08.03.2007 — A new testing program aims to take the pain out of HDMI cables by ranking them on a scale of 1 to 5 based on variety of characteristics.

You know the drill: You need a 60-foot HDMI cable to use for a project. So you think, hey, the package says the cable is certified for HDMI Version 1.3 Category 2 and delivers a 1080p resolution at that length. You're safe, right? Of course not.

Even if you do your due diligence — you know, comparing specs — you still can't get the darn things to work. But who knows what the specs mean? You simply cannot do a worthwhile comparison without bench-testing the cables yourself.

Imagine how the traditional consumer must feel.

Ranking System to the Rescue

At the CEDIA Expo, we'll see the first viable HDMI rating system ever introduced. It's called the Digital Performance Level (DPL) Ranking System and is the brainchild of CE Pro columnist Jeff Boccaccio, and his crew at InVisions Technology, a leading consulting and development firm well known for its HDMI expertise.

The DPL Ranking System aims to convey mathematically how HDMI cables actually will perform based on all of the known electrical characteristics of an HDMI cable. The system consolidates the complex interactions of all of the various aspects down to a single number from 1 to 5.

Imagine you need to purchase an HDMI cable. Once the industry endorses DPL, all you need is the length, a price window, an acceptable DPL number, and a preferred manufacturer. It is that simple.

ADI on Board

A ranking system is no good, of course, if no one adopts it. ADI, North America’s leading wholesale distributor of security and low voltage products, has been supporting Boccaccio and his team on this initiative for a long time and starting in 2008, ADI will direct its suppliers of HDMI cables to incorporate the DPL Ranking System logos and numbers into their packaging. As an advocate for the program even outside its own vendor base, ADI’s goal is to have all key HDMI vendors on board with program by the end of year.


Doug Hoerle, director of systems business for ADI, says the distributor — a strategic partner in the program — receives about 1,100 HDMI-related support calls a day, with a dramatic increase over the year. ADI, like other vendors of HDMI cables, spends an inordinate amount of troubleshooting time determining if it is the cables or the equipment causing the problems.

“We find installers are reluctant to use HDMI because of the problems they have seen," Hoerle says. "With this program, the installer will feel more confident that they have chosen the correct cable for the application. … If the cabling is done correctly, it is easier to turn to the [component] manufacturers for help if we can eliminate a lot of the finger pointing.”

Boccaccio, who consults with manufacturers on HDMI, knows the problems all too well. "Many vendors punt when it comes to the actual performance of their HDMI cables and accessories," he says. "They ‘claim’ a cable meets certain specifications, but they cannot supply proof. What specifications are they using? To what tolerances are the products manufactured and tested? Why do vendors have so much trouble quantifying how their cables will perform? Now we will have a way to know.”

The program will bring clarity to HDMI performance. Boccaccio says that if you have a choice between two HDMI cables — one that is rated 1 and the other rated 4 — naturally you would buy the higher rated cable if they are comparably priced.

The Trouble with HDMI

For the majority of cables on the market, no one knows exactly what they're buying. For those that want to know, it is impossible to find out.

In addition, few consumers, or vendors for that matter, know what electrical characteristics constitute a well performing HDMI cable. It requires a complex interaction of all of the pieces working in unison to end up with a reliable system. If one electrical characteristic is off, marginal, or out of some unknown tolerance, it may appear to work well under certain conditions, but still may fail for no apparent reason. If the requirements change and higher resolutions are needed, it may not work at all or the quality will be unusable.

On top of that, there is no consistency in how the vendors list their specifications, if they actually know what they are. This constitutes a huge problem for an industry that is growing at such a rapid pace.

The DPL system cuts through the esoteric claims and provides a simple way to determine the actual Digital Performance Level of an HDMI cable or an accessory. Cables are selected from a random sampling of products purchased from a variety of sources, and they are tested several times per year.

“We will present a detailed engineering report to the participating vendors on how their cables perform in real life," Boccaccio says. "This way, they can confirm what they think they are selling. We are committed to providing them reliable and reproducible test results so they can learn where they need to improve. Eventually, the market will force everyone to take care of their problems.”



Now the manufacturers of HDMI products need to get on board and ADI says its primary vendors are ready.

Several have already lined up.

"We’re certain the DPL Program will not only raise the bar for performance among HDMI manufacturers, but also make it easier to identify and sell the best HDMI cable for the job— and that means a real win for distributors, dealers and end users," says David Coleman, program manager of CE cable products for Honeywell's Genesis Cable.

He adds that the program "will demystify the buying decision for dealers, helping them choose a HDMI cable based on how well the cable was manufactured, not marketed. We expect and look forward to industry-wide adoption among all professional manufacturers and distributors of HDMI cables."

Not surprisingly, Honeywell is "confident" that its cables will score high.

Another cable vendor, Ethereal Home Theater says bring it on. "The performance ranking system will clearly separate the level of integrity between the high and low quality products," says chief operating officer Bill Fornino. "We are eager to get our products through the testing process and to show our stripes."

Boccaccio says that the ultimate goal is to expand the DPL rank past five. "Integrators will finally be able to deliver systems that perform," he says. "We finally are going to turn the industry in the direction it needs to go. As soon as the vendors realize their competition is getting all the business because they are delivering a product with a higher DPL, they will be forced to make better cables or get out of the business. Support problems will be reduced dramatically. This is a real boon for the customers."

More information on DPL will be available at http://www.DPLRating.org.

A Word on HDMI

HDMI often gets a bad rap because products in an HDMI chain often don't work. Boccaccio says not to blame the spec.

“The problems we see today are not with the HDMI specification itself," he says. "They are due to poor implementations of the standard by the vendors."

