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UHD, Much more than 4K (II). Colour and speed
When we talk about Ultra High Definition -or UHD, for short-, we only tend to think about the increase in definition that this implies, but UHD is not just more pixels; instead, it is better pixels. In this series of articles, TM Broadcast explores what UHD really means and the most important aspects of this new technology.
Text by Yeray Alfageme, Business Technology Manager, Olympic Channel
We shall be addressing two crucial aspects in this article, colour and frame rate.
Within the set of standards that compose the UHD ecosystem, one seems to go unnoticed more often than not but represents a very noticeable change in the quality of the image. It also means drastic changes in how we transport the signal and produce it. We are referring to the colour space.
Until now, the standard used for both SD and HD signals was either Rec.709 or BT.709, which were limited to the number of colours that could be represented by the old CR cathode ray tube monitors. The colour space had to be restrained to avoid errors in its representation when being transmitted to a monitor that could not actually represent them. This led to a really limited colour space.
We can see (Picture 1) how the 709 colour space is delimited by the triangle drawn in the picture and its corners bear the three primary colours of this standard. The dot defined as “white” is at D65, which means that it is a 6500K white. The good thing about this colour space is that it only needs 8 bits per pixel to be transmitted.
However, as can be seen, much of the visible colour spectrum -everything that is outside the triangle- is outside of what can be represented by signals within this format.
Along with the introduction of the new UHD standard came a new standard concerning colour space, the Rec.2020 or BT.2020. This new colour space was called WCG (Wide Colour Gamut) (Picture 2). The white dot remains at D65, but the three primary colours are stretched to the maximum to cover a larger colour space and practically reach the limits of what is visible.
Naturally, this type of signals can no longer be transmitted on only 8 bits per pixel, so the number of bits had to be extended to 10 or 12, depending on whether we include HDR in the signal too, which we will talk about in the last edition of this series. There is one problem, however, and that is that there are virtually no monitors on the market capable of representing this vast colour space. Only Canon and Dolby laboratories -perhaps there is another prototype or last-minute model out there- have monitors capable of representing such a large number of colours. These are, obviously, out of reach for consumers and most production houses.