At present, TV equipment is gradually transitioning from analog to digital. SONY, Panasonic, JVC and other companies have launched the latest digital camera, recording and editing equipment. In today's television technology, digital recording and editing has been widely used.
Digital video recording formats are uncompressed and compressed. The uncompressed recording format of D1, D2, D3, D5, etc., which directly records the input signal at the original signal rate, maintains the original level of the signal, and is a lossless recording. The recording method is divided into digital components (D1, D5) and digital composites (D2, D3), which represent the highest standard of video equipment, the highest image quality, the smallest signal loss, but at the same time due to the large amount of image signal data, the machine hardware The requirements are extremely demanding and the price is very expensive. Although the products have been available for several years, there are only a few video production companies that have extremely high image quality requirements. The compression format refers to video using digital compression technology. Commonly, there are DV, MPEG-2, M-JPEG, etc., and various data compression standards are formulated, so the corresponding digital Betacam (DVW), DV, DVCPRO, DIGITAL appear. -S, DVCAM, Betacam-SX and other digital video recorders, which compress and re-record the image signal on the tape. The purpose is to reduce the amount of image signal data, reduce the size of the device, and reduce the image quality. The amount of tape used to achieve the best possible results with minimal signal loss, thereby reducing equipment costs.
1. International standards for digital video technology
(1) CCIR 601 recommendation
In order to facilitate international program exchanges, eliminate the differences between the digital devices, and enable compatibility between the 625-line television system and the 525-line television system. In February 1982, the International Radio Consultation Committee (CCIR, now changed to ITU Radio) At the 15th Plenary Session of the Ministry of Communications, ITU-R), Recommendation No. 601 was adopted to determine the component coding based on the luminance component Y and the two color difference components RY, BY, as the digital coding of the television studio. International standards.
(2) H.261 standard
H.261 is abbreviated as p&TImes; 64. The standard is for video telephony and video conferencing. The image encoding algorithm is real-time processing with minimal delay time, allowing images and voice to work closely together to achieve full-color real-time motion video transmission with high compression ratio. The standard was adopted by the International Telegraph and Telephone Consultative Committee (CCITT) in 1990.
(3) JPEG standard
The Joint Photographic Experts Group (JPEG), the Joint Photographic Experts Group, is a collaboration between the International Standards Organization (ISO), the International Telegraph and Telephone Consultative Committee (CCITT) and the International Electrotechnical Commission (IEC). It was completed in 1991. by. JPEG is both an ISO standard and a recommended standard for CCITT. Its goal is to compress still color picture data, which is used for satellite and news picture transmission and storage, as well as graphics and image document processing.
(4) MPEG standard
With the wide application of digital audio and digital video technology, ISO's Moving Picture Expert Group proposed the draft of the ISOll172 standard in November 1991, commonly known as the MPEG-1 standard, which was adopted in November 1992. The MPEG-1 standard is applicable to applications with a digital rate of around 1.5 Mbps, which is also defined for video storage and playback of CD-ROM discs.
MPEG-2 is the second standard developed by MPEG and was officially established as an international standard in November 1994. MPEG-2 is the "Generic Coding of Moving Figures Associated Audio Informa TIon" standard. As a recognized compression scheme, the standard has the advantages of openness, low technology cost, interoperability and flexibility, optional scalability of bit rate and support from many vendors. It is adopted in the fields of network, communication, satellite, etc. .
MPEG-4 was established in 1993 and was finalized in October 1998. It officially became an international standard in early 2000. The standard has many compelling features, including object-based video object access, scene content-based scalability, video access, error correction capabilities, and more. The MPEG-4 video standard not only provides a new type of multimedia information transmission standard with more compression efficiency, but also has good interactivity and all-round survivability. Its name is 1+2+4= 7 (MPEG-1, MPEG-2, MPEG-4). It extends the limited capabilities of existing content-aware solutions and includes more data types. MPEG-7 is a standard set that specifies a descriptor for describing various types of multimedia information.
2, digital video compression
Based on the above criteria, the three compression methods currently widely used in the television field are MPEG-2, M-JPEG and DV. They are all based on discrete cosine transform (DCT), and run-length encoding after transforming the transform coefficients.
(1) JPEG Introduction JPEG is used for continuously changing still images, including continuous changes in both grayscale and color. JPEG contains two basic compression methods: the first is lossy compression, which is a DCT (DiscreteCosineTransform)-based compression method; the second is lossless compression, also known as predictive compression. The most commonly used is the former, the DCT compression method, also known as the Baseline SequenTIalCodec method, which is advanced, efficient, simple, and easy to communicate, and is therefore widely used.
(2) Introduction to M-JPEG
M-JPEG is based on the normal required speed of the moving image. Each frame is compressed in the intraframe compression mode of 25 frames per second. Each frame is treated as an independent signal. The series of frames is actually one. JPEG signal stream. The benefit of this design is that it is easy to edit and can be edited randomly for any frame, making it an excellent choice for nonlinear editing applications. The compression and decompression of M-JPEG is symmetrical and can be implemented by the same hardware and software. The disadvantage is that it requires too much bandwidth and storage space, because it does not compress the line, but only the intraframe compression, the compression efficiency is not high enough.
