With the development of the computer, the peripheral digital video equipment has gradually been widely used. Individuals can also play video, audio production VCD, SVCD, DVD, and even online video. These things have been in contact with the need for video image compression. Why do you need to be compressed? Let's take a look at the following two reasons!
First, the bandwidth required to transmit digital images is far narrow than the uncompressed image. For example, the NTSC image is transmitted at approximately 640 x 480, and the data rate reaches 28m bytes / second or 221 m bits per second when transmitting about 640 x 480. In addition, the NTSC sound signal also adds a bit rate of the uncompressed image to increase. However, single speed CD-ROM (1x) drive can only transfer data at a rate of 1.2 m / sec. The second reason is to take up the 420m bytes of memory space at a rate of 28m bytes / second, which will occupy a 420m byte memory space, which is unacceptable for most desktop computers that can only handle small image pieces. Today's key issues that add images to electronic signals are compression. There are several different compression methods, but MPEG is the most market potential compression.
MPEG's full name is Moving Pictures Experts Group, which is jointly established by ISO (International Standards Organization, International Standardization) and IEC (INTERNATIONAL ELECTRONIC COMMITTEE) in 1988, dedicated to moving images (MPEG video) and its Compound (MPEG audio) standardization work. There are 4 versions of MPEG, with the first two versions of MPEG-1 and MPEG-2 applications, while MPEG-4 has been launched for nearly two years, but the application of it is not active until recently, MPEG-7 is It belongs to the future standard.
MPEG-1 Standard (ISO / IEC11172) was developed in 1992, an international standard for digital storage media motion images and its sound coding design for data transfer rates below 1.5Mbps, mainly for CD-ROM (including video-cd, CD-i, etc.) Store color synchronous motion video images, which compresses images of SIF (standard exchange format) standard resolution (NTSC system 352 × 240; PAL system 352 × 288), can play 30 frames per second The picture is available in the sound quality of the CD (referring to the laser disc). At the same time, it is also used in digital telephone networks, such as non-symmetrical digital user lines (ADSLs, video on demand (VOD), educational networks, etc. Its purpose is to compress the 221 mbit / sec NTSC image to 1.2 Mbit / second, the compression ratio is 200: 1. Using the MPEG-1 compression algorithm, a 120 minute film can be compressed to about 1.2GB, so it is widely used in VCD production and some video clips, currently 90% of VCDs are mPEG -1 is compressed. At present, some CCD collecting compressed cards, like SNAZII DVC, MP 10, and white exchanges, etc. are MPEG-1 compression standards.
MPEG-2 is used for the image of broadband transmission, and the image quality reaches the standard of TV broadcasts and even HDTV. Compared to MPEG-1, MPEG-2 supports a wider range of resolutions and bit rate ranges, which will become a digital image disc (DVD) and digital broadcast TV compression. These markets will be intertwined with the computer market, so that MPEG-2 becomes an important image compression standard for computers. MPEG-2 Standard ISO / IEC13818) Developed in 1994, an international standard for moving images and their audio coding for data transfer rates of 3 to 10 Mbps. MPEG-2 can provide a wide range of variable compression ratio to accommodate different picture quality, storage capacity, and bandwidth requirements. It implements low-rate and multi-channel extensions on the MPEG-1 compatibility: MPEG-2 can compress one 120-minute movie to 4 ~ 8GB (it provides the quality of the DVD we usually ), Its audio coding provides left and right and two surround channels, an aggravated bass channel and up to 7 parties (so DVD can have 8 languages). In addition to the specified standards of DVD, MPEG-2 can also be used to provide broadcast, cable TV, cable networks, etc. to provide broadcast level digital video. However, for ordinary users, due to the limitations of TV resolution, the high-definition picture quality (such as DVD screen) is not obvious on TV, and it is its audio characteristics (such as aggravating bass, Multi-sound channels, etc.) have been widely used. MP3 is an audio compression technology standard applied to MPEG-1, and the full name of English is MPEG-1 Audio Layer3. The basis for making this definition is: First, MPEG officials have clearly stated that MP3 and MPEG-1 Audio Layer3 refers to one thing. Second, Layer's Publisher Fraunhofer IIS-A [Note 1] The official technical document also mentioned that MP3 is MPEG-1 Audio Layer3. In addition, many well-known manufacturers such as Sony, some of the related technical documents of Philips also illustrate the problem of MPEG-1 Audio Layer3 (refer to the corresponding technical part). MPEG-3 is the coding and compression criteria for ISO / IEC initially developed by HDTV (high-definition television broadcast), but since MPEG-2's excellent performance can be applied to HDTV, the MPEG-3 standard is not formulated, we usually say MP3 refers to MPEG Layer 3, just an audio compression standard