Component Video Cables - The Definitive Guide - page 3

2.4 Flawed Cable Designs or Manufacturing Techniques

There is no substitute for good engineering. But even beyond good engineering, there has to be quality manufacturing to assure that all the engineering principles were correctly implemented. There are numerous 75-ohm video cables on the market and at times, it is challenging to determine which include good engineering/manufacturing, and which are only comprised of 'good' marketing.

Pursuing the Truth: The audio and video cable market is filled with a number of scams and fallacies that create confusion among consumers (us). Some ideas to consider when shopping for a 75-ohm cable are as follows:

Both cables were hand soldered during manufacturing, yet it is clear that the cable on the left did not alter the diameter of the white colored dielectric while the cable on the right did. Also, look at the quality of the solder joint in the left side cable. There are no solder bulges, dielectric melting, or exposed wires. The result is the creation of as near perfect a 75-ohm termination as possible. The cable on the right is another story. It seems that little care was taken to protect the dielectric as seen by the noticeable melting near the end of the green jacket layer. Also, there are exposed conductor wires, no termination protection and the solder joint displays extremely poor quality. With inconsistencies and the multiple diameter changes of the conductor/dielectric on the right, it is obvious this manufacturer created a poor termination and this cable will likely have significant signal loss at certain video frequencies.

Pursuing the Truth: Don't be afraid to remove the RCA barrel and inspect these solder joints for yourself. The connector barrel usually unscrews and can be slid down the cable without damage. Some manufacturers hard mount the RCA barrels by placing shrink sleeve over them or implementing other methods so they cannot be removed. These manufacturers may be so bold as to express elaborate reasons for doing this, but the bottom line is that if you cannot inspect their solder joints, you should use caution when selecting their cables.

3.0 PRACTICAL APPLIED CABLE LOGIC

A basic component video cable is an assembly that consists of a coaxial cable and RCA connectors that have been soldered together at the termination. The coaxial cable is also an assembly of its own. At a minimum, this assembly includes a conductor, ground, dielectric and jacket. Coaxial cables with improved quality also contain a foil shield, friction reducer (not shown in diagram and not used in all coaxial cable) and at least one braided shield (to serve as a ground and shield from EMI. Each part of these subassemblies, play an important role in the quality of the component video cable and is governed by basic engineering principles, as discussed below.

Pursuing the Truth: Most, if not all, component video cable Manufacturers, purchase, not make, their coaxial cable. The component video cable Manufacturers specify the type/quality of coaxial cable they purchase (and there are many) match it up with a specified RCA connector (another purchased item) and solder them together. After the assembly is made, they neatly package the end product and market it to the Home Theater World.

3.1 Conductor and Ground

The inner core of coaxial cable is called the conductor. For component video cables, the conductor is usually made with stranded wire for improved flexibility and increased surface area, but solid conductor works just as well although it is a bit stiffer. The conductor wire is comprised of either Silver Plated, Oxygen Free High-Conductivity (OFHC) Copper, pure OFHC Copper, or pure Silver, depending on what the component video cable Manufacturer specified and purchased. The use of different materials for the primary conductor becomes apparent with video cables above 30 meters, as signal losses increases due to conductor resistance. However, the first order effects that attribute to signal loss of runs this long at the bandwidth of these video frequencies is transmission line effects resulting in impedance mismatching if the characteristic impedance of the cable is not 75 ohms.

