Component Video Cables - The Definitive Guide - page 4

3.3 Dielectric Insulator

The purpose of the dielectric is to separate and insulate the center conductor from the ground shielding. As indicated in section 1.2, there is a specific relation between the diameter of the conductor (d) and the diameter of the dielectric (D) that must be maintained to ensure a true 75-ohm cable. The equation is also dependent on the dielectric characteristics of the material. It is helpful to know about some of the standard materials used in the industry so you can make logical decisions when considering a component video cable.

A number of different "standard" materials are used for creating a dielectric barrier. They include PVC, Polyethylene, foamed (gas injected) Polyethylene, Polypropylene, Nylon and Teflon. Each of these materials have their own unique dielectric rating directly related to the materials ability to store energy. A perfect dielectric will store zero energy and therefore, not take away any signal from the conductor. The closest to a perfect dielectric is air with a dielectric value of one, but it is not practical to use air as a dielectric as the entire assembly would have to be hermetically sealed and the cable would be more like a balloon. The practical solution is to use industry standard materials such as those listed above. From the possibilities, the order starting with the best dielectric (keep in mind that air is the best with a dielectric constant of 1) is as follows: Teflon and/or foamed Polyethylene (nitrogen, air·), Polyethylene, Polypropylene, Nylon and PVC.

Pursuing the Truth: It is important to note that the significance of injecting gas into Polyethylene during formation of the dielectric is to create air pockets within the Îfoamedâ Polyethylene to decrease the dielectric constant. As indicated above, air is the best possible dielectric so having air pockets in the material somewhat improves the dielectric characteristics of the material. During manufacturing, nitrogen or other inert gases are injected into the liquid material in order to create bubbles. When the material cools and hardens the pockets and gaps from the bubbles remain resulting in the formation of a foamed material, but the nitrogen itself does not remain within the cable. The dielectric constant for foamed Polyethylene (1.5 to 2.1) is a variable dependant on the manufacturing process. For this reason, it may or may not be as good a dielectric as Teflon. Teflon is a more consistent and uniform material and has a predictable and repeatable dielectric constant.

With the exception of Nylon and PVC, the materials listed above have closely matched dielectric constants and the differences in their dielectric characteristics only become significant in cable lengths far exceeding typical 2 meter installations. As seen by their constants, neither Nylon (4.0 to 4.6) or PVC (3.0 to 8.0) make effective dielectrics for any cable length, yet they are still used by some component video cable manufacturers due to their low cost.

3.4 Termination

The termination of the cable is the region where the coaxial cable is attached to the RCA connection. Most cables use solder, and in almost all soldered applications, they are done by hand. As discussed in section 2.4, soldering techniques can vary, and poor soldering will result in a poor quality cable that does not meet the requirements for 75-ohm.

Another important aspect of termination involves minimizing damage of the cable that can result from crimping or kinking. The solder joint is an area of high stress since it must support the weight of cable at the connection point. When a cable is hung from the back of a TV or DVD player, the weight of that cable will cause it to bend within termination region. Over time, this bend will fatigue the solder joint resulting in the potential for wire strand breaks, solder breaks, kinking or crimping, thus decreasing the life of the cable. In order to minimize this effect, some cable manufacturers use a strain relief at the termination as seen in the picture below.

In this assembly, notice how the solder joint is completely surrounded by a metal ring at the end of the yellow jacket, with full support to the RCA connection and protection of the solder joint. There are two reasons for this design. This ring helps prevent fatigue on the solder joint and also increases the solder joint area for the grounding shield. Under this ring, the braided shield has been combed straight. After combing, the wire is carefully soldered to the inside of the ring in a full 360 solder. This is an effective design and well manufactured cable termination as it increases the contact area therefore decreasing the resistance in the ground. This method also preserves the diameter ratio of the dielectric and conductor at the end of the cable where it has been cut and soldered onto the connector housing, thus preserving the 75-ohm impedance of the cable/connector combination from tip-to-tip.

