HDMI: A Closer Look Under the Jacket
There are some basic guidelines companies must follow when developing HDMI cables. But each manufacturer has the power to build its own concoction of “Magic Sauce” and still manage to fall within HDMI’s compliance standards.
Have you ever used an HDMI cable that looked beautiful, but when installed it didn’t work?
HDMI cables are not just thrown together. Yet there are those who do buy HDMI products from off-the-shelf inventory in the Far East and then - poof - they find themselves in the cable business. I have a phrase for this: “Manufacturers that design their products and manufactures that just sell them.”
Let’s focus on those companies that indeed design their own products. It’s these types of companies where you should be able to get the answers to the questions that will surface here.
Building quality HDMI cables has its challenges, no doubt, so how do companies cope with these challenges and end up with a deliverable that they can stand behind? There are some basic guidelines that must be followed. But at the end of the day, each cable manufacturer has the power to build its own concoction of “Magic Sauce,” which allows for the cables to take on their own personality and still manage to fall within HDMI’s compliance standards.
HDMI Licensing uses revisions to define the specifications required for products to function reliably. These revisions offer more features and benefits that become available by way of technological advancements. This is exactly what the HDMI interface is about - adaptability. And as revisions change and get more intense, it will undoubtedly increase the cabling challenge that much further.
Figure 1 demonstrates what a basic HDMI cable looks like edge on. A total of 19 wires and shields are needed for this system to work. That’s a lot of wires to cram into a single jacket, not to mention certain attributes for how they are in placed within the jacket. So where do you start? How does one determine placement, gauge, insulators, shields, size, etc? This is where that “Magic Sauce” can enter the design phase.
Each design has to be defined before it starts. Decisions such as bandwidth, with/without Ethernet, length, and cable size must be carried out first. A precise selection process now takes place not only on the wires specification themselves, but the challenge of just how to proportionate them all to fit into a limited jacket, kind of like the “rob Peter to pay Paul” scenario. Favoring video and not DDC could put the system in an HDCP deficit. Reverse that order and the video can go south. Or is a fair and balanced approach the answer for HDMI integrity?
We’ll Report, You Decide
Video, DDC (HDCP&EDID), 5 volts, Hotplug, CEC and a spare wire take up most of the real estate inside the jacket. For discussion purposes, we will use a relatively short cable length of 5 meters to analyze. With this form factor a small 28-AWG wire gauge is all that is necessary for the 5-volt function to perform reliably.
The video, in this case, holds a little more weight in order to produce a solid eye at its rated maximum frequency. The eye patterns in Figure 2 demonstrate what happens to video signal integrity between wire gauge sizes at 1080p. Here a 28-AWG versus a 24-AWG is shown; the larger the gauge, the better the integrity. But there are other ingredients that can be added to improve the signal, like material and topology. In this case a large eye opening is ideal.
Jeff Boccaccio is president of DPL Labs. Jeff can be reached at firstname.lastname@example.org. Have a suggestion or a topic you want to read more about? Email Jeff at email@example.com
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