A Discussion On Video Cable Construction

The question comes up repeatedly about the price of cables making a difference or not. Why spend big bucks for no apparent gain? Many people do not find any difference in the PQ of expensive cables and the lower priced brands and so forth.

So I thought a thread that focused on the engineering look at the construction of cables and the reasons behind the differences would be in order.

The bulk of the information required to get a pretty complete technical knowledge of the details is located here: Component Video Cables - The Definitive Guide and it is one of the best series of articles I have seen on the subject. However it is very technical and difficult for many laymen to comprehend.

Here are some excerpts of the paragraphs that are very technical in the article I pulled out that hits the main points of the paragraph:

Quote:
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.

2.0 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.

2.1 Internal Impedance of 75-ohms
In a standard audio cable for example, the internal impedances are between 35-ohms and 50-ohms.

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.

2.2 Electromagnetic Interference (EMI)
Electromagnetic Interference (EMI) is around us at all times. It comes from radio towers, sun spots, cell phones, modems, remote controls, computers and a number of other signals that are generated from most electronics used in every day life…
…

It is possible for stray signals from EMI to find their way into a video cable and therefore, create a false signal or internal noise
within the component video cable. The result can range from very minor to significant depending on the noise origination and strength. A quality 75-ohm shielded video cable implements several methods to minimize EMI from entering into the cable.

2.3 Mismatched Impedance
Thus for component video cable, transmission line effects of any cable lengths beyond 3 meters (remember 1/10 of 30 meters) must be considered.

Since the source is a 75-ohm impedance, this reflected signal is sent directly back to it creating a delay effect at certain frequencies. This delay, for example, can show up as a ghost in the picture. Multiple ghosts resulting from multiple frequency reflections can look like ringing around the original image. These reflections can also cause partial signal cancellations at various frequencies corresponding to a partial signal loss resulting in a loss of picture detail or color.

2.4 Skin Effect
In fact, if you continue with these calculations you will find that it doesn’t even become an issue until over 1GHz which is significantly higher then audio, component video and HDTV signals.

2.4 Flawed Cable Designs or Manufacturing Techniques
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:

  1. Good Engineering: Try to recognize a design feature that makes sense for its purpose over ones that seem too good to be true and may actually be a marketing scam. For example, there is a new component video cable hitting the market that doesn’t use conventional RCA connectors. It actually has a “Bullet Connector” derived for audio cables. Based on the RCA connectors geometry and the governing equations discussed in this article, its design cannot produce near a 75-Ohm termination and is no better than a standard audio rca termination. Since the manufacturer choose to use this type of termination, one must wonder if they choose the correct dielectric and spacing requirements to maintain a cable chacteristic impedance of 75 ohms. If not, this may cause impedance mismatching for cable lengths 3 meters or greater resulting in the reflections and signals loss addressed in Section 2.2.
  2. The package alone does not define the cable: Some cable manufacturers like to package their product in fancy, designer packages. Just remember that the package ends up in the trash and the signal the cable transmits is what you will be living with. The primary purpose of the package is to protect the cable from damage during shipping and receiving from the manufacturer to the end user. It should prevent kinks and bends in a cable which can result in diameter changes that affect the impedance.
  3. Bad, inconsistent manufacturing: There are a number of small cable manufacturers to be found on the Internet. Nothing is wrong with a small manufacturer as many of the larger companies once started small. However, it is important that these small manufacturers produce a consistent product. Most cable manufacturers (large and small) hand-solder the RCA connectors onto the cables. This is a valid procedure provided it is done by experienced staff and the solder joints are carefully inspected upon completion. Problems that can occur during soldering are melting of the dielectric, excessive solder, incomplete solder joints and so on. As an example of how soldering can damage a cable, it is worth considering the dielectric. If the low melting point dielectric material is overheated during the soldering process, it can easily melt resulting in a change in diameter. Based on the governing equation for internal impedance, by changing the diameter of the dielectric the impedance of the cable has effectively been changed. Some Manufacturers may have a great design, but if they are inconsistent with their soldering or manufacturing techniques, the cables will also be inconsistent. This can mean that from batch to batch, or from cable to cable, the customer is not guaranteed they are purchasing a quality cable. The following pictures show two forms of solder joints on actual 75-ohm component video cables. These cables were taken right out of their package and the RCA barrels were removed to expose the solder joints. Note the lack of quality in the one on the right defined by the inconsistencies in the solder joint.

 

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.

This website has many other articles on cables, see: Cable Info Center

The remainder of the series is pretty easy to follow and I would recommend you read it if you are interested in information about cables.

A very good alternative to pre-made component cables that is used by many installers that pre-wire homes for home theater applications is RG6 quad coax cables. This cable is considered overkill for many, but considering the cost and the ruggedness of the cables, it is some of the best cables to use. We use a standard F connector attached to a F to RCA adapter to plug into the equipment. Many people do not like the RG6 because of the stiffness of the cables, but once it is routed neatly, it is fine.

Probably the most reconized brand of cables is Monster. If you want Monster cables they can be ordered from amazon.com here: Monster composite cables.

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