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High Dynamic Range Video– Software is the Solution

Today, there is much debate about Ultra High Definition (Ultra HD) focused on three issues: more pixels (4K or 8K); more video images or higher frame rates (HFR); and better video image quality or high dynamic range (HDR) and extended color gamut.

HDR is a ready way to increase perceived image quality, and this article reviews how it can be implemented. Technologies exist to deliver HDR, although standardization remains unresolved. Any implementation will require the flexibility found with software-defined video (SDV) solutions.

In 1990, the colors available to HD television were defined by the International Telecommunications Union (ITU) as ITU-R Rec 709. Shown as a triangle superimposed on the CIE 1931 color space diagram, the limitations of Rec 709 are clear. It was defined to reflect the capabilities of technology at the time, specifically the limitations of color CRT displays.

In 2012, the ITU issued ITU Rec 2020, a color gamut better suited for today’s high-performance flat screen displays. Current HDR video efforts are working towards implementing the expanded color space available in Rec 2020.

Both a technical and a business case consensus around HDR are needed. Sub-sectors in the industry – such as pay TV providers and the content producers who serve them – will thrive or be challenged, depending on the set of specifications, standards and practices that are ultimately adopted.

Every Bit Helps

Elemental_HDR_White_Paper_8-bit_10-bit_Color-StackThe number of colors that can be displayed in video is directly controlled by bit depth. In an RGB system, the bit depth of each color determines the total number of colors available. An 8-bit bit system offers 256 levels per color, or 256*256*256=16,777,216 different colors. In a 10-bit system, there are 1024 levels per color, or over a billion different shades.

A billion colors may seem unnecessary, but adding more subtlety to shades enables smoother color transitions within color families. In a video image with large areas of a similar color – a blue sky or a green forest, for example – an 8-bit color environment can lead to distinct stepping artifacts where the color splits into visible bands. 10-bit color makes the difference between adjacent shades much finer, reducing visible and intrusive banding.

More colors leads to a perceived improvement in picture sharpness: HDR can make a video image look like it has more pixels. This is because there is a finer, more sharply delineated difference between each color. To the human eye, HDR more closely approximates what is visible in the natural world.

Finally, HDR can extend the dynamic range by making the blacks blacker and the whites whiter while still retaining smooth, linear color transition in the mid-tones, if the screen has the capabilities to handle HDR.

Display Technology

CRTs are limited in terms of light level ranges they can generate, which is why the original ITU Rec 709 establishedElemental_HDR_White_Paper_Color_Gamut_Chart a strictly restricted color space and only 8-bit color. Attempts to improve brightness from a CRT results in increasingly smeary pictures, which reduces rather than increases the perceived resolution.

Liquid crystal displays (LCDs) are also limited in dynamic range. Even when a pixel is notionally set to black, some light leaks through, and when set to white, it is not perfectly transparent.

The emergence of the OLED screen has raised the potential for HDR. OLED is an emissive technology: the pixel itself generates light. In the best OLED screens, the black is remarkably black, and high output devices are being made that can generate bright white levels. Between the two extremes, OLED screens are remarkably consistent.

This linearity raises another benefit. Historically, a gamma curve was used to make the most of the available dynamic range of the CRT. It ensured best use of available bandwidth, enabling colorists to make artistic judgements about how the limited range of colors could best be used.

The gamma curve has now been replaced by a new formula, the electro-optical transfer function (EOTF), sometimes called perceptual quality (PQ). This provides a more granular way of mapping luminance while retaining creative control. EOTF allows some existing distribution architectures to carry HDR content, and is incorporated into the high-efficiency video coding (HEVC/H.265) compression standard.

Compatibility

Elemental_HDR_White_Paper_Solution_Comparison_Chart_FinalWhen color TV was first introduced it was backwards compatible: one signal could be viewed on black and white and color receivers, where the black and white receiver ignored the color information. Today, many industry including Dolby, Technicolor, Philips and BBC/NHK,  advocate a backwards-compatible approach to HDR.

Others, especially those working in over-the-top (OTT) streaming and Blu-ray 2, are less concerned with backwards compatibility. Implementing their proposals for broadcast could mean swapping out encoders, decoders and set-top boxes.

The other structural difference between approaches is the single- or dual-layer question. In a dual-layer approach, standard dynamic range (SDR) and HDR video streams are carried separately through the workflows, and the receiving device selects which stream it can accommodate. This adds complexity, increases required bandwidth and raises challenges for internal workflows, especially when splicing SDR commercials into an HDR program, or inserting emergency broadcast system messages.Elemental_HDR_White_Paper_Parade_SDR_HDR

In a single layer approach, one workflow carries the SDR signal, plus additional metadata telling HDR-equipped devices how to extend the dynamic range. This provides full backwards compatibility – non-HDR devices will not understand and therefore ignore the additional metadata. Introducing additional processing may limit the benefits of extending dynamic range.

Encoder designs will also need to ensure HDR can be achieved in real time, as live sport is a big driver for HDR. It should not impose too much latency.

Software

Currently, there is a genuine interest in HDR as a means of delivering more engaging content to consumers. HDR can deliver a real boost to perceived video quality at a modest additional budget per bit. Both industry and consumers want HDR sooner rather than later.

The practicalities, however, are far from finalized. Competing proprietary solutions are on the market, with standards bodies racing to keep pace. Content owners could be asked to deliver HDR content in different formats for different distribution platforms, making workflows more cumbersome and increasing processing and storage costs.

Elemental provides software-defined video solutions for HDR implementation, allowing broadcasters, service providers and content owners to create, manage and deliver content. This can be done without forcing expensive decisions and the outlay of a substantial amount of capital for video infrastructure, and does not require the maintenance of parallel hardware HDR silos. Elemental offers a scalable, flexible approach to future proof video workflows kept current through software updates.

This solution aligns with the trend towards software-defined architectures, and a readiness to take advantage of virtualization and the cloud. It reduces both capex and opex, minimizing risk during the transition. Elemental’s flexible and scalable software allows organizations to embrace HDR quickly, lead the market, and meet the expectations of their audiences.

 

About Keith Wymbs

Keith possesses 20 years of business leadership experience in the video, IPTV and telecommunications industries. As Chief Marketing Officer, Keith leads the development and implementation of Elemental’s worldwide marketing strategy and oversees the company’s marketing functions, ranging from product marketing to outbound communications. Keith has a proven track record of consistently generating growth businesses, building and restructuring organizations, and developing new business ventures and products. Prior to joining Elemental, Keith led the marketing team of IPTV pioneer Tut Systems (acquired by Motorola), the business development efforts of Viagate Technologies (acquired by Tut Systems), and most recently, a wellness product company as CEO. Earlier in his career, Keith managed a variety of growth businesses for AT&T and was a strategy consultant for wireline and wireless communications companies at PricewaterhouseCoopers. He currently serves on the board of directors of Coherence Resources. Keith earned a B.S. in engineering from Cornell University and an M.B.A. from New York University.

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