Wired for HD

A look at how HDMI technology brings High Definition to the viewer's TV screen.
Analysis, Delivery & Transmission


A look at how HDMI technology brings High Definition to the viewer's TV screen.

If you have recently bought a plasma or LCD high-definition display, you might have noticed an odd connector on the back. This connector is very small but has nineteen pins in it. This is an HDMI connection, High-Definition Multimedia Interface.

It is currently the only way you can feed a fully digital HD image (16x9, 1080i or 720p) signal to this display. Now you might argue and say "I see some RGB (red-green-blue) connections on my display."

Yes, you might, but those are strictly analogue, not digital. It is by design that the HDMI input is the only way to get digital into your consumer display.

And if you are a broadcast engineer (or talk to any), they will tell you how ridiculous this connector is. Broadcasters can do the same thing, or even more, on a single coax. But the difference is they own the content.

The consumer does not own the content. And the programme originators and distributors are all backing HDMI because it is the best way to make sure that consumers do not steal the programme or movie that they are watching.

You cannot go out and buy a box that will record an HD programme with this HDMI connector on it. Sure, you can get a cable box with an HD-DVR built into it. However, try and open that box!

One would need special tools just to get the cover off. Then try and take the hard drive out and go sell it. No, that's pretty close to impossible. Try and make a copy of the programme you have recorded on that DVR? Not possible.

And the cable between boxes, that HDMI cable, is very much a cable designed primarily to protect the programme owners and not to be a great way to transfer the material. Inside that cable are four twisted pairs (three data pairs plus a clock pair).

There are seven other wires for various things such as HDCP, High Definition Copy Protection. The source box (cable receiver, BluRay player, PS3 game console) has to talk to your display, and they have to agree on what is happening or nothing works.

There are two ways to test an HDMI cable, and it can "pass" either way. But there is no requirement to test any cable. Like any signal cable, a short HDMI cable will probably work, as long as it is wired up correctly.

A long cable is much more problematic. Then things like the gage size of the individual wires come into play, and their precise delivery of the clock and three data pairs ("delay skew") are important. They even have a specification for the length of the wires inside each pair ("intra-pair skew") that are new to the commercial and professional cable business.

This is why using Category 5e or 6 to carry HDMI is a potential problem. Category cables have never been tested for some of these HDMI tests.

So either you are 'praying' that the Category cable you have is really good, or you are spending a lot of money on boxes that will make these cable perform as required, such as having active EQ and skew compensation.

In the HDMI standards, there is no mention of cable length. If the cable passes, it passes. Of course, small gage cables won't go as far as large gage cables (all other things being equal). But sometimes, you don't need to go very far.

Three metres (10 ft.) covers a lot of home theatre distance. It's only those commercial users, the plasma or LCD at the airport, or in the shopping mall, that might need a really long cable.

Then it would be a very good idea to get some test results from the manufacturer or distributor. It is unfortunately common for vendors to show you the two-metre test results, when they are selling you a 10-metre cable.

The longest unamplified copper cable has been tested to 15 metres (50 feet). Many cables, longer and shorter, are subjectively tested. That is, plug it in and see if it works.

Considering that a battery of standard tests on an HDMI cable costs around $3000 or more, many cables have only subjective testing. But you have to ask more questions. Is this tested at 1080i or 720p? Or is it the coming 1080p standard?

And, if this is subjective testing, what kind of 1080p source and what kind of display? Of course, each of these makes a difference too, so be careful. For really long runs, and a lot of money, you can convert to fibre optic cable.

Like every manufactured product copper, HDMI cables can be good, average, or bad. Often their market price does not reflect quality at all, so price can be a poor guide to choosing the best performance.

They're already talking about future improvements such as 1440-line resolution, or high refresh (frame) rates up to 120 Hz. And there's already another competing cable design, called DisplayPort, on the horizon. It's going to be an interesting future!

Steve Lampen works as Multimedia Technology Manager with Belden.

Belden unveils new HDMI cables

Belden produces bonded-pair HDMI cables in the USA and offers this in three different gage sizes. The largest gage bonded-pair cable holds the record for fully compliant HDMI testing at 15 metres.

The manufacturer verifies HDMI cable performance, among other methods, with a technique known as an "Eye-Pattern" test. Other critical electrical testing parameters include Impedance, Attentuation, Crosstalk, and Skew.

HDMI digital video/audio cables can be used to connect HDMI sources like broadband/cable set-top boxes, digital video recorders, DVD players, and Blu-ray players to displays like plasma screens, digital light projection (DLP), projectors, liquid-crystal displays (LCDs) and liquid crystal on silicon (LCOS) or digital image light amplification (D-ILA) devices, among others.

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