(edit: rewrote it)
MPEG-4 is a vast set of standards. It's still being developed, so what is allowed in a standard MPEG-4 file can change over time. Player applications typically implement some subset of the MPEG-4 standard depending on their intended uses.
The .mp4 filename extension officially refers to a file in the container format defined in part 14 of the MPEG-4 specification, superseding the older format found in part 1. MPEG-4 container files may contain audio tracks, video tracks, still images, VRML data, subtitle tracks, embedded fonts, DRM information, metadata, and anything else.
.m4a is used to refer to an MPEG-4 container which contains only audio tracks (more or less.) The MPEG-4 core standard includes many audio formats, some of which are compressors and others of which are sound synthesis standards. AAC compression is the default, but AAC is itself a complex standard. AAC features are typically grouped into profiles, or subsets aimed at particular applications, and the profiles can have several different layers of implementation, from basic to complete.
MPEG-4 files can use any audio codec at all, and the filename extension does not give any information as to which codec a file actually uses. This may be related to why some people call .m4a files "annoying."
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Last edited 09/12/31(Thu)17:15.