MP4 Mpeg4

MP4

MPEG-4 Part 14, 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 audio and digital video streams, especially those defined by MPEG, but also can 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. The official filename extension for MPEG-4 Part 14 files is .mp4, thus the container format is often referred to simply as MP4. Devices that play .mp4 files are referred to as MP4 players.

History

MPEG-4 Part 14 was based on Apple's QuickTime container format.[1] Although MPEG-4 Part 14 still resembles the QuickTime format, it has been improved upon in many different ways.

[edit] MP4 versus M4A

The existence of two different file extensions for naming audio-only MP4 files has been a source of confusion among users and multimedia playback software. Since MPEG-4 is a container format, MP4 files may contain any number of audio, video, and even subtitle streams, making it impossible to guess what type of streams are present in a MP4 file based on its filename extension alone. In response, Apple Computer started using and popularizing the .m4a file extension. Software capable of audio/video playback should recognize files with either .m4a or .mp4 file extensions, as would be expected, as there are no file format differences between the two. Most software capable of creating MPEG-4 audio will allow the user to choose the filename extension of the created MPEG-4 files.

[edit] Data streams

Almost any kind of data can be embedded in MPEG-4 Part 14 files through private streams; the widely-supported codecs and additional data streams are:

Video: MPEG-4 Part 10 (also known as H.264 and MPEG-4 AVC), MPEG-4 Part 2, MPEG-2, and MPEG-1.
Audio: AAC (also known as MPEG-2 Part 7), MP3 (also known as MPEG-1 Audio Layer 3), MPEG-4 Part 3, MP2 (also known as MPEG-1 Audio Layer 2), MPEG-1 Part 3, CELP (speech), TwinVQ (very low bitrates), SAOL (MIDI).
Subtitles: MPEG-4 Timed Text (also known as 3GPP Timed Text).

Some private stream examples include Nero's use of DVD subtitles (Vobsub) in MP4 files. They are however not a part of the MPEG-4 file format standard and programs are not required to support them.

[edit] File extensions

While the only official file extension defined by the standard is .mp4, various file extensions are commonly used to indicate intended content:

Audio-only MP4 files generally have a .m4a extension.
- MP4 files with audio streams encrypted by FairPlay Digital Rights Management as sold through the iTunes Music Store use the .m4p extension.
- Audio book and podcast files, which also contain metadata including chapter markers, images, and hyperlinks, can use the extension .m4a, but more commonly use the .m4b extension.
MP4 files with audio and video generally use the .mp4 and .m4v extensions, occasionally .mp4v.
3G mobile phones use 3GP, a simplified version of MPEG-4 Part 12 (a.k.a MPEG-4/JPEG2000 ISO Base Media file format, MPEG-4 Part 14 is a derivated standard from ISO Base file format too), with the .3gp and .3g2 extensions. These files also store non-MPEG-4 data (H.263, AMR, TX3G).

The common, but non-standard use of the extensions .m4a and .m4v is due to the popularity of Apple Computer's iPod and the iTunes Store.

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