No one could accuse ACT boss Siggi Loch of going for the easy sell. Knut Rössler and Johannes Vogt's previous collaboration for the label was the unpromising-sounding but ultimately triumphant Between the Times (2007), a series of improvisations largely based on 17th-century French lute music. The successor to that project, Octagon, moves even further from obvious jazz terrain by focusing on musical ideas associated (sometimes rather tenuously) with the famously cosmopolitan Sicilian court of King Frederick II (1194–1250). The three ‘Octagon Interludes’ are inspired by the eight-sided Castel del Monte in Apulia; ‘Zirafkand’ is an original composition based on an Arabic eight-note scale; ‘Caritas abundat in omnia’ is an elaboration of a well-known piece of plain chant by the Christian mystic Hildegard of Bingen; and so it goes on. It's musicologically fascinating – there's a wealth of detail in Vogt's liner notes – and gorgeously sung with monastic purity by the soprano Ute Kreidler. If you've ever wondered what medieval jazz might have sounded like, this is the place to find out.