Posted by: Bob Ferguson on March 03, 2017
Recent discoveries have been made about HDBaseT™ signals over the last few years, impacting the types of cables that are best suited to carry these new signals.
As recent test results show, category cables – which are optimized for Ethernet traffic – often struggle with the higher bandwidth requirements of a 4K/UHD HDBaseT signal. The characteristics of category cables are, at times, overkill for HDBaseT applications; other times, category cable characteristics are not sufficient to guarantee a good 4K/UHD signal. HDBaseT is a different type of signal, and 4K video is pushing that limit even further.
To help you better understand the capabilities of category cables when it comes to transmitting 4K/UHD HDBaseT, we want to share the results of Belden’s in-house cable testing. But, first, it’s important to know what standard we tested to.
HDBaseT uses HDMI signal for its main video transport. The HDMI standard has become the gold standard for residential and commercial high-definition video delivery because HDMI offers exceptional-quality, high-definition video. In addition, there are more than 4 billion devices using HDMI cables today – so it’s not going away anytime soon. The HDMI standard defines acceptable picture quality as one that has less than one error per 1 billion pixels, guaranteeing that the picture you see is the picture that was sent. It only makes sense that the video image be tested to the HDMI gold standard for the HDBaseT 4K/UHD signal.
Visual lossless or lossy pictures (nice ways of referring to pictures with errors that you will more than likely never notice) seem to be a topic of concern. Who is going to notice a few errors with more than 8.3 million pixels in every frame, and 30 to 60 frames per second? If you are going to settle, why consider a 4K image when you can get a great deal on a 720p monitor? But people are not settling; they are willing to invest in a system that delivers the best picture possible.
Before testing, the first thing we evaluated was the HDBaseT signal itself. We looked at the frequency response curve, which is the power level by the frequency of the signal. Simply put, this shows the maximum frequency of an HDBaseT signal. The HDBaseT signal has a frequency response curve that is not more than 500 Mhz, with 94% under 425 MHz. Cables tested over this frequency are of very limited value. (See chart below.)
The goal of our testing was to figure out which electrical parameters matter for HDBaseT systems. Which category and style (shielding) performs the best? Each category cable level specifies a set of electrical parameters for performance, such as capacitance, crosstalk, impedance, attenuation, return loss and several others. How do these parameters measure up when carrying an HDBaseT signal? Which levels meet the challenge and which do not?
The original specification by the HDBaseT Alliance called for Category 5e cabling, which was the most dominant cable used for network installations at the time; however, from the beginning, issues arose with standard cables being used for 4K/UHD video.
A vast majority of U.S. network installations use unshielded twisted-pair cable (UTP). Depending on budgets and network demands, some customers upgrade to Category 6, and a few futureproof their systems with Category 6A cables (almost all UTP cables).
While the HDBaseT signal works great in the lab for a short distance on a Category 5e UTP cable, alien crosstalk becomes an issue when installed out in the field. Alien crosstalk is electrical noise from outside the cable; often, the worst source for this noise comes from adjacent cables carrying the same signal because the frequencies match (bundling).
Much of the HDBaseT industry has resolved this issue by using a screened or shielded cable. Thus, the search for a cable to support the new signal began. What started out as an attempt to achieve infrastructure convergence (using the same cable for multiple systems) failed because equipment manufacturers were being forced to select a different cable to get their products to work in the field.
The HDBaseT Alliance currently recommends that users consider upgrading to Category 6, ideally moving to Category 6A or Category 7/7A for better performance when transmitting 4K/UHD. While this does help with HDBaseT signal transmission, our recent testing shows that it is still not enough.
Some consider Category 7 or 7A cables as an option for HDBaseT 4K/UHD signals. First of all, Category 7/7A cables are not recognized standards by ANSI/TIA-568 documents (the Telecommunications Industry Association writes category specifications for U.S. markets); it’s part of the ISO/IEC 11801 international standard only.
Category 7/7A is an individually shielded 4-pair cable, usually with 22- or 23-gauge copper conductors and with an overall shield, which is often a braid. In order for individual shields to maintain effectiveness, they need to be maintained through the connector, which is not possible with RJ45 connections used for HDBaseT equipment.
Category 7A is tested to 1000 MHz and Category 7 is tested to 600 MHz, which is overkill when compared to the maximum 500 MHz requirement of the HDBaseT signal.
Furthermore, our shielding testing proves that most of the issues come from alien crosstalk – not crosstalk within the cable. Shielding pairs from each other is not an advantage. To make sure you get the performance you need and expect, however, we don’t offer Category 7 for HDBaseT applications; testing has proven that it is not an ideal fit.
If category cabling is not the answer, what is?
Belden took the information it learned from testing and developed a new cable specifically for 4K/UHD HDBaseT transmission. We took performance parameters specified by TIA for category cable and modified them to better fit the HDBaseT signal. We wanted to make a cable that is easy to install and price competitive, but still designed to offer the best transmission possible for the 4K/UHD HDBaseT signal.
We also realized the industry was struggling with an infrastructure convergence message that was not working. Instead of buying category “something” cables, look for a cable that is designed specifically for HDBaseT.
My next blog will explain more about how Belden’s testing lead us to this conclusion. Or, if you want to learn more now, or download our white paper at http://info.belden.com/ecos/4k-uhd-wp.