In the beginning there was Herc. Thirty-one years ago, in a sweltering community recreation room at 1520 Sedgwick Avenue in the Bronx, the original B-boy uncovered the fundamental ingredient of rap music. ''Let's get this straight,'' says Herc. ''Two turntables been out there, but they wasn't using the same two records obscure records and prolonging [them]. That's my innovation.'' Born in Jamaica, Herc immigrated at age 12 and soon found himself immersed in the local music scene. He had been experimenting with his new technique at home before finally testing it out on his first paying crowd. And the response? ''Everybody went into a frenzy,'' he says.