Organising a music library: plan storage, formats and tags

Do the maths first, then plan

Before settling on formats, it helps to know the order of magnitude. The formula is simple: megabytes per minute are roughly bitrate in kbps times 0.0075. That gives these reference values for a four-minute song:

A real-world example: on a 128 GB iPhone, after the system, apps and photos, 40 GB are often still free. That fits about 1,600 songs in FLAC, around 5,200 in 256 kbps AAC, or over 10,000 in Opus. With a collection of 8,000 songs, the decision is almost made: the full archive won't fit on the phone losslessly, an AAC copy will.

Where you can't hear a difference, storage decides. Why 256 kbps AAC is indistinguishable from the original for most people is explained in music file formats explained.

The two-tier model: archive and working copy

Most well-kept collections follow the same pattern:

• The archive holds every recording in the best quality you have: CD rips and purchased downloads as FLAC or ALAC, MP3s and AAC purchases as they are. It lives on a computer, an external drive or a NAS and is backed up regularly. This is where you maintain the tags.
• The working copy is what goes on your phone: either the archive itself, if it fits, or an automatically generated AAC or Opus version. It's replaceable. Lose the phone and you simply generate it again.

The decisive advantage: you always encode from the lossless source, never from one copy to the next. And when a better codec arrives in five years, you just regenerate the working copy.

Conversion tools

• XLD (Mac, free) reads FLAC and writes AAC with Apple's own encoder, whole folders at once, tags and artwork included.
• The Music app on the Mac can convert songs to AAC. Choose the target format under Settings › Files › Import Settings, then use File › Convert. It doesn't read FLAC, though.
• foobar2000 (Windows and Mac) and fre:ac (free, all systems) batch-convert to AAC, MP3 or Opus.
• dBpoweramp (Windows and Mac, paid) is the comfortable classic for ripping and batch conversion.

Apple itself notes in its guide that converting between compressed formats, for example from MP3 to AAC, can reduce quality. That's exactly why the lossless source belongs in the archive.

What that looks like with Luma

Luma for Mac adds your music folders in place without copying files, so the archive on your Mac doubles as your Mac library. To get music onto the iPhone, Luma Transfer sends either the originals or a folder of AAC copies, and skips songs already on the iPhone automatically. On iPhone and iPad, Luma copies imported files into its own storage. You can delete the originals in the Files app after importing; otherwise they take up space twice.

Folder structure and file names

Players like Luma sort by tags, not folders. The folder structure still matters: for you when browsing in a file manager, for backups, and for the day you switch players. A proven layout:

Album Artist/Album (Year)/01 Title.flac

Multi-disc albums get a disc prefix: 1-01 Title.flac, 2-01 Title.flac. Compilations go under a shared folder such as Various Artists. A few rules save trouble later:

• No characters that other systems forbid: / \ : * ? " < > |. An album called "AC/DC: Live" becomes "AC-DC - Live" in the file name.
• Keep paths short. Some Windows programs still trip over paths longer than 260 characters, for example with classical music and long work titles.
• Track numbers with a leading zero (01, 02 … 10) so file managers sort correctly.
• Maintain the tags, not the folders. Programs like Mp3tag or MusicBrainz Picard can rename and file songs automatically based on their tags, so the two always stay in sync.

Tags: the data players sort by

Tags are text fields inside the audio file: title, artist, album, artwork and much more. Each format stores them differently. As a user you rarely see that, but it helps when moving between programs:

• MP3 uses ID3v2. The common versions are 2.3 (from 1999) and 2.4 (from 2000). Version 2.4 supports UTF-8 and multiple values per field; 2.3 is more compatible, for example with older car stereos and Windows programs. If unsure, use ID3v2.3 with UTF-16, which is also Mp3tag's default.
• FLAC, Opus and Ogg use Vorbis comments: simple fields like ARTIST=…, freely extensible and well documented. In FLAC, artwork sits in its own picture block.
• M4A (AAC and ALAC) uses the tag scheme Apple established with iTunes, which practically every program reads.
• WAV and AIFF have no consistently supported tag format. One more reason to convert such files to FLAC for your collection.

The fields that really count

Album artist is the most important field

The most common problem in any library is albums falling apart into several tiles. It's almost always the album artist. An example: on a Daft Punk album, one song lists "Daft Punk feat. Pharrell Williams" as artist. Without an album artist, the player files that song as a separate album by a different artist.

The fix: guests go in the artist field, and the album artist stays the same for every song, here "Daft Punk". For compilations it's "Various Artists" for every song, or whatever you prefer, as long as it's consistent.

Luma builds an album from exactly the songs with an identical album name and identical album artist. "Deluxe" versus "Deluxe Edition", or one extra space, makes two albums. How to merge such cases in Luma is in the help center under Edit song and album info.

