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Unit 7 Video and Audio Compression
There is a total of 48 occurrences of the most common characters(10 spaces,9 "o",7 "e",etc.)and 25 occurrences of the less common ctharacters (3 "a", 3 "m", etc.). The result is(483)+(259)=319 bits,which is a reduction of 45 -)ercent from the original. (Plus, this reduction is lossless; see the discussion in the following ection.) An even more efficient VLC scheme called Huffman coding,would result in a reduction of over 50 percent; this is why Huffman coding is used in MPEG.
Unit 7 Video and Audio Compression
The input of compression systems can be a computer file, such as a document, an image, and/or a video/audio recording. Or the input can be a continuous stream of data, such as a digital video or audio signal. Either way this information is fed into the compression engine.
Unit 7 Video and Audio Compression
Compression allows a 2-hour movie to fit onto a 4-inch DVD in place of an 812-foot long (248-m) VHS tape. Satellite television uses compression to allow hundreds of video channels to be distributed economically to viewers.An understanding of video and audio compression is essential to understanding and using modern video transport systems, including video over IP networks. VHS：video home system 家用录放机 VOIP:video over Internet Protocol 网络电视
Unit 7 Video and Audio Compression
Since there are only 22 different characters in this sample sentence, we can easily encode this data using 5 bits. By doing this we can compress the file from 584 bits down to 365 bits, a 37 percent reduction.
Unit 7 Video and Audio Compression
The output of the compression engine is a second data file or data stream that contains fewer bits than the input file or stream. This output can be storec or transmitted over a network. Before the data file can be used, it must be restored to its original size, via a decompression engine. Note that a compression engine is often called an encoder, and the decompression engine is commonly called a decoder.
Unit 7 Video and Audio Compression
Compression technology is a continuing field of research. As better mechanisms are developed, more information can be carried in fewer and fewer bits. Plus, as processing power increases, these techniques can be implemented on ever-faster, ever-cheaper processors. So compression is constantly being used in more and more applications. In this section, we'll look at the reasons for using compression as well as its benefits and drawbacks.
Unit 7 Video and Audio Compression
Many communication systems that have become commonplace in the past few years depend on compression technology. For example, digital mobile telephones use compression to increase the number of users who can be served and to increase the battery life of mobile handsets. Digital TV and HDTV delivery to homes would be impossible without compression to fit content into consumer delivery systems.
Unit 7 Video and Audio Compression
To make a compressed version of this sentence, we need to analyze the data. The first two columns list all of the characters in the sample sentence and shot\ the count for each character. Note that space is the most common character, with 10 occurrences. There are 9 occurrences of "o," 7 of both "e" and "n," and so on down to the final period, which has one occurrence. Also note that capital "T" is counted separately from lowercase "t".
Unit 7 Video and Audio Compression
Digital cameras use data compression to fit more pictures into a fixed amount of storage. MP3 players and Apple iPods use compression to take files from audio CDs and make them small enough to fit into the memory of a portable player.
Unit 7 Video and Audio Compression
The goal of compression is to reduce the size of the incoming data file without removing useful information. In most real-world data files, some data patterns will repeat. For example, consider the following sentence: "The job of compression is to remove redundant or unnecessary information." In a typical computer file, each character in this sentence would be represented by an 8-bit code. There are 73 characters, including spaces and punctuation, in the sample sentence, which would occupy 584 bits.
Unit 7 Video and Audio Compression