Musicbrainz has used several audio fingerprinting systems over its lifetime.
All of them (so far) work in essentially the same way. It is generally a two-step process of submission and lookup. First, the raw audio is used to create a fingerprint, which is then submitted to a third-party server. This server analyzes the fingerprint, compares it to other fingerprints, and decides whether it is sufficiently different from known fingerprints as to issue a new ID.
Once this step is done, a fingerprint can be calculated for any file and this can be used to look up the corresponding ID.
This ID is associated with a given track (pre-NGS) or recording (post-NGS), and metadata can be gathered from there.
TRM (TRM Recognizes Music) IDs were MusicBrainz’ first audio fingerprinting system. This system was created by Relatable, and added to Musicbrainz in 2000.
This system was used in the original musicbrainz tagger application.
This system worked reasonably well for finding duplicate music files on a local system, but had problems with collisions (different-sounding audio which got the same ID) and duplicates (same-sounding audio which has different IDs). Apparently the server also did not scale well
PUIDs are Musicbrainz’ second audio fingerprinting system. This was initially operated by MusicIP, and bought by Gracenote in June 2011. Gracenote is expected to discontinue the public service soon, and it already appears to be largely non-functional.
This system was better than TRM, but still has several major problems:
- The fingerprint submission system is not open source, and as such cannot be included in Picard. (the lookup system is, and was included in Picard as of version 0.7.0, released in July 2006.)
- The fingerprinting process is slow, both on the client side and the server side
- Over time, the operators have become less and less interested in running the server, to the point where today it is barely working, if it works at all.
PUIDs are also quite opaque, being nothing more than a unique number referencing a database outside of MusicBrainz’ control. If/when that database goes away, they become useless.
It has several immediate advantages:
- It is open source.
- It is actively developed, along with supporting software.
- It gives the ability to visually compare music
- AcoustID fingerprints have their duration recorded, making it easy to discard certain incorrect links between recordings and acoustIDs.