effects
of feedback
6
modulation sounds different than positive-only
FM. Frequency shifting by increasing negative
amounts has a distinctive effect that at some
point, the original frequencies approach zero,
cross it, and then start to increase again (see
fig. 3). For example, with the incoming signal
frequency being 300Hz, turning the shift
knob counterclockwise to the -300Hz position,
i.e., 30 with the x10 switch engaged (while lis-
tening to the up shifted outs), compensates
the original frequency, so the result reaches
0Hz (300-300=0). Beyond this point, as we
hear it, the frequency appears to be increasing,
even though it actually moves towards -inf.
Interestingly, shifting a harmonic signal down
moves its fundamental to an inaudible DC. At
the same time, all the overtones move down by
one position, thus producing a harmonic signal
again. Further shifting, however, causes its fun-
damental to appear in increasing (negative)
frequency, as opposed to its other components,
which still decrease toward zero; thus, the sig-
nal becomes increasingly inharmonic.
the effectS Of feedBAcK
Feeding a frequency-shifted signal back to the
input of the shifter creates a cascade of shifts
because part of the signal is shifted multiple
fig. 4: time-frequency response of frequency shifting with feedback
20 000
Frequency (Hz)
Time (s)
Gain
0
0
2
4
6
8
10
5 000
10 000
15 000
0
0.2
0.4
0.6
0.8
1