The M4V file format was developed by Apple. It is very similar to the MP4 file type, with the main difference being that M4V may contain Apple's DRM protection, Fairplay. Video content downloaded from the iTunes store is likely to be in M4V format. If M4V file does have DRM enabled then the iTunes account used to procure the file will need to be authenticated on the computer playing the file.
The M4V file is a container that allows audio and video. In certain circumstances where Apple's DRM has not be applied to the file, it is possible to play the file from a different unauthenticated computer by amending the file extension from .m4v to .mp4. Additionally m4v provides the functionality to create chapter information which is not possible with MP4.
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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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