but this comes at the cost of increased ohmic losses and lower phase velocity. The outer conductor can also be made of (in order of increasing leakage) wound foil, woven tape, or braid. Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. Unfortunately, all dielectrics have loss associated with them, which causes most coaxial lines to have more loss than open wire lines. Most cables have a solid dielectric; others have a foam dielectric which contains as much air as possible to reduce the losses. Foam coax will have about 15% less attenuation but can absorb moisture — especially at its many surfaces — in humid environments, increasing the loss. Stars or spokes are even better, but more expensive. Furthermore the lower dielectric constant of air allows for a greater inner diameter at the same impedance and a greater outer diameter at the same cutoff frequency, lowering ohmic losses. Connectors From the signal point of view, a connector can be viewed as a short, rigid cable. The connector usually has the same impedance as the related cable and probably has a similar cutoff frequency although its dielectric may be different. High-quality connectors are usually gold or rhodium plated, with lower-quality connectors using nickel or tin plating. Silver is occasionally used in some high- end connectors due to its excellent conductivity, but it usually requires extra plating of another metal since silver readily oxidizes in the presence of air. One increasing development has been the wider adoption of micro-miniature coaxial cable in the consumer electronics sector in recent years. Wire and cable companies such as Tyco, Sumitomo Electric, Hitachi Cable, Fujikura and LS Cable all manufacture these cables, which can be used in cellular phones. Important parameters to Co-Ax • The characteristic impedance in ohms (Ω) is calculated from the ratio of the inner and outer diameters and the dielectric constant. Assuming the dielectric properties of the material inside the cable do not vary appreciably over the operating range of the cable, this impedance is frequency independent.