Changes are being made behind the scenes to iTunes Music's library matching and streaming algorithms, allowing for more accurate determination of what songs the user owns, and preventing inadvertent destruction of music libraries.
Apple's rollout of the acoustic fingerprinting to Apple Music subscribers started on Monday. The new method of determination eschews the tag-based matching that Apple Music used since launch in 2015, in favor of how iTunes Match has always performed track identification.
Starting shortly, iTunes will look at the user's library again, and will re-match incorrectly matched songs with the correct song, assuming it exists in Apple's iTunes catalog. As always, unmatched songs will be uploaded to a user's iCloud account.
Apple Music will not replace songs that have been downloaded to a device, correct or not, so if a user has an incorrect song in a library, it must be manually deleted for the match to take place.
According to Jim Dalrymple from The Loop, Apple will be migrating between one and two percent of its users of Apple Music over to the new method of identification per day. With the the addition of acoustic fingerprinting to Apple Music, subscribers need not also pay for the iTunes Match service.
Apple Music's matching feature was widely criticized after launch for replacing live tracks with studio recordings, as well as possibly deleting large swathes of user's music sourced from CD. The same problems never manifested with the earlier iTunes Match feature, as it used acoustic fingerprinting to identify users' tracks, and uploaded music that wasn't readily identifiable as being able to be streamed from Apple-hosted sources.
An acoustic fingerprint is a digital summary of a larger audio signal, similar to a checksum, that can be used to uniquely identify an audio sample. Acoustic fingerprinting algorithms are compression-independent, meaning that it is not dependent on the method of encoding of the file, and uses the music itself as the source.
in 2011, iTunes Match debuted with the acoustic fingerprinting technology, which allowed users to "store your entire collection, including music you've ripped from CDs or purchased somewhere other than iTunes" according to Apple. A "separate but complimentary" system to do roughly the same thing, deployed alongside Apple Music, with many refinements and revisions along the way responding to user complaints.