The Cable Game
You could spend $20 or less for a cable to connect your DVD player or cable box to your television--or you could spend hundreds. But how do those cables differ?
While conventional wisdom says a wire is a wire, a poorly constructed cable can seriously degrade video quality. Used with a digital connection, a bad cable can mangle pixel data, resulting in a speckled image; with an analog link, it can distort the signal, resulting in faded colors or blurred details. So the question is not whether a good cable is essential, but whether you need to spend a lot to get a good one.
To find the answer to that question, the PC World Test Center evaluated analog and digital video cables from five vendors: budget-cable companies CableWholesale.com and StarTech.com, industry leader Monster Cable, and boutique brands AudioQuest and Kimber Kable. For analog connections, we tested the three-wire component-video cable--the only common analog interconnect that can carry high-definition TV or progressive-scan DVD signals. For digital connections, we used High-Definition Multimedia Interface (HDMI), which carries digital video and audio signals (though we tested only the video portion).
Prices for the component cables ranged from $18 (CableWholesale.com) up to $275 (AudioQuest). The HDMI cable prices ranged from $31 (CableWholesale.com) to $300 (Monster Cable). And since cable length is an important factor in testing--each additional foot has the potential to worsen the signal--we chose cables that measured about 4 meters (12 to 15 feet) each, long enough for the majority of home theater setups.
Judging cable quality by eye is tricky. It's hard to know, for example, whether a problem stems from the cable, the TV, the signal source (such as a DVD player), or the content. But a signal analyzer can isolate the cable from the mix and measure its ability to transmit data. Video generators produce top-quality images for diagnosing problems such as incorrect colors or blurriness.
We began our evaluation by checking the cables on precision test equipment. Then we used generators to send pristine video signals through the cables so we could view the results on TVs. We tested at least two cables of each model examined. Some cables came to our test center direct from the vendors; others, we bought.
All cables are designed to transmit signals well, but certain tests can be used to determine how well an individual model performs. For the digital cables, we tested the three color components, the copy protection signal, and the device identification--all parts of the HDMI specification. In the case of analog cables, we measured impedance, return loss, and insertion loss.
Our conclusion: You don't need to spend a fortune on cables. The HDMI cables performed comparably in both our instrument tests and our visual tests. And with analog cables, the analyzer revealed some degree of variation in quality, but the variances did not translate into noticeable differences in our visual tests.
Digital technology promises to transmit a perfect signal nearly all the time, thanks to the exacting standards incorporated in the HDMI specification. HDMI Licensing, which oversees the HDMI spec and ensures that companies comply with it, requires that no more than one pixel per billion be lost in transmission. "Even if you lost one out of a thousand pixels, you wouldn't notice it," says Leslie Chard, HDMI Licensing president.
To see for ourselves, we first used Quantum Data's 802BT Signal Generator/Analyzer to determine each cable's ability to handle video at resolutions of 480 progressive, which DVD players use; 720 progressive, popular among HDTV stations; and 1080 progressive, a possible format for future high-definition DVD discs and HD broadcasts.
We ran these resolutions at the 1.0-volt signal standard that HDMI sources (such as DVD players and cable set-top boxes) should provide. But we also challenged the cables with less-than-ideal voltage levels, ranging from an anemic 0.15 volt (to simulate a weak signal) up to an overdriven level of 1.56 volts. All but one cable performed well down to 0.15 volt and up to 1080 progressive for the main video signal elements: sync and InfoFrames (which allow the source and the TV to maintain a proper connection), plus three components that carry color or brightness information. Even the exception--a 4.5-meter AudioQuest HDMI-X cable that we purchased at Best Buy--would work fine in real life, where sources are more powerful than 0.15 volt and current resolutions are less demanding than 1080p. (Two other HDMI-X cables from AudioQuest performed well.)
