20 Nevertheless it can rightly be said that Our predecessor of immortal memory, St. Pius X, made as it were the highest contribution to the reform and renewal of sacred music when he restated the principles and standards handed down from the elders and wisely brought them together as the conditions of modern times demanded. Finally, like Our immediate predecessor of happy memory, Pius XI, in his Apostolic Constitution Divini cultus sanctitatem (The Holiness of Divine Worship) , issued December 20, 1929, We ourself in the encyclical Mediator Dei (On the Sacred Liturgy) , issued November 20, 1947, have enriched and confirmed the orders of the older Pontiffs.
Source: Musicae Sacrae (Vatican.va)