HDMI Corner: Avoid Surprises with Legacy Cables and 4K 60Hz

The expansion to 18Gbps provides integrators with a choice: do the necessary tests with your legacy cables in the field, or leave yourself open to new challenges.

Jeff Boccaccio

There's an old saying that seems to come up often when working within the digital video environment: “Problems are opportunities in disguise.”

We’ve had quite a few opportunities over the past 10 years as each new HDMI revision brings on new challenges. The most recent revision left us with a few surprises, which eventually turn into opportunities:

  • The bandwidth practically doubled from 9Gbps usable to 18Gbps. This brought on an opportunity and challenge for all manufacturers to step it up a bit. From electronics and accessories to cables and long distance solutions, the expanded bandwidth would affect all.
  • A new HDCP revision that had some compatibility issues. One more task that had to be added to list of system integration design, to be sure all the products are compatible with HDCP.
  • Poor public outreach providing an easier way to learn the necessary facts about some of the limitations of 4K as its deployment expanded.
  • A big surprise last spring when HDR was announced under what now is Rev 2a, exposing the need for greater bandwidth that should have been addressed with the 4K 60Hz announcement.

Integrators must deal with every one of these “opportunities” through education, experience, or by just punting. We at DPL Labs hear you all the time, so I am going to hit you with another one that may just sneak up on you.

As with any technology there are rules in place to try to get everyone building products on the same page. Take audio, for example. When audio products are constructed they follow some basic rules so they can work in harmony. One in particular is the reference level (amplitude) — whether it was a phono cartridge, tape recorder, preamp or even a power amp there are rules when it comes to levels. Otherwise everyone would invent their own level and some products might be too sensitive while others would be too low in gain. Got it?

With the expansion to 18Gbps,
actually being able to see the signal
after traveling through a worst-case transmission line provides better test data.

Back to digital video … rules emerged involving amplitude and time. The relationship is a bit more complex than the signal we would expect out of an audio preamp, but it’s amplitude (referencing volts) and time (base referencing jitter) that basically govern digital video output. A standard measurement practice generally used and accepted by all digital testing firms is that of waveform polygons called eye patterns, with limits set by its eye mask. They have both vertical (amplitude) and horizontal (time) attributes, as in pictured above. Judging performance of the test device depends on how the eye mask is set up — the larger the eye mask, the more difficult to pass.

DPL uses a special mask developed to emulate worst-case scenarios we’ve encountered. However, under the current 4K 60Hz environment this has changed to some extent. Before, we always knew what the device’s output was by a set eye mask. But today the preferred method with test equipment is to measure the eye after it is sent through a predetermined transmission line that is said to be worst case. This makes good sense in many ways, except when it comes to legacy transmission lines that may not have been tested to a worst-case comparison.

In the past when testing sources we were only concerned about what came out of the source’s output connector, not out of a predetermined transmission line. Now, with the expansion to 18Gbps the eye becomes that much more critical, and actually being able to see the signal after traveling through a worst-case transmission line provides better test data.

So for those of you that have lots of legacy cables in the field, make sure to do the necessary tests with 18Gbps products. Otherwise running the newer 4K 60Hz sources may become just one more “opportunity.”