Martin Fröst has just released a new album for Sony Classical entitled 'Vivaldi'. On it he plays a handful of Vivaldi 'clarinet concertos', works created with the assistance of the composer and arranger Andreas Tarkmann from arias drawn from the composers operas. James Jolly spoke to Fröst at his home in Stockholm about the project and the challenges of performing at an unusual pitch and working with the period-instrument ensemble Concerto Köln.