It is important to clarify the reason why OFHC copper is used in place of pure, unalloyed copper. Unalloyed copper is an important metal for cables because of its high electrical conductivity. Electrolytic tough-pitch (ETP) copper is an inexpensive industrial copper used for the producing wire, rods and strips. ETP copper has a nominal oxygen content of about 0.04%. Oxygen is almost insoluble in ETP copper and forms interdendritic Cu20 when the copper is cast. For most applications the oxygen in ETP copper is an insignificant impurity. However, when ETP copper is heated to a temperature above around 400C / 752F (such as when using a high temperature solder) in an atmosphere containing hydrogen (found in air), the hydrogen can diffuse into the copper and react with the internally dispersed Cu20 to form steam according to the chemical reaction:

Cu20 + H2 (dissolved in Cu) --- 2Cu + H20 (steam)

The large water molecules formed by the reaction do not diffuse readily and therefore form internal holes, particularly at the grain boundaries, which makes the copper brittle. Brittle copper at solder joints (the area that has been heated) will decrease the life of the cable since this area is usually under high stress from supporting the weight of the cable at the connector point. To avoid hydrogen embrittlement caused by Cu20, the oxygen can be reacted with phosphorus to form phosphorus pentoxide (P205), but this is not practical when hand soldering RCA connectors onto a coaxial cable. Another way to avoid hydrogen embrittlement is to eliminate the oxygen from the copper by casting the ETP copper under a controlled non-oxygen reducing atmosphere. The copper produced by this method is called oxygen-free high-conductivity (OFHC) copper and is alloy C10200. OFHC can be heated and soldered in an open environment without becoming brittle at the heat effect zone (solder joint).

Pursuing the Truth: The fact of the matter is that OFHC copper and pure unalloyed copper, both oxidize at around the same rates. Some cable manufacturers use the OFHC as an advertisement that it will not oxidize, or it will oxidize less than other copper conductor materials, but there is no truth to these claims.

There are valid methods that help minimize, but never totally prevent oxidation. Silver plating is the most common, but it cannot totally prevent oxidation. Silver does indeed oxidize, but at a much lesser rate than copper. It is cost prohibitive to use in pure form for component video cables, especially if it is over 2-meters. By adding a silver-plating over copper, it forms a layer of protection on the copper minimizing its direct exposure to air, thus resulting in a reduced rate of oxidation at an economical price.

3.1.1 Conductor

There are many types of conductors used in coaxial cables including various forms of stranded wire and solid wire. Component video cables tend to be made from stranded conductors for additional flexibility, but both solid and stranded wires produce quality conductors. There are many methods and procedures to manufacture a stranded conductor.

Pursuing the Truth: Be forewarned that some cable manufacturers use nonstandard conductors along with a caveat that it prevents 'Strand Jumping.' Through clever marketing schemes and tactics, this made up term is promoted as if stranded wire has a problem that creates a loss of signal, especially in audio frequencies, due to electrons jumping between strands. By using this made up term, they attempt to justify the use of 'proprietary' cables for their audio and component video cable assemblies to justify a higher price. Be fully assured that 'Strand Jumping' is not a real engineering principle used to define any level of performance, induced distortion, or loss for coaxial, or any types of cables. In fact, this made up term is not found in any engineering or physics textbooks related to AC signals, cables and so forth. You will also not find any technical papers in AES, IEEE or any other Engineering Organization that speaks any relevance about this fallacy as the perpetuators of this fallacy fear peer review from real engineers and scientists. When you see this term, ask the manufacturer or AV Forum host to quantify the loss with a measurement or calculation and see what creative (mostly philosophical), but nonrealistic and fictitious answer they provide. The bottom line is at the frequencies we are dealing with, there are no multipathic effects within the cable. Even if there were, the results cannot cause diode rectification as some vendors claim.

The truth of the matter is that current flows through the path of least resistance. Resistance through the strand of reasonably short cables is significantly less than resistance between air gaps or metal oxide layers found between two strands therefore, the signal will not jump. Secondly, each strand within the bundle is at relatively the same electrical potential and thus the signal has no reason to jump. If there is any logic to strand jumping (which there isn't) it is like saying that if you change lanes on I4, you will not end up in Tampa.

There are really no significant advantages or disadvantages between the use of solid and stranded wires for a cable conductor outside of flexibility and a slight increase in surface area. For the purpose of understanding stranded wires, it is helpful to review the assortment of types they come in. Some of the more common methods for stranding the conductor include Bunch, True Concentric, Equilaly, Unidirectional Concentric, Unilay and Rope.

