High Resolution Over Long Cables – A Challenge For Display Interface Ecosystem

MHL LLC Staff Writer
November 28, 2016

When I grew up, many teenagers were upgrading their car stereos.  I watched my brother and his friends install huge power amplifiers and subwoofer boxes in their trunks.  That same brother taught me how a coil of copper could be used as a low pass filter, cutting out the higher frequencies from the subwoofer. We did not know how it worked, it just worked.  It was not until years later, when in college, that I learned about inductors, RLC crossover networks, and transmission lines.  Fast forward to today.  The same principle that a cable is a low pass filter is appearing again.  This time, as display interfaces need to deliver increasing resolution.  Read on to learn more.

Today’s Display Interfaces Are Digital, That Helps, Right?

Modern display standards digitize video signals, which changes the way we experience attenuation.  One advantage of digitization is that subtle distortions and degradations do not occur.  However, attenuation is still present and once the signal has been attenuated sufficiently it will not be received correctly and the interface will stop working.  Techniques such as Forward Error Correction (FEC) can help make links more reliable by improving immunity to noise but they cannot remove the impact of attenuation.

Higher Resolutions Increase Attenuation

When digitized display interfaces were first introduced they seemed to solve all of our display connectivity problems.  Strange distortions at the edge of the screen, ghosting and color variations all disappeared.  And at the resolutions in use at the time, attenuation was only a problem at many tens of feet.  However, over the past 15 years display resolutions have increased rapidly.  To move all this additional information frequencies have been increased.  This takes me back to what I learned in my car audio installation days.  Copper cables act as a low pass filter.  So as frequency goes up so does attenuation.  This reduces the maximum cable length.

Quantifying Impact on Cable Length

There is no one answer as to how far a digital interface can drive.  It depends on the standard and the quality of the cable.  For example with HDMI 1.0 we see that  30 feet cables are fairly common, while with HDMI 2.0 cable length seems limited to about 20 feet due to the higher frequencies.  USB is also interesting to review; it has undergone similar changes in maximum cable length as frequencies have increased:

• USB 2.0     5 meters (480 Mbps)
• USB Type C    2 meters (5 Gbps)
• USB Type C    1 meters (10 Gbps)

Delivering Longer Cable Length

There are three basic techniques that are used to improve video and data losses due to cable length:

• Active Electrical Cables, which amplify the signal at one or more points in the cable.
• Active Optical Cables, which convert the electrical signals to optical for transmission and then back to electrical at the receiver.
• Using more data conductors which results in higher bandwidth without higher frequencies.

As the road to 8K displays is paved over the next few years, and bandwidth continues to increase, the display interface ecosystem will need to consider each of these solutions when determining how to deliver more data over longer cable lengths.  Clearly the display interface ecosystem has lots of decisions and tradeoffs before it.  One thing is for sure, it will be an exciting few years as we collectively chart a course forward.