Active Optical Cables vs. Traditional HDMI cables

According to HDMI.org "The longest HDMI cable runs seen to date have been achieved using fiber-optic cable, which is far less susceptible to attenuation and interference than copper. Electrically it is similar to an active cable or Cat 5/6 solution, the principal difference being the higher-quality optical cabling between the transmit and receive connectors. HDMI over fiber is a highly robust solution, effective in cable runs up to 100 meters or more."

But why?

First, let’s start with a refresher on how video signals work. Your source (DVD player, game console etc.) generates values for each pixel to be displayed and sends them to an encoding unit. The encoding unit takes these values that specify the address and color each pixel and turns them into numerical representations using binary code. On the other end of the cable in your TV is a decoding unit that will translate these 1s and 0s back into pixel coordinates and colors. The encoding and decoding units are somewhat like telegraph operators, converting words into dots and dashes which are sent to another operator who receives them and turns them back into words. These 1s and 0s are transferred as “packets” which are series of electric pulses through the wire (1 represented by a charge, 0 represented by no charge). The receiver doesn’t care about the strength of the charge in a digital signal, just that there is one or isn’t one at any given point in time. It can seem complicated at first but the underlying concept is, in many ways, similar to Morse code. Optical cables use a concept similar to copper cables but take it one step further. Optical takes the electric pulses and converts them into pulses of light. Again, there are only two “words” than can be used with this method “On and Off” (1 and 0). Again the pulses are transferred as packets down the cable until being decoded and displayed. The reason optical HDMI cables can work at a greater distance than copper HDMI cable is due to the material and type of signal which passes through it. Earlier we mentioned that data is decoded in “packets” of 1s and 0s and now that becomes important. Due to the nature of copper and design of HDMI cables, these packets can get disrupted in route, preventing the data in the packet from arriving at the correct intervals. This can result in what are often referred to as “sparkles” which are just random pixels displaying the wrong color/brightness or not displaying at all. If enough packets get disrupted the entire signal will fail completely. A good way to think of this concept can be thought of as spelling a phrase if the spaces aren’t placed at the right points the letters no longer make sense.

The problem is in the material

A copper HDMI cable consists of 19 internal wires all carrying various pieces of data. The tightly packed wires exchange and disrupt adjacent charges, the result is interference created by interaction among all those small wires. These interferences aren’t noticeable on short runs of HDMI cable but become more evident in longer cables. Not only is there interference within the cable, the environment can’t disrupt the electric pulses in the wire. Other cables prevent this environmental disruption with heavy shielding, but due to the 19 internal wires in an HDMI cable, it would be far too thick if it was covered in a heavy shielding.

An optical HDMI cable consists of a single fiber of pure glass at the core and a layer of slightly different glass surrounding the core called cladding. Light passes through the core and the cladding prevents it from escaping. There is nothing to interfere with the signal on the way because it is just one single strand. The light just reflects off the walls of the cable back and forth while it travels until it reaches its destination. This is similar to how in a hall of mirrors; someone around a corner can appear to be directly in front of you. Light is also much more resistant to electromagnetic interference from external sources than electricity.

Optical cables are immune to the problems faced by their copper counterparts due to the physics of light and the single fiber strand through which the signal passes. The resistance from the glass experienced by the light is negligible so the signal maintains its fidelity for a much longer distance. Packets always arrive at the right intervals so there is no loss of data along the way.

Now if your head is spinning after all that HDMI talk we don’t blame you. Feel free to give this article another read and check out the accompanying diagrams and it will probably make a lot more sense.

If you’re looking for a great solution for a long run HDMI cable check out the Light-Link our Active Optical HDMI Cable.