Which innovative recommendation algorithm technique did Spotify implement in 2015 that dramatically improved their music discovery capabilities?

Streaming platforms have revolutionized how we discover music through sophisticated recommendation systems. These algorithms analyze your listening patterns and introduce you to new artists you might enjoy. But how much do you know about the technology powering these music discovery engines? Test your knowledge about the music industry's recommendation systems that shape our playlists and influence which songs become hits.

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Neural Network Processing (NNP)
Collaborative Filtering with Deep Learning
Discover Weekly with Convolutional Neural Networks
Echo Nest's Taste Profiles with audio analysis