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Digital Perfection

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.

Real-World Digital

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.

Digital Reliability

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.

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