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INTRODUCTION
The Moving Picture Experts Group and the Video Coding Experts Group (MPEG and VCEG) have developed a new standard that promises to outperform the earlier MPEG-4 and H.263 standards, providing better compression of video images. The new standard is entitled.’ Advanced Video Coding’ (AVC) and is published jointly as Part 10 of MPEG-4.
The advent of H.264 (MPEG-4 part 10) video encoding technology has been met with great enthusiasm in the video industry. H.264 has video quality similar to that of MPEG-2, but is more economical with its use of bandwidth. Being less expensive to distribute, H.264 is a natural choice for broadcasters who are trying to find cost effective ways of distributing High Definition Television (HDTV) channels and reducing the cost of carrying conventional Standard Definition channels. In fact, the use of bandwidth has been reduced to the point that it has captured the interest of telephone and data services providers, whose bandwidth limited page link to the subscriber had previously not allowed for delivery of bandwidth thirsty television services. H.264 has the potential to revolutionize the industry as it eases the bandwidth burden of service delivery and opens the service provider market to new players.
Fig1: Block diagram of H.264
CHAPTER 2
GOALS OF H.264
The main objective behind the H.264 project was to develop a high-performance video coding standard by adopting a “back to basics” approach with simple and straightforward design using well known building blocks. The ITU-T Video Coding Experts Group (VCEG) initiated the work on the H.264 standard in 1997. Towards the end of 2001, and witnessing the superiority of video quality offered by H.264-based software over that achieved by the (existing) most optimized MPEG-4 part 10 based software, ISO/IEC MPEG joined ITU-T VCEG by forming a Joint Video Team (JVT) that took over the H.264 project of the ITU-T. The JVT objective was to create a single video coding standard that would simultaneously result in a new part (i.e., Part 10) of the MPEG-4 family of standards and a new ITU-T (i.e., H.264) recommendation. The H.264 standard has a number of advantages that distinguish it from existing standards, while at the same time, sharing common features with other existing standards. The following are some of the key advantages of H.264:
1. Up to 50% in bit rate savings: Compared to MPEG-2 or MPEG-4 Simple Profile, H.264 permits a reduction in bit rate by up to 50% for a similar degree of encoder optimization at most bit rates.
2. High quality video: H.264 offers consistently good video quality at high and low bit rates.
3. Error resilience: H.264 provides the tools necessary to deal with packet loss in packet networks and bit errors in error-prone wireless networks.
4. Network friendliness: Through the Network Adaptation Layer, that is the same as for MPEG-2, H.264 bit streams can be easily transported over different networks.
The above advantages make H.264 an ideal standard for offering TV services over bandwidth restricted networks, such as DSL networks, or for HDTV. Figure 1 shows a block diagram of the H.264 encoding engine.
CHAPTER 3
H.264 TECHNICAL DESCRIPTION
The main objective of the emerging H.264 standard is to provide a means to achieve substantially higher video quality compared to what could be achieved using any of the existing video coding standards. Nonetheless, the underlying approach of H.264 is similar to that adopted in previous standards such as MPEG-2 and MPEG-4 part 2, and consists of the following four main stages:
a. Dividing each video frame into blocks of pixels so that processing of the video frame can be conducted at the block level.
b. Exploiting the spatial redundancies that exist within the video frame by coding some of the original blocks through spatial prediction, transform, quantization and entropy coding (or variable-length coding).
c. Exploiting the temporal dependencies that exist between blocks in successive frames, so that only changes between successive frames need to be encoded.
This is accomplished by using motion estimation and compensation. For any given block, a search is performed in the previously coded one or more frames to determine the motion vectors that are then used by the encoder and the decoder to predict the subject block.
d. Exploiting any remaining spatial redundancies that exist within the video frame by coding the residual blocks, i.e., the difference between the original blocks and the corresponding predicted blocks, again through transform, quantization and entropy coding.
On the motion estimation/compensation side, H.264 employs blocks of different sizes and shapes, higher resolution 1/4-pel motion estimation, multiple reference frame selection and complex multiple bi-directional mode selection. On the transform side, H.264 uses an integer based transform that approximates roughly the Discrete Cosine Transform (DCT) used in MPEG-2, but does not have the mismatch problem in the inverse transform.
In H.264, entropy coding can be performed using either a combination of a single Universal Variable Length Codes (UVLC) table with a Context Adaptive Variable Length Codes (CAVLC) for the transform coefficients or using Context-based Adaptive Binary Arithmetic Coding (CABAC).