One of the glorious things about the traditional analog video formats is their robustness over distance. Our customers have run analog component video for hundreds of feet without so much as a booster box to keep the signal together. When DVI and HDMI first hit the market, many people had trouble running signals over even modest distances; 15 feet would be reliable, and anything longer was a gamble. That situation has improved, though HDMI will never rival analog video for reliability over distance. Here, we address why that is, and what you can expect if you need to run HDMI cable over distance.
What Does the HDMI Spec say about Length?
A tale has been drifting around the Internet for some time to the effect that the DVI and HDMI specs give a maximum distance of 15 feet. This isn't actually true. The HDMI spec, in fact, gives no length limit explicitly at all, but the requirements of the spec implicitly give rise to some length limitations for "compliant" cables. A cable isn't permitted to degrade the signal past a certain point--that point is a bit hard to put one's finger on, because a cable is deemed spec compliant if it meets either of two tests: an "eye-pattern" test which measures the overall shape of the HDMI waveform at the cable output, or a set of parametric tests which measure the attenuation and other losses in the signal.
This "implicit" limitation on cable length, of course, is dependent on the limits of what can be done in the way of cable design. HDMI cable testing by the HDMI Authorized Testing Centers results in issuance of Compliance Testing Certificates, which are something of a guide to available cable lengths. The longest cable we have ever seen a compliance test certificate for is our own Series-1, which passed ATC testing at 45 feet under HDMI 1.3a (CTS 1.3b1); apart from that, the longest we've seen, after seeking these from all of the many vendors who try to sell us HDMI cable, are some 40-foot certificates issued under HDMI Version 1.2, which was a somewhat easier test to pass because of changes to the test protocols which came into effect with 1.3.
"Now, wait," you may be saying after that last paragraph, "your 45-foot cable can't be the longest compliant cable. I see 50-foot and longer cables around all the time." It's true that there are a lot of 50-foot cables on the market, and there are a lot of vendors who play fast and loose with the compliance issue. If you really think your vendor has a 50-foot HDMI-compliant cable, ask him for a copy of his compliance testing certificate, which will show the distance for which he's passed testing, and under what spec version. If you find a 50-foot compliant cable (with no booster or EQ unit; more about that below), let us know; we have never found a single one, and we are pretty sure there's no such thing.
There are, of course, "active" cables on the market also. These typically involve use of a powered amplifier which may or may not incorporate an EQ unit to compensate for the loss of high-frequency information. We don't know what the longest lengths passing compliance testing are in products of that type, though we have seen active connectivity solutions which run considerable distances, the longest of them using fiber optic cable and costing a bundle. Our attitude toward active solutions has generally been that it's better to do without, if one can. These amplifiers and EQs provide new potential points for failure in signal delivery, and make diagnosis of problems more complex; still, if you need to run extreme distances, these types of solutions may work for you.
So, If I Want To Go More Than 45 Feet, Do I Need an Amp?
Fortunately, connections which are not quite spec-compliant frequently work just fine. The spec is written with the intent of ensuring that any compliant source, hooked to any compliant display through any compliant cable, will work; in practice, this means that while one isn't guaranteed success with a non-compliant cable, there normally is some headroom to work within. The signal coming out of the source is probably better than the minimum signal required by the spec, and the data-recovery characteristics of the display circuitry are probably better--often, much better--than required by the spec. So, while it appears that every 50-foot passive cable on the market is noncompliant, most of them work, on most source and display combinations, just fine.
But what will work is certainly tied to the bitrate being run through the cable, and the difference between what will work at 480p and 1080p may be extreme. Because HDMI is an uncompressed signal, the bitrate running in the cable corresponds directly to the amount of information in the picture. Standard HD resolutions (720p, 1080i) use nearly three times the bitrate of standard-definition 480p; 1080p uses double what 720p and 1080i do; and 16-bit color depth, if and when it becomes available on the market, will double the bitrate for any given resolution.
When we brought in our first test reel of Belden HDMI cable, we found that while we had perfect 480p at 180 feet, we had to shorten the cable up considerably to get perfect 1080i and 720p. With improvements, we've narrowed the difference considerably, but it it still the case that we can run 480p longer (175 feet worked fine in our in-use testing; we didn't try anything longer) than 1080p (125 feet worked perfectly on our source and display), and that 720p and 1080i fall in the middle (150 feet worked perfectly). The hardest thing to get right in HDMI cable is high-frequency performance, and so generally speaking, the lower the cable quality, the more the working distance will fall as resolution or color depth rises.
Another factor in these distances is the headroom provided by both the source and the display. Obviously, if the source signal isn't very good, or the display's data-recovery characteristics aren't very good, or the input or output impedances are meaningfully off-spec, then distances will be more limited than if these things are all performing well above spec. This is one of the maddening things about HDMI: it's not really possible to say with perfect confidence that a long cable will or will not work in a given application because once one is in "non-compliant" territory, it all depends upon the characteristics of the devices in use. It's not uncommon, with long cables, to find that they work with one source or display and not with another.
What Does This Mean in Practical Terms?
In practical terms, today, for distances 50 feet and shorter, even economical cables are usually reliable at 720p, 1080i and (though this is less consistently so) 1080p. For very short runs--all those 3 and 6 foot cables out there in the world, at least when not being used as part of a much longer signal chain--it's best not to worry about it at all. But for those long runs, the future is still very unclear. Low-cost 50-foot cables which are near their performance limit at 1080p today may not work with 16-bit color 1080p tomorrow.
If you're in the longer-than-50-foot category, it gets dicier. We have had good consistent results with the Series-1 cable out to 100 feet, with reports of no trouble in 90% of installations. Cable quality starts to be a real concern at these distances, and performance always is hard to predict, especially because the cable that works on one source/display pair may not work on another. There is, unfortunately, no really good way to know what will work without plugging it in.
So what should you buy? We get a lot of questions from customers who are not sure whether to buy our best cable or our cheapest. There's not one consistent "right" answer to this; if the cheapest cable will do everything you ask of it, there's no picture improvement to be had in going to the best cable (this is, after all, a digital signal), but if what you ask of it may change, the answer may change, too. We strongly encourage people who are installing cable behind walls, in ceilings, and the like to weight their choices heavily in favor of buying the very best cable possible, simply because the cost of revisiting an installation later can be much higher than the cost of the best cable. On the other hand, if your installation will not restrict your access to your cable, and if the inexpensive cable works well with your gear today, there's no compelling need to have the best possible cable quality.
Last update: 01:01 PM Sunday, January 11, 2009