for MPEG. The self-contained MPEG-4MPEG-4 was announced in November 1998. The international standard MPEG-4 that is expected to use is a video, audio coding, more focus on multimedia system interactive and flexible for a certain bit rate. To this end, MPEG-4 introduces Audio / Visual Objects, making more interactions possible: "AV object" can be an isolated person, or a voice or a background music of this person. It has high efficiency encoding, efficient storage and propagation and interactive features. MPEG-4 operators to the AV objects mainly: use AV objects to represent audiscations, visual, or audio-visual combination content; combine existing AV objects to generate composite AV objects, and generate AV scenes; flexible data for AV objects Multi-channel synthesis and synchronization in order to select the appropriate network to transmit these AV object data; allow the user to interact with the AV object in the AV scene. The MPEG-4 standard is composed of 6 main parts:
1 DMIF (The Dellivery MultiMedia Integration Framework) DMIF, Multimedia Transport Overall Framework, which mainly solves the operational issues of multimedia applications in the interactive network and the multimedia applications in disk applications. Establish the interaction and transmission of the client and server side by transmitting multi-channel synthesis bits. With DMIF, MPEG4 can establish channels with special quality service (QoS) and bandwidth for each basic stream. 2 The data plane in the data plane MPEG4 can be divided into two parts: the transfer relationship section and the media relationship section. In order to appear in the same scene in the same scene, MPEG4 references the concept of an object description (OD) and flow graph (SMT). OD transmission information flow diagram related to special AV objects. The desktop connects each stream with a CAT (Channel Assosiation Tag), and CAT can implement the smooth transmission of the stream.
3 Buffer Management and Real-Time Identification MPEG4 define a system decoding mode (SDM) that describes an ideal decoding device for processing a schematic symptoms of a bitcar, which requires special buffers and real-time modes. By effectively managed, limited buffer space can be better utilized.
4 The superiority of audio encoding MPEG4 is - it supports natural sounds not only, but also supports synthetic sounds. The audio portion of the MPEG4 combines audio synthesis coding and coding of natural sounds and supports audio object feature.
5 Video codes Similar to audio coding, MPEG4 also supports encoding of natural and synthetic visual objects. The synthetic visual object includes 2D, 3D animation and human face expression animation.
6 Scene Description MPEG4 provides a range of tools for forming a set of objects in the scene. Some necessary synthetic information constitute a scenario, which describes the Binary Format for Scene Description, and BIFS is transmitted, encoded with the AV object. Scene description is mainly used to describe the various AV objects in a specific AV scene coordinate, how to organize and synchronize. At the same time, there is also a problem such as an AV object and the intellectual property protection of AV scenes. MPEG4 offers us a rich AV scene. MPEG-4 application
Compared to MPEG-1 and MPEG-2, MPEG-4 is more suitable for interactive AV services and remote monitoring, which makes it more adaptive and scalability: MPEG-4 transmission rates at 4800-64000bps Between the resolution 176 × 144, it can be compressed and transmitted by frame reconstruction technology using a narrow bandwidth, so that optimal image quality can be obtained at least the least data. Therefore, it will be greatly launched in digital TV, moving images, Internet, real-time multimedia monitoring, mobile multimedia communication, Internet / INTRANT, with video streams on video streams, interactive multimedia applications on DVD.
Of course, for ordinary users, MPEG-4 is the most attractive place in the current most attractive place that it can basically implement the quality of DVD in the ordinary CD-ROM: ASF (Advanced Streaming Format) with MPEG-4 compression algorithm. Advanced Format Flow) The 120-minute film is compressed into a video stream of about 300MB; DivX video coding technology using the MPEG-4 compression algorithm can compress the 120-minute movie to about 600MB, or a DVD movie can be compressed to 2 sheets. CD-ROM! In other words, there is MPEG-4, you don't need to buy DVD-ROM, you can enjoy the quality of the video! Playing this coded film is not high on the machine: as long as your computer has 300MHz (no matter which model) CPU, 64MB memory, 8MB graphics card can play smoothly. However, compared to DVD, MPEG-4 belongs to a high ratio loss compression algorithm, and its image quality is always compared with the MPEG-2 of the DVD. After all, the storage capacity of DVD is relatively large. In addition, to ensure that the image picture of the high-speed movement does not matter, there must be enough code rate. At present, although the code rate of MPEG-4 can be adjusted to DVD, the overall effect has a small gap. Therefore, the current MPEG-4 can only facilitate entertainment, appreciation market, and those professional video areas with higher image quality are still unable to use.
