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.

Figure 1

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.

The DDC also carries some weight for guaranteed HDCP and EDID. Without this you’re dead in the water. Depending on wire size, and topology, the integrity of this channel can go south also. Figure 3 depicts two I²C waveforms that ride these wires. In a perfect world the square wave would have a sudden rise on the leading edge.

However, increase the length, lower the gauge, increase or decrease the twist rate, change the dielectric and these results can change. Poor rise time will limit integrity.

Figure 3

The Hotplug operates with a relatively high impendence load; so again, this wire can be limited in size. The spare wire is a funny one – it is unnecessary for non-Ethernet cables, but with Ethernet this wire has to handle 200 MHz. Since this feature requires two wires, the Hotplug wire was chosen to be the Siamese twin. This increased bandwidth typically increases the gauge and yields a better Hotplug current.

This is where careful planning needs to be orchestrated not only by the manufacturers but also by integrators. In a basic system environment we have a few unknowns. We don’t know the level of integrity that comes in or what goes out of each product within the system. We do know that they should have at least passed HDMI compliance, but what happens if the majority of these products came out on the low side of the compliance window?

Always remember that although we are passing digital information, these integrity levels we are discussing are strictly analog in nature. The level of failure is an analytical unknown to you. For example, we’ve measured some products that just barely meet these minimums. These are some of the reasons why some products work in harmony when others don’t. So a cable’s “Magic Sauce” may make or break a low-integrity system.

The firms that do design their own products should be able to brief you on their secret ingredient and provide technical support for “tuning” a system to achieve perfection.

HDMI Magic Sauce Examples