Component Video Cables - The Definitive Guide
Component video cables are a key interconnect element to any Home Theater System. To better understand how these 75-ohm cables can affect a video signal from a DVD player, it is helpful to cover some fundamental engineering principles that define them. To begin with, the primary purpose for 75-ohm component video cables is to conduct an AC video signal from a source (DVD player) to a load (TV monitor) with as little change to that signal as possible. There are numerous electronic textbooks and Internet sites referenced at the end of this article that discuss cables and wires in terms of equations and engineering principles. When applying textbook equations and engineering principles to component video cables it becomes clear that a well engineered and manufactured cable will help keep the source signal within the cable conductor to minimize loss, while keeping out noise from other outside sources.
1.0 Engineering 75-ohm Interconnects
Unlike audio cables, which only conduct low frequency data on the order of 20Hz to 20,000Hz, video cable must transmit higher frequencies up to levels of around 8MHz to 10MHz for NTSC and for over 35MHz for HDTV. At these frequencies, there are a number of factors that can create interference, signal loss, and signal degradation, all of which can be identified and minimized by applying fundamental electrical engineering principles. This article is intended to discuss these basic engineering principles, provide information on various manufacturing techniques of component video cables and coaxial cables, and explain the different types of component video cables on the market. There are a number of concepts and misconceptions about component video cables that will be addressed in this article, many of which will be proved or disproved mathematically. As you read this article, it is not necessary to focus on the calculations as much as the conclusions that are drawn by them.
1.1 Impedance Defined
Impedance is a measure of the ability of an AC network to impede the flow of charge or current through a network. For our application, the network is the combination of the source (DVD player), load (TV monitor) and component video cables. There are many derivative equations that apply to impedance calculations, however at the higher frequencies of video signals, typical cable laws are not entirely applicable. At frequencies relative to component video (10MHz maximum), impedance is more closely related to specific resistance for electro-magnetic waves. It can actually be defined as the load a cable poses at high frequencies.
For sinusoidal AC signals of video frequencies flowing in one end of the cable, the signals travel as an electrical wave at the same potential energy down the cable. If the cable length is an extremely large number of wavelengths at the frequencies of that AC signal, the ratio of the AC voltage to AC current in that traveling wave is defined as the characteristic impedance of the cable as defined by the following equation.
wavelength (in meters) = v / f
where f is the frequency of the signal in Hz, v is the velocity of the signal = 3x10^8 meter/second
For example, at 60-Hz the wavelength is 3100 miles; at 10-MHz it is 30 meters (max frequency for component video).
1.2 75-Ohm Impedance of Component Video Cables
Component video cables are composed of coaxial cables. As discussed in the section above, since most component video cable lengths are shorter than 30 meters, they are considered short compared to the wavelength (10-MHz max for component video) of the signal therefore, coupling between circuits can be represented by lumped capacitance and inductance between conductors. With this in mind an electrically short coaxial cable ( < 1/10 wavelength) can be analyzed by using principles outlined in basic network theory.
As indicated in Section 1.1, the characteristic impedance of the coaxial cable is primarily determined by the size and of the conductor and the type and size of dielectric.
The following formula can be used for calculating the characteristic impedance of coaxial cable: (formula taken from Reference Data for Radio Engineers book published by Howard W. Sams & Co. 1975, page 24-21).
|Characteristic Impedance (Zo ):||
d = outer diameter of inner (center) conductor (approximate value for stranded)
D = outer diameter of dielectric
ε = dielectric constant (ε=1 for air)
This equation supports the fact that the characteristic impedance of a coax cable is directly related to the diameter of the conductor and the dielectric. For component video cables, this characteristic impedance should be 75-ohms. With characteristic impedance (Zo) held at a constant 75-ohms, the variables are the diameters and dielectric constant.
Once an understanding of the engineering principles behind 75-ohm cables is established, practical issues that apply to designing, or considering the purchase of a 75-ohm video cables include conductor material and diameter, dielectric material and diameter, grounding shield, noise protection, termination (solder joints) and RCA connector design. Each of these topics will be discussed within the sub-Sections of 3.0.
Check out our Transmission Line Effects of Component Video Cables Supplemental for a more detailed look into this topic.
2.0 Signal Loss
Prior to reviewing basic 75-ohm cable design principles, it is helpful to understand what causes signal loss. Signal loss or degradation, can occur from a number of factors which include internal impedance, Electro Magnetic Interference (EMI) including EMI in the Radio Frequency range (RF noise), mismatched impedance, flawed basic cable designs or inconsistent/bad manufacturing. Terms that are often misleading and thought to cause signal loss are skin effect and strand jumping. As you will see in Section 2.3, skin effects are minimal and not significant at the frequencies related to video cables. Strand jumping is a term a few manufacturers use for audio cables due to "skin effects," but since skin effect is related to frequency, the same myth can be applied to the higher frequencies of video. Section 3.1.1 will discuss these issues in more detail.
2.1 Internal Impedance of 75-ohms
A properly designed and manufactured component video cable should have an internal impedance of 75-ohms, which is design impedance for most video components, especially in Home Theater.
However, because of form factor limitations of the RCA connector, it is nearly impossible to design a true 75-ohm connector. Since the connector contact area is very small, relative to wavelength, maintaining a true 75 ohm termination is not that critical, however it is good engineering practice for the manufacturer to attempt to come as close as possible to nullify any unwarranted losses due to impedance mismatches or inconsistant manufacturing techniques.
One method of considering impedance is being comprised of reactive and resistance values and therefore related to resistance and capacitance. In a standard audio cable for example, the internal impedances are between 35-ohms and 50-ohms. Since they are designed to carry low frequencies (20Hz to 20kHz), resulting in much longer wavelengths than the length of the cables themselves, audio cables by their design, aren't concerned with characteristic impedance as much as lumped R,L,C parameters. However, if audio cables are used in place of component video cables, or poorly constructed component video cables are used that are not a true 75-ohm characteristic impedance, the lower impedance value of these cables may result in a partial signal reflection do to a mismatch in impedance, depending on the length of the cable. Detailed information on mismatched impedance and internal impedance are provided within the sections 2.2 and 3.0.
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