A video codec library following the MPEG-4 standard, specifically MPEG-4 Part 2 Advanced Simple Profile (ASP). And AC3 is a file extension for surround sound audio files used on DVD s format.  
A file with the XVID file extension is a XviD CODEC file. Other types of files may also use the XVID file extension. If you know of any additional file formats that use the .XVID extension, please let me know so I can update this information. AC3 is a file extension for surround sound audio files used on DVD s format. The AC3 file format was created by Dolby Labs for use in a Dolby Digital audio on DVD, Blu-ray and other digital video formats. AC3 stands for Audio Coding 3. Convert XVID to AC3. XVID to AC3 Conversion Software converts XVID files to AC3 fast. The converter is a multiple channels encoder. The software supports 2.1, 4.0, 4.1, 5.1, 6.1, and 7.1 channels encoding for AAC, AC3, AIFF, AU, FLAC, OGG, and WAV files. If you find that an application on your PC does try to open the XVID file but it's the wrong application or if you would rather have another installed program open XVID files, see my How To Change the Default Program for a Specific File Extension guide for making that change in Windows. Xvid (formerly XviD) is a video codec library following the MPEG-4 standard, specifically MPEG-4 Part 2 Advanced Simple Profile (ASP). It uses ASP features such as b-frames, global and quarter pixel motion compensation, lumi masking, trellis quantization, and H.263, MPEG and custom quantization matrices. XviD is a very popular codec and as such, there are many thousands of files available that contain XviD video. It is fairly unlikely that you will have a file with the extension.xvid. In all likelihood, your file is named something along the lines of Filename.XviD.AVI . When you install the XviD codec it basically provides your system with specific instructions how to compress and decompress video in the XviD format. Once you've installed the XviD codec on your system you will be able to play XviD videos using Windows Media Player or any other XviD-enabled player. The Xvid codec compresses video files to make them smaller. It can compress video at a ratio of 200:1 or more compared to uncompressed video. This makes transmitting the video over a network speedy or saves you a lot of space on your computer's hard disk.
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