4K UHD resolution in ProAV

In video signal transmission from sources to displays we deal with digital data that needs to be transferred with maximum accuracy, but how exactly high definition 4K video is sent to displays?

And especially how should we carry high resolution videosignal, as now we hear about various formats of 4K video as 4:4:4, 4:2:2, 4:2:0? Which of them is better in digital video transmission and what’s all about?

A native television resolution of 3840 pixels by 2160 lines is referred to as 4K UHD (Ultra High Definition) when viewed on a TV. 4K UHD is a format with an impressive 8,294,400 pixels per frame, which is more than four times the pixel count of a Full High Definition 1080p format (1920×1080=2,073,600). Additionally, it requires a 16×9 aspect ratio at a minimum, and it can refresh at 24 frames per second (23.98), 30 frames per second (29.97), or 60 frames per second (59.94).

Due to the fact that 4K UHD sets are capable of exceeding most available bandwidth that we have available through most common connections such as HDMI and HDBaseT, we are required to carefully define the colors and bit sampling depths used.

For the purpose of postproduction and transmission, 4K UHD will undergo a similar process of color subsampling in the TV studio, just like any other TV resolution format.

4:4:4 Format

As a rule of thumb, all images from 4K UHD TV studio cameras are sampled in 4:4:4 format, so that all images are sampled in this format as well. Below is a grid of 4×4 pixels that explains this format in more detail. In RGB format, there are two color difference pixels for each pixel: Cr (red differential) and Cb (blue differential). The matrix that converts YCrCb to RGB is transparent. Because of this, the 4:4:4 format represents the most color information available on the camera sensor.

4:2:2 Format

In broadcast studios and postproduction facilities, 4:4:4 formats are typically subsampled to a format called 4:2:2, in which half of the Cr/Cb samples are used horizontally. Because our retina has considerably less color resolution than black & white, such sub-sampling is well suited to human vision. Therefore, for video material, there is little difference between 4:4:4 and 4:2:2.

4:2:0 Format

It is also recommended that 4:2:0 be used as the transmission format of compressed television programs for further data rate reduction. 4:2:0 takes advantage of the fact that our retina reduces color resolution in all directions. 4:2:2 only reduces color resolution in horizontal directions. As shown below, 4:2:0 reduces color resolution both horizontally and vertically:

When it comes to regular video material, it is very difficult to distinguish any quality differences between the 4:4:4 and 4:2:0 formats.

Almost all TV programs today are encoded 4:2:0 and 8 bits/pixel before being transmitted via Terrestrial, Cable or Satellite and Internet streaming. The savings are evident. For example, 4K UHD 4:4:4 8 bit/pix at 60 F/s requires 12 Gb/s data rate. In contrast, 4K UHD 4:2:0 8 bit/pix at 60 F/s requires 6 Gb/s data rate. For a change that is difficult to see on the TV screen, that is a reduction by two.

A table below shows that HDBaseT can support 4K UHD 4:2:0 8 bit/pixel at 60 F/s, which is the most widely used digital connectivity format over twisted pair today.

The HDMI 2.0 specification supports 4K UHD 4:2:0 8 bit/pixel at 60 frames per second as shown in the table below, where Black represents HDMI 1.4 and Orange represents HDMI 2.0.

Based on articles By Mike Tsinberg