Transmission Line Effects in Video Cables - page 2

With HDTV at 30 MHz, and 480p video at about 12 MHz, the wavelengths in question are 10 meters and 25 meters. Using a solid dielectric, with approximately the same dielectric constant as PE, we're running at around 66% velocity of propagation. This cuts the physical wavelengths in question to 6.6 meters and 16.7 meters. A quarter wavelength, which is often used as the benchmark for where the characteristic impedance of a transmission line becomes critical, is then 1.65 meters or 4.16 meters. Many think a quarter wave is a bit too long to use, and prefer to go with 1/10 wavelength or so. But staying with ¼ wave, and converting to feet, we get 5.41 feet and 13.7 feet as "critical" distances for impedance. Bear in mind that many people switch video, so this may represent a couple of cables linked together through a receiver on the way from the source to display. Do you know anybody who doesn't have at least 5 1/2 feet of cable between his source and display?

Again, if we are considering ultra high performance, and use more conservative estimates to determine the point at which the cable's electrical length becomes long (with respect to a wavelength) to consider transmission line effects for video cables, we use 1/10 wavelength as our critical distance.

In this case, a cable of greater than 2.25 feet becomes critical NOT 24 feet as the Exotic Cable vendor states!

"In this case, a 50 ohm characteristic impedance may be better due to less signal loss."

Pursuing the Truth…
This claim almost implies that the vendor is confusing characteristic impedance with resistance, incorrectly inferring that the lower the ohms, the lower the loss. Not so!

The biggest factors for loss of image quality in a transmission line are:

• Impedance match, for which 50 ohm cable is unsuited in a 75 ohm circuit,
• Capacitance. 50 ohm cables generally have nearly twice the capacitance of 75 ohm cables. 75 ohm cables generally run anywhere from about 16 to 21 pF/ft, with the best ones at the lower end of the range, while 50 ohm cables run from about 24pf/ft to 32pf/ft.

Video Cable Measurement Comparison

As you can see, the exotic video cable has nearly 63% higher capacitance and 66% higher resistance than the free OEM patch cord. It is interesting to note that the free patch cord has a lower shield resistance than the $1200 exotic cable which is exactly opposite of the cable vendors concern that added shielding will do more harm then good.

First, it's easy enough to come up with a central tube; there are video coaxes which use a hollow tube around the center conductor, with a spiral thread of PE wrapped around to help maintain distance. If one were to ask a cable supplier (such as Belden) to build a cable like this, it'd be easy enough for them to produce the tube without the PE and the inner conductor. Apart from the missing center conductor, this exotic cable really looks like a very low-grade Triax cable in construction; low-grade because Triax normally would have a braided shield, with or without foil, while this has two "serve" or "spiral" type shields around the nonexistent center. The material cost and labor for such a cable would be, on the order of $0.50/ft for long runs of about 25K feet. I'm hard pressed to think of any real world application for it, though I suppose that something weird like a microwave transmission cable might be built that way to maximize center conductor skin area. The problem with that, though, would be that a serve shield can act as an inductor, and you wouldn't want a big unpredictable inductance, varying as you flex the cable, on a transmission line carrying extremely high frequencies.

"The wire is considerably more advanced than other cables used for this purpose using OHNO Continuous Cast Single Crystal Copper and is one of the most expensive raw wires."

Pursuing the Truth…
I am uncertain what the exotic cable vendor depicts as "OHNO Continuous Cast Single Crystal Copper". The only features of copper that have much to do with performance are annealing (which makes it less brittle) and purity (which, up to a point, makes it more conductive and less corrosion-prone). I checked with several cable supplies and E&M experts such as Henry Ott , and nobody could make any sense of this.

The fact that the "monofilament" in the center is PE tends to make me think, if that tube is hollow, that it might indeed be a modified triax design, because PE is very commonly used as a dielectric.

