A Brief on Visual Acuity and the Impact on Bandwidth Requirements

Transcription

1 Managing The Future Cable of Ultra TV Migration High Definition to IP TV Part (UHDTV) 1 A Brief on Visual Acuity and the Impact on Bandwidth Requirements Series Introduction: Market Drivers and Tech Challenges Sean McCarthy, Ph.D., Fellow of the Technical Staff

2 Contents Introduction...2 4K and UHDTV defined...2 Can you see a difference?...2 Higher Resolution...2 Higher Frame Rate...3 Enhanced Color Space...4 Conclusions...5 Related Reading...6 Appendix...7 Relative visual experience & bandwidth factors for various UHDTV scenarios ARRIS All rights reserved.

3 Introduction UHDTV is more than just higher resolution: it can be thought of as a step towards enabling digital media to seem almost as real as the real world. When the International Telecommunication Union (ITU-R) set out to define the parameters of UHDTV, they were motivated by the desire to provide viewers with an experience that virtually covers all of the human visual field. Covering the visual field means going beyond HDTV resolutions, which is where most conversations about UHDTV begin and end. But ITU-R also imagined an enhanced color space that could match real world colors better than even digital cinema, and higher frame rates that would make motion feel real even for high action sports. Each of these components resolution, color space, and frame rate can impact the viewer s quality of experience, and an operator s use of bandwidth. The big question is: Will UHDTV be worth it? Will people find the UHDTV experience a compelling improvement over HDTV? How much will it cost operators? Will UHDTV be good business? This brief provides some information that might help us answer those questions. 4K and UHDTV defined The terms 4K and UHDTV are often used as synonyms. Though it is a lot easier to say 4-Kay than UHDTV, we sometimes need to be more specific to avoid confusion. When a television professional says 4K, they usually mean a spatial resolution of 3840x2160 and frame rate up to 120 fps. When a digital cinema professional says 4K, they typically mean DCI 4K, which is 4096x2160 and frame rate up to 24fps. (DCI stands for Digital Cinema Initiative) In addition to 4K UHDTV, there is also an 8K UHDTV. In an attempt to simplify terms, the European Broadcasting Union (EBU) has defined 4K as UHD-1, and 8K as UHD-2. The latter is effectively the same as NHK s Super Hi-Vision with a resolution of 7680x4320 up to 120fps. Can you see a difference? Higher Resolution The ability to notice fine details is called visual acuity. Broadly, visual acuity may be thought about in terms of 3 levels of refinement. Snellen acuity, which corresponds to the well-known concept of 20/20 vision, is the lowest level and relates to our 2 ARRIS All rights reserved.

4 ability to read and identify symbols. Simple acuity is the middle level and relates to our ability to discern individual pixels it is a result of the sampling density of photoreceptors in the retina. Hyperacuity is our finest level and relates to our ability to localize features (and discontinuities in features) such as edges and lines it is a result of the brain processing input from groups of retinal neurons. When we as an industry made the jump from SD to HD, we crossed an enabling point. For the first time, we were able to display images above the Snellen acuity threshold (reading) with a pixel density above the Simple acuity threshold at typical viewing distances. In moving to UHDTV, we may or may not cross another visual acuity threshold, depending on how people decide to watch UHDTV. On one hand, viewers might choose very large displays that fill their field of view (FOV) and create a sense of immersion. In this wide-fov scenario, the acuity equation changes little compared to HDTV. The number of pixels per visual angle remains the same, and a person s sensitivity to fine detail would remain about the same. On the other hand, viewers might chose UHDTV displays that are about the same size -- and same field-of-view -- as typical HDTVs in the home today. This narrow-fov scenario would cross the Hyperacuity threshold, and viewers will have more of a sense of looking through a window The bandwidth requirements for the wide-fov immersive scenario are higher than for the narrow-fov though-the-window scenario. The bits-per-pixel required for wide-fov UHDTV is about the same as it is for HDTV because they have the same visual acuity level. Thus, for example, 4Kp60 would in principle need about 4 times the bandwidth of 1080p60 (using the same compression algorithm, frame rate, and color precision). The bandwidth requirement for the narrow-fov through the window scenario is more flexible. We would need only to deliver the same bit rate (the same bits-pervisual-angle) to the UHDTV to match the visual-acuity level of HDTV today. Any additional bandwidth would cross into a new level of visual acuity Hyperacuity and be perceived as a level of video quality that could not be matched by HDTV. Higher Frame Rate There are 3 main temporal artifacts that relate to frame rate: the stroboscopic effect (jerkiness), which is particularly important for high-action sports; flicker caused by too low of a display refresh rate; and motion blur caused when pixels remain on for too much of the on-off duty cycle. UHDTV is more prone to creating temporal artifacts for wide-fov scenarios because peripheral vision responds to temporal change different from central vision. 3 ARRIS All rights reserved.