He explains that if all of the parameters measured by that DPL meet or exceed the specifications, and are kept within close tolerances, HDMI works just fine. Things like HDCP handshaking issues, DDC (VESA Display Data Channel) corruption, Hot Plug Detect failures, synch loss, and incorrect EDID information being passed will go away (see tech details below).

Boccaccio says, "Our ranking system quantifies what it takes to get all of the electrical factors orchestrated to deliver the maximum HDMI performance. You no longer will need to wade through the specification quagmire just to purchase a cable that works. Of course, there still is the possibility an installer could ruin it all by putting it in wrong, bending it, or crimping it. Better training can solve this, but that is doable.”

He claims, "The HDMI interface has more potential than any other interface ever brought into our industry."

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For Video Geeks: The Tech Behind HDMI Testing

There are many aspects influencing the performance of any given HDMI cable or accessory, such as an HDMI switch or distribution device. Put your geek glasses on! The following represents just a few of the measured elements that comprise the Digital Performance Level (DPL) ranking system:

Bandwidth — The aggregate bandwidth requirement for HDMI Version 1.3a Category 2 is listed at 10.2 Gbps. This value is the sum of the individual color, data, and audio lines contained within the HDMI cable. More HDMI “dynamic headroom” will be needed above that to take it to even higher resolutions. Low bandwidth decreases the available resolution and or the color depth.


Capacitance/Dielectric loss — High capacitance and dielectric loss causes video frequency response issues and corrupted data on the data communication channels (DDC). This is caused by the proximities of the individual wires within the cable to each other and the dielectric materials between them.

Current for the supply line — The current required to pass the Hot Plug and EDID (Extended Display Identification Data) information between the source and the display. A weak current causes this handshaking to fail.

The HDMI standard specifies a minimum of 55 mA drain on the voltage supply lines for handling the Hot Plug feature and the passing of EDID information between the source and the sink. Some of the “active repeaters” built into HDMI cables draw their power from this same line. If the amplifier circuitry draws 100mA or more, which most do, and is used with a weak source current, the handshaking used during the creation of the connection and in maintaining it likely will fail due to the fact that it is not a requirement for all manufacturers to provide the high current required by such devices.

Display Data Channel (DDC) Corruption — EDID and HDCP (High-bandwidth Digital Content Protection for digital rights management) data are passed as part of the constant handshaking process between the source and the display. DDC data corruption is the largest cause of the connection having problems or not working at all.

Video quality can downgrade or even change in color if DDC is corrupted. With some systems, any DDC corruption can cause the unit to fail totally. HDCP needs to refresh about every 2 seconds. If the system does not get this refresh, it surely will die.

DDC-I2C (Inter-Integrated Circuit) — This electrical bus supplies all of the intelligence to the system. The clock signals used for the data transfers must match the timing of the data precisely.

Delay — The timing of the serialized signals varies for the individual conductors within an HDMI cable. If these individual delays fall outside of the “wiggle room” for the specification, the clock timing fails to operate correctly and failures occur.

Eye Pattern — This is the popular “Cat’s Eye” pattern shown in ads for many of the better cables on the market. The pattern visually depicts the electrical characteristics of the entire data transmission: output voltage, frequency response, jitter, noise, timing, and even the probability of its working well. It is a good indicator of the overall quality of a cable and how well it actually will work.

A “mask margin” is inserted into the actual signal reflecting the HDMI minimum compliance spec as a base for comparison. This margin is a percentage measurement. The key is to see how much performance over and above the spec’s margin is achieved and, in some cases, negative margin (or non-compliance).


The left photo shows a near perfect eye at 1080i for one of the vendors tested at the DPL labs -- notice how wide the white space is. The middle photo shows the eye for the same length at 1080i for a different manufacturer. Although both products pass the eye test with flying colors, the right cable would be the first one to fail when pushing for higher resolution (1080p) or longer distance (or if it is installed improperly). The left cable would get a higher DPL ranking than the left. If they cost about the same, which one would you buy? Needless to say, the cable on the right is "in the dirt.” It may work, but there is a high probability it will be unreliable. It would receive a low DPL rank.

Impedance — All wires exhibit resistance to the voltage being carried across it. To produce a perfect non reflective transmission line, load impedance must match the HDMI impedance of 100 Ohms. If there is not constant 100-Ohm impedance, the longer the cable, the more the performance of the signal is degraded.

Intra Pair Skew — The individual pairs of cables within the HDMI cable itself are twisted to eliminate noise and establish the operating impedance (balanced pair signaling). The tolerance of the twists drastically affects the timing of the signals running across them. If they are not completely symmetrical, one wire ends up being the slightest bit longer than the other. This means that the signal from one wire arrives the tiniest bit later than the other one, which could cause enough duty-cycle distortion to create bit-errors.

Inter Pair Skew — The timing between each of the balanced pairs of wires within the cable. They all must be twisted, laid, and cut precisely to reduce any length differences. Again, if the signal of one pair arrives even the tiniest bit later than another, it could destroy the signal.



Intra- and inter-pair skew: Some cables tested at the DPL labs were over 300 thousandths of an inch off in timing, meaning the individual wires vary too much in length.

Jitter — Changes in the timing of the signals over time. Due to tolerances and variances in the signal propagations, the signal characteristics vary, which affect the timing. Jitter is measured in picoseconds and has a very low tolerance.

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Derek R. Flickinger is vice president of R & D for Interactive Homes, Inc. He provides consulting and implementation services for manufacturers and integration companies on new technologies, products, and strategies related to standards-based Distributed Audio, Video, Communications, and Control (DAVCC) systems. Derek’s long-term goal is to be instrumental in the development and deployment of entertainment systems on space stations and space colonies.