As consumers increasingly view live and VOD television on a broader range of IP-connected devices, broadcasters are struggling to deliver these high-bandwidth services. However, a new video compression standard called High Efficiency Video Coding (HEVC) promises to improve upon the current compression standard H.264, also known as MPEG-4 AVC, easing broadcasters’ pain. Using HEVC, broadcasters can reduce the data rate needed for high-quality video coding by approximately 50 percent, enabling them to deploy higher quality OTT video services using the same amount of bandwidth, or half the bandwidth at the same quality. In addition to improving OTT delivery, HEVC also has the potential to support a broad range of current and future applications, including 4K x 2K Ultra HD Television (UHDTV), making it an extremely exciting technology that pushes the consumer experience to the next level. (For more information about 4K UHDTV, read the ITU recommendation on the technology, available online at www.itu.int/net/pressoffice/press_releases/2012/31.aspx.)
Evolution of video compression
An uncompressed HD video signal consumes approximately 1.5Gb/s to 2Gb/s of bandwidth. In the 1980s, when video compression was in its infancy, the goal was to compress this down to 130Mb/s. Today, content providers are able to encode HD video between 5Mb/s to 10Mb/s using H.264/MPEG-4 AVC, depending on the quality target resolution and encoding scheme (CRB or VBR). HEVC is designed to push that envelope even further by reducing the bit rate of MPEG-4 by 50 percent. (See Figure 1.)
But, before discussing the improvements of HEVC, it is important to understand the key technologies that HEVC builds on. MPEG-2, in the broadcast and entertainment TV world, was a huge success. It offered full SD quality at 4Mb/s, but the industry quickly realized that MPEG-2 could also enable HD for bandwidth-sensitive networks. Using MPEG-4 AVC compression, it became affordable to transmit HD to the home. In addition to featuring tools for interlace, MPEG-2 added more adaptive coding functionality such as multiple patterns to scan transform coefficients, linear or nonlinear quantization, and changeable coefficient weighting matrixes.
The H.264/MPEG-4 AVC standard refined motion estimation with quarter-pixel precision. It enabled a 50 percent bit-rate reduction compared with MPEG-2, enabling broadcast SD at very low bit rates and broadcast HD at shockingly low bit rates for the time. H.264/ MPEG-4 AVC also added new tools, including sophisticated intra prediction (whereby a prediction block is formed based on previously encoded and reconstructed blocks within the same frame), a better de-blocking filter, new transforms, improved motion compensation interpolation, multiple motion estimation references, weighted prediction and context-based adaptive binary arithmetic coding (CABAC).
To profile or not to profile?
A significant portion of the standardization effort is spent debating whether the compression gains of various compression tools justify the tools’ complexities. What we learned during the development of H.264/ MPEG-4 AVC technology is that while a Baseline profile (no CABAC and no B frame) was recognized as a good technology trade-off in 2002, those restrictions didn’t pay off in the future. In 2012, when the standard was in full swing, some phone and tablet silicon manufacturers decided to implement only that profile, preventing additional savings of approximately 20 to 30 percent in the mobile network, a substantial amount of wasted bandwidth. Additionally, it forced operators to encode, store and serve two different profiles, depending on the device capability, adding significant infrastructure cost or reducing the efficiency of the delivery system.
HEVC achieves improved coding efficiency by introducing additional tools to exploit spatial and temporal correlations. Specifically, HEVC incorporates enhanced motion-compensated filtering, multiple coding block sizes and expanded loop filters, including de-blocking, sample adaptive offset and an adaptive loop filter.
Some of the key improvements in HEVC are due to the use of larger block sizes, making the standard well-suited for 4K x 2K UHDTV. The larger block sizes enable more efficient coding of large images, especially of regions with few changes in the picture content. Improved intra frame prediction enables better prediction of pixels by exploiting redundancy within the current frame. The proposed tools offer more prediction directions than AVC, and a more sophisticated way of predicting and coding the intra mode selected.
HEVC also directly addresses the banding problem that can be seen with H.264/MPEG-4 AVC, whereby contouring artifacts appear when coding flat or smooth image backgrounds. An internal increase in precision (greater bit depth) adds greater accuracy to internal calculations. These extra bits help to prevent the banding.
In addition to these benefits, HEVC offers support for interlaced video so that content providers can efficiently compress, store and transmit decades of legacy content. In the future, HEVC may include support for multiview video coding or stereo 3-D video, combined with scalable video coding, enabling a video stream, sequence or image to be represented in multiple ways and multiple formats. This ensures that content can be prepared in different resolutions, frame or bit rates, for viewing on any device, such as an HDTV or smartphone, all while retaining a high level of coding efficiency.
The potential for HEVC spans a wide gamut of applications, including home and digital cinema, surveillance, broadcast, videoconferencing, mobile streaming, video storage and playback, and VOD. HEVC has the ability to significantly affect next-generation HDTV displays and content capture systems that feature progressive scanned frame rates and display resolutions up to UHDTV. Through HEVC, content providers can more efficiently deliver high-quality UHDTV
content using today’s networks.
Another application for HEVC is a fixed-point contribution environment, such as newsgathering, live events, sports and concerts, where bandwidth is typically restricted. By replacing today’s MPEG-2 or AVC equipment with HEVC technology, broadcasters can reduce the bit rate to cost-effectively deliver more content at a higher quality.
Possibly the biggest application for HEVC is OTT video delivery. Video is expected to represent more than 50 percent of Internet traffic by 2016, so a reduction of 50 percent of the video bandwidth will have a tremendous impact on the video experience and on the video business model. (See Figure 2.) Most OTT video services are actually being viewed at a quality level below HD and are not available in full 1080p resolution.
Yet despite this reduced video quality, OTT services are hugely popular with today’s viewers who are looking for that anywhere, anytime television experience. This challenges operators, content owners and consumer electronics manufacturers to find a more efficient way to deliver an ever growing amount of content at a high picture quality. Using HEVC, viewers can watch high-quality OTT video on a wide range of devices at a reduced cost for content operators.
The HEVC standard is currently under joint development by the ISO/IEC Moving Picture Experts Group (MPEG) and ITU-T Video Coding Experts Group (VCEG). MPEG and VCEG have established a Joint Collaborative Team on Video Coding (JCT-VC) to develop the HEVC standard. A final version of the HEVC standard is expected in early 2013, with approval following in mid-2013. With regards to international adoption of the standard, French media regulator CSA has announced that it will mandate HEVC for 4K x 2K video services starting in 2016.
The first deployments for HEVC are expected to be used for the delivery of HD OTT, and will take place after encoding and decoding equipment is commercially available, approximately in 2014. UHDTV and contribution are other attractive applications that will take more time, as it is a more radical change on the broadcast ecosystems.
The HEVC standard has the potential to have a revolutionary impact on OTT video delivery, as it will dramatically reduce the bandwidth compared to existing H.264/MPEG-4 AVC-based delivery systems. Video can be delivered at a higher quality or at the same quality using a lower bit rate. OTT is expected to be the primary application for HEVC, with full HD now possible. UHDTV and contribution are two other attractive applications for the HEVC standard. Relying on HEVC, broadcasters have the power to cost-effectively deliver a more high-quality, expanded and differentiated service than ever before.
—Dr. Paul Haskell is VP R&D, Harmonic, and Thierry Fautier is Senior Director of Convergence Solutions, Harmonic.