MPEG-2 standard

The MPEG organization introduced the MPEG-2 compression standard in 1994 to achieve the possibility of interoperability between video/audio services and applications. The MPEG-2 standard is a detailed specification for compression schemes and system layers for various digital televisions and high-definition televisions in various applications. The coding rate is from 3 Mbits/s to 100 Mb/s, and the standard formal specification is ISO/ In IEC13818. MPEG-2 is not a simple upgrade of MPEG-1, MPEG-2 has made more detailed regulations and further improvements in system and transmission. MPEG-2 is particularly suitable for the encoding and transmission of broadcast-grade digital televisions and is regarded as an encoding standard for SDTV and HDTV.

The principle of MPEG-2 image compression is the use of two features in the image: spatial correlation and temporal correlation. These two kinds of correlation make a large amount of redundant information in the image. If we can remove these redundant information and only keep a small amount of non-related information for transmission, we can greatly save the transmission band. The receiver uses these non-related information and according to a certain decoding algorithm, can restore the original image under the premise of guaranteeing certain image quality. A good compression coding scheme is to remove the redundant information in the image to the maximum extent possible.

The MPEG-2 coded pictures are classified into three types, which are referred to as I-frames, P-frames and B-frames, respectively.

The I-frame image is intra-frame coded, that is, only the spatial correlation within a single frame image is utilized, and no temporal correlation is utilized. The P-frame and B-frame images adopt an inter-frame coding method, which utilizes both spatial and temporal correlations. P-frame images only use forward time prediction, which can improve compression efficiency and image quality. The P-frame image may include an intra-coded portion, that is, each macroblock in the P-frame may be forward-predicted or intra-frame-encoded. The B-frame image adopts bidirectional time prediction and can greatly increase the compression ratio.

The MPEG-2 coded stream is divided into six levels. In order to better represent encoded data, MPEG-2 specifies a hierarchical structure using syntax. It is divided into six layers, from top to bottom are: image sequence layer, image group (GOP), image, macroblock, macroblock, block.

The main applications of the MPEG-2 standard in the field of broadcast television are as follows:

(1) The preservation of audiovisual data TV programs, audiovisual materials, etc. have all been preserved on tape. This method has many drawbacks: it is fragile, it occupies a large area, and it is costly and difficult to reuse. What is more important is that it is difficult to save for a long time, it is hard to find and difficult to share. With the development of computer technology and video compression technology, high-speed broadband computer networks and large-capacity data storage systems provide possibilities for networked storage, query, sharing, and exchange of television programs.

The use of MPEG-2 compression-encoded DVD video discs brings new hope for data preservation. TV programs, audiovisual materials, etc. can be encoded by the MPEG-2 encoding system and stored on low-cost CD-R discs or high-capacity rewritable DVD-RAMs. DVD authoring software (eg Daikin Scenarist NT, Spruce DVDMaestro) can also be used. And so on) to make a standard DVD video disc, can save money, but also can save storage space.

(2) Non-linear editing systems and networks of television programs In non-linear editing systems, program materials are stored, produced and broadcast in digital compression. Video compression technology is the technical foundation of non-linear editing systems. At present, there are mainly two digital compression formats, M-JPEG and MPEG-2.

M-JPEG technology is a motion still image (or frame-by-frame) compression technique that enables frame-accurate editing, but compression efficiency is not high.

MPEG-2 adopts the inter-frame compression method, which requires only intra-frame compression processing of I-frames. B-frames and P-frames are obtained by detection. Therefore, most of the transmitted and computed data are obtained from the temporal correlation between frames. In terms of data volume, high compression ratios can be achieved. With the resolution of the frame-by-frame editing problem, MPEG-2 will be widely used in non-linear editing systems, and greatly reduce the cost of editing. At the same time, the decompression of MPEG-2 is standard. Compressed data from different manufacturers can be compressed by other devices. Manufacturers designed the decompressor to decompress, which ensures that all manufacturers' devices are fully compatible.

Due to the adoption of MPEG-2 IBP video compression technology, the amount of data has been reduced exponentially, reducing storage costs, increasing data transmission speed, reducing the pressure on the computer bus and network bandwidth, and using pure Ethernet to form a non-linear editing network system. It is possible, and Ethernet is the most mature network at present, the system management is relatively complete, and the price is relatively low.

MPEG-2 based nonlinear editing system and nonlinear editing network will become the future direction of development.

(3) Satellite transmission MPEG-2 has passed ISO recognition and has been widely used in the broadcasting field, such as digital satellite video broadcasting (DVB-S), DVD video disk, and video conferencing. At present, there are tens of millions of DVB-S users around the world. DVB-S signals are encoded in MPEG-2 compression format and transmitted via satellites or microwaves. They are decoded at the user end by MPEG-2 satellite receiver decoders for viewing by users. . In addition, MPEG-2 compression coding technology can also be used for remote TV news or program transmission and exchange.

(4) The broadcast of television programs is broadcasted throughout the television technology. It is a necessary link to digitally transform the broadcast system. One of the most critical steps is to build a hard disk broadcast system. MPEG-2 hard disk automatic broadcast system is favored by people due to its advantages such as simple editing and broadcasting, large storage capacity, and high video index. However, in the past, MPEG-2 playout equipment was very expensive, and only a small amount was used. With the development of MPEG-2 technology and the reduction of related product costs, the MPEG-2 hard disk automatic system broadcast is expected to be popularized.