Education
Born in Lucerne, Diethelm studied at the Conservatory and the School of Church Music there (now both divisions of the Lucerne Musikhochschule). He studied composition privately with Johann Baptist Hilber (1891–1973) and Albert Jenny, and took master classes with Paul Hindemith and Arthur Honegger, which had a significant influence on him, in addition to summer courses in Darmstadt with Karlheinz Stockhausen and Luigi Nono. He completed his education as a conductor with Ernst Hans Beer and Alexander Krannhals.