ARP 2600 - Filter Resonance and Oscillation Effects

56
Still
with
all
inputs closed,
lower the resonance
control to
just
below
the
point
where the filter
ceases to
oscillate.
At
this point
the
filter
delivers the
sharpest
"peaked"
output
it is capable
of;
open
the
NG-Filter
input
again and use
the
keyboard this time to play up and
down
on the
filter
F
c
. For this
experiment use pink
noise.
5.1071
The
noise
signal has
energy
at all
frequencies;
it can
be
thought
i
of
as
the sum
of
an infinite
number
of sine waves
at an infinite number
of
frequencies. The
filter,
set
for
peaked
response
and
swept across its
frequency range,
simply
"picks
out" and
selectively
emphasizes
these
components
of
the
noise
signal.
The periodic waveforms
from
the VCO's on the other
hand,
distribute their energy across a
definite
"spectrum"
of
frequencies.
Close
the NG-Filter input
and open
the VC03-Filter input, with
F
c
at
maximum and
resonance
still
set
for
peaked
response from
the filter.
Tune
VC03
down
to
about
200Hz.
Now
flip
the
F
c
control to minimum
and
start
moving the con-
trol
up
slowly from
the
left.
When
the filter F
c
reaches 200Hz
you
will
hear
a
fairly
loud sine
tone
from the
speakers;
this is
the
funda-
mental frequency component
of
the sawtooth waveform
you are
feed-
ing
into the
filter,
lust
past
this
point
the
filter
output drops to almost
nothing again, until
F
c
reaches
4G0Hz, exactly
one
octave
above
the
fundamental
frequency of the sawtooth.
Again the
filter output
will
increase
sharply
and
then fall
off
as
the F
c
reaches
and crosses the
fre-
quency
of the sawtooth wave
2nd
harmonic component.
The
next tone
will
appear at 600Hz,
an octave
and
fifth above
the
fundamental,
and
the
next at
800Hz,
two
octaves up.
This is
the
4th harmonic. The
4th,
5th and 6th harmonics outline
a
major triad: first, third, and fifth
res-
pectively. The
7th harmonic
is a
minor
seventh up
from the 4th,
and
the
8th is
three
octaves above
the
fundamental. The
8th
through
the
1 6th harmonics
are
not
quite a major scale; several
of the
notes
in
this
series
are
sharp or
flat. And
so on.