As for the dielectric between the center and the outer spirals, polypropylene is an unusual choice. I know that it does have a good dielectric constant, but it's not used nearly as much as PE. Why that is, I'm not sure. It may be that it's more costly, or it won't foam, or it's harder to produce in consistent density, or it's harder to extrude to a consistent size, or it's more brittle, or something else I haven't thought of....nothing wrong with using PP, so far as I know, but it's an unusual choice. As for the assertion that "cheap cables use PVC," I have seen PVC dielectric cables, but rarely. Cheap-and-poorly designed Chinese molded cable assemblies sometimes have "coax" cables with PVC dielectric, but PVC has a terrible dielectric constant and isn't used as a dielectric in any product of even halfway decent quality. It is, of course, commonly used as a jacket material, where its low cost, flexibility and durability are all benefits and its lousy dielectric constant is irrelevant.

Adding an extra shield would have nothing to do with altering the performance in the time domain. The velocity of propagation of the signal in the center conductor is purely a function of the dielectric material with which it's in contact. When we're talking about audio, too, any discussion of the time domain vis-a-vis cable construction is pretty silly. The only way you'd get a time domain issue would be by having enough capacitance and resistance to start having serious R/C network issues.

Industry Experts in Electromagnetics and Signal Propagation Assessments

I submitted pictures of the exotic cable in question to E/M & EMC Industry experts (John Escallier and Henry Ott , respectively) and both arrived at very similar conclusions regarding the geometry and shield effectiveness of the cable.

According to John Escallier
First, the frayed outer shield is frayed only after you cut it and splayed it out. Prior to it being dissected, what did the shield look like? Ten to one it was a nicely spirally wrapped, uniform coverage shield...Not 100%, mind you, but certainly uniform.

Second, by spiraling the inner wire around a plastic core, they eliminate the internal inductance of the inner conductor. For all non tubular wires, the internal inductance is 15 nanohenries per foot...this inner wire construction eliminates that internal component. The inner wire inductance will actually be 15 nanohenries divided by the number of strands the inner wire is composed of. (that correction factor accounts for the non zero thickness of the tube).

However, there is one thing that is wrong in the design, one which should probably not be used for hf signals, even though it may be ok for audio stuff.

They wound each conductor in a spiral fashion..In doing so, they have created a solenoid component of magnetic field. The outer spiral is opposite pitch from the inside. What this does is create a solenoid field component that is not cancelled by the coaxial geometry. And it causes deviation from the ideal equation L*C=1031*DC. In other words, this cable will have external sensitivity; will have too much inductive storage... It may be of the correct dimensions for 75 ohm impedance, but by spiraling both (either) of the conduction paths, they have a horrible high frequency response, consistent with your ghosting experience.

Measurement Update: Based on phsically measuring the inner and outer conductor spacing of the cable in question, it was found that the characterstic impedance was somewhere between 50-55 ohms which also correlates to the calculated characteristic impedance based on the measurements above.

Closing Comments

So what have we learned from all of this? With respect to cables, you don't always get what you pay for. Sometimes free patch cord supplied by the manufacturer is BETTER than the expensive exotics. Care must be taken when choosing cables, especially video cables. When you see an exotic cable manufacturer repackaging a typical 50 ohm audio cable for HD video use, be very cautious as you may be buying into snake oil . The three basic essentials in component video cables to look for are:

• 75 ohm characteristic impedance
• Good double braided / foil combo shielding
• Quality soldered or crimped terminations

If the component video cable you are considering doesn't have these basic three elements, consider looking elsewhere. If you are concerned with the best video quality, leave the snake oil at the hifi store and buy a cost effective cable that will do no harm to your precious high performance home theater equipment that you worked so diligently to assemble.

We sincerely hope the exotic cable manufacturer in question will view this article as constructive feedback in efforts to improve their product performance which benefits the end user. We don't have a problem with an exotic cable vendor asking high $$$'s for their cables unless the actual cables in question don't perform to our minimum established standards. Component Video cables costing as little as a few dollars per foot meet our minimum design criteria. The exotic cable vendor in question can simply repackage a decent inexpensive shielded 75 ohm component video cable and charge more for their added cosmetics and marketing appeal.