Not all significant TV technical improvements require more bandwidth.
The race to ever higher pixel counts never seems to end. One result is that consumers now believe that the path to higher quality images is through more pixels. Yet, other technologies like HDR, WCG and HFR can enhance every TV pixel by adding clarity, depth, and realism without requiring more bandwidth or expensive new production and broadcast workflows. The path forward depends choices made by television set makers as well as broadcasters.
A couple of NAB Shows ago, a big telecom exhibit housed a demonstration consisting of three 50-in displays. Each screen was separated by about two feet behind a common generic bezel, and each anonymous screen displayed the same feed from a helicopter 4K wide shot of a nighttime baseball game from Los Angeles' Dodger Stadium.
All three screens were impressive, but the one in the middle showed a remarkably superior picture. The stadium was significantly brighter than the others, and detail under the brightest stadium lights and in the darkest streets outside the stadium was significantly more visible than the other two screens.
Attendees were told the screens displayed either HDTV or UHD, and the viewers were asked to guess which screens displayed which format. During the presentation I attended the group agreed that the center screen displayed UHD and the other two displays were HD. In fact, the center screen displayed a 1080p with HDR (High Dynamic Range) image while the outer screens were UHD images with standard dynamic range (SDR). That demonstration convinced me that more pixels don’t necessarily make better pictures.
TV HDR today
HDR TV isn’t exactly new. Netflix added it in 2016 and HDR channels are available from numerous OTT sources. Momentum has been building for HDR in the OTT world, primarily because a server can handshake with a display, recognize the it as HDR or SDR, and stream accordingly.
However US TV broadcasters must transmit one stream for all viewers, whether that is HDR, SDR, WCG, SCG, HFR or SFR. ATSC 3.0 does just that, except HDR and WCG do not work with interlaced formats. And WCG and HDR require only 5-20% more bandwidth to create an HDTV channel, depending on systems and settings. Test viewers also typically find that high frame rates (HFR) provide a visible improvement and, when compressed in HEVC, do not add that much bandwidth. All this suggests there are other ways to improve the visual image instead of simply throwing more pixels at viewers. A genuine downside of the more pixel solution is that the required bandwidth for a single UHD channel will eliminate some digital subchannels and the revenue they produce.
The good news is that HDR is built into many new 4K TVs, partially thanks to brighter screen technologies. HDR visual content improvements are not subtle with HDR sources. HDR is typically more noticeable than 4K's added pixels, and the benefits of HDR, WCG and HFR can be seen from across the room. Some say HFR is a bit too realistic, but that is another story beyond the scope of this article.
Attention is currently focused on HDR because it is already available in many television products, ranging from server to off-the-shelf TVs. Conversely, WCG displays are rare, expensive, and not likely to be market-ready until 2020. Even so, HDR, WCG and HFR technologies are all supported by the ATSC 3.0 standard with Video-HEVC A/341. About the only places one might see real WCG today is at the Consumer Electronic Show and and the NAB Show.
Benefits of HDR
In his Technical Guide to High Dynamic Range & Wide Colour Gamut, author Peter Schut, CTO & VP of R&D at Axon says “HDR can produce a dynamic range of 200,000:1 (or 17.6 stops in camera terms) when shown on a 2,000 cd/m2 display with a bit depth of 10-bits per sample. This compares to the 64:1/approximately 6 stops from standard dynamic range (SDR) on a conventional gamma curve with a bit depth of 8-bits per sample.
Figure 1: BT.709 colorspace compared to BT.2020 colorspace. Image: Axon
By extending the dynamic range, more information can be accommodated in an image, bringing with it more luminance subtleties. In a similar way WCG allows more vibrant colours to be displayed because it can store a wider range of colour values than established red, green and blue colour spaces.
According to the ATSC, the full Rec.2020 color gamut can only be achieved with laser projectors in the cinema. Yet home TV devices are improving rapidly and by the year 2020 they may be able to display all of the Rec.2020 colors. In the interim, expect content to be created in the so-called D65-P3 color gamut, which today's TVs can display and is the current specification for digital cinema. Although not as wide as full Rec.2020, D65-P3 does offer a significant improvement over the Rec.709 standard.
Travelxp 4K, was the world’s first 4K HDR channel, launched in October on Eutelsat’s HOTBIRD satellite. Travelxp 4K is encoded in HEVC with HDR using the Hybrid Log Gamma (HLG) standards developed by the BBC and NHK. The HLG standard enables SDR TV sets to display an Ultra HD image (in SDR mode). In mid-December, AT&T announced that DIRECTV will begin its first 4K High Dynamic Range (HDR) live TV broadcasts in U.S. with a full slate of sports, music and events.
