In the 90s, hip hop mixtapes were used by artists trying to get a name for themselves on the streets. The aim of the mixtape was to get the attention of the big company execs.
The model has completely changed lately with artists such as Drake, Kanye and Wiz Khalifa, who already have massive following releasing mixtapes between albums. The mixtapes are self released and stick two fingers up at their record labels who sometimes put unreasonable demands on their artists.
The label/ artist relationship keeps changing; artists have more control then ever as to when and where they release their albums. Mixtapes are a way for artists to stay in contact with the fans, give them free music and try out their material before putting their proper album out.
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