Contrast this with, for example, standards for professional digital video coax for SDI (Serial Digital Interface) applications. The SMPTE (Society of Motion Picture and Television Engineers) has established published standards for SDI cable which any engineer with the proper test gear can test cables against. So, when Belden or some other cable vendor says that a certain cable will carry a certain resolution of SDI a certain distance in conformity with the SMPTE standard, (1) everyone knows exactly what is meant by that claim, in terms of measurable attributes (e.g., “attenuation at a frequency 1.5 times the bitrate”), and (2) Belden’s claim to conform to the spec can and will be tested by customers and competitors, and if the product doesn’t live up to the promise, there’ll be trouble. Standards like that are useful, meaningful, and above all, testable.
And, of course, as we’ve pointed out, the capability of an HDMI cable to carry a particular data speed will depend very much on the cable’s length. The result of any proper speed rating system will not be to say, “model A rated for bitrate X, model B rated for bitrate Y, model C rated for bitrate Z.” Instead, it will be something like, “model A rated for 480p up to 75 feet, 720p/1080i up to 40 feet, 1080p up to 30 feet, 1080p/12 bit up to 20 feet,” and so on, with each model being rated for higher speeds in short lengths than it is rated in long lengths. This is, in fact, exactly what one finds if one consults Belden on the suitability of cables for SDI; 7731A (RG-11 precision coax) will run any given resolution farther than 1855A (mini-RG-59 type precision coax), but 1855A will run the highest resolutions just fine, at shorter distances. If we needed to run very high-resolution HDMI for a distance of only one inch, a bundle of nineteen rusty coathangers would do the job (but would be very difficult to solder to the connectors!)despite the poor quality of the cable and the very high bitrate; if we need to run it three feet, a decently-made but unremarkable HDMI cable will work just fine; it’s primarily when we need to run longer distances that the differences in cable quality start to become important.