Which major innovation did Spotify introduce to its recommendation algorithm in 2015 that dramatically improved music discovery?

The music industry has been revolutionized by streaming platforms and their sophisticated recommendation algorithms. These systems analyze listening habits, preferences, and even the acoustic qualities of songs to create personalized experiences. Behind these seemingly magical recommendations are complex machine learning models that have evolved dramatically over the years, shaping how we discover new music and redefining artist promotion strategies.

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Collaborative Filtering based on user listening patterns
The Echo Nest acquisition and audio convolutional neural networks
Mood-based classification using sentiment analysis
Genre-clustering technology from the Pandora acquisition