Why otherwise good HDMI cables won't always work with modern devices and how to test them

 

Why otherwise good HDMI cables won't always work with modern devices and how to test them.

A friend had trouble connecting a modern 4K TV streaming dongle, Chromecast with Google TV, to an older 4K TV. The problem lay with the HDMI cable. It had worked with the previous device, a DVD player, I think, but not the newer 4K dongle. I was mystified by how the cable did not work, so chased it up.

HDMI cables are very high-tech and work at very high data rates and bandwidths. As such, the possible length of a cable has always been an issue, something I discovered trying to run long leads to home cinema projectors. The physical construction of the cable has a big impact on the data rate it can handle.

Like network cables, HDMI cables use shielded twisted pairs of wires, including a shield ground for each pair. There are three signal pairs and a clock pair. As well there are other wires for digital control signals. There are, or should be, nineteen wires in total.

The HDMI standard has been around for 20 years, as have HDMI cables. Early HDMI standards and cables have much lower data rates and don't always implement all of the current digital control signals. 

A big problem with HDMI cables is that the standard bans the printing of the version, 1.4, 2.0 or 2.1, on the cable, so it is hard to tell what version the cable is designed for. The standard does allow the notional speed of the cable to be printed on the cable, Standard (10.2 Gbps), High Speed (18 Gbps), Premium High Speed, and Ultra High Speed (48 Gbps).

Network data cable runs at 1 Gbps by comparison, hence is able to have longer cable runs. Newer Cat 5 or 6 cables can run faster, primarily through the use of shielding, as HDMI also implements.

Older or cheap cables can be missing some of the shield grounds or digital control wires.

The problem comes when using old cables for new devices. I have a squillion old HDMI cables, some of which are more than 10 years old. How can I check them?

Good article about the different HDMI versions and other issues with cables.

https://au.pcmag.com/home-entertainment/60751/slaying-the-cable-monster-what-you-need-to-know-about-hdmi-cables

HDMI 2.1 & TMDS Crash Course - ENMU EET 457 - Presentation 

https://www.youtube.com/watch?v=5acgSK0kWTE

An explanation of the HDMI signals and wires

Testing HDMI cable: Continuity

A simple HDMI continuity tester will detect if all the wires are present. Cheap or old cables may not have the ground shield wires for all four high-speed channels or may be missing some of the control wires. All nineteen wires need to be checked for continuity, some testers only do a subset of the most critical. I have a UNI-T UT681 HDMI cable tester from Amazon ($48).

Testing Cheap HDMI Cable Testers

https://www.youtube.com/watch?v=di4xRSxNcG8

Some HDMI continuity testers are better than others. Explains how some cheap cables are missing wires, usually the ground screen wires for the main data pairs. Missing these ground screen wires will limit bandwidth and hence performance.

Testing HDMI cable: Performance

If a cable tests ok for continuity, the next thing is to check for the desired performance. Like other digital transmissions, HDMI cable has a digital cliff where performance suddenly drops off. As such, it becomes a pass or fail at the desired data rate, effectively the display resolution and frame rate.

The most simple test is to check the cable between the two devices at the desired rate, pass or fail.

https://au.pcmag.com/home-entertainment/60751/slaying-the-cable-monster-what-you-need-to-know-about-hdmi-cables