The UHD, an inevitable challenge: bandwidth and SDR-HDR coexistence

Xavier Bonet, realization in TV3, coordinator of the postproduction group of UHD Spain and responsible for the Department of Technology of the ESCAC, open the doors to two interesting challenges that the industry will face in its transformation to the UHD: the question of bandwidth and the coexistence of material in SDR with HDR.

Since the beginning of the audiovisual industry, he has struggled to make the user experience as realistic and immersive as possible, with the will that the spectator can be isolated from his real environment and enter the audiovisual experience. To achieve this goal, there have been many efforts and technological improvements throughout history, and now It is the turn of the UHD.

With the purpose of establishing a unique criterion among all operators, within the Foro UHD Spain working on the definition of the UHD, and it agrees that only An increase in resolution UHD cannot be considered. Most forum members believe that, in order for a product to be considered UHD, it must contain all technical characteristics, WCG, HFR Y NGA, defined in the standards. This does not mean that until a full implementation of all concepts linked to the UHD is achieved cannot be dealt with very attractive intermediate situations, such as the HD signal in HDr.

An important idea of remembering, is that working at UHD does not mean that the characteristics should be carried out to its maximum performance. For example, using an expanded color space (WCG) does not mean that an audiovisual in black and white cannot be produced, using immersive audio techniques (NGA) is not at odds in producing in stereo, or that by using a high temporary resolution (HFR) a fiction can not be performed at 25p. Technology must be at the service of creativity, but it is important to clarify technological standards to achieve maximum compatibility between systems.

Canal UHD Spain - Carnavales

The impact of bandwidth

But the great challenge in implementing the UHD appears to manage your bandwidth. And this generates some concern, especially in the field of contribution, where you work with small compression factors. Increase spatial and temporal resolution, by example from a 1080@25p format to another 2160@50p, means multiply by eight in bandwidth, so you have to be very prudent when choosing which of the improvements offered by the UHD must be used in each project. Fortunately, one of the greatest contributions is the HDR, and its impact on bandwidth is small. So in this sense, the main problem for operators is to decide if they bet on the HLG hybrid format or by any of the commercial options based on the PQ such as the HDR10, HDR10+, Dolby Vision…

A concept that always goes hand in hand with HDR is WCG, And fortunately they do not have a strong impact on bandwidth. However, professionals must Learn to manage transformations between color spaces. The UHD establishes as a reference gamut the BT-2020, but the cameras has been widely exceeding this color space for years, and most new screens that are in the market cover almost 100% of a space DCI-P3. Consequently, in postproduction it is necessary to decide what are the most appropriate workflows so that these transformations do not distort the creative intention, while the colorimetry is adjusted to the desired regulations and limits.

One of the aspects that has a clear impact on the bandwidth is HFR And, for this reason, it is important to assess when it is convenient to use it. The opinion that a material produced in HFR produces the viewer is generalized a feeling of realism, that on some occasions, as in sporting events, informative programs, it can be very valuable. But on some occasions, for example in fiction or certain documentaries, this sense of reality can move the viewer to the purpose of alienation of the product, and this is due to the CULTURAL BACKGRESS OF THE CONSUMER. But the new generations are accustomed to consuming video games and social networks with an audiovisual narrative far from the classical narrative, and consequently the differentiation between reality and fiction can be different. That is, surely The audiovisual culture of the company will be the one that will end up marking the rhythm of implementation of the HFR in the audiovisuals.

Vimeo SDR / HDR

Can SDR material mix with HDR?

There are many television producers and chains that have multitude of files encoded in SDR and that now they find that they must combine it with New material in a project in HDR. And some of the questions that are usually generated are: "Show the master's degree in SDR and subsequently the HDR version or vice versa?", Or "is it possible to combine these two types of signal?"

The UHD Spain postproduction group has asked these questions to a Specialized Colorist Series And, from your experience, it can be concluded that the best practices to have two versions of the same product, a Sdr y otra HDR, it consists of Make a Master HDR first and then generate an SDR version, by changing the ODT (Output device transform) in the event that they work in ACES, or in those cases that require it, a Trim Pass to SDR through the adjustment tools offered by color correction programs.

When what is required is to generate a master in HDR that includes material, for example file, which is coded in SDR, the process You should not suffer any special treatment. It must be indicated by a IDT (Input device transform) to postproduction software that the material comes from a color space That bt.601 or That- bt.709, and then treat it as if it were recorded in HDR. What is highly recommended is that, as far as possible, it is avoided using SDR images that the whites “trimmed” have, since a signal that does not exist cannot be recovered.

Limitations and opportunities

Due to the destructive nature of LUT, its use in creative color correction environments is not recommended. Even so, experience shows that some of the solutions in the market capable of conducting technical conversions of SDR a HDR And vice versa are very satisfactory. Of course, it will have to be caution with small details such as graphics, flies, all that material that is over-impression and will have to be evaluated how to treat the diversity of content to adjust them to the final emission.

One of the main limitations that television operators have is that, if they broad Receptor Park is not fully adapted to the UHD. The best alternative would be to perform a simulcast en SDR y HDR, but the availability of radio spectrum does not allow it. For this reason in Spain, most content diffusers in UHD are the platforms that distribute online. The availability of bandwidth, the possibility of adapting the quality of the signal to each user, as well as the flexibility of implementing new codecs, offer clear advantages with respect to traditional diffusion channels, which do not leave indifferent to television operators. Even so, It will be necessary to see what tour can have solutions such as 5G or DVB-I.