Which of these fields Luma uses for what, and how to make them consistent step by step, is shown in the metadata tutorial.

Set conventions

Which spelling you choose matters less than using it everywhere. Decide once and stick with it:

• Guest appearances: "feat." in the artist field or in the title? One way, consistently.
• Year: original release year or the remaster's year? For playlists by decade, the original usually makes more sense.
• Genre: ten to twenty genres rather than two hundred. "Indie Rock", "Indie-Rock" and "Indie / Rock" are three different genres to any player.
• Articles: "The Beatles" or "Beatles, The"? Many players skip a leading "The" when sorting anyway, so the natural spelling is usually better.
• Editions: "(Remastered 2011)" and "(Deluxe Edition)" either always in the album title or never.

Artwork: embedded, but not huge

Artwork can be stored two ways: as an image file in the album folder (cover.jpg or folder.jpg) or embedded in each audio file. Embedded artwork is more robust because it travels with the song: when copying to the phone, sharing, or importing into another player. Luma reads the embedded cover. When you export the library, Luma also writes a cover.jpg into every album folder.

The catch: the embedded image is stored again in every song. A 3,000 × 3,000 pixel cover, as some stores deliver, easily weighs 1.5 to 3 MB as JPEG. Across twelve songs that's 18 to 36 MB per album, just for images. Across 500 albums it adds up to 9 to 18 GB, more than a complete AAC copy of 1,000 songs.

On a phone or laptop display you can't see a difference between 1,000 and 3,000 pixels, especially since players show artwork scaled down. Recommendation: embed 1,000 to 1,500 pixels as JPEG at quality 85 to 90. If you want to keep the high-resolution version, put it in the archive folder as cover.jpg as well. PNG covers are needlessly large for photos and scans.

Avoiding duplicates

Duplicate songs almost always arise in the same three ways: the same album in two formats (old MP3, new FLAC), a folder imported twice, and several editions of an album. A few habits help:

• One format per album in the library. When you replace old MP3s with FLAC, delete the MP3s in the archive instead of keeping both.
• Import from one folder, not from several copies of the same folder.
• Separate editions deliberately: to keep the original and the deluxe version, give them different album titles.

Luma detects duplicates on import by title, artist, album, track number and length, and skips them regardless of format. A FLAC and an MP3 with identical tags count as the same song. How the matching works in detail is in the help center under How Luma recognises and matches songs.

Tools for cleaning up

• MusicBrainz Picard (free, Windows, Mac, Linux) matches your files against the open MusicBrainz database, optionally by acoustic fingerprint (AcoustID). Ideal for fixing large, messy collections in one go.
• Mp3tag (free on Windows, paid on Mac) is the Swiss army knife of tagging: edit many files at once, build file names from tags and vice versa.
• Kid3 (free, all systems) offers similar features as open-source software.
• beets is a command-line tool for very large collections that automates import, tagging and folder structure.
• Luma itself edits tags right on iPhone, iPad and Mac and writes changes into MP3, M4A and FLAC files without re-encoding the audio.

Keeping iPhone storage under control

On the iPhone, a second factor joins the collection itself: the backup. Like any app, Luma is included in iCloud Backup together with its music. With a large collection that can fill your iCloud storage quickly. Whether you want that depends on your archive:

• If you have an archive on a computer, you don't also need the music in iCloud. Under iOS Settings › your name › iCloud › iCloud Backup you can exclude Luma, and back up your playlists as a file.
• If the iPhone is your only copy, iCloud Backup is your safety net. Keep it on, but better still, start a real archive. How is covered in the article on building a digital music archive of your own.

More levers: Settings › Library & Storage in Luma shows how much space it uses. The iOS feature "Offload App" removes only the app and keeps the music, while deleting the app also removes the imported music. Details are in the help center under Storage, backup and a new iPhone.

Checklist

• Estimate your collection: how many songs, how much space in which format?
• Choose an archive, lossless where possible, with at least one backup.
• Decide for the phone: the archive itself or a 256 kbps AAC copy.
• Make album artist and album title consistent across all albums.
• Write down conventions for guests, year, genre and editions.
• Resize artwork to 1,000 to 1,500 pixels and embed it.
• Resolve duplicate formats in the archive.

Frequently asked questions

Sources

• ID3.org: ID3 tag version 2.4.0, Native Frames (field definitions for MP3). id3.org/id3v2.4.0-frames
• Xiph.Org: Ogg Vorbis I format specification, comment field and header specification. xiph.org/vorbis/doc/v-comment.html
• IETF: RFC 9639, Free Lossless Audio Codec (FLAC), section on metadata blocks and pictures. rfc-editor.org/rfc/rfc9639.html
• Apple Support: Convert a song to a different file format on your Mac with the Apple Music app. support.apple.com/en-us/HT204310
• MusicBrainz Picard, official website and documentation. picard.musicbrainz.org

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