How to Choose the Right HDMI Cable

For all the questions that could be asked about High-Definition Multi-Media Interface (HDMI), one thing is certain: its popularity is surging.

More than 500 manufacturers have adopted the HDMI standard -- a 60 percent increase since January 2006. Furthermore, a communications research firm predicts that more than 130 million devices featuring HDMI will be shipped this year.

Customers view HDMI as a product that offers the best video and audio quality on the market. It's also regarded for easy installation, particularly for its ability to replace up to 13 analog lines with a single cable.

At the same time, though, integrators and installers have much to live up to when it comes to delivering the HDMI quality customers expect. So how can custom installers ensure their clients are getting the most bang for their HDMI bucks?

Even the best HD system can grind to a screeching halt if a bad HDMI cable is used. Installers can take a few simple, but critical, steps to make sure they provide the best HD possible, while also avoiding having to troubleshoot a $100,000 system because a $100 cable isn't working.

Going the Distance

More than with any other type of audio or video cable on the market, HDMI signal quality degrades over distance. Achievable distance depends heavily on the quality of the cable, which can run anywhere between five to 50 feet.

When dealing with longer cables, it's important to understand that HDMI signals might not make it to the other end on their own. The cables are comprised of 19 relatively high-gauge conductors, which are terrific at transmitting a lot of information very quickly, but not without elevated capacitance at distance.

Peripheral devices can be used to "boost" or "restore" signals. Devices that boost the signal can add several dB to the signal strength at the beginning of the line, while restoring devices actively repair lost bits at the end. Installers, though, must be careful to make sure that any peripheral devices are running on external power sources. Borrowing power from the bus is noncompliant and often results in signal degradation.

Additionally, for applications where HDMI transmissions are distributed throughout a house, building or a particularly long home theater, devices are available to transcode the HDMI signal for distribution across a different medium, like Cat 5E or fiber.

Proof Is in the Eyes

With such a sophisticated design (19 conductors) and stringent length tolerances (0.5mm), manufacturing HDMI cables can involve hundreds of critical steps. It is, however, up to each manufacturer to decide how they will build their particular cable. Consequently, not all HDMI cables are created equal.

Despite clever marketing, a consumer can have no confidence that one HDMI cable is better than another without some sort of proof. Eye Pattern Tests can provide it. Eye Diagram Test results graphically depict transmission of a digital signal -- as its name implies, a good test result resembles an eye.

A wide opening, thin lines and a symmetrical shape are all indicators of a high-quality cable. Any area of the pattern that touches a shape -- called a mask -- inside the eye represents a failure. In the case of an HDMI cable, the amount of headroom (the distance between the mask and the pattern) might determine whether an HD system will work properly.

Because manufacturers have not implemented HDMI the same way in their respective devices, different systems put different levels of stress on the cables. In fact, a system may exert stress to the point that a cable, which previously passed an eye-pattern test, fails.

Integrators should therefore look for cables that passed with as much headroom as possible at the resolution and frequency they plan on displaying (1920x1080p at 60 Hz, for example).

Certain cable manufacturers include their eye-pattern test results with their packaging. Others will provide them upon request. Installers must be careful to ensure the test results displayed are relevant -- for the correct length, resolution and frequency.

Why Won't This Work?

HDMI cables can be damaged during installation. However it's not very easy to see if conductors are bent to the point of failure. Installers can use HDMI testers to verify full continuity after installation. If the problem is somewhere else in the system, more sophisticated diagnostics units are available and can save time pinpointing the cause of failure.

Certain diagnostics units can monitor the 5-volt line, Hot-Plug Detection, EDID and Highbandwidth Digital Content Protection (HDCP) activity in a HDMI system. With this information, the installer can determine whether the problem is upstream or downstream, whether the problem can be fixed easily (like changing the TV to the correct input) or if the problem is likely to require a firmware upgrade.

Cables certainly aren't the only critical element in a sophisticated HD system. Non-compliant sources and sinks have also shouldered blame for failures. In an effort to not stand in the way of adoption, the HDMI specification had allowed manufacturers leeway in programming their devices with code that often led to a lack of interoperability. Additionally, HDMI Authorized Testing Centers previously were not required to test for the HDCP repeater function, which opened the door to a lot of common problems.

These conditions led several consumers to conclude that HDMI was unreliable. However, the standard has matured and today's HDMI systems offer improved interoperability, and the testing centers recently began testing for HDCP compliance.

As HDTVs improve, there is an increasing demand on the cables. The jump from 1080i to 1080p requires a cable that can support twice the bandwidth.

How can an installer know if his cable of choice is capable? With test results available, he'll have to "keep an eye out."

David Coleman is the program manager for Honeywell's CE Cable Products.