m4v M4V is a file format developed by Apple for the iTunes store's content and is very similar to the MP4 file extension. M4V is supported exclusively by Apple devices. Other video formats like MP4 and MOV can be converted to M4V in order to run on Apple devices. The most common difference between the M4V and MP4 formats is that M4V offers DRM-Copyright Protection which prevents the copyright of media content. To play a DRM-protected M4V file the computer needs authorization by opening the iTunes account used for buying the media file. However, if the files are not protected by DRM, the user can just rename the file extension from M4V to MP4 in order for it to be recognized by non-apple devices.
ac3 Files with the .ac3 filename extension refer to the proprietary audio file format Dolby digital AC-3, developed by Dolby Laboratories. The AC-3 format is from the lossy family of audio compression formats. It supports a variety of audio channel configurations and was the first of its kind to support the full surround sound experience. Because of this, the format has been commonly used in the development of motion picture features designed for theaters and in consumer homes through DVDs and surround sound home theater systems. The AC-3 format is one of the supported audio codecs of the ATSC standard ' a set of standards developed for digital television transmission in several affiliated countries.