The song's words were written in Chinese, so Japanese during the Edo period noted down the pronunciation of the southern Chinese in "Katakana" (a square form of "kana" [Japanese syllabaries]) - the notes were called "toon" at that time - and they sang according to these notes.
清楽の歌詞は中国語で、江戸時代の日本人は、南方の中国語の発音をカタカナで写し(当時、これを「唐音」と言った)、そのまま唱った。