Hdmi Cable All about HDMI

What’s the Matter with HDMI?

How the designers of the HDMI standard screwed up, and what’s to be done about it.

HDMI, as we’ve pointed out elsewhere, is a format which was designed primarily to serve the interests of the content-provider industries, not to serve the interests of the consumer. The result is a mess, and in particular, the signal is quite hard to route and switch, cable assemblies are unnecessarily complicated, and distance runs are chancy. Why is this, and what did the designers of the standard do wrong? And what can we do about it?

The story begins with another badly-developed standard, DVI. A few years ago, there was a movement within the computer industry to develop a new digital video display standard to replace the traditional analog VGA/RGBHV arrangement still found on most computer video cards and monitors. Interested parties grouped together to form the Digital Display Working Group (DDWG), which developed the DVI standard.

DVI had all the earmarks of a standard designed by committee, and it remains one of the most confusing video interfaces ever. DVI could run analog signals, digital signals, or both, and it could run digital signals either in a single-link configuration (in a cable using four twisted pairs for the signal), or in a dual-link configuration (using seven). Identifying which DVI standard or standards any particular device supported was not always easy, and the DVI connector came in various flavors and was never really manufactured in any form that wasn’t well-nigh impossible to terminate.

But the worst thing about DVI was something that the computer-display professionals involved in its development really didn’t give much thought to: distance runs. Most computer displays are mounted at most a few feet away from the CPU, so it didn’t seem imperative that DVI work well over distance. This lack of concern for function at a distance, coupled with common use of twisted-pair cable (e.g., CAT 5) in computer interconnection, led to a decision that DVI would be run in twisted-pair cable.

Had the DVI standard been designed by broadcast engineers rather than computer engineers, things probably would have turned out very differently. In the broadcast world, everything from lowly composite video to High-Definition Serial Digital Video is run in coaxial cables, and for good reasons, which we’ll get to in a bit. Long-distance runs of VGA, in fact, are always handled in coaxial cable (though there may be a number of miniature coaxes in a small bundle, rather than something which obviously appears to be coax).

DVI lacked a couple of things which the consumer audio/video industry wanted. It was implemented on a variety of HD displays and source devices, but it was confusing for the consumer because of the many variants on the standard and different connector configurations, and it didn’t carry audio signals. A consortium to develop and promote a new interface, HDMI, was formed; the idea was to come up with a standard which could be implemented more uniformly, was less confusing, and offered the option of routing audio signals along with video.

Here, again, was an opportunity to avoid problems. The difficulties of running DVI-D signals over long distances were well known, and the mistakes of the past could have been avoided by developing HDMI as a wholly new standard, independent of DVI. Instead, the HDMI group elected to modify the DVI standard, using the same encoding scheme and the same basic interface design, but adding embedded audio and designing a new plug. Instead of many DVI options, analog, digital, single and dual link, there was one “flavor” of HDMI (actually, there is also a dual-link version in the HDMI spec–but you won’t find it implemented on any currently available device). This provided the advantage of making HDMI backward-compatible with some existing DVI hardware, but it locked the interface into the electrical requirements of the DVI interface. Specifically, that means that the signals have to be run balanced, on 100 ohm impedance twisted pairs.

We’re often asked why that’s so bad. After all, CAT 5 cable can run high-speed data from point to point very reliably–why can’t one count on twisted-pair cable to do a good job with digital video signals as well? And what makes coax so great for that type of application?

First, it’s important to understand that a lot of other protocols which run over twisted-pair wire are two-way communications with error correction. A packet that doesn’t arrive on a computer network connection can be re-sent; an HDMI or DVI signal is a real-time, one-way stream of pixels that doesn’t stop, doesn’t error-check, and doesn’t repair its mistakes–it just runs and runs, regardless of what’s happening at the other end of the signal chain.

Second, HDMI runs fast–at 1080p, the rate is around 150 Megapixels/second. CAT5, by contrast, is rated at 100 megabits per second–and that’s bits, not pixels.

Third, HDMI runs parallel, not serially. There are three color signals riding on three pairs, with a clock circuit running on the fourth. These signals can’t fall out of time with one another, or with the clock, without trouble–and the faster the bitrate, the shorter the bits are, and consequently the tighter the time window becomes for each bit to be registered.

Consider, by contrast, what the broadcast world did when it needed to route digital video from