Video Compression: More Bang with Fewer Bits
Video compression was one of the more ubiquitous topics at last month’s NAB Show. Perhaps equally ubiquitous was 4K and 8K. Seldom was one of either topic discussed without reference to the other.
Fortunately, in the world of digital television and digital video, a video-specific variation of Moore’s Law seems to be at work. With umpteen years of incremental NTSC picture quality progress that many viewers could barely see — if at all — the industry was ripe for the change to HDTV and self-contained big-screen displays.
Until Moore’s Law made it possible for common computers to handle digitized SD video, few analog engineers could envision what was in the future of the digital television. Some might remember the days of 1125/60 in the early 1990s. It was, more or less, the original analog HDTV format. Many design engineers were trying to compress it into the standard 6MHz broadcast television channel bandwidth using a variety of hardware-intensive systems. The industry even formed the 1125/60 Consortium before DTV was invented.
DTV, which was originally conceived to shoehorn HDTV into a standard 6MHz TV channel, has flooded the industry and viewing public with myriad unforeseen changes. Among many was progressive scan, streaming video, file-based video, the 1920 x 1080 raster and, more recently, 4K, 8K and beyond. While current DTV and ATSC transmission standards have capped the resolution TV stations can broadcast, computers, DVDs, the Internet and a host of video compression schemes and standards have eliminated barriers to delivering higher definition images to viewers from non-over-the-air sources. Current state-of-the-art video compression standards can reduce the bandwidth of baseband video by a factor of approximately 100.
The Codec
The first digital video codec standard was H.120. It was published in 1984 and revised in 1988, but the quality was so poor that there were very few users. H.120 was followed later in 1988 by H.261, an ITU-T video coding standard. The primary use for H.261 was for video transmission over ISDN lines at a resolution of 352 x 288 or 176 x 144.
MPEG-2, aka H.262, was first published in 1994. It paved the road to DTV, OTT and ATSC transmission, and it continues to be the standard used to create DVDs.
MPEG-4, aka H.264, was published in 2003 and is currently the most commonly used video codec and the standard for Blu-ray discs and HDTV.
Today, numerous compression standards are being used in a variety of ways to create and deliver content. The latest standards with the highest quality are JPEG 2000 (J2K) and more recently HEVC (High Efficiency Video Coding).
J2K
J2K is designed for compressing individual images, not video sequences. It is primarily used in production and video feeds with high bit rates up to approximately 120Mb/s. The higher bit rates make artifacts and blocking virtually invisible. It is hardware-intensive, and it can be accomplished in real time.
In J2K, each frame is compressed individually and, therefore, stands alone. In video compression terms, each individual complete frame is an I-Frame. While this feature is advantageous in maintaining video quality, its high bandwidth doesn’t lend itself well to distribution.
On April 15, 2013, the Video Services Forum issued a Technical Recommendation that defines profiles for streaming of JPEG 2000 Broadcast Profile in a MPEG-2 Transport Stream over IP with optional Forward Error Correction (FEC). The recommendation is for unidirectional transport of SD-SDI, HD-SDI and 3G-SDI signals, encapsulated in an RTP stream and transmitted via IP to a receiving device that will decode the output to an SDI signal.
HEVC
On the other hand, HEVC, aka H.265, and its modern predecessors H.264, MPEG-4, MPEG-2, Advanced Video Coding (AVC) use I-frames, P-frames and B-frames to encode moving images. To quickly review, the I in I-frame stands for Intra-coded and is a fully specified still image. The P in P-frame stands for Predicted picture, and it contains only the changes from the previous frame, saving unchanged data from having to be repeated. The B in B-frame stands for Bi-predictive. It saves more space than a P frame because it specifies the differences between it and the frame not just before it, but after it as well. Images are usually segmented into macroblocks, where prediction types can be determined based on the movement within each macroblock.
HEVC contains 33 directional modes for intra block prediction. MPEG-4 uses only eight directional modes. These modes use information from previously decoded neighboring prediction blocks. H.265 motion vector prediction is a 16-bit range for both H and V motion vectors (MVs) with quarter pixel precision. This gives HEVC a dynamic vector prediction range 16X greater than H.264.
Most pre-H.265 codecs independently encoded 16 x 16 pixel macroblocks. In HEVC, the image is split into Coding-Tree Units (CTUs), each up to 64 × 64 pixels. The root of a quadtree data structure. A Quadtree contains four branches that are used to partition a two-dimensional space which uses a recurring algorithm to subdivide it into four quadrants. The quadtree can then be sub-divided into leaf-level coding units (CUs), as illustrated in the following figure.
HEVC (H.265) represents the natural progression of Moore’s Law in video compression. In essence, the H.265 codec is twice as efficient as the H.264 codec. It increases the use of parallel processing, improves compressed video picture quality and supports higher resolutions such as 8K Ultra high definition television (UHDTV).
As early as 2004, the ITU-T Video Coding Experts Group began work toward a new video compression standard to take H.264 to the next level. The Group was considering two approaches. One was to create extensions to H.264. The other was to create a new standard.
Three years later, the ISO/IEC Moving Picture Experts Group (MPEG) began work on a similar project called High-performance Video Coding. Its goal was to achieve a 50-percent bit-rate reduction without affecting subjective picture quality. The works of both groups evolved into the HEVC joint project in 2010.
In February 2012, a Committee Draft of the HEVC standard was written. In July, an International Standard was drafted. In January 2013, the Final Draft International Standard was introduced. One month before the Committee Draft was completed, Qualcomm demonstrated an HEVC decoder operating on a dual-core 1.5GHz Android tablet at the Mobile World Congress. In August, Ericsson showed the SVP 5500, the world’s first HEVC encoder at IBC. From that point until the opening of last month’s NAB, approximately 20 manufacturers introduced new encoders. Several more new HEVC systems were introduced last month at the NAB Show.
HEVC is the Future
As of now, HEVC is simply a bit-stream structure and syntax standard but not an MPEG Transport Stream, although that is expected to happen in the next few months. It will take about a year for HEVC to be reduced to a silicon chip set. Some are predicting HEVC will be incorporated into some new set-top boxes (STBs) by 2014 or 2015. This will likely be the first opportunity for proud owners of new 4K TVs to receive 4K programming.
H.265 may also be the successor to H.264 for the next generation of 4K DVDs. A standard dual-layer Blu-ray disk contains up to 50GB of data, enough for typical feature-length movies. Typical feature-length movies encoded with HEVC in 4K will contain up to 100GB.
Some experts seem to agree that 4K with HEVC codec most likely won’t significantly penetrate the market until at least 2017. Given the universal population and long life cycle of STBs, the administration of the wholesale replacement of existing STBs is staggering.
A similar situation exists for the population of ATSC HDTV sets in the United States. With the apparent failure of 3-D TV to catch on, many are questioning the eagerness of the market to replace their existing HDTV display devices as they did with the shutdown of NTSC. Thus, the future of HEVC in broadcast transmission is unknown.
On the other hand, if it turns out 4K doesn’t catch on with consumers, HEVC can double the number of existing HDTV channels using the same amount of bandwidth. Either way, over-the-air broadcast adoption of HEVC is dependent on upcoming ATSC 3.0 specifications.
By Ned Soseman, Broadcast Engineering
