I know that a lot of emcees like to rhyme over established instrumentals. The drawback to rhyming over those beats includes copyright infringement and the inability to profit from having the vocals over the beat. Rapping over original production makes the most sense plus it forces an artist to present his or her best material. Emcees have been notoriously known for wanting free beats (I have been guilty of this as well) and Soundclick has made this possible (shout out to Sho Down, Anno Domino, Kajmir Royale, Engineer, and more). I really despise hearing a rapper rhyme over a tagged beat. It is disrespectful and it shows that the artist is selfish. I like that beat makers are making it easier for emcees with a lower budget to rhyme over their beats. Non-exclusive leasing options gives the beat makers leverage and the ability to keep the exclusive rights to the beat. I believe that the backbone for most hip-hop songs is the instrumental. Finding production is not that hard. A few years ago I utilized Myspace to make the connections that I have today. Myspace collapsed and I have utilized SoundCloud, Twitter, Blogs, BeatKings, YouTube, Last.fm, and more. As an artist you must be willing to search and find hidden talent.
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