HDMI cables 101

An easy way to understand fact from fiction with HDMI cables is to look at is close relative, twisted pair cables. If you don’t know anything about twist pair, let me introduce you; you are using it right now (even if you are using a Wi-Fi enabled device to read this story). Twisted pair is the ubiquitous network cable used in data networks. Since business, industry, governments and military communications are dependent on this type of wire for data transmission, there has been a lot of research and testing conducted. What is known to be true in the world of twisted pair cables is that all cables are not created equal.

HDMI transfers digital data (1s and 0s) from sender to receiver just like networking cable does. The ability of twisted pair networking cabling to effectively sustain a data transfer rate is broken into categories established by the ANSI/EIA (American National Standards Institute/Electronics Industries Association) Standard 568. The standards cover things like connectors, insulators, installation lengths and material used to construct the wire, all of which play a part in the ability of the wire to carry a signal from point A to point B. The most common type of network wiring in use today is category 5 or category 5E for data local area networking applications, and category 3 for voice and data applications. To achieve a particular standard the cabling must be able to sustain a particular transfer rate when tested, 10 Mbps for category 3 and 100Mbps in the case of category 5. Things like how many twists per inch of wire length and shielding affect the amount of data throughput that a cable can effectively sustain. The category that twisted pair wire is certified for ensures that it will perform within the guidelines for the category in question at minimum. But it is important that you understand that the cable may be able to achieve sustained data transfers at a much higher throughput than it has been certified for. For example, it is possible to achieve a transfer rate of 100Mbps using category 3 cable. I know this to be true because I have firsthand knowledge of this.

I was testing network wiring run into a hotel boardroom a few years back, and one of the wires I tested was failing the 100Mbps test. When I investigated the situation, I found that wire that was installed was category 3 (with category 5 ends), so that explained the failure. 4 pair Category 3 and 4 pair Category 5 wire look the same, when you can’t see the twists of the pairs; so if both type of wires are being used, the installer must be careful to read the jacket so that they use the correct wire. Obviously the installer on this job wasn’t so careful. The unexpected thing was I tested 7 other runs going into the boardroom, (all the same length as the cable that failed). Each passed the 100Mbps test however, all 7 runs had category 3 wire installed instead of the category 5 wires I expected.

HDMI cables perform the same function for AV components that twisted pair wiring does for networked equipment; get a digital signal from point A to point B. Manufacturers of HDMI cables have to wrestle with impedance, crosstalk, EMI and attenuation just like manufacturers of twisted pair cables. Manufacturers of HDMI cables (like manufacturers of twisted pair cables) have a choice, make and sell cables that just meet the minimum standards or make cables to outperform the standards. Changes to HDMI specifications and cable requirements have advanced rapidly, so cables designed to meet the minimum standards, may not be adequate for use with newer components.

The HDMI organization has adopted the practise of ANSI/EIA and certifies cables into categories. Currently there are two categories of cables, category 1 and category 2 (Standard Speed and High Speed respectively). Category 1 cables must be able to perform at 75 MHz at minimum, which is a throughput of about 2.23 Gbps of data. Category 2 cables must perform at 340 MHz at minimum, a throughput of 10.2 Gbps. One thing you should note when reading the specs on HDMI data throughput, is the throughput is sometimes stated differently. HDMI transfers data on 3 different channels and the data throughput rate is usually stated as an aggregate of all three channels. Manufacturers of HDMI cables and HDMI switching equipment will sometimes state only a single channel throughput, since all channels run at the same speed. So you may see an HDMI category 2 cable listed with a data transfer rate of 3.4 Gbps, which equates to an aggregate transfer rate of 10.2 Gbps across the 3 channels. According to the HDMI organization, cables tested for category 1 compliance can effectively transfer enough data per second to handle a 1080i signal. Category 2 cables can handle a 1080p signal, at increased color depths (color depth above 8 bit) and at higher refresh rates (refresh rates above 60Hz). Category 1 and 2 cables are tested up to distances of 15 meters and 7.5 meters respectively. Going forward as higher HD resolutions are adopted (such as 1440p), and color depth and refresh rates increase, new categories of HDMI cables will be forthcoming.