Bunch - Strands of any number are twisted together or "bunched" with the same lay direction but without a defined geometric configuration. The "bunched" construction will have a variable cross-section and does not allow a well-extruded product to be produced because of its fluctuating construction and diameter.

Smooth bunch constructions, in recent years, have solved the major problems by arranging the strands in geometric configurations and by providing a consistent diameter.

True bunching is the lowest cost method of stranding with smooth bunch being slightly more expensive.

True Concentric - A central strand surrounded by well-defined layers of helically laid strands. Each layer has reversed lay direction and an increasing lay length in each succeeding layer.

Equilay - A central strand surrounded by well-defined layers of helically laid strands. Each layer has reversed lay direction but the lay length is the same in each layer.

Unidirectional Concentric - A central strand surrounded by one or more layers of helically laid strands with the same direction of lay and increasing lay length in each succeeding layer.

Unilay - A central strand surrounded by one or more layers of helically laid strands with the same lay direction and the same lay length in each succeeding layer.

Rope - Cable stranded groups of any of the before mentioned into a very large cable. Rope construction is used to create a flexible conductor, typically 8 AWG and heavier, but in some cases is used to create very flexible fine wire constructions.

As indicated in Section 1.2, a 75-ohm cable is dependent on the ratio of the diameter of the wire bundle (for stranded conductors) and the diameter of the dielectric (d/D). Therefore, there are many possible diameter combinations that will produce a 75-ohm cable. Wire bundle diameters are measured as American Wire Gauge (AWG). Although the diameter ratio needs to remain constant for a 75-ohm cable, as shown in Section 1.2, the conductor bundle diameter can be changed. The importance of the conductors bundle diameter is related to the resistance of the cable. A smaller diameter conductor such as the 18-AWG wires (0.04ä diameter) found in an RG6 cable, will have a higher resistance than the 16-AWG wires (0.06ä diameter) found in an RG11 cable. Resistance values of cables become significant in lengths well over 2-meters. If you are installing a custom Home Theater System and must run lengths of coaxial cable that greatly exceed 2-meters, component video cables made from higher gauge wire are typically recommended as they will have a lower resistance and therefore, will minimize video signal loss.

3.1.2 Braided Shield

The importance of the shield is its ability to complete the circuit and protect the conductor by minimizing signal leakage from the conductor and minimizing EMI from entering the conductor.

Aside from serving as a signal return path, the shield when properly braided around the conductor, also helps prevent signal leakage (resulting in signal loss), and EMI noise from entering into the conductor. Braided shields are usually made from thin OHFC copper wires that are tightly braided and rated by the percentage of that braid. In theory, the higher the percentage of braid, the less leak paths exist. A quality coaxial cable will use no less than a single 95% braided shield. But even with 95% braiding, there can be leakage of the video signal as it is conducted through extremely long cables (well over two meters). For custom home theater installs that require long cables, cables with two 95% braids in an overlapping design are recommended. This double braid increases the random closing of gaps and can reduce signal leakage in the longer cables.

Single braided and double braided shields also help reduce EMI noise from entering into the conductor path. Depending on the percentage of coverage and the length of the cable, braided shields can drastically reduce a broad range of EMI, but are not very effective for EMI in the RF range above 50-MHz. This is why some component video cable manufacturers, choose a cable design that has additional forms of shielding, which will be discussed in Section 3.6.

3.2 Friction Reducer

As discussed above, a coaxial cable is actually made up of an assembly that includes the center conductor, the dielectric, the grounding shield, protective shielding, and friction reducers. Friction reducers include materials such as thin paper or Mylar. They are found sandwiched within some coaxial cable to help reduce friction that occurs in bending, especially during installation. By reducing friction between these layers, it minimizes the potential of damage to the internal conductor and/or ground braid layers.