What makes a video cable suitable for HDTV?
We're often asked about HDTV cables; specifically, what makes a cable suitable for HDTV? What technical specs or requirements are applicable to HDTV cables, as against cables for standard-definition TV? And what criteria are best used to judge one set of HDTV cables against another? This article will briefly address those subjects in order to help you shop and compare HDTV cables.
What Cables Don't Support HDTV?
First, it's important to note that, regardless of whether your devices handle HDTV signals, not all connections to those devices actually support HDTV resolutions. Composite video, s-video, and RF outputs on consumer gear do not handle HD resolutions; instead, they support only downconverted signals, in 480i (480 lines, interlaced) resolution--and so composite and s-video cables are not "HDTV cables" at all, even when used in connection with an HDTV display. An RF cable (that is, an F-connector type cable, used for modulated RF signals) will support HDTV if the source--antenna, or CATV input--contains HDTV signals, but a modulated RF output on a consumer device, at least at this date, is always a standard-definition output.
What Cables do Support HDTV?
HDTV connections between devices can be analog or digital, and different HDTV cables are used to make these different connections. We'll address first the analog, and then the digital, types of HDTV cables.
Analog HDTV Cables:
Analog HDTV signals are ordinarily component video (Y/Pb/Pr), but can be RGB (usually RGBHV, but there are other varieties). Either way, HDTV cables for these applications take the form of 75 ohm video coaxial cable, usually terminated with RCA plugs but sometimes using BNCs or HD15 (VGA monitor) plugs.
The most important quality consideration for HDTV analog cables is that the cables should maintain a consistent impedance, as close to 75 ohms as possible. HDTV analog component video signals range up to about 35 MHz, as compared to NTSC composite video's 3.5 MHz, and the imporance of correct impedance increases both with frequency and with cable length; for long runs to projectors and the like, cables which are badly out-of-spec for impedance will result in ghosting or "ringing" of the image as a portion of the signal reflects back and forth within the cable rather than being delivered cleanly to the display.
Cable should be sweep-tested by the manufacturer to guarantee that it does not show excessive or uneven attenuation within the required frequency range, or within the first couple of harmonics--in the case of HDTV cables for analog video, that would mean that cable should be sweep-tested at least up to 105 MHz. Most of our Belden cable products have been sweep-tested at least to 3GHz, or about thirty times the required bandwidth.
When shopping for an analog HDTV cable, then, one should look principally for these two things. Does the manufacturer sweep-test the cable, and to what frequency? And what is the impedance tolerance? The best precision video cables will guarantee 75 ohms within +/- 1.5 ohms. If a manufacturer doesn't reveal these specs, or even produce so much as an attenuation chart, chances are that the cable isn't up to snuff.
Digital HDTV Cables:
In the professional world, digital HDTV cables are SDI -- Serial Digital Interface -- and are 75 ohm video coax. Unfortunately, the highly robust, uncompressed SDI standard isn't readily available to the consumer as a means of routing video, and we have to put up with the poorly-designed DVI and HDMI interfaces for digital HDTV cables.
DVI and HDMI exist in their current form not so much for the good of the user as for the good of content providers; both are designed to accommodate HDCP, a copy-protection scheme. These standards were designed for very short interconnect lengths, because it seems to have escaped the imagination of the developers of DVI and HDMI that anyone would ever want to run video over any distance. Instead of using coaxial cable, with its superb impedance control, they used twisted-pair cable; and instead of making up for the deficiencies of the twisted-pair cable by providing error-correction protocols, they left the system to simply fail when a sufficient amount of information is dropped. DVI and HDMI will work flawlessly over short distances; but some of the less-well-made cables start to fail at as little as ten feet, and practically none work beyond about fifty feet.M
uch is made of the attributes of DVI and HDMI, and we often read that the data stream in these formats is "uncompressed." That statement is a bit deceptive. First, it suggests that the analog outputs of devices are compressed somehow, and that therefore there's something in the digital signal that's not available on the analog side, which is simply not true. Second, it ignores the massive compression that goes on in converting raw digital video to the standard received by the DVI or HDMI device--there's no such thing as uncompressed HD video in an ATSC broadcast signal, or a Quam-encoded CATV signal. Uncompressed HD is a gorgeous thing, but it's not something you're likely to see outside of a production environment.
Practically none of the DVI and HDMI cables on the market offer any meaningful performance specs, but fortunately performance is easy to judge by plugging them in. There really are only three possible conditions for a DVI or HDMI cable connection: (1) it works, without apparent problems; in this case, it is working perfectly, or very nearly so; (2) it works, but the screen is peppered with little "sparkly" bit errors; this means the connection is near the threshold of failure; or (3) it doesn't work at all, and there's no image to view. If a connection is performing perfectly, as in (1) above, then there's no room for improvement by swapping out the cable; if it's not, as in (2) or (3), another cable may be able to do the job due to better internal construction (in particular, better impedance control).
We hope this article has helped answer some of the questions you may have about HDTV cables. We handle a variety of HDTV cables, which you can view by visiting our online store; if you're looking for more articles and information about video and audio cable characteristics, see our articles index.