The music you are listening to is "Il Combattimento Di Tandredi
E Clorinda - Ma Ecco Homai" by Claudio Monteverdi's "Seconda
prattica". This music was considered to be the beginning of
modern opera, as now poetry could be set to music with a new,
Humanist theory of rhetoric tacens, and not the old,
inflexible, strictly hierarchical Scholastic view of reality.
Cantus (sometimes spelled cantos) comes from the Latin for
"singing"; derived from cantera. Used with respect to polyphonic music,
cantus has several meanings:
Cantus firmus ("fixed song", sometimes canto fermi) is a pre-existing
melody forming the basis of a polyphonic composition. The term
was not used until the 14th century, but cantus firmus was considered
the norm in polyphonic composition from roughly the 10th through the
early 15th centuries; almost all of the music of the St. Martial and
Notre Dame schools employ cantus firmus, as well as most 13th century
motets. (Many of these motets were written in several languages, with the
cantus firmus in the lowest voice.) Gregorian chants and plainchant
employed cantus firmus.
Cantus durus (hard), cantus naturalis and cantus mollus (soft)
were overlapping hexachords that in mediaeval music formed the basis
of musica recta, also called musica vera (the correct or
true music). All three hexachords encompassed the three most common semitone
steps: from B natural to C, from E to F, and from A to B♭.
Cantus durus was the hexachord that included B natural, which was regarded
as "hard"; cantus mollus was the hexachord that included B flat, which was
regarded as "soft"; cantus naturalis included neither B nor B♭.
All three hexachords bore the pattern TTSTT; the semitone was always the
third note of the scale. The place where the scale moved from full tone to
semitone was called the "mi-fa" step.
It was permitted to change from one of these hexachords to another; the
point of change was termed a mutation (similar to the modern
"modulation").
In the 14th century the hexachord system was expanded, permitting the use
of semitones in positions other than mi-fa. This entailed transposing
hexachords to 'alien pitches' or 'unaccustomed places. Music so created
was called musica ficta (false or contrived), because the hexachord
was specially created for the sake of the semitone.