The Cable Game

The Analog Story

Senior performance analyst Jeffrey Kuta (left) and author Se?!n Captain use a Tektronix TDS8200 to measure cable properties. The device displays results in graphical form, making them easier to evaluate.
Photograph: Marc Simon
Someday every TV connection will be digital. But today, because many people still have devices that use analog connectors, they must contend with some signal loss and distortion. So do expensive cables help reduce such problems? To find out, we tested our cables on the Tektronix TDS8200 Digital Sampling Oscilloscope, and then we analyzed them with the company's IConnect 3.5.1 waveform analysis software.

We started by measuring characteristic impedance--the extent to which a cable hinders the flow of a signal. The standard impedance for each wire in a component-video cable is 75 ohms. If the impedance in any one wire is far off the mark, it produces an impedance mismatch with the devices it connects to; as a result, some of the signal may be lost in transmission, or it may bounce back along the cable to the source, producing smeared colors or blurriness in the picture's fine details.

Monster's M500CV was the winner here, as all three wires inside the cable varied within a negligible 1 ohm of 75 ohms. Translation: This cable imposes as little distortion as possible. Other cables didn't do as well. The three wires included in the CableWholesale.com cables hovered between 63 and 64 ohms, while the Kimber Kable's wires measured between 85 and 86 ohms. The AudioQuest's wires varied from about 71 to 75 ohms. And the StarTech.com's varied from about 67 to 69 ohms.

But here's the rub: Virtually every consumer component cable uses RCA-style jacks. Originally used for analog audio connections, RCA plugs have an impedance of about 50 ohms, creating unavoidable impedance mismatches at both ends of a cable. How well a cable manages the impedance at every point of the cable, not just at the connectors, affects its performance. But the impedance mismatch between a cable's wires and its RCA connectors has far more impact on performance than any other attribute.

We next calculated return loss, a measure of how much of the signal bounces back down the cable. According to the experts at Tektronix we consulted, 14 decibels is ideal. CableWholesale.com's product exhibited the least amount of signal bounce in our tests, at 13 decibels. The other four brands did worse (all at about 8 decibels). The Tektronix experts explained to us that practically all signal bounce is a result of the impedance mismatch between the wires and their RCA connectors.

We conducted one final test: Insertion loss, measured in decibels, gauges how much of the video signal gets lost as it runs through the cable. Four of the cables managed roughly equivalent performance. The worst performer in the group, the Kimber Kable V21, lost less than 2 decibels--an insignificant amount.

Eyeballing Analog

So what does all of this mean in a real-life setting? Not much, we discovered.

Working with an AccuPel HDG-3000 HD/SD/DVI Component Video Calibration Generator, we sent 720p test patterns through our cables to the Epson PowerLite 500 projector. None of the cables transmitted a perfect signal, but the imperfections were minor. In crosshatch patterns (a grid of fine horizontal and vertical lines), some lines displayed slightly smeared edges or shadows, rather than sharp pixel-for-pixel transitions from white to black. But we had to get within a foot of the screen to see any of this, and we saw the same problems regardless of which cable we used.

Another set of test screens displayed multiburst patterns, featuring several swaths of parallel vertical lines that get progressively finer from left to right. On every screen, the finest swath--where the lines were just a single pixel wide--looked blurry for each cable, indicating that even a good display might smear small details. The other swaths were sharp, with well-defined transitions, regardless of the cable.

Bottom Line: Though the analog cables varied slightly in our instrument tests, they did not produce distinguishable differences in transmitting real video content.

The Big Picture

Whether you hook up your TV via digital connections, analog connections, or both, you are unlikely to detect any difference in picture quality between a cable with a moderate price and a luxury brand. The only difference you're likely to notice is how the cable looks behind your TV.

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