There's a not-very-subtle distinction between treating a folk music tradition as a fragile heritage to be sheltered against corruption and treating it as a tough, grizzled system of roots that serves as a sturdy foundation for perpetually changing bursts of inexplicable greenery. Living traditions, oral or otherwise, change over the years like any living thing, and Robin Williamson--singer, songwriter, multiinstrumentalist, poet, and storyteller--understands this, knowing when to yield to the weight of history, as on his extremely listenable 1984 Legacy of the Scottish Harpers, and when to throw himself headlong into a more idiosyncratic mode of expression. Williamson's 1988 LP Ten of Songs is perhaps the most convincing example I've heard of bringing traditional Celtic music into today's world without trashing its roots or turning it into some kind of bastardized rock and roll thing. Williamson, who first came to prominence in the 60s as half of the Incredible String Band, writes songs that show how deeply he has internalized the old idioms. In his arrangements synthesizers and citterns blend in a way that makes complete sense--a luminous, electrifying setting for Williamson's phonetically rich lyric sense and burring tenor, which sails over the chords with tough vitality. It's real Scottish balladry for the 21st century. In this weekend's concert at the Old Town School of Folk Music Williamson will be working with the highly esteemed, if somewhat less edgy, British folk-jazz guitar virtuoso John Renbourn; the program will emphasize a traditional repertoire. Friday, 7 and 10 PM, 909 W. Armitage; 525-7793.