MPEG-4

MPEG-4 , 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 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. The official filename extension for MPEG-4 Part 14 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 part 14 format. To the makers of these players, the "MP4" designation simply means that they play more than just MP3. This can get rather confusing for potential buyers.

.MP4 versus .M4A file extensions 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 Part 14 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. 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, a simplified version of MPEG-4 Part 12 (a.k.a MPEG-4/JPEG2000 ISO Base Media file format), 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’s iPod, iPhone, and iTunes Store, and Microsoft's Xbox 360 and Zune. Without mods, Sony's PSP can also play M4A. M4A generally delivers better audio quality than the older MP3 format at the same bit rate.

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 (or H.264, also known as MPEG-4 AVC), MPEG-4 Part 2, MPEG-2, and MPEG-1. Audio: AAC (also known as MPEG-2 Part 7), Apple Lossless, MP3 (also known as MPEG-1 Audio Layer 3), MPEG-4 Part 3, MP2 (also known as MPEG-1 Audio Layer 2), MPEG-1 Audio Layer 1, 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.