It is not far away that small display screens use data streaming and digital video technology. Content creators, service providers, suppliers and equipment manufacturers are eagerly looking forward to the arrival of high-speed 2.5G and 3G mobile networks and the progress of efficient, high-performance, interconnectable portable devices. At all levels, this industry will accelerate the realization of video dreams for consumers in the next five years.

Video performance has exceeded the newly designed compression and decompression (codec) technology. The next generation of encoders and decoders will be more advanced, or more "smart", requiring more efficient and processing solutions to compensate for stable and clear viewing on handheld or other mobile devices The ultimate is the computationally intensive operation required by interactive video images. RISC processors, DSPs, and various new technologies are being used to cope with increasing loads. Although digital data sounds simple, is it actually the case? Streaming video technology is not that simple. The realization of streaming video on mobile devices requires multimedia and corresponding support technologies, which are still in their infancy. Among these efforts, the most promising is the MPEG-4 standard, which is increasingly favored by global suppliers and service providers to send and receive video images on low-power, low-bit-rate wireless systems.

The Moving Picture Experts Group (MPEG) is a committee of ISO / ICE responsible for developing standards for digital video and audio code representation. MPEG-4 is an internationally recognized standard used to handle the transmission, synchronization, encoding and reproduction of audio / visual multimedia content, including various specifications for the Internet and mobile applications.

MPEG-4 main functions include efficient compression and decompression; error repair; bandwidth scalability (covering from HDTV and PC to cellular phones and PDA); network and transmission protocol independence, built-in IP protection; target-based interactivity for processing images .

Recently, the second version of MPEG-4 has been optimized for level 3 transmission: low speed, bit rate below 64 kbits / s; medium speed, 64 to 384 kbits / s; high speed, 384 kbits / s to 4 Mbits / s. The standard applies to both constant bit rate (CBR) and variable bit rate (VBR), both of which are important for mobile devices. These three levels can also describe the maximum target size of the video, the macroblock per second for decompression, and the buffer size.

For the currently used 14.4 kbits / s mobile phone data rate, it can send approximately 1 to 3 fps (frames per second) video, while TV is 30 fps. fps is affected by the number of moving objects in the scene, the image size, and the available bandwidth.

MPEG-4 not only specifies video, it also includes audio, computer-generated images, and content-based interactivity. It supports YUV4: 2: 0 color format; asymmetric compression (in order to save space, the compressed video takes longer than decompression); and includes instantaneous scalability (when decoding a subset of the total bitstream generated by the encoder, to reduce The instantaneous resolution is reconstructed and the video image is displayed). The MPEG-4 video coding scheme is based on the DCT (separate cosine transform) algorithm.

The MPEG-4 standard supports eight main functions in the following three categories: interactivity, compression, and universal access. The 8 functions are: target-based multimedia access tool; target-based bit stream processing and editing; target-based random access; mixing of natural data codes and synthetic data codes; improved video compression efficiency; robustness to prevent information errors and loss; target Identify scalability; and target scalability. Mobile devices only support a part of all the above functions.

There are 6 different target types for representing natural video image information: simple type, simple expandable type, advanced simple expandable type, core type, main type, and N-bit type. The static expandable structure is the target type of static natural visualization information; the simple surface is the target type of synthetic surface animation; the mixed information target type is the 2D animation grid and the basic animation structure. Simple, advanced, simple expandable and static expandable structures may be the most common target type for future mobile applications.

Character shape refers to the most complex visual target type that can be restored in the scene. For example, for mobile video phones, the most suitable character shape may be a simple appearance. The only target allowed in this form factor is the simple type. This form factor is also suitable for products that store images in a memory card or embedded memory.

In January this year, while engaging in other work, MPEG completed and passed the expansion of its MPEG-4 video encoding with fine-grained scalability and an advanced simple shape. The advanced simple shape is the shape optimized for Internet data streams. The advanced simple form factor can be used with fine-grained scalability to improve the quality of Internet data streams under various conditions.

The Wireless Multimedia Forum (WMF) recently announced support for MPEG-4 simple form factor. The organization consists of approximately 60 industry members, including Emblaze Systems (formerly GEO Interactive Media), Hitachi, Intel, Luccent Technologies, Matsushita Communication (Panasonic), PacketVideo, Parthus, Samsung Electronics, Sanyo, TI, Toshiba America Electronic Components Zucotto Wireless.

In general, MPEG-4 defines tools for advanced and efficient compression algorithms that process audio and video information on different system layers. There are two official MPEG-4 video codec implementation tools: one is the C language provided by the European project ACTS-MOMUSYS; the other is the C ++ language provided by Microsoft. Any MPEG member can download the source code of these two tools from the MPEG Web site (www. Cselt.it/mpeg).

One element of the wireless application of the MPEG-4 standard involves an error repair technique called Newpred. It uses the upstream channel from the decoder to the encoder to provide fast error repair in real-time encoding applications. In order to respond to various error conditions of the network, the encoder can switch from the reference frame to the adaptation frame. According to MPEG reports, Newpred can achieve an Internet video packet loss rate of about 5%.

Motion assessment is another element of the standard in low-power, low-bitrate applications. Because MPEG-4 relies on content-based encoding, using an object-based motion evaluation tool to evaluate each object in the video scene can achieve high-efficiency compression. Motion estimation can be used to reduce the amount of data required to regenerate the entire video image at the receiving end. The evaluation is done by analyzing the first frame of the video transmission and storing the still part of the image in the buffer (that is, the scene in the scene); redundant data is placed in subsequent frames, and only the moving part of the video is processed Create moving images. The motion evaluation algorithm used by MPEG-4 uses the minimum number of cycles to calculate the motion vector (without reducing the quality) to reduce the evaluation time and power consumption.

Motion evaluation is the most computationally intensive function in video encoding and decoding, and it is also the main factor for slow encoding speed. Many semiconductor companies are developing adaptive DSP and RISC microprocessors, or creating new architectures to handle the heavy workload of multimedia applications on wireless devices and Internet appliances, while maintaining low power consumption and high performance.

The architecture provides the overall structure of the video data stream transmission, and the codec performs the actual compression / decompression function. The file structure created by the codec in the architecture is called the format.

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