By Jonathan Abrams
Dolby, founded over 50 years ago as an audio company, is elevating the experience of watching movies and TV content through new technologies in audio and video, the latter of which is a relatively new area for their offerings. This is being done with Dolby AC-4 and Dolby Atmos for audio, and Dolby Vision for video. You can read about Dolby AC-4 and Dolby Atmos here. In this post, the focus will be on Dolby Vision.
First, let’s consider quantization. All digital video signals are encoded as bits. When digitizing analog video, the analog-to-digital conversion process uses a quantizer. The quantizer determines which bits are active or on (value = 1) and which bits are inactive or off (value = 0). As the bit depth for representing a finite range increases, the greater the detail for each possible value, which directly reduces the quantization error. The number of possible values is 2^X, where X is the number of bits available. A 10-bit signal has four times the number of possible encoded values than an 8-bit signal. This difference in bit depth does not equate to dynamic range. It is the same range of values with a degree of quantization accuracy that increases as the number of bits used increases.
Now, why is quantization relevant to Dolby Vision? In 2008, Dolby began work on a system specifically for this application that has been standardized as SMPTE ST-2084, which is SMPTE’s standard for an electro-optical transfer function (EOTF) and a perceptual quantizer (PQ). This work is based on work in the early 1990s by Peter G. J. Barten for medical imaging applications. The resulting PQ process allows for video to be encoded and displayed with a 10,000-nit range of brightness using 12 bits instead of 14. This is possible because Dolby Vision exploits a human visual characteristic where our eyes are less sensitive to changes in highlights than they are to changes in shadows.
Previous display systems, referred to as SDR or Standard Dynamic Range, are usually 8 bits. Even at 10 bits, SD and HD video is specified to be displayed at a maximum output of 100 nits using a gamma curve. Dolby Vision has a nit range that is 100 times greater than what we have been typically seeing from a video display.
This brings us to the issue of backwards compatibility. What will be seen by those with SDR displays when they receive a Dolby Vision signal? Dolby is working on a system that will allow broadcasters to derive an SDR signal in their plant prior to transmission. At my NAB demo, there was a Grass Valley camera whose output image was shown on three displays. One display was PQ (Dolby Vision), the second display was SDR, and the third display was software-derived SDR from PQ. There was a perceptible improvement for the software-derived SDR image when compared to the SDR image. As for the HDR, I could definitely see details in the darker regions on their HDR display that were just dark areas on the SDR display. This software for deriving an SDR signal from PQ will eventually also make its way into some set-top boxes (STBs).
This backwards-compatible system works on the concept of layers. The base layer is SDR (based on Rec. 709), and the enhancement layer is HDR (Dolby Vision). This layered approach uses incrementally more bandwidth when compared to a signal that contains only SDR video. For on-demand services, this dual-layer concept reduces the amount of storage required on cloud servers. Dolby Vision also offers a non-backwards compatible profile using a single-layer approach. In-band signaling over the HDMI connection between a display and the video source will be used to identify whether or not the TV you are using is capable of SDR, HDR10 or Dolby Vision.
Broadcasting live events using Dolby Vision is currently a challenge for reasons beyond HDTV not being able to support the different signal. The challenge is due to some issues with adapting the Dolby Vision process for live broadcasting. Dolby is working on these issues, but Dolby is not proposing a new system for Dolby Vision at live events. Some signal paths will be replaced, though the infrastructure, or physical layer, will remain the same.
At my NAB demo, I saw a Dolby Vision clip of Mad Max: Fury Road on a Vizio R65 series display. The red and orange colors were unlike anything I have seen on an SDR display.
Nearly a decade of R&D at Dolby has been put into Dolby Vision. While Dolby Vision has some competition in the HDR war from Technicolor and Philips (Prime) and BBC and NHK (Hybrid Log Gamma or HLG), it does have an advantage in that there have been several TV models available from both LG and Vizio that are Dolby Vision compatible. If their continued investment in R&D for solving the issues related to live broadcast results in a solution that broadcasters can successfully implement, it may become the de-facto standard for HDR video production.
Jonathan S. Abrams is the Chief Technical Engineer at Nutmeg, a creative marketing, production and post resource.