MP4 files (MPEG-4 Part 14) are multimedia files. MP4 is a container format that can store video, audio and subtitle data (as well as more depending on content). Since stores like iTunes uses this container format, and it is used with iPod and PlayStation Portable (PSP), MP4 files have become more common.
MPEG-4 Part 14 or MP4, formally ISO/IEC 14496-14:2003, is a multimedia container format standard specified as a part of MPEG-4. It is most commonly used to store digital video and digital audio streams, especially those defined by MPEG, but can also be used to store other data such as subtitles and still images. Like most modern container formats, MPEG-4 Part 14 allows streaming over the Internet. A separate hint track is used to include streaming information in the file. The only official filename extension for MPEG-4 Part 14 files is .mp4.
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AC3 is an audio file specifically developed by Dolby Digital to provide surround sound audio. The format is used for audio on many DVD and Blu-Ray media. In order for the file to be correctly decoded and output, a DVD player with a Dolby Digital compatible home cinema system is required. It became the de facto standard for surround sound, and has held that position since its inception in 1992, with it coming to home cinema in 1995.
AC3 allows for separate audio to be defined for each of the 6 speakers in the standard cinema and home cinema setup. This produces the effect of surround sound to the person listening to the audio file. The six channels consist of right front, center, left front, rear right, rear left and a sub woofer.
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