All of the cables, however, had some trouble when we degraded the high-definition copy-protection signal, a dialogue between the source and the display that is required for decrypting copy-protected material. If the HDCP information isn't transmitted, the device won't deliver a signal. Every cable we tried failed to transmit the HDCP data at between 0.32 and 0.29 volt as we progressively lowered the voltage in our test setup. You're unlikely to encounter a signal as weak as that, however.
Next we used the Quantum Data 802BT to generate 720p test patterns and send them to an Epson PowerLite Cinema 500 LCD projector--a $5000 model that displays crisp high-definition signals in their native resolution of 1280 by 720 pixels, without scaling the image to higher or lower resolutions and thereby introducing artifacts. If the cables we tested could handle video signals specifically designed to illustrate problems, they could handle The Aviator or Lost.
We used a number of tests, including ramp screens, which present a smooth horizontal progression from black to full brightness. In viewing 720p ramps for gray, red, green, and blue, we looked for blinking or incorrectly colored pixels. Blue speckles appearing on a red ramp, for instance, would have indicated that the cable had delivered incorrect data; with real video, such errors might translate to colored flecks in shadows. But all the cables, including the AudioQuest HDMI-X one that had some difficulty in our instrument tests, had no problems.
One obstacle we did face involved establishing solid connections with our devices' HDMI ports. In some cases, we connected the cable but no image appeared. Sometimes wiggling the cable fixed the problem, and sometimes it didn't. But the trouble seems to stem from the the standard HDMI connector design used by all cable vendors.
"If you jiggle an HDMI connector, one or two electrical lines in the connector might have intermittent connections," says Chad Nelson, an engineer with Maxim Integrated Products, which manufactures chips for digital and analog signaling.
"We don't believe that there is a fundamental problem with the design of the HDMI connector," says HDMI Licensing's Chard. However, he notes that his organization is evaluating proposals for a connector that latches in place.
For now, the best strategy is to position the cable carefully so that it applies no stress to the connector. In our tests, we had the most trouble when trying to attach Monster's $300 M1000HDMI cable to the Epson's HDMI port. Easily the thickest, stiffest, heaviest model we reviewed, the Monster cable pulled away from the projector's HDMI port, often causing the screen to go blank.
Once you get a good HDMI connection, our tests indicate, you can expect flawless performance from any 4-meter cable, regardless of price. "That is what I would expect from the HDMI cables," says Maxim's Nelson. "It is not too difficult to make them work perfectly at 4 meters."
Digital cables are inherently more dependable than analog ones. Both transmit data by controlling the voltage levels in an electrical signal. With analog, slight shifts in voltage correspond to precise values in the final picture. Thus, if the signal carrying blue color information loses voltage as it travels down the cable, the blue objects on screen will appear weaker than intended. (Think faded skies.)
For its part, digital carries just ones and zeros. In HDMI, if the signal voltage is high, it encodes a one; if low, a zero. The voltage encoded as a one can drop a fair amount and still be distinguishable from voltage encoded as a zero. After a certain point, however, the signal voltage drops so low that ones and zeros look alike, and the TV's receiver chip attempts to guess their value. So rather than gradually diminishing in accuracy, the way an analog signal does, a digital signal may remain perfect up to a critical level and then fail catastrophically. According to the experts, such problems are likelier to occur with an 8- to 12-meter copper cable (which is significantly longer than most users need) than with a 4-meter cable of the same type.
The Analog Story
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.
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.
Cable Confidential (chart)
Prices vary far more than performance does.
|AudioQuest HDMI-X||4.5 meters||$183|
|CableWholesale HDMI-01115||15 feet (4.57 meters)||$31|
|Kimber Kable HD19||4 meters||$146|
|Monster Cable M1000HDMI-4M||4 meters||$300|
|StarTech HDMIMM15||15 feet (4.57 meters)||$64|
|AudioQuest YIQ1-1||4.5 meters||$275|
|CableWholesale 10R4-03112||12 feet (3.66 meters)||$18|
|Kimber Kable V21 RGB||4 meters||$265|
|Monster Cable M500CV-4M||4 meters||$150|
|StarTech CPNTHQ12||12 feet (3.66 meters)||$42|