Part 2 - HDMI Signals

Part 2 - HDMI Signals

Our last post described how compatibility for HDMI cables and switches is ultimately determined by the amount of data in the HDMI signal. To understand how different HDMI options affect the data rate of the signal, it helps to know a bit about how video is actually sent through an HDMIcable, and how that data is displayed on your TV screen.

TV screens are made up from millions of individual rectangular cells called "pixels". When you buy a 4K TV, the term "4K" is there to tell you the number of pixels in the screen, which determines its clarity. A 4K TV contains 3,840 columns of pixels (~4,000, or 4K), and each column contains 2,160 individual pixels, for a total of 8,294,400 pixels. Each of those pixels is usually composed of three even smaller cells - subpixels - each of which is lit to a different brightness to produce the overall color for that pixel.

When you use HDMI, video is sent in three channels. One sample is needed from each channel to color one pixel on the screen, but there are two different three-channel formats you can choose between:

Normally, the samples for each channel use 8 bits of data. HDMI 1.3 added a feature named Deep Color. Deep Color just means that each channel is using 10, 12, or 16 bits for each sample, allowing each pixel on your TV to take be colored more precisely. The obvious downside is that 12-bit sample require 50% more bandwidth than an 8-bit sample, and that increase applies to the entire signal.

As an example, a Premium High-Speed HDMI cable, rated for 18Gbps, must be able to pass 4K video at 60 frames per second using 8 bits per channel. Such a signal will use about 17.82Gbps, which is very close to the 18Gbps testing limit of the cable. If the same 4K video is sent at 10 bits per channel, it will require 20.05Gbps, which exceeds the speed rating of the cable, and is likely to fail.

The framerate, usually displayed in hertz (Hz), is the number of different images a TV can display in one second. A higher framerate allows smoother motion, while a low framerate can look choppy. If that same Premium High-Speed HDMI cable is used to send a 4K video at 30Hz (30 frames per second), the bandwidth requirement will be cut in half. At 30Hz, there is plenty of bandwidth available for 10 bits per sample, or even 12 bits per sample.

As you can see, it's hard to give a complete answer to the compatibility question in one sentence. An HDMI switch may support 4K video at 60Hz and support 4K video with Deep Color, but not support both at the same time.