The Broadcast Approach
Axon’s Schut suggests, “While this (4K HDR) is relatively easy for OTT providers such as Netflix, the existing infrastructure of linear television poses more of a challenge. With TV you've got a multicast running over a pipe, with one signal for everybody. That's a much more difficult proposition. And you don't want to jeopardize your current standard viewers.
If you can start from scratch with an UHD/HDR/WCG channel things are less messy, and not a lot more difficult than doing a normal production. The challenge is to produce and transmit both the new and the old format at acceptable cost and in an easy plug-and-play way so the end user doesn’t need to be a colour grading specialist.”
Of course, the key to future of broadcast UHD, HDR, WCG and HFR TV (in the U.S.) requires ATSC 3.0.
The technical challenge behind this is changing from the HDTV colour space (BT.709) to the colour space for UHD plus HDR and WCG (BT.2020). This requires moving from the relatively small color space of BT.709 (See Figure 1) which has a maximum brightness/luminance of 100 nits, typically at 10-bit YUV, to BT.709, BT.2020 has a much larger colour space to work in with luminescence measuring 1000/10,000 nits.
In effect, it may appear the industry is pounding a square peg in a round hole. This is normally impossible without brute force but with the correct forms of conversion and compression, it can fit.
“The whole issue becomes even more complicated if WCG is part of the equation. Higher dynamic range with some sort of backwards compatibility is only possible when everything is working and remains in the same colour space. But because the broadcast market must be part of the real world, services will have to include WCG, which will mean the whole issue of backwards compatibility in practice is down the drain,” says Schut.
Figure 2: CIE 1931 color space, defined in 1931. Image Axon.
“Worldwide standards for colour and light are drawn up by the CIE, the Commission Internationale de I'eclairage or International Commission on Illumination. Founded in 1913 and headquartered in Vienna, the CIE defined the quantitative links between physical pure colours - otherwise called wavelengths - in the electromagnetic visible spectrum in 1931. The CIE 1931 RGB and CIE 1931 XYZ colour spaces laid down the basis for colour technologies that followed and still have an influence today.
"The colour space chromaticity diagram shown in Figure 2 forms part of these spaces. Its horseshoe shape encompasses everything the human eye can see, starting at 460 nanometers, just before ultraviolet, up to 630 nanometers. It has a narrow spectrum from blue to green to red. Other colours are created in the middle by mixing the three primaries. In the very center is D6500 (a colour temperature of 6500 Kelvin), which corresponds to natural daylight at midday in northern/western Europe."
Three HDR Curves
Three new curves have been developed, each with a unique approach to improving dynamic range. In general, the different curves determine different peak brightness levels.
Perceptual Quantisation (PQ) is a curve optimised for the human eye through extensive research carried out by Dolby Laboratories and is a component of Dolby Vision, a 4K video display technology that includes both HDR and WCG. PQ has been standardised by SMPTE as ST 2084, and it requires a license.
The PQ curve goes all the way up to 10,000 nits. This is probably overkill for the average household TV but it takes about 1000 to 4000 nits to produce a quality HDR viewing experience.
The second curve is HLG (Hybrid Log Gamma), developed jointly by the BBC and NHK. It has some backwards compatibility, although only in BT.709, and increases the dynamic range of the video more than conventional gamma curves. This is accomplished by applying a logarithmic curve to the upper half of the signal values.
BBC White Paper WHP 309 describes how the HLG approach “provides a ‘display independent’ signal that can produce high quality images, which maintains the director’s artistic intent on a wide range of displays in diverse viewing environments.” The log-gamma HDR signal may be viewed in a production suite, home theater, living room, or on a laptop or mobile device. On a conventional SDR display, HLG provides a high quality “compatible” image.
A HLG-HDR signal may be mixed, re-sized, compressed, and generally “produced”, using conventional tools and equipment. The only specifically-HDR gear needed for HLG is cameras and displays for quality monitoring. HLG does not require a license.
The third curve, S Log 3, is purely a production/acquisition tool and features on Sony digital cameras. The Sony F65, F55 and F5 use two new grading spaces, GGamut3.Cine/S Log3 and SGamut3/S-Log3, which are intended for Log Base Grading.
The Future of HDR and WCG
Broadcasters face different challenges in transmitting HDR than do streaming companies or with physical media. Issues include live production environments, regional opt-outs and interstitial advertising. Such broadcast requirements result in operational demands that are different from offline-produced content. The bottom line is that broadcasters may be concerned about fragmented HDR solutions entering the marketplace, that might confuse consumers thereby lowering its pace of adoption.
Plenty of HDR and WCG products will be displayed at this year's Consumer Electronics Show. And, we can expect lots of HDR and WCG demonstrations at the 2018 NAB Show. Combined, these two events may set the place of the technology's adoption.
Editor's note: A technical guide on HDR & WCG is available from Axon.
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