The last metadata format in this comparison series will be metadata for AAC audio files,
which are usually included in an MP4 container (see MPEG-4 Part 14).
MP4 audio files can have various file extensions, most commonly m4a, but also mp4 or m4b (for audio books).
So let the confusion continue ...
MP4, AAC and iTunes-style Metadata
The MPEG-4 container format is based on Apple's QuickTime container and each MPEG-4 file must have a major file brand. For example, an AAC audio file typically lists M4A as its major file brand. See AtomicParsley for a description of the MPEG-4 container and MP4 Registration Authority for a list of official file brand identifiers.
Unfortunately no official metadata or tagging standard exists for MP4.
The MPEG-4 specification just defines a standard place to put metadata, but there is no standard on any fields, except cprt for copyright :).
Therefore different metadata styles are possible: iTunes-style metadata, 3gp-style metadata, MP4 ID3v2 tags and more. Most podcasts and audio files use iTunes-style metadata.
So far so good, but iTunes-style metadata isn't defined in any publicly available document.
Its format is determined by the types of files that iTunes and the iTunes Music Store produce and provide (see AtomicParsley, MPEG4 Files, bottom).
Some open source projects assembled a list of currently known iTunes tags: iTunes Metadata (mp4v2) and Known iTunes Metadata Atoms (AtomicParsley)
Helpful tools to analyze MP4 audio are mp4v2 (including the tools mp4chaps, mp4art, mp4file, mp4info, mp4track) and faad, the Free Advanced Audio Decoder, can list AAC codec details and metadata tags.
But now let's look at some real life MP4 files. The following table lists iTunes-style metadata tags and the encoding/tagging software of a few podcasts:
|Elementarfragen 9||GarageBand||name, artist, composer, encoded with, date, album, genre (Podcast), comments (for URL), description, lyrics (for show notes)|
|Fanboys Podcast 49||Hindenburg Journalist||name, artist, encoded with, album, track, genre (Podcast), comments (for show notes), description (for show notes), lyrics (for show notes)|
|Engadget Podcast 270||GarageBand||name, artist, encoded with, date, album, track, genre (Podcast), comments (for URL)|
|Newz of the world 1||iTunes||name, artist, encoded with, date, album, track, genre (Podcast), BPM, album artist|
|Naked Scientists||unknown||name, artist, encoded with, date, album, description (for show notes), genre (Podcast), BPM|
|Pressrecord 16||GarageBand||name, artist, composer, encoded with, album, comments (for show notes)|
|Disney Land News Today||GarageBand||name, encoded with|
|Inside Home Recording||GarageBand||name, artist, composer, encoded with, date, album, track, genre (Podcast), lyrics (for show notes)|
|Fubar Podcast 6||Hindenburg Journalist||name, artist, encoded with, album, genre (Podcast), album artist, lyrics (for show notes)|
AAC Details, Chapter Marks and Images
It's also interesting to analyze which AAC flavours (LC or HE AAC, for a short description see
AAC Details) and bitrates are used.
Furthermore we compared mostly Enhanced Podcasts, which contain chapter marks. And there are of course multiple ways to embed chapter marks in an MP4 audio stream (e.g. QuickTime chapters, Nero chapters), but basically all podcasts use QuickTime chapters.
The next table shows AAC details, bitrates, cover image size, number of chapter marks and length of the analyzed podcasts:
|Elementarfragen 9||LC AAC, 128kbps||jpeg, 1024x1024||15||02:04:22|
|Fanboys Podcast 49||HE AAC, 56kbps||png 320x320, jpeg 1024x1024||24||01:08:17|
|Engadget Podcast 270||LC AAC, 80kbps||jpeg, 300x300||6||01:26:35|
|Newz of the world 1||HE AAC, 49kbps||jpeg, 600x600||6||57:52|
|Naked Scientists||LC AAC, 48kbps||jpeg, 300x300||26||01:07:32|
|Pressrecord 16||LC AAC, 128kbps||jpeg, 300x300||No||01:20:48|
|Disney Land News Today||LC AAC, 192kbps||jpeg, 300x300||4||01:58:22|
|Inside Home Recording||LC AAC, 128kbps||jpeg, 300x300||8||01:02:08|
|Fubar Podcast 6||HE AAC, 80kbps||jpeg, 1200x1200||33||04:04:30|