Editing MP3 (section)

=== File structure ===
{{Panorama
|image   = File:Mp3filestructure.svg
|height  = 400
|alt     = Diagram of the structure of an MP3 file
|caption = Diagram of the structure of an MP3 file (MPEG version 2.5, not described here, changes the last bit of sync word to "0" as an indication, effectively moving one bit to the version field<ref name="MPEG-2.5-2" />).
}}

An MP3 file is made up of MP3 frames, which consist of a header and a data block. This sequence of frames is called an [[elementary stream]]. Due to the "bit reservoir", frames are not independent items and cannot usually be extracted on arbitrary frame boundaries. The MP3 Data blocks contain the (compressed) audio information in terms of frequencies and amplitudes. The diagram shows that the MP3 Header consists of a [[sync word]], which is used to identify the beginning of a valid frame. This is followed by a bit indicating that this is the [[MPEG]] standard and two bits that indicate that layer 3 is used; hence MPEG-1 Audio Layer 3 or MP3. After this, the values will differ, depending on the MP3 file. ''ISO/IEC 11172-3'' defines the range of values for each section of the header along with the specification of the header. Most MP3 files today contain [[ID3]] [[metadata]], which precedes or follows the MP3 frames, as noted in the diagram. The data stream can contain an optional [[checksum]].

[[Joint stereo]] is done only on a frame-to-frame basis.<ref name="Limitations"/>