FinalScratch already does this (Score 0, Redundant)

FinalScratch does the same thing, without any special sensors or electronics. You just use your standard turntable, a vinyl LP with a special audio signal, and plug the output into your computer. Software does the rest.

IMHO this is the best solution, because software (and disk space) is much easier to upgrade. Plus, you don't need to find space for One More Gadget. Instead you can slake your gadgethirst by purchasing a niftier computer.