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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AAC is similar to MP3. It essentially cuts out, or compresses, information that the human ear cannot pick up thereby making digital files smaller and more easily managed and therefore quicker to download. AAC is about half the size of MP3 but with better quality. It is at least one tenth the size of CD digital data. Apple are the biggest users of this format and if you have an iPod or iPhone then you will have come across this format as it is iTunes default audio format.
AAC is similar in concept to MP3 but goes further. It too compresses digital audio files but to a bigger degree. It is also part of the MPEG-4 standard, it is most widely used to create small digital audio files. The current variant is specified in ISO/IEC standard 14496-3. Like MP3, AAC is a lossy algorithm. The human hearing system cannot hear quiet sounds in the presence of loud sounds of a similar frequency; for example, a voice conversation cannot be heard while an aeroplane flies low overhead. This is known as auditory masking, this allows the discarding of data with minimal loss of quality.
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A combination of Fraunhofer IIS, AT&T Bell Laboratories, Dolby and Sony Corporation