MPEG's upgrade
Nov 1, 2008 12:00 PM, By Aldo Cugnini
Take a tutorial on MPEG-4’s much-enhanced toolkit.
Although MPEG-2 continues to enjoy widespread deployment over broadcast and DVD media, MPEG-4 is rapidly catching up, thanks to advances in algorithms, processing power and storage. And while the original video codec of MPEG-4 — called Part 2 or Visual — delivered the ability to perform object-based coding, the feature was quickly superseded by the efficiency improvement provided by Advanced Video Coding (AVC), i.e., MPEG-4 Part 10 (ITU H.264).
Toolkit
MPEG-4 provides a more enhanced toolkit than its predecessors, yielding up to HDTV resolution with as much as a 50 percent reduction in bandwidth. Some of the key features of MPEG-4 are listed in Table 1.
MPEG-4 Visual first introduced the concept of video objects and video object planes. In contrast to coding an entire picture, a background image can be coded over which a foreground video object plane is separately coded. One typical application of this is in synthetic or computer-generated images (e.g., animation), where objects routinely move in front of a fixed or slowly moving background image.
In fact, a sophisticated encoder can make use of this technique when coding live-action video as well, with scene-detection algorithms capable of detecting moving foreground objects such as people and faces. However, while this type of coding has relevant applications for video conferencing systems, its widespread use has not grown, perhaps in part because of the amount of processing horsepower needed.
AVC features
AVC introduces various new picture coding tools for interlaced pictures, such as picture-adaptive frame/field coding and macroblock-adaptive frame/field coding. These allow several choices: coding each field separately; combining two fields together and coding them as a frame; or combining two fields into a frame while splitting pairs of vertically adjacent macroblocks into either field macroblocks or frame macroblocks. AVC thus enables different macroblocks to be coded as either field or frame within the same picture. In addition, the prediction method for each coded macroblock can be one of several different types, such as intra, inter, forward, backward and bidirectional. Macroblocks can also be partitioned into smaller horizontal and/or vertical submacroblocks.
Figure 1. MPEG-2 is limited to showing each frame in succession. AVC, however, can use multiple reference frames.
While MPEG-2 uses fixed anchor frames to predict the intercoded frames, AVC introduces the concept of multiple reference frames for interprediction. A reference frame index identifies one of several lists of reference frames so that predictions can efficiently account for back-and-forth scene cuts and object occlusions. (See Figure 1.)
AVC also adds error resilience tools that can help to reduce the visibility of artifacts when errors occur. These include additional picture and slice types, such as switching intraframes (SI-frames) and switching predictive frames (SP-frames), and flexible macroblock ordering. - and SP-frames (or slices) are coded to allow efficient switching between different pictures, allowing correct pictures to be built up in a decoder (even when there is missing information) by using a choice of different reference frames (or slices). SI- and SP-frames can also be used to facilitate bit stream switching.
Number of cable subscribers who have used VOD increasing
In 2008, 43 percent of cable subscribers utilized video on demand.
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AVC also expands on the way that the various symbols, such as motion vectors and residual data (transform coefficients) are coded, by adding techniques for variable length coding (VLC). VLC is a reversible, lossless procedure for entropy coding that assigns shorter bit strings to frequent symbols and longer bit strings to less-frequent symbols. Three types of coding are available in AVC: Exponential-Golomb (or Exp-Golomb) coding, context-adaptive variable-length coding (CAVLC) and context-based adaptive binary arithmetic coding (CABAC).
Exp-Golomb is the baseline entropy coding method of AVC, which relies on a single code word set that is used for all syntax elements except for the residual data. The fixed tables of Exp-Golomb, however, as well as those used in MPEG-2 and other codecs, do not allow an adaptation to the actual image transform statistics, which may vary over space and time, as well as for different source material and coding conditions. With CAVLC, intersymbol redundancies are exploited by switching VLC tables for various syntax elements, depending on already-transmitted coding symbols. In this way, CAVLC is more efficient than the MPEG-2 VLC.
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