This is the global file extension for the MPEG-4 container format including all kinds of multimedia content (video, natural and synthetic audio, 2D and 3D graphics, animated avatars, user interactivity etc.) that can be streamed over any kind of connection or network. The definition of this file format was handled in the Systems part of the MPEG-4 standard, i.e. ISO/IEC 14496-1 (chapter 13). New extensions have led to separate parts for the ISO media format, i.e. ISO/IEC 14496-12 and 14.

You can obtain the full specifications of these parts from the ISO for a small fee on a CD or download the publicly available standards for free from their website and/or the FTP servers of the MPEG and MPEG Audio Subgroup. ISO/IEC 14496-5 contains the MPEG-4 Audio, Visual and Systems parts of the source code, but remember to look for the latest version, because there are several listed, also split up in reference software, conformance streams and documents.

The patent pool administrator for the MP4 file format is MPEG LA, because it belongs to the Systems part of the standard. They also take care of license issues related to the Visual part of MPEG-4, but not for the AAC format which is handled by Via Licensing, see the Licensing Information on this web site.

Since MP4 is based on the QuickTime MOV format, the resources available from Apple give a thorough insight into its internal structures (atoms etc.), too. The developer documentation's "Inside QuickTime: QuickTime File Format" and "What's New in QuickTime 6" can be downloaded as PDF files.

The additional file extensions *.m4a and *.m4p (as well as *.m4v) are used by Apple to differentiate between MP4 audio files ripped with iTunes (no DRM protection) or downloaded from their new Music Store (copyright protected).

The latest Apple creation is *.m4b for bookmarkable *.m4a files with speech content, i.e. audio books in one big file that will be automatically restarted at the point where you left the last time either in iTunes or on an iPod. FAAC can encode to this file extension, too, but there are no other decoders which know this file extension yet except a patched input module for foobar2000 (losing *.mp4 support in the process).

Apple also uses the *.m4a file extension for their proprietary lossless format (not related to a future MPEG-4 Audio Lossless standard), so files with this compression method cannot be played back by other MPEG-4 audio applications.

On the other hand MP3 audio tracks in the MP4 container (valid combination in the MPEG-4 Audio specifications) will neither play in QuickTime nor in iTunes, but in some open source decoders instead like mp4player from MPEG4IP, GPAC/Osmo4 or foobar2000, also in M4Play from the IBM Toolkit for MPEG-4.

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In the context of audio compression MP4 normally means "MPEG-4 AAC" audio files that do not differ in sound quality from "older" MPEG-2 AAC files, because the raw bitstream is exactly the same, only the file headers are different (i.e. if they have been encoded with the same codec and the same settings, e.g. with PsyTEL AACEnc or Nero's AAC plugin). But being a container format, an MP4 file can also contain an MPEG-2 AAC or even an MP3 bitstream, either alone or together with e.g. an MPEG-2 video bitstream. These different "Object Types" are always signaled in the header of an MP4 file, see e.g. the ADTS and ADIF headers of an AAC bitstream or take a look at an MP4 header with the -list option of mp4creator or mp4info (another MPEG4IP tool).

All MPEG-4 software players (Winamp 2.x and foobar2000 with their appropriate plugins, QuickTime 6, Windows Media Player with EnvivioTV or 3ivx DirectShow filter etc.) should play these MP4 audio files the same, because the AAC decoder part has been exactly defined by the MPEG since several years now.

Nevertheless this is only true for the Low Complexity / LC profile, so if you have encoded your AAC files (old or new) with another profile (HE AAC, Main, LTP, whatever), decoders that do not know these profiles like QuickTime 6 will either refuse to play them or only decode the LC part if there is one like in HE AAC files. So it's not recommended to use these other profiles for encoding if you know that your hardware or software player doesn't understand them.

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If you have encoded many MPEG-2 AAC LC files in the past and wish to convert them to MP4 now, you can easily do this in one batch operation with mp4creator (part of the MPEG4IP tools, available on RareWares) and a GUI called Ivan & Menno.

