De institutione musica I.12: De divisione vocum earumque explanatione.
But enough of these things. Now let us gather the differences of voices. Every voice is either continuous (synechés), or divided by intervals (diastématiké), which is said when it is held suspended with an interval. The continuous is that by which, speaking or reading prose, we run through the words. For the voice then hastens not to rest on high and low sounds, but to run through the words as swiftly as possible, and the impulse of the continuous voice works to unfold the meanings and to express the speech. The diastématiké, on the other hand, is that which we hold suspended in singing, in which we serve not so much words as melodies; and the voice itself is slower and, through the varieties to be modulated, makes a kind of interval, not of silence but rather of a suspended and slow song. To these, as Albinus asserts, a third difference is added, which can include the middle voices, namely when we read a heroic poem, neither in a continuous course, as prose, nor in the suspended and slower manner of the voice, as a song.