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Cutting Edge Broadcast Solutions
High Dynamic Range (HDR) Solutions
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.
How Does HDR Work?
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.
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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. It is flexible to adapt to a wide range of these parameters making it a versatile choice for your production workflow.
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