MPEG-4

MPEG-4 is a multimedia container format standardwhich is most commonly used to store digital audio and digital video 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 allows streaming over the Internet. A separate hint track is used to include streaming information in the file. The official filename extension for MPEG-4 files is .mp4, thus the container format is often referred to simply as MP4.
Some devices advertised as "MP4 players" are simply MP3 players that also play AMV video and/or some other video format, and do not play MPEG-4 format. This can become rather confusing for potential buyers.

MPEG-4 Part 14 is based upon ISO base media file format which is directly based upon Apple’s QuickTime container format.MPEG-4 is essentially identical to the MOV format, but formally specifies support for Initial Object Descriptors (IOD) and other MPEG features.MPEG-4 revises and completely replaces , in which the file format for MPEG-4 content was previously specified.

The MPEG-4 file format specification was created on the basis of the QuickTime format specification published in 2001. The MP4 file format defined some extensions over ISO Base Media File Format to support MPEG-4 visual/audio codecs and various MPEG-4 Systems features such as object descriptors and scene descriptions. Some of these extensions are also used by other formats based on ISO base media file format (e.g. 3GP).List of all registered extensions for ISO Base Media File Format is published on the official registration authority website www.mp4ra.org. The registration authority for code-points (identifier values) in "MP4 Family" files is Apple Computer Inc. and it is named in Annex D (informative) in MPEG-4 Codec designers should register the codes they invent, but the registration is not mandatory and some of invented and used code-points are not registered. When someone is creating a new specification derived from the ISO Base Media File Format, all the existing specifications should be used both as examples and a source of definitions and technology. If an existing specification already covers how a particular media type is stored in the file format (e.g. MPEG-4 audio or video in MP4), that definition should be used and a new one should not be invented.

MP4 versus
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, MPEG-4 files may contain any number of audio, video, and even subtitle streams, making it impossible to determine the type of streams in an MPEG-4 file based on its filename extension alone. In response, Apple Inc. 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.
While the only official file extension defined by the standard is .mp4, various file extensions are commonly used to indicate intended content:
MPEG-4 files with audio and video generally use the standard .mp4 extension.
Audio-only MPEG-4 files generally have a .m4a extension. This is especially true of non-protected content.
MPEG-4 files with audio streams encrypted by FairPlay Digital Rights Management as sold through the iTunes Store use the .m4p extension. iTunes Plus tracks are unencrypted and use .m4a accordingly.
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. An .m4a audio file cannot "bookmark" (remember the last listening spot), whereas .m4b extension files can.
The Apple iPhone uses MPEG-4 audio for its ringtones but uses the .m4r extension rather than the .m4a extension.
Raw MPEG-4 Visual bitstreams are named .m4v.
Mobile phones use 3GP, an implementation of MPEG-4(a.k.a MPEG-4/JPEG2000 ISO Base Media file format), similar to MP4. It uses .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’s iPod, iPhone, and iTunes Store. Without modification, Nintendo's DSi and Sony's PSP can also play M4A.

Data streams
Almost any kind of data can be embedded in MPEG-4 files through private streams; the widely-supported codecs and additional data streams are:
Video: MPEG-4 Part 10 (or H.264, also known as MPEG-4 AVC), MPEG-4 Part 2, MPEG-2, and MPEG-1.
Audio: MPEG-2 Part 7 (also known as AAC), Apple Lossless, MPEG-1 Audio Layer 3 (also known as MP3), MPEG-4 Part 3, MPEG-1 Audio Layer II (also known as MP2), MPEG-1 Audio Layer I, 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