Tidal is here, and it aims "to forever change the course of music history." That's how rapper and entertainment mogul Jay Z put it earlier this week, when he announced he'd bought the online music streaming service for a cool $56 million. The idea is for Tidal to be the answer to complaints that free music streaming services like Spotify cut musicians out of earning a return for their work. A normal subscription will cost $9.99 a month, and premium access will cost $19.99.