''One day, some underprivileged kids in the Bronx are going to have a ball with this.'' That is exactly what Thomas Alva Edison was not thinking in 1877 when he developed a way to record and play back sound with metal cylinders. Nor was it on Emile Berliner's agenda when he redesigned it as a flat disc and patented the gramophone 10 years later. Even so, the late '70s brought a turntable revolution: The simple device suddenly evolved from a way of playing back sound into a tool used to create music. It became the foundation of hip-hop production and, therefore, of hip-hop itself.