5 According to subjective video quality testing reported by ITU-R, people preferred 120 fps sports content over 60fps by a significant margin for both narrow- and wide-fov. Other data showed that viewer sensitivity to flicker depended strongly on field of view. For a 30-degree FOV about the same as a typical HDTV viewing scenario 60fps were sufficient to avoid flicker. But for a 100-degree FOV, the results depended on both frame rate and motion blur related to the pixel duty cycle. For low duty cycle (pixel is on for a short period and motion blur is minimal), 120fps were needed to avoid flicker. For high duty cycle, only 60fps were needed but at the expense of more motion blur. The consequent bandwidth requirements again depend of field of view. Wide-FOV scenarios are likely to need 120fps, which would mean up to twice the bandwidth of 60fps. Narrow-FOV scenarios are likely to be satisfactory at 60fps to start, with the option of gradually increasing to 120fps over time to improve the viewer s quality of experience. Enhanced Color Space Two of the dominant color artifacts in digital television are posterization and outof-gamut colors. Posterization occurs when the precision of the luma and chroma components is too low. In 8-bit color HDTV, for example, posterization might occur in regions of smooth gradation such as sky or flesh. Out-of-gamut colors occur when a real world color lies outside of the domain of colors that can be represented by a particular color space. Viewers tend to be more forgiving of out-of-gamut colors than they are of other kinds of artifacts because they contribute to the overall sense of the unrealness of television rather than serving as immediate distractions. UHDTV incorporates an enhanced color model usually referred to as Rec (ITU-R Rec actually covers all UHDTV parameters, not just color). Besides emphasizing 10-bit and 12-bit color precision to reduce posterization, Rec introduces a wide color gamut that encompasses almost all real world colors thus effectively eliminating out-of-gamut artifacts. On first glance, moving from 8-bit color to 10- or 12-bit color might seem like a 25% to 50% increase in bit rate required; but it is not likely to be that onerous. Most uniquely discriminate colors are already encompassed within the HDTV color space (ITU-R Rec. 709). Rec simply expands the gamut to capture the less frequent color outliers. From a video quality standpoint, there should be an incremental gain, particularly for posterization. From a compression standpoint, the entropy of color representation should not be very different in UHDTV than it is in HDTV. Thus, moving from 8-bit Rec. 709 HDTV to 10-bit Rec UHDTV, for example, could be 4 ARRIS All rights reserved.

6 introduced gradually over time and should be expected to increase bit rate requirements only marginally. Conclusions The way in which consumers decide to watch UHDTV is the key issue in predicting future bandwidth requirements. The wide-fov UHDTV scenario could be compelling to consumers. It would provide an immersive IMAX-like experience in the home, provided the living room wall is big enough. But immersion comes at a steep bandwidth price. To maintain the level of visual acuity people have come to expect from HDTV, and to avoid peripheral flicker artifacts by frame rates below 120fps, operators might need to dedicate up to 8-times the bandwidth they dedicate today to 1080i30 even accounting for the 50% improvement in compression efficiency afforded by the new High Efficiency Video Coding (HEVC) standard. The alternative narrow-fov scenario has a more attractive bandwidth cost, but it is not clear if the advantages of incrementally finer visual detail would compel consumers to buy a new television display. Narrow-FOV UHDTV would fit into living rooms as well as HDTV does today. The advantage to operators is that HDTV video quality could be delivered to UHDTV at half the bandwidth using HEVC instead of H.264. And over time, operators could gradually enhance the viewer s quality of experience by incrementally increasing coded resolution, frame rate, and color precision. If we were to look into the crystal ball, the future might look something like this: UHDTV will be a feature of your next flat panel display rather than the primary reason for purchasing a new television. A future UHDTV display will have higher resolution, support for higher frame rates, support for a wide color gamut and precision, and a suite of sophisticated sharpening filters that can make HDTV content look a bit like true UHDTV content. This is great news for operators. At first, we will be able to leverage HEVC to save bandwidth without sacrificing quality. And, over time, we will have the opportunity to leverage the full UHDTV tool set to improve the consumer s quality of experience continually in step with advances in compression and network efficiency. 5 ARRIS All rights reserved.

7 Related Reading UHDTV: Avoiding the Pitfalls of 3DTV this paper examines several considerations for service providers as they contemplate adding UHDTV capabilities to their networks. It examines the benefits of this new technology, investigates the differences between UHDTV and 3DTV from both the deployment and adoption perspectives, and provides practical steps to help guide service providers as they create phased UHDTV rollout plans. Quantitative Evaluation of Human Visual Perception for Multiple Screens and Multiple CODECs - this article examines how principles of vision science can be used to predict the bit rates and video quality needed to make video on devices, ranging from smartphones to ultra high definition televisions, a success. Transcoding Choices for a Multiscreen World This paper explores the applications for home- and network-based transcoding, and previews some of the innovations that are emerging to help providers transcode their content more efficiently and effectively in the multiscreen world. 6 ARRIS All rights reserved.

8 Appendix Relative visual experience & bandwidth factors for various UHDTV scenarios HDTV UHDTV UHDTV Display Resolution 1080p 4k 4k Coded Resolution(s) 1080i p p p p30 4kp30 4kp60 4kp30 4kp120 Field of View (FOV) Narrow Narrow Wide "Through the Viewer Experience HDTV HDTV + HDTV "Immersion" Window" Acuity Level Simple Simple Hyperacuity Simple Minimum Frame Rate Optimal Content Class TV Film Sports Progressive TV & Film Interlace TV Film Sports Film Sports CODEC H.264 HEVC HEVC HEVC HEVC HEVC HEVC HEVC ~Bandwidth Factor 100% 100% 50% 75% 200% 400% 200% 800% ARRIS Enterprises, Inc All rights reserved. No part of this publication may be reproduced in any form or by any means or used to make any derivative work (such as translation, transformation, or adaptation) without written permission from ARRIS Enterprises, Inc. ( ARRIS ). ARRIS reserves the right to revise this publication and to make changes in content from time to time without obligation on the part of ARRIS to provide notification of such revision or change. ARRIS and the ARRIS logo are all trademarks of ARRIS Enterprises, Inc. Other trademarks and trade names may be used in this document to refer to either the entities claiming the marks and the names of their products. ARRIS disclaims proprietary interest in the marks and names of others. The capabilities, system requirements and/or compatibility with third-party products described herein are subject to change without notice. 7 ARRIS All rights reserved.