With the development of radio and television technology and the updating of radio and television equipment, a series of dramatic changes have taken place in the television field. New technologies and systems such as conference TV, VCD, DVD, digital TV and high definition television (HDTV) are rapidly moving. Enter our lives. Compared to traditional analog TV, these new systems feature an all-digital image/sound processing technology. A series of corresponding digital video and audio coding standards have been rapidly developed and improved for different application fields, including: H.261 for conference television and video telephony, JPEG for still image compression, MPEG-1 for VCD, MPEG-2 for broadcast TV, DVD and HDTV, MPEG-4 for Internet TV, etc. At the same time, digital studio standards and digital TV quality evaluation standards were also developed.

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Digital Television

Digital TV can be interpreted from the content of the program, the technical point of view, and the user's perspective. From the perspective of user viewing, the program that the user uses IRD or digital television receiver (DVB interface) to watch is a digital TV program in the true sense; according to the content source of the program, the digital TV program can be a TV program or a movie. From a technical point of view, digital TV programs can be digitally filmed, produced, stored, broadcast and transmitted movies and televisions, as well as movies and televisions that have been digitally processed from previously stocked data sheets.

Digital TV transmission

Digital TV programs can be transmitted to the user's receiving end using different technologies in different propagation media. The transmission means of digital television mainly includes satellite, terrestrial transmission, HFC network, SDH, etc., and SDH is mainly used for long-distance transmission of digital television programs.

Since China's digital TV standard is based on the European Digital Video Broadcasting Standard (DVB, described by ISO/ICE 13818), currently, DVB-C, DVB-T and DVB-S are being promoted. . DVB-S (QPSK modulation) is adopted for digital television satellite broadcasting; DVB-T (OFDM modulation) is adopted for terrestrial wireless transmission of digital television broadcasting; and DVB-C (QAM modulation) is adopted for terrestrial HFC network digital television broadcasting. China has completed the digital transformation of radio and television satellite transmission, and has formed a digital TV satellite broadcasting network with DVB-S technology framework. The experimental broadcast of DVB-T is being prepared, and it is the main coverage of digital TV broadcasting in China. The means of DVB-C has been vigorously promoted in the HFC network.

Digital television coding technology

Currently, MPEG has promulgated four official international standards for moving pictures and sound encoding, called MPEG-1, MPEG-2, MPEG-4 and MPEG-7.

1.MPEG-1 encoding

The MPEG-1 standard is: compression encoding of moving pictures and sounds in a digital storage medium with a code rate of up to 1.5 Mb/s and a standard formal specification in ISO/IEC 11172. The input image format supported by MPEG-1 is the SIF format. SIF has two formats, 525/625: 352 x 240 x 30 and 352 x 288 x 25. MPEG-1 is an open, unified standard that has achieved great commercial success. Although its image quality is only equivalent to the quality of VHS video, it can not meet the requirements of broadcast level, but it has been widely used in home video products such as VCD.

2.MPEG-2 encoding

The MPEG-2 standard is: a detailed specification for the compression scheme and system layer of standard digital television and high definition television in various applications, the coding rate is 3Mb/s to 100Mb/s, and the standard formal specification is in ISO/IEC13818. . MPEG-2 is not a simple upgrade of MPEG-1. MPEG-2 has been more detailed and further refined in terms of system and transmission. MPEG-2 is particularly suitable for the encoding and transmission of broadcast-grade digital televisions and is recognized as the encoding standard for SDTV and HDTV. MPEG-2 also specifies the multiplexing method for multiple programs. In addition, MPEG-2 also takes into account the problem of adaptation to ATM cells.

The MPEG-2 video coding standard is a hierarchical series that is divided into four "Levels" according to the resolution of the encoded image; it is divided into five "Profiles" according to the set of coding tools used. Several combinations of "levels" and "classes" constitute a subset of the MPEG-2 video coding standard in a particular application: for a certain input format image, a specific set of compression coding tools are used to generate the code within the specified rate range flow.

We know that the current analog TV has the problems of coexistence of PAL, NTSC and SECAM. Therefore, the input format standard of digital TV tries to unify these three systems to form a unified digital studio standard. This standard is CCIR601, now called the ITU-RRec.BT601 standard. The four input image formats "levels" in MPEG-2 are based on this standard. The pixels of the Low Level input format are 1/4 of the ITU-RRec.BT601 format, ie 352×240×30 (representing the image frame rate is 30 frames per second, and the effective scanning lines per frame of the image are 240 lines) , the effective pixels per line is 352), or 352 × 288 × 25. The input image format of the Main Level above the lower level is fully compliant with the ITU-RRec.BT601 format, ie 720 x 480 x 30 or 720 x 576 x 25. Above the main level is the HDTV range, which is basically 4 times that of the ITU-RRec.BT601 format. The 1440 Advanced (High-1440Level) image has an aspect ratio of 4:3, the format is 1440×1080×30, and the advanced (High) Level) has an image aspect ratio of 16:9 and a format of 1920×1080×30.

