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DIGITAL CCTV A Security Professional’s Guide phần 4

Một trong những điều quan trọng để nắm bắt về video kỹ thuật số là nó chỉ đơn giản là một cách thay thế mang cùng một thông tin video như là một hệ thống tương tự. Video kỹ thuật số là một loạt chuỗi số đại diện cho các cấp điện áp của một tín hiệu video analog. Một hệ thống kỹ thuật số lý tưởng | More on Digital Video Compression 83 Correlation between neighboring pixel values Spatial redundancy as explained earlier removes repetitive information composed of adjoining pixels. Correlation between different color planes or spectral bands Spectral redundancy consists of color spectra or brightness due to the fact that the human eye distinguishes differences in brightness more readily than it sees differences in pure color values. Correlation between different frames in a video sequence Temporal redundancy is the similarity of motion between frames. If motion redundancy did not exist between frames there would be no perception of realistic motion. Level 3 of MPEG-1 is the most popular standard for digital compression of audio commonly known as MP3. MPEG-2 MPEG-2 published as a standard in 1994 is a high bandwidthencoding standard based on MPEG-1 that was designed for the compression and transmission of digital broadcast television. This is the standard used by DVD players. MPEG-2 will decode MPEG-1 bit-streams. MPEG-2 was designed for high bit rate applications and like MPEG-1 it does not work well at low bandwidths. The main difference between MPEG-1 and MPEG-2 is the encoding of interlaced frames for broadcast TV. MPEG-1 supports only progressive frame encoding and MPEG-2 provides both progressive frame and interlaced frame encoding. MPEG-4 MPEG-4 an open standard was released in October 1998 and introduced the concept of Video Object Planes VOP . The result is an extremely efficient compression that is scalable from low bit rates to very high bit rates. MPEG-4 is advanced audio and video compression that is backward compatible with MPEG-1 and 2 H.261 and H.263. 84 Digital CCTV MPEG-4 is designed to deliver video over a much narrower bandwidth. It uses a fundamentally different compression technology to reduce file sizes than other MPEG standards and is more wavelet based. Object encoding provides great potential for object or visual recognition indexing .