The Quick HDMI Guide

The High-Definition Multimedia Interface, almost always abbreviated HDMI, is a video and audio cable standard meant to unify and simplify audio/video cable systems. It is increasingly the de facto standard for A/V equipment, and largely succeeds at its goal of making things simpler. But there are still a few gotchas to watch out for.

HDMI in a nutshell

HDMI is an all-digital communications link that carries both audio and video in a single cable. The video standard used is fully compatible with DVI, the standard used by many computers. Simple pass-through adapters, which merely change the pinout of the cable, may be used to convert DVI to HDMI, or vice versa, with essentially no penalty. A variety of audio standards are supported. Up to 8 channels of audio may be carried.

There have been several iterations of the standard, but from an end-user perspective these are mostly of no consequence. The only revision of particular note is HDMI 1.3, which introduced the new "category 2" cable described below.

Shopping for HDMI cables

If you have a question about HDMI, odds are it involves buying an HDMI cable. The good news first: it is much simpler than it might look. In fact, this one section covers everything most people need to know on the topic. The bad news: it is not quite as simple as sometimes indicated, as the section after this describes.

As of HDMI 1.3, there are two standards for HDMI cables. "Standard" ("category 1") cables must be able to handle signals up to the limits of the older versions of the standard (fine all the way up to 1080p video). "High Speed" ("category 2") cables can handle signals that push even more data, including 2560 x 1600 computer monitors. If you are shopping for a cable today, you should be buying a Category 2 cable; they are quite affordable if you buy from a good merchant. And they provide that little extra assurance of future compatibility.

There is also now a "mini" connector, introduced in HDMI 1.3. If you have a device that uses that connector, buy such a cable. Otherwise, you do not need to care about the mini connector.

Beyond that, all you really need to do is buy a cable that is adequately long and of adequate quality. "Adequate quality" means, in essence, a cable that will have actually been properly tested by the manufacturer and certified under the HDMI Compliance Test Specification. Any reputable manufacturer will adhere to this standard; no-name cables from a flea market might not. Any two "category 2" cables that pass this testing may be considered functionally equivalent for nearly all ordinary applications.

Cable quality

In the cable industry, much is made of wire thickness, gold-plated connectors, and other special features. In the world of analog cables, such as most audio or component video cables, there can be something to that. For a standard like HDMI, not so much.

HDMI is different from some standards like USB, which place very specific constraints on the cable (USB cables are not permitted to be more than 10 m in length, due to timing issues). HDMI requires that, given specific inputs, a cable must produce outputs that meet specific criteria. And that is all. The physical implementation can vary widely.

Because of this, and because HDMI is entirely digital, in most home applications there is little performance-based reason to favor one cable over another if they both passed the same testing regimen. As long as they will both get all of the bits from point A to point B with an acceptable (undetectable by humans) bit error rate, it makes no difference. This is radically different from some analog standards, where an improvement in cable quality can produce a substantive improvement.

But as always, there are caveats. If you one day want to run your cable out of spec (for instance, trying to push data through at a higher data rate than it was tested to support), cable quality might suddenly be very important. A better-quality cable may lead to a slightly more "open" eye diagram, which could be the difference between acceptable and unacceptable performance. In that case you might also benefit from a cable with superior shielding, which would also be valuable in an unusually noisy environment. In short, there are potential benefits to spending more on a cable.

Quirks

HDMI has some interesting quirks that you may not be aware of.

First, as alluded to above, HDMI places very few restrictions on the physical implementation of the cable. Ordinary shielded copper cabling is normal, but Cat 5 or Cat 6 cabling (the sort of twisted-pair cables commonly used for Ethernet) may also be used, and is beneficial for longer cable runs (twisted-pair cables have superior performance in rejecting noise from the environment). Fiber optics are also perfectly valid, and allow for extremely long cable runs.

Second, the standard does actually support two-way communications in addition to one-way audio and video transmission. This feature, known as Consumer Electronic Control (CEC), essentially allows the devices in a home theater to form a simple local area network. Pansonic was the first to demonstrate the technology in practice, giving it the name Viera Link. In Panasonic's demonstration, a user uses a television remote control to send commands to the television, which in turn may relay commands to another device such as a DVD player.

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