These intrepid early music singers compare their sound to a string quartet’s – a match made in heaven for the ACO.

The Hilliard Ensemble sing Byzantine chant and thirteenth-century polyphony with otherworldly purity and precision that astounds audiences around the world. But they know their limits. “Presentation has never been a strong point of the Hilliard. We wouldn’t win any beauty competitions standing up there,” says countertenor David James, somewhat sheepishly.

Since 1974, the all-male English group’s core repertoire has been sacred a cappellafare from the Middle Ages and Renaissance and new music specially composed for the foursome, but over the past two decades they have brought collaborators into the mix to liven up their own performing tradition. It’s a gamble that’s paid off: Officium,their 1994 album of ancient hymns and chants accompanied by saxophonist Jan Garbarek was a surprise sensation, selling more than 1.5 million copies.

“We’ve since branched out in more collaborations with instrumentalists,” says James. “It’s amazing how you can generate a special electricity between two groups with similar ideas but who come from a different angle.”

It is in this spirit of shared creative ideals that the compelled the Hilliard Ensemble to join forces with...