For the sake of the end users, almost all AVI files that you might have downloaded would have been compressed already by the time it reaches your computer. Unless you're the type of person who likes to recieve a whole lot of data and bytes just for one file. Compressing the file reduces the time needed to download the file as well as reducing the burden in your PC. Differing codecs can provide differing rates of compression between the quality of the video and the compression size
The first one is XviD which is an MPEG-4 compliant video codec most used as of this date. It can handle digital video compression so well that it is most used by a wide audience number. It showcases high quality, high compression rate that makes this codec the "in-thing" to consider when dealing with the movie market.
The next codec that is noted for its absurd compression rate is the MJPEG codec. MJPEG stands for the term Motion JPEG, and like its namesake, offers the best compression rate anywhere else. But the standard penalty for this is like with its awesome compression, the quality is not quite what you'd expect. There's also a noted loss of quality that leaves it's viewers unsatisfied. Still, a lot of editors use this because of the compression rate.
DivX also offers high quality compression. The shining feature of this codec is that it offers multiple language support, competent visual quality, speed and compression. The only debacle with this codec is its synchronization issues. A lot of compatibility problems arise from DivX as well, which is why most users go for its counterpart, the XviD codec.
The Microsoft MPEG-4 Video codec comes with the default Windows Operating System. MPEG stands for Moving Pictures Experts Group and MPEG is probably a household word anywhere you go. This proves MPEG's popularity. By default, Microsoft MPEG-4 gives outstanding quality compression, accessibility and compatibility that is needed in today's market. MPEG comes to a close second against DivX in terms of quality.
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