Among the five "classes" of MPEG-2, the higher "class" means that more coding tools are used, and the encoded image is processed more finely, and better image quality will be obtained at the same bit rate. Of course, the cost of implementation is also greater. Higher class encodings use some additional tools that are not used by lower classes, in addition to the lower class encoding tools. Therefore, in addition to decoding images encoded by this type of method, higher-class decoders can also decode images encoded with lower-class methods, that is, backward compatibility between "classes" of MPEG-2. The Simple Profile uses the fewest coding tools. The Main Profile adds a bidirectional prediction method in addition to the coding tools for all simple classes. The SNR Scalable Profile and the Spatially Scalable Profile provide a multi-level broadcast method that divides the encoded information of an image into a basic information layer and one or more secondary information layers. . The basic information layer contains information that is crucial for image decoding. The decoder can decode based on basic information, but the quality of the image is poor. The secondary information layer contains the details of the image. The basic information layer is strongly protected during broadcast, so that it has strong anti-interference ability. In this way, in the case that the distance is relatively close and the receiving condition is good, the basic information and the secondary information can be received at the same time, and the high quality image can be recovered; and under the condition that the distance is long and the receiving condition is poor, the condition can still be received. To the basic information, the image is restored, and the decoding is not interrupted. The High Profile is actually applied to higher bit rates and requires higher image quality. In addition, the first four classes process the color difference signals line by line when processing Y, U, and V. Provides the possibility to process color difference signals simultaneously.

The current standard digital TV uses the MP@ML main class and the main level, while the HDTV uses the MP@HL main class and advanced.

3.MPEG-4 encoding

The MPEG-4 standard is: Video coding has undergone changes in H.261, MPEG-1 to MPEG-2. It is already a fairly complete system standard for broadcast, but it is still insufficient for communication and computer applications. So the MPEG-4 system layer extends the Transmapping Layer (TransMux) based on the original ES Stream Multiplexing Layer (FlexMux), which includes almost all multimedia, storage media and communication interfaces, such as (RTP) UDP IP. , PES MPEG-2 TS, AAL ATM, H223 PSTN, DABMux, etc. The system of MPEG-4 is extremely widely used. In terms of source, instead of simply compressing successive images and sounds, it decomposes and describes the images and sounds in detail, introducing the concept of "Objects" in computers into MPEG-4, for audio and video objects. It is compressed separately from the background and text, and even the image and sound are restored by means of parameter-controlled synthesis, which greatly improves the efficiency. Such as the synthesis of the announcer's face animation, text-to-speech speech synthesis, etc., so that the decoding process can be extended to the object's zoom, with a channel's object transparency adjustment and other complex scenes. That is to say, in addition to natural images, artificial traces of creation, creation and processing are added. The classes and levels of MPEG-4 have also changed a lot. For video content, it is divided into natural video content, natural and synthetic mixed image content. The categories of natural video content are further divided into five categories: simple video classes for mobile communications; simple scalable video classes for software decoding of quality-rated Internet; core video classes for simple video supplements Encoding of arbitrary shapes and objects scaled over time for Internet multimedia applications; primary video class, complementary interlaced, translucent and sub-picture object encoding for core video classes, for interactive multimedia quality broadcast and DVD applications; N - Bit-view class, which adjusts the sample quantization depth of the core video object, and has a 4 to 12-bit quantized core video class for monitoring applications. There are four other types of composite video content for composite natural images: simple facial animation, fast hierarchical video, basic animated 2D texture, and hybrid video. There are two types of graphics classes: 2D graphics classes and full graphics classes. There are five types of scene description classes: simple scene classes, 2D scene classes, virtual reality module language (VRML) scene classes, audio scene classes, and full scene classes. The types of audio are: voice class, low bit rate synthesized audio class, scalable audio class and main audio class. The level is the classification of bit rate, sampling rate, image resolution and complexity. There can be no class with no level, but some classes have only one level. MPEG-4 currently has version 1, version 2, which will include object-based spatial scalability. Examples of MPEG-4 version 2 applications are the HomeNet Processing Laboratory and the progressive scan definition television encoder, which encodes 1080 lines of 60-frame progressive scan HDTV using MPEG-4 space, MPEG-4 space scalable The performance is usually better than MPEG-2/4 single-layer encoding, and the required frame memory is 12.5% ​​less, and 1080P can be easily down-converted to 1080I and 720P. This experiment will reduce the transmission rate of high-quality 1080P/60. To 18Mb/s.

4.MPEG-7 encoding

The MPEG-7 standard is: MPEG-7 is not a source coding standard, but a description standard for content and objects in order to better manage and reuse content, which requires an accurate description of the content. It is Meta Data. The so-called metadata is the data that describes the characteristics of the data, because in the transmission system and processing system, whether video, audio and data are data, it must be described. For example, the SMPTE/EBU bitstream program material exchange coordination standard is a content description standard for describing audio and video material for program exchange. OCI (Object Content Identifiers) in MPEG-4, used in audio and video databases to describe data such as materials. The standard used by MPEG-7 to describe content can be divided into formal elements and informal elements. Its formal basic elements are: Descriptors, Description Schemes, Descriptions, and Coded Descriptions. The basic elements of informality are: Descriptor Value, Features, Description, and Coded Description. The current applications mainly include program production management and program resource management, and related materials in the database until the retrieval of related audio and video objects, business-to-terminal interaction services, copyright recognition, self-generated databases, and the like. The production and application of metadata will advance the construction of programming tools. A little further away from the content is the Application Programming Interface (API). The API is an interface between the underlying devices, such as a set-top box with CPU, input and output devices, memory, graphics engine, MPEG decoding, and so on. These underlying devices are controlled by application software such as transport protocols, virtual machines, browsers, etc., while application software implements human-machine dialogue. The implementation of the set-top box function and its application range are directly related to the API, so countries and standards organizations attach great importance to it. The earliest effort in this area was MHEG (Multimedia and Hypermedia Information Coding Experts Group), an interpreted language that provides classes for objects that can be used in set-top boxes, allowing them to display frequently used shapes and text, providing interaction or timing. , or connect to another scene from one scene, or have a part of a scene to launch an application, etc.

Understanding and mastering the coding technology of digital TV is a knowledge that every TV staff must be familiar with. In particular, engineers and technicians should master the development of TV technology to update concepts, familiarize with equipment, and guide daily work.

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