Expert: Jeff Laatsch - 9/15/2006

Question
Are MP4 audio files compressed, or are they a lossless format like FLAC?

Answer
Chris,

Compressed.

MPEG4 is a file format that is commonly used to store media types defined by the ISO/IEC Moving Picture Experts Group, though it can store other media types as well. The files of this format usually have extension .mp4.
The start point design of the *.mp4 file format was Apple's QuickTime file format. Having been improved in many ways, today MPEG-4 differs markedly from its predecessor. MPEG-4 allows each streaming over the internet, multiplexing of multiple video and audio streams in one file, variable frame- and bit-rates, subtitles and still images.

Being quite often used as the alternative to *.mp3 on an Apple iPod and in iTunes, *.mp4 is still not as widely used in computer and hardware players as the *.mp3 in spite of the higher quality of the AAC codec.

The following kinds of data are recommended (for compatibility reasons) to be embedded in MPEG-4:

Video: MPEG-4, MPEG-2 and MPEG-1
Audio: MPEG-4 AAC, MP3, MP2, MPEG-1 Part 3, MPEG-2 Part 3, CELP (speech),TwinVQ (very low bitrates), SAOL (midi)
Pictures: JPEG, PNG
Subtitles: MPEG-4 Timed Text, and/or xmt/bt text format (means that subtitles have to be translated into xmt/bt)
Systems: Allows animation, interactivity and DVD-like menus
Here are some file extensions used on files that contain data in the *.mp4 format:

.mp4: official extension, for audio, video and advanced content (see above) files
.m4a: for audio-only files; can safely be renamed to *.mp4, though opinions differ on the wisdom of this.
.m4p: FairPlay protected files
.mp4v, .m4v: video-only (sometimes also used for raw mpeg-4 video streams not in the *.mp4 container format)
.3gp, .3g2: used by 3G mobile phones, may also store content not specified directly in the *.mp4 specification (H.263, AMR, TX3G)



Kind Regards,
Jeff