Apple Lossless (MP4A) Metadata


Where can I find information on the way that Apple lossless files are tagged? We've been using FLAC for our radio station's music library, in part because tagging (vorbis comments) is simple to do and understand. It's also easy to create 'custom' vorbis comments for fields like STYLE and PERFORMER. It also seems like it would be relatively easy to develop a new player that we could use for radio, computer and home theater that could read whatever custom tags we create.

We converted a number of tracks from FLAC to ALAC as a trial and were surprised to see that Mp3tag correctly reported the field names for custom vorbis comments we had added to the original FLAC files. A few of the standard vorbis comment field names, like ORGANIZATION, were converted to an equivalent field name, in this case to LABEL, and a non-standard Vorbis comment, TRACKNUMBER, was left the same. Another non-standard vorbis comment field name, Style, was converted to CONTENTGROUP.

Now, I realize that the format and tag conversion was done within dbpoweramps' Music Converter, but I was hoping you could shed some light on where I could learn more about mapping vorbis comments to equivalent MP4A tags used for ALAC and the iTunes media player. Since you know how to read these tags, I thought maybe you could point me in the right direction.

Are there any significant limitations on the number and type of applications that can read ALAC tags?

I've also heard that Apple is prone to changing the way they tag Apple lossless files. Is this a moving target, even though ALAC is now supposed to be open-source?

Can mp3tag.exe be used to create custom tags in MP4A files with the same ease and reliability, even though iTunes' media player can't display them?

Thanks in advance for whatever insight you can provide.

Dennis Brunnenmeyer, aka "d2b"