3.5 RCA Connections

As explained in section 1.2, any cables ability to transmit a 75-ohm signal is dependent on its center conductor diameter and its dielectric diameter. An RCA connection is no exception as it too, falls under the laws of physics governed by this basic equation. Interestingly enough, RCA connectors are not a native video connection.

Pursuing the Truth: Prior to the onset of AV Receivers, DVD players and HDTV monitors, the world of television and movie cameras created the standards for video interconnects. In the television/movie world, most video equipment is connected with the use of BNC (locking bayonet-type) connectors. When AV Home Theater Equipment became more readily available, products were developed to transmit and pass through high frequency video signals. Given the size and cost of BNC connectors, it was not practical to include this type of connector to cost sensitive and tightly packed back panels of AV receivers, DVD players, TV's and other Home Theater equipment. In addition, the average user is not familiar with a BNC connection. For these reasons, the Home Theater market adapted RCA connectors for all video interconnects. Interestingly enough, RCA connectors were developed for audio, and based on their diameters, the internal impedances are between 35-ohm and 50-ohm. They were never intended to have an internal impedance of 75-ohm.

Now, this doesn't mean all DVD players and TV monitors have improper 75-ohm terminations. It is true that some do have this problem, and for the average consumer, it is difficult, if not impossible, to tell the difference as it is not something that is readily advertised or discussed. This information is provided as a precaution to those in the market for new products.

In the case of Home Theater equipment, it is possible for engineers to compensate for impedance by using a resistor, usually on the order of 25-ohms within the DVD player or TV. By doing so, the resistor allows the connector to act like a 75-ohm connection by absorbing reflections from the impedance mismatch. The absorption of this reflected energy results in heat and a partial signal loss, but it is typically compensated for by increasing the output signal of the DVD player.

Pursuing the Truth: It is not practical or necessary for cable manufacturers to include resistors within the cables due to manufacturing issues and weight. That being the case, there are no known component video cables who have a true 75-ohm RCA connectors, regardless of the handful of suppliers that make this claim. Some companies use words like "75-ohm type" RCA to make it sound as if it is, but in reality, it is only about 55-ohms or slightly higher. To compare the connectors in order to determine which is closer to 75-ohms it is helpful to compare the diameters of the dielectric between two connectors. The picture below shows the RCA's of two different component video cables.

Note the differences in the RCA connectors in this picture. For starters, the gold plated inner conductors of both RCA's are the same, but the diameters of the RCA's ground shield are slightly different. It is noticeable by comparing the diameters of the white dielectric and the thickness of the shield. If you remember in Section 1.2, the inner diameter of the ground shield is approximately equal to the outer diameter of the dielectric. The diameter of the white dielectric in the RCA connector on the left is slightly larger than the diameter of the white dielectric in the RCA connector on the right. Don't be fooled by the gold covering of the dielectric on the right. The dielectric in this connector actually goes beyond that dimension. The critical outer diameter of the white dielectric is what defines the RCA connector's impedance, as defined in Section 1.2. It turns out that the RCA connector on the left has a very slightly larger dielectric diameter and is a bit more robust with the shield diameter, and is therefore a bit closer to 75-ohms, although not fully. Oddly enough, this manufacturer claims it is a true 75-ohm connection even though it can not meet the form factor. Audioholic's has contacted this supplier and made them aware of this discrepancy and we are awaiting their "measurements" to justify this claim, but have not see any to date. Regardless, the differences between the two are likely to be too insignificant to alter performance, especially with resistors in the circuit (DVD player and TV) absorbing most reflections. The fact is, both RCA connectors are adequate even though they are not true 75-ohms. There is truth to the statement that reflections due to impedance mismatch will increase substantially with much longer cables, but for the average 2-meter installation, it is not a significant issue provided it is a quality RCA and termination. The bottom line here is for long cable runs ( > 3meters), it would be wise to choose cables from a manufacturer that explicitly states the characteristic impedance of their cables is 75 ohms and not worry too much about the RCA connector impedance. Note that the electrical length of the RCA termination is significantly shorter than the actual cable itself. Thus at video bandwidth frequencies it's more important to maintain a 75ohm cable characteristic impedance then losing sleep over the RCA termination impedance.