| Popis: |
The demand for multimedia services is increasing rapidly in the last years. Therefore, efficient video compression has become a very important research for the multimedia communication. H.264 is the state-of-the-art digital video coding recommendation, which is also known as H.264/MPEG-4 Advanced Video Coding (H.264/AVC) (ITU-T, 2003). The standard is a joint collaborative effort between the ISO/IEC Moving Picture Experts Group (MPEG) and the ITU-T Video Coding Experts Group (VCEG). The team responsible for the development and evolution of the standard is known as the Joint Video Team (JVT) and officially the standard is known as H.264 by the ITU-T and MPEG-4 Part 10 by ISO/IEC. The H.264 standard can achieve much higher coding efficiency than the previous standards such as MPEG-1/2/4 (LeGall, 1991; ISO/IEC, 1994 & ISO/IEC, 1999) and H.261/H.263 (CCITT, 1990 and ITU-T, 2003). In addition to coding efficiency, the rate control also plays a key role in a video encoder for multimedia services, especially for real-time communication such as video streaming, video conference and video surveillance. The number of bits required for encoding a video sequence varies with time to provide consistent visual quality because complexity of each frame generally differs from the other frames in the input sequence. Therefore, a rate control scheme which meets a constrained channel rate by controlling the number of generated bits is necessary in an encoder. Nowadays, real-time video streaming scenarios requiring very low end-to-end delay are getting more and more popular. However, it is very difficult to adjust the encoding parameters directly to obtain fixed bits for every encoded frame in the constant bit rate channel. Therefore, it is necessary that the buffer to regulate the bit stream before transmission. With a good rate control technique, it should adjust the output rate to prevent the buffer from overflow and underflow. If the buffer suffers from overflow and underflow, it will cause frames skipping and wastage of channel resource, respectively. Furthermore, the size of buffer is usually very small to achieve low end-to-end delay requirement for real-time communication. It causes the buffer overflowing and underflow easier. So, the low delay video communication requires more accurate bit allocation and encoder parameter adjustment to achieve a suitable rate control. There are two parts that should be considered when designing a rate control scheme. One is about the bit allocation for each basic unit according to its complexity. The other is the adjustment of the encoder parameter, i.e., quantization parameter (QP) to encode each basic unit to match target bits. Rate control scheme have been widely studied in video standards, such as TM5 for MPEG-2 (ISO/IEC, 1993), TMN8 for H.263 (ITU-T, 1997), and VM-8 for |
