AVI stands for Audio Video Interleave. Files of this format have an .avi extension. Developed by Microsoft in 1992, it has become so widespread that many people consider it the de-facto standard for storing video and audio information on PC. AVI combines audio and video into a single file in a standard container to allow simultaneous playback. Its advatage is a simple architecture, due to which AVI runs on a number of different systems like Windows, Mac, Linux, Unix and is supported by all of the most popular web-browsers.'
AVI is a derivative of the Resource Interchange File Format (RIFF), which divides a file's data into blocks, or chunks. Each chunk is identified by a FourCC tag. An AVI file takes the form of a single chunk in a RIFF formatted file, which is then subdivided into two mandatory chunks and one optional chunk. The first sub-chunk is identified by the hdrl tag. This sub-chunk is the file header and contains metadata about the video, such as its width, height and frame rate. The second sub-chunk is identified by the movi tag. This chunk contains the actual audio/visual data that make up the AVI movie. The third optional sub-chunk is identified by the idx1 tag which indexes the offsets of the data chunks within the file.
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MP3 is a digital music format which allows CD tracks to be reduced to around a tenth of their normal size without a significant loss of quality. MP3 gets rid of a lot of the information recorded in a song that our ears are not able to hear and then uses complex algorithms to reduce the file size. This then enables you to get hundreds of songs on to a CD and it also has opened up a new market over the internet - the download market as download times have been significantly reduced.
The MP3 format is a lossy format. That means that an MP3 file does not contain 100% of the original audio information. Instead, MP3 files use perceptual coding. In other words, that means it removes the information that your ear doesn't notice thereby making the file smaller. The reason lossy formats are used over RAW is that RAW audio files are too large to travel over the internet at any great speed. By using lossy formats it enables even dial up users to download mp3 files at a reasonable speed. RAW file formats generally require 176,000 bytes per second compared to a lossy format which requires 17,600. The difference is massive and so are the download times.