(3) Introduction to DV
The DV compression method is an expandable format designed for home video recorders for standard definition televisions and high definition televisions. DV compression was first developed by a consortium of standard and high definition home VCR manufacturers. The DV uses a 13.5 MHz sampling rate, 4:1:1 encoding, and uses an 8-bit code to increase the signal-to-noise ratio with a spatial compression ratio of 5:1. The DV extends this from the standard 25 Mb/s to 50 Mb/s in the DVCPRO 50 by promoting the compression of substantially inactive video images. Sony offers DVCAM series VTRs based on DV compression. There are two main types of DVCPR0 cameras for broadcast TV: one is DVCPR025 and the other is DVCPR050.
(4) Introduction to MPEG
The basic steps of digital compression of the MPEG standard are: first convert analog video into digital video and then group by time. Each image group (GOP) selects a reference image, uses motion estimation to reduce temporal redundancy between images, and finally reduces the reference image. And motion estimation errors are performed by discrete cosine transform (DCT), coefficient quantization, and entropy coding (VLC & RLC) to eliminate spatial redundancy. The MPEG expert group initially had three tasks: 1.5Mb/s, 10Mb/s, 40Mb/s compression coding standards, namely MPEG-1, MPEG-2, MPEG-3, of which MPEG-3 was revoked in 1992. .
The MPEG-1 standard is used for moving pictures and their audio coding standards, with a high compression ratio, low bandwidth and low resolution, and a video rate of approximately 1.5 Mb/s. The basic algorithm is for the spatial resolution of the compressed horizontal direction (360 pixels) and the vertical direction (288 pixels), and has a good effect on moving images of 24 to 30 frames per second. The MPEG-l standard also provides the functions of some video recorders: forward, image freeze, fast forward, fast reverse and slow release, as well as random storage. The MPEG-l standard also uses a series of techniques to obtain a high compression ratio: 1 sub-sampling the color difference signal to reduce the amount of data; 2 using motion compensation technology to reduce line quantization, discarding unimportant information, and quantizing the DCT component Reordering according to frequency; 5 converting the DCT component into variable length encoding; 6 performing predictive differential encoding on the DC component (DC) of each data block.
The MPEG-2 standard is similar to the MPEG-1 standard, and its bit rate is much higher than the latter, thus requiring higher bandwidth and resolution. The MPEG-2 standard can define bit rates up to 400 Gb/s and images of 16000&TImes; 16,000 pixels. It adopts intraframe coding and quality sword?? GOP (Group of Images) is divided into I frame (intra coded frame), P frame (forward predictive coded frame), B frame (bidirectional predictive frame), and they are different. Compression coding. In January 1996, the International Munich Group Conference confirmed the MPEG-2MP@ML standard with high broadcast quality and high editing precision, which allows for shorter GOPs to be adapted to the precise editing of program production. MPEG-2 automatically converts the compression ratio in the "simple" and "complex" areas of the image, which is more efficient by using different compression ratios within the same frame. Under the condition of compression to the same image quality, the space occupied by the MPEG-2 image is only 10% to 15% of the M-JPEG image. In various fields such as image acquisition, production, transmission, and broadcast, MPEG-2 has been widely adopted.
In addition to fully supporting all the video functions that MPEG-1/2 already provides, MPEG-4 includes effective encoding of standard rectangular area image sequences at different input formats, frame rates, and bit rates, and adds new features: Transmission efficiency, MPEG-4 uses "sub-picture" prediction and encoding technology, which uses the still background as a "sub-picture", first sent to the receiving end, as the first frame is stored in the encoder and decoder at the same time, and then the camera is used. Move, rotate, and zoom, capture the video objects that appear before the background, and then encode them separately to form a video sequence for transmission, and then reconstruct the original image. This technology is very beneficial for the realization of multimedia databases, while also improving image quality. MPEG-4 is a highly efficient encoding standard with a minimum bit rate of 5 to 64 kbps. For a specific video, its interactivity and operability can be compatible with various encodings in the field of multimedia applications. MPEG-7 is based on MPEG-4 and retrieves information content with few features. For example, for graphics, as long as a few lines are drawn, corresponding graphics, trademarks, etc. including the feature can be found.
Comparison of 3 and 3 commonly used digital compression formats
M-JPEG, DV and MPEG are the three main compression technologies in the video industry, all based on DCT, and their recorded images are converted into frequency domain coefficients for uniform, quantized and variable length coding. Digital video coding technology commonly used in broadcast television, in which DV and typical M-JPEG are intra-frame coding, using intra-frame compression, without degrading. For MPEG-2, when using MPEG-2 for editing, it is generally limited to I-frame coding, and it is possible to perform frame switching at the edge without knowing the previous frame and the next frame. In addition, DV and MPEG-2 compression use motion adaptive processing to achieve efficient intraframe coding, but DV can only perform fixed bit rate (CBR) encoding, while MPEG-2 and M-JPEG can perform CBR and mutable. Bit rate (VBR) coding.
Digital video compression technology enables us to obtain high-quality video at a lower cost, making it possible to transmit video signals with high quality, and with the continuous development of TV technology, this technology is constantly updated and improved. When TV technology and Internet technology are combined, digital video technology is more reflective of its communication and communication advantages.
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