|MP4 (MPEG-4 Part 14)|
|Filename extension||.mp4 +|
|Internet media type||video/mp4 +|
|Developed by||International Organization for Standardization|
|Audio-only MP4 (Apple iPhone)|
|Protected AAC MP4|
|Bookmarked audio-only MP4|
MPEG-4 Part 14 (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 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.
- Main article: Wikipedia:MP4 player
Some devices advertised as MP4 Players are not capable of playing MP4 files.
History of MP4[edit | edit source]
MPEG-4 Part 14 is based upon ISO/IEC 14496-12:2005 which is directly based upon Apple’s QuickTime movie container format (.mov). MPEG-4 Part 14 is essentially identical to the MOV format, but formally specifies support for Initial Object Descriptors (IOD) and other MPEG features.
.MP4 versus .M4A file extensions[edit | edit source]
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.
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[edit | edit source]
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.
References[edit | edit source]