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modern operating systems 2nd edition phần 6

khôi phục cơ sở dữ liệu (lưu trữ Public Folder / cửa hàng Mailbox ') sẽ được tự động gắn trên MS Exchange System Manager Trao đổi Mailbox gạch cấp sao lưu | 464 MULTIMEDIA OPERATING SYSTEMS CHAP. 7 All compression systems require two algorithms one for compressing the data at the source and another for decompressing it at the destination. In the literature these algorithms are referred to as the encoding and decoding algorithms respectively. We will use this terminology here too. These algorithms have certain asymmetries that are important to understand. First for many applications a multimedia document say a movie will only be encoded once when it is stored on the multimedia server but will be decoded thousands of limes when Lt is viewed by customers . This asymmetry means that it is acceptable for the encoding algorithm to be slow and require expensive hardware provided that the decoding algorithm is fast and does not require expensive hardware. On the other hand for real-time multimedia such as video conferencing slow encoding is unacceptable. Encoding must happen on-the-fiy in real time. A second asymmetry IS that the encode decode process need not be invertible. That is when compressing a file transmitting it and then decompressing it the user expects to get the original back accurate down to the last bit. With multimedia this requirement does not exist. It is usually acceptable to have the video signal after encoding and then decoding be slightly different than the original. When the decoded output is not exactly equal to the original input the system is said to be lossy. All compression systems used for multimedia are lossy because they give much better compression. 7.3.1 The JPEG Standard The JPEG Joint Photographic Experts Group standard for compressing continuous-tone still pictures e.g. photographs was developed by photographic experts working under the joint auspices of ITU ISO. and IEC. another standards body. It is important for multimedia because to a first approximation the multimedia standard for moving pictures MPEG is just the JPEG encoding of each frame separately plus some extra features for .