Since v1.7 this GUI can even convert non-compliant AAC files encoded with the outdated PsyTEL AACEnc_MPEG4 to a correct MP4 file using the option "old AAC format" (if no other PsyTEL switches like -profile 1 or 2 had been used during the encoding of course). This conversion would also make sense if you want to use the MP4 tagging and ReplayGain options of foobar2000 (Look up Software Audio Players for Windows) or just load many MP4 files in Winamp faster than the older AAC files with ADTS headers. Foobar2000 also has a plugin for AAC->MP4 conversion and vice versa called foo_mpeg4u.dll.

The other way round (MP4->AAC) is also possible with "Ivan & Menno" without any difficulties now, because mp4creator has been updated with a new -mpeg-version switch that enables to override the MPEG-x version flag in the MP4 and/or AAC file (since v0.9.9 this switch has been renamed to -aac-profile). So if e.g. a QuickTime 6 Pro (= MPEG-4 AAC) user also owns a Philips Expanium portable (= MPEG-2 AAC), he can choose the appropriate conversion option from the drop-down list in "Ivan & Menno" now as well as a PsyTEL AACEnc or AACEnc_MPEG4 user who wants to play the converted MP4 files in QuickTime 6.x.

"Ivan & Menno" also integrates several command line encoders (PsyTEL AACEnc and FastEnc as well as FAAC) and FAAD2 for writing decoded WAV files to your HDD or play them through your sound card (v1.8) and can also show the header infos of AAC and MP4 files in a row in case you have to inspect many of them.

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The following table gives an overview of several AAC/MP4 codecs and their different header options as well as the recommended switch in mp4creator and/or "Ivan & Menno" for a conversion. As stated above, the possibility for a re conversion to a file e.g. with an *.aac extension and a MPEG-2 AAC LC header luckily no longer depends on the defaults or settings of the individual encoder. The values given below all refer to the Low Complexity profile / LC of course, since another profile like e.g. LTP would change the Object Type to MPEG-4 AAC (LTP only exists in this version of the standard, not in MPEG-2 AAC).

	MPEG-2 header	MPEG-4 header	conversion to MP4 file with MPEG-4 header	reconversion to AAC file with MPEG-2 header
PsyTEL AACEnc	X		use "MPEG-4" option	not needed
PsyTEL AACEnc_MPEG4		X	use "old AAC format"	use "MPEG-2" option
PsyTEL FastEnc	X		use "MPEG-4" option	not needed
Nero AAC	with "ISO" option	default mode	not needed	not needed
FAAC	default mode	with --mpeg-vers 4 switch	not needed	not needed
QuickTime 6 Pro		X	not needed	use "MPEG-2" option

"Not needed" in this table refers to the inherent possibilities of the different codecs, e.g. Nero AAC can produce both standard-compliant MPEG-2 AAC files and MPEG-4 AAC files in an MP4 container, so it is not necessary to use mp4creator or "Ivan & Menno" for that. The same is true for FAAC, because it can encode *.aac as well as *.mp4/*.m4a/*.m4b files with either MPEG version flag using the command line switches -o plus appropriate output file extension, -w or --mpeg-vers X, see the FAAC Wiki page and/or the verbose console help screen for more.

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It should be mentioned that encoding directly to MP4 with Nero or FAAC enables gapless files, i.e. with no audible pauses between live, concept album or classical music tracks ripped from a CD, because the necessary information for the decoder is stored in the MP4 file header. QuickTime, iTunes or the iPod cannot use this additional information though, so only players based on FAAD2 are able to translate them into gapless playback yet. These infos will also be lost when converting MP4 files to AAC, because the file muxer mp4creator neither recognizes nor translates them to an AAC header, since an equivalent field for this information does not exist there.

Furthermore MP4 encoding of audio-only content enables direct tagging with the method used in iTunes, so it has some advantages vs. the ADTS AAC format which is rather used in hardware players like the Philips Expanium, DivaGEM and mobile phones or for Shoutcast internet streaming, see e.g. Tuner2 for a list of radio stations using aacPlus for their low bitrate ADTS streams via HTTP (48 kbps and lower). The additional licensing costs on the MP4 file format for software companies may also have influenced this decision, and AAC internet streams are free of royalty fees unlike MP3 for example.