MP4 File Format

• The copy-protected audio tracks that were sold through the iTunes Store, were named with the extension .m4p.
  
  
• The non-copy-protected iTunes Plus audio tracks currently sold through the iTunes Store, use names with the extension .m4a.
  
  
• Audio books and podcasts (which usually contain chapter markers, and other options such as images and hyperlinks) usually have file names with the extension .m4b, although sometimes .m4a is also used.
  
  
• Apple's iPhone, although it uses standard MPEG-4 audio for its ringtones, requires file names with the extension .m4r.
  
  
• The .m4v extension is sometiems used for videos in the MP4 format.

1. The MP4 file format is closely related to the 3GPP and 3GPP2 file formats used by mobile phones, and although 3GPP and 3GPP2 files usually use the extensions .3gp and .3g2, occasionally you may encounter a 3GPP or 3GPP2 with a .mp4 extension.
   
   
2. Some hardware devices, advertised as "MP4 players", are in fact MP3 players with support for the AMV video format (or other video file formats), but do not in fact support MPEG-4 Part 14.

• Choosing Video Format Conversion Software
  
  
• Video File Formats

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