APE tag

An APE tag is a tag used to add metadata, such as the title, artist, or track number, to digital audio files.

APEv1
The APEv1 tag was designed for the Monkey's Audio format.

APEv2
The creator of the Musepack format Frank Klemm extended the original format to add a header, allowing APE tags to be at the beginning of files, and also allowed metadata values to be Unicode rather than simply ASCII. Because of its simplicity and flexibility, the WavPack and OptimFROG formats also adopted it as their primary tag format. Version 3.99 of Monkey's Audio also switched from using APEv1 to APEv2.

The foobar2000 player can also tag MP3 files with APEv2 tags instead of ID3 tags, because they are considerably easier to write and more flexible.However, because ID3 was designed with the MP3 format in mind and APEv2 was not, there are some complications. For example, the string APETAGEX signals the start of an APEv2 tag, but the string TAG signals the start of an ID3v1 tag. If the TAG in APETAGEX ends up where an ID3v1 tag is expected, it may be read incorrectly. ID3 also has an “unsynchronization scheme” to ensure players do not try to play the tag data as audio; APEv2 has no such scheme, and APEv2 tags may result in errors or static at the end of files.

Features
APE tags are closer to Vorbis comments than ID3 tags. Like Vorbis comments, they are unstructured key/value pairs. However, unlike Vorbis comments, they do not allow for inter-key ordering. This is because they store a list of values with each key, rather than a key per value.

APE values can also be flagged as “text”, “binary”, or “external” types. Because of this, tag editing software can avoid presenting blocks of unreadable data to users. Binary data cannot be easily stored in Vorbis comments for this reason (though this is by design).

APEv2 supports Unicode using UTF-8 for key values. For key names an ASCII subset (control characters from 0x00 to 0x1f are not allowed) must be used.