Compression method is the core technology of DVR. Compression method largely determines image quality, compression ratio, transmission efficiency, transmission speed and other performances. It is an important part of evaluating DVR performance. With the development of multimedia technology, many compression coding standards have been introduced one after another. Currently, there are mainly JPEG/M-JPEG, H.261/H.263 and MPEG standards.
1, JPEG/M-JPEG
①. JPEG is a compression standard for still images, which is a standard intra-frame compression coding method. When the hardware processing speed is fast enough, JPEG can be used for video compression of real-time moving images. It can provide quite good image quality under the condition of small changes in the picture, the transmission speed is fast, and the use is quite safe. The disadvantage is that the amount of data is large.
②. M-JPEG is derived from JPEG compression technology. It is a simple intra-frame JPEG compression. The compressed image quality is better and there is no mosaic under the condition of picture changes. However, due to the limitation of this compression technology, large-scale compression cannot be achieved. , When recording about 1-2GB space per hour, network transmission requires 2M bandwidth, so regardless of recording or network transmission, it will consume a lot of hard disk capacity and bandwidth, not suitable for long-term continuous recording needs, not practical for video Network transmission of images.
2, H.261/H.263
① The H.261 standard is usually called P*64. H.261 can achieve a higher compression ratio for full-color, real-time transmission of moving images. The algorithm is composed of intra-frame compression plus inter-frame compression and encoding to provide video Fast processing of compression and decompression. Since only the last 1 frame is predicted in the inter-frame compression algorithm, it has an advantage in duration, but the image quality is difficult to achieve high definition, and it is impossible to achieve large compression ratios and variable rate video recording.
② The basic coding method of H.263 is the same as that of H.261, both of which are mixed coding methods. However, H.263 has made improvements in all aspects of coding in order to adapt to extremely low bit rate transmission, such as saving codes. To improve the quality of coded images, H.263 also absorbs MPEG’s two-way motion prediction and other measures to further improve the prediction accuracy of inter-frame coding. Generally speaking, when the bit rate is low, the use of H.263 is only half the rate. Image quality comparable to H.261 can be obtained.
3, MPEG
MPEG is a video and audio coding standard for compressing moving images and their accompanying audio. It uses inter-frame compression and only stores the differences between consecutive frames to achieve a larger compression ratio. MPEG has three versions, MPEG-1, MPEG-2 and MPEG-4, to meet the requirements of different bandwidths and image quality.
① The video compression algorithm of MPEG-1 relies on two basic technologies, one is motion compensation based on 16*16 (pixel*line) blocks, and the other is compression technology based on transform domain to reduce spatial redundancy, and the compression ratio is similar. Higher than M-JPEG, better image quality can be obtained for video signals with less intense motion, but when the motion is intense, the image will have a mosaic phenomenon. MPEG-1 transmits video and audio signals at a data rate of 1.5Mbps. MPEG-1 is equivalent to the image quality of a VHS video recorder in terms of video image quality. The color mode of video recording resolution is ≥240TVL, and the quality of two-channel stereo audio is close to that of CD. Sound quality. MPEG-1 is a compression algorithm for multi-frame prediction before and after frames. It has great compression flexibility and can compress video at variable rates. Depending on different recording environments, different compression quality can be set, ranging from 80MB to 400MB per hour, but The amount of data and bandwidth are still relatively large.
②, MPEG-2 It is to obtain higher resolution (720*572) to provide broadcast-level video and audio coding standards. As a compatible extension of MPEG-1, MPEG-2 supports interlaced video formats and many advanced features including support for multi-level adjustable video coding, suitable for occasions with multiple qualities such as multiple rates and multiple resolutions. It is suitable for real-time images with large motion changes and high image quality requirements. The video signal with a resolution of 30 frames per second and 720*572 is compressed, and the data rate can reach 3-10Mbps. Due to the large amount of data, it is not suitable for long-term continuous recording.
③, MPEG-4 is a low-rate, high-compression video and audio coding standard for mobile communication devices to transmit video and audio signals in real time over the Internet. The MPEG-4 standard is an object-oriented compression method. It does not simply divide the image into blocks like MPEG-1 and MPEG-2, but separates the objects (objects, characters, background) according to the content of the image. , Perform intra-frame and inter-frame encoding separately, and allow flexible allocation of code rates among different objects, allocate more bytes to important objects, and allocate fewer bytes to secondary objects, thereby greatly improving The compression ratio can achieve better results at a lower bit rate. MPEG-4 supports most of the functions in MPEG-1 and MPEG-2, and provides different video standard source formats, bit rates, and frame rates for rectangular graphics images. Effective coding.
In short, MPEG-4 has three advantages:
①, with good compatibility;
②, MPEG-4 provides a better compression ratio than other algorithms, up to 200:1;
③, while MPEG-4 provides high compression ratio, the loss of data is very small. Therefore, the application of MPEG-4 can greatly reduce the video storage capacity and obtain higher video clarity, which is especially suitable for the needs of long-term real-time video recording. At the same time, it has excellent network transmission capabilities at low bandwidth.
H.264 is a new digital video coding standard developed by the joint video team (JVT: joint video team) of VCEG (Video Coding Experts Group) of ITU-T and MPEG (Moving Picture Coding Experts Group) of ISO/IEC. It is part 10 of ITU-T's H.264 and ISO/IEC's MPEG-4. The solicitation of drafts started in January 1998. The first draft was completed in September 1999. The test model TML-8 was developed in May 2001. The FCD board of H.264 was passed at the 5th meeting of JVT in June 2002. . The standard is currently under development and is expected to be officially adopted in the first half of next year.
H.264, like the previous standard, is also a hybrid coding mode of DPCM plus transform coding. However, it adopts a concise design of "return to basics", without many options, and obtains much better compression performance than H.263++; it strengthens the adaptability to various channels and adopts a "network-friendly" structure and syntax. Conducive to the processing of errors and packet loss; a wide range of application targets to meet the needs of different speeds, different resolutions, and different transmission (storage) occasions; its basic system is open, and no copyright is required for use.
Technically, there are many highlights in the H.264 standard, such as unified VLC symbol coding, high-precision, multi-mode displacement estimation, integer transformation based on 4×4 blocks, layered coding syntax, etc. These measures make H.264 algorithm have very high coding efficiency, under the same reconstructed image quality, it can save about 50% of the code rate than H.263. H.264's code stream structure has strong network adaptability, increases error recovery capabilities, and can well adapt to the application of IP and wireless networks.
In fact, most of the current H.264 is H.263 + + through the improved algorithm, the compression rate has become a bit smaller (including some individual manufacturers now, my colleagues have seen their source code)! If it is compared from the definition of a single screen, MPEG4 has an advantage; from the definition of action continuity, H.264 has an advantage!
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