High Dynamic Range (HDR) and the companion technology Wide Color Gamut (WCG) combine to provide better images. HDR offers greater dynamic range, giving brighter highlights and deeper, clearer shadows. WCG expands the palette of available colors to reproduce the world we see more faithfully. This is achieved without significant changes to existing signal standards, so it does not require a complete change of infrastructure.
From the inception of television in the first half of the 20th century, until the development of digital television (DTV) in the 1980’s, television standards have advanced to produce better pictures, including the standardization of black and white broadcasts, the introduction of color and transition to digital creation, processing, and delivery. However, since then, the most noticeable most noticeable impact has been to create bigger images through the development of high-definition (HD) and ultra-high-definition (UHD) television.
However, the individual pixels have been limited to similar restrictions that existed over 70 years ago. HDR has now introduced techniques and technology that allows us to make better pixels which produce better images including brighter highlights, more detailed shadows, and a much broader palette of colors. These improvements have a greater impact on the viewer experience than bigger images in most cases.
HDR introduces a change to the curve that maps the brightness of light to digital code words that are transmitted and processed. These transfer curves are optimized to provide brighter highlights and better shadow detail. HDR imposes a minimum of 10-bit coding, which is the norm for most broadcast and professional production, so much of the existing infrastructure can handle HDR signals. It is important to avoid 8-bit codecs as these will show much more distinct artifacts in HDR. Similarly, BT.2020 Wide Color Gamut employs optimized primaries and maps broader range to encompass about 99% of colors the human eye can see in nature. This wider color gamut is mapped into the same 10-bit codewords as are employed in traditional BT.709 coding. Although the dynamic range (HDR) and wide color gamut (WCG) are defined independently, they are used together in most situations.
There are two primary standards in common use for HDR in broadcast production: Perceptive Quantization (PQ) and Hybrid Log-Gamma (HLG). These are more than just different “gamma curves”, but each represent a different approach to HDR. Both HLG and PQ can be (and usually are) paired with BT.2020 colorspace for wide color gamut. It should be noted that Sony’s S-Log3 comes into the conversation here; S-Log3 is not considered an HDR production format (and never a transmission format), but very common in live workflows and does offer extended dynamic range compared with traditional SDR. Fortunately, most HDR-capable converters, such as Gator-Toolbox, can also deal with S-Log3, so an HDR-capable workflow usually can accommodate S-Log3 sources as well. This has enabled HLG to become preferred in many HDR live productions.
HDR and WCG can pass through most existing SDI and IP infrastructure, since video is encoded into the same 10-bit transport as SDR. The use of embedded metadata through SMPTE ST 352 Video Payload ID in SDI, and colorimetry metadata in SDP for ST 2110 environments can communicate to a receiver how to interpret the pixel data. It is important that HDR processing equipment be able to receive and interpret this metadata and have the ability to override it in cases where upstream equipment may not have properly preserved it. Equipment should also provide the capability to accurately author this metadata based on the current production workflow so that downstream equipment can properly detect the video encoding.
Note that HDR/WCG video cannot be directly combined with traditional SDR signals. Just as it is necessary to normalize to a common resolution and frame rate, all media must be normalized to a common color standard in order to combine them in a production setting, so HDR conversion is required in these workflows. There are many factors to consider when converting between SDR and HDR/WCG. A complete conversion strategy should be developed before just throwing converters at the system. You need to have a good understanding of what your source material is (SDR? HLG? PQ?), and what your final deliverables are (are you producing exclusively HDR? Mixed SDR/HDR? How will your content be consumed?). Consider how your cameras will be shaded. Additionally, white levels can be more flexible in HDR production, so you need to understand how that impacts your end-to-end workflow. You will also need to consider whether display-light conversion (for a rich HDR look) or scene-light conversion (for a more natural look) are appropriate at each stage of your production chain. The Ross Gator-Toolbox is flexible to adapt to a wide range of these parameters making it a versatile choice for your production workflow.
In some simple cases, you may need to incorporate a small number of HDR feeds into your SDR production (or vice versa). In these situations, a simple direct-mapped conversion will easily achieve this. The capability is built into certain Acuity and Carbonite production switchers and Gator-Toolbox. For example, a sport stadium in house production transitioning to HDR benefits from the simplicity of a direct mapping utility conversion function to help monitor HDR signals, drive legacy SDR LED displays or incorporate a small number legacy SDR sources into the HDR LED screens during the transition period.
In many situations, it is necessary to produce both HDR and SDR versions simultaneously to serve both the emerging HDR-capable viewers and the larger established SDR audience. It is often not feasible to run separate productions for HDR and SDR, so hybrid workflows have been developed to create HDR and SDR content from a single production. In these situations, the conversions need to be matched so that HDR and SDR content appear matched, and that round-tripping (i.e., SDR->HDR->SDR) retains the look of the original material. These workflows provide recipes on dealing with SDR cameras, HDR cameras, graded SDR and HDR inserts. They offer guidelines on camera shading and monitoring throughout the process and indicate the type and position in the signal chain for HDR conversion devices.
These advanced workflows usually implement 3D lookup tables (3D-LUTs) to craft the overall look of the tone mapping between SDR and HDR. A 3D-LUT provides a customized RGB map of source to destination color spaces, providing complete flexibility in the tone-mapping process. They allow very precise tuning of the colors when tone-mapping between standards is performed (for example, how does a particular purple in BT.2020 colorspace map to BT.709, where that purple does not exist?). They are packaged as a set to include conversions between each of the supported formats. Gator-Toolbox offers optional 3D-LUT support, bundled with LUT packages from BBC and NBCU. Additionally, you can upload you own 3D-LUT to support the conversions required in your workflow. It also provides options for normal and extended range (allowing you to leverage the super-white range of the transport for greater dynamic range) and supports both scene-light and display-light conversion modes.
Ross produces live production solutions to build your HDR workflow including tools for HDR content creation and conversion. Ross Carbonite and Acuity production switchers will work cleanly in your HDR production, with the ability to pass and process HDR video. Certain models also incorporate input and output conversion for simple HDR workflows. For more advanced and hybrid SDR/HDR workflows Gator-Toolbox provides a complete set of HDR conversion tools including 3D-LUTs to match your chosen HDR workflow. These can be combined with our award-winning HDR-capable graphics solutions including XPression, Piero, and Voyager.