How did Bach and Vivaldi play their own music? Until somebody builds a time machine à la H.G. Wells, we’ll probably never know for sure. But classical fans who attend some of this weekend’s various early music concerts in Nashville may insist they know exactly how the old masters played, no ifs, ands or sackbuts about it. This week, some of the country’s finest period-instrument specialists are converging in Music City for the inaugural Nashville Early Music Festival.