[Martin Taylor 931124 15:45]
(Bruce Nevin Wed 931124 13:25:04)
To avoid confusion spreading too widely, I'll correct Bruce's conceptual
typo for him, though I'm sure he will notice it himself soon enough.
Formant enhance the harmonics within their band, rather than "damp" them.
"Damp" is an unfortunate word in this context, because damping is related
to the width of a formant. Wide formants damp fast, narrow ones damp
slowly. But they all enhance rather than diminish the signal within
their pass-band. Damping refers to how fast an oscillation dies down,
not to enhancement or suppression.
A Formant is the effect of a resonator on a signal. To see the formant
in a spectrum, the signal that it affects must be of sufficient bandwidth.
Bruce's illustration is correct, except that his dotted lines should show
frequencies of greater rather than missing energy. The same arguments
apply. The soprano signal does not have the components to define the
formants, as he showed them, so long as she keeps a constant pitch.
Singers are trained not to keep a constant pitch. Their fundamental
fluctuates above and below the notional target pitch. One of the effects
of that fluctuation is to drift the harmonics, which are multiples of the
fundamental, up and down the frequency scale, so that they are likely to
move across the band of any formant resonator. The shift in relative
amplitudes of the harmonics allows the ear to hear the formants.
Incidentally, this effect is even more important for the tone of a violin.
The body resonances of a violin are quite sharp and there are many of them.
As the player's vibrato moves the harmonics across the peaks and valleys
of the violin's body spectral filter, the instantaneous "quality" of the
tone changes rapidly. This change is heard as the richness of the violin
tone, and is hard to simulate electronically. Too much fluctuation, or
body formants poorly located relative to one another, make for a poor
violin. Same, I suppose, for a singer.
As for the original query about changing the voice quality by changing
the pitch, this is one of the standard kinds of transformation that
speech researchers do. They also may change utterly the excitation
waveform, replacing the periodic glottal pulse with something else.
One of the weirder demonstrations of this kind was something Melvyn Hunt
did some years ago: He made the Berlin Philharmonic sing "We were away
a year ago" as the first bars of Beethoven's Fifth Symphony. I don't
mean that the orchestra members sang. The instruments did. Melvyn
took the formant structure of himself saying the phrase, and fed the
recording of the music through it. A fascinating concept and a