ARP 2600 - Tremolo and Amplitude Modulation (AM) Effects

63
The control
input attenuator, by
governing
the
strength of
the
control signal allowed into the
amplifier, governs
the amount of
con-
trol
obtained. In this
case, opening
the attenuator all the way allows
the full 1
0V
p-p
voltage of the
sine wave in. This is
enough to drive
the
VGA
all the
way from maximum to
minimum
gain. Overloading
the control input, which
may occur
with
some
input
signals, can
be
recognized
by
the discontinuities it
causes
in the VCA output;
it
does
no harm and can
be
used
deliberately
if
you
wish, (In general this
is
true of ail the 2600
functions.)
Since the voltage
signal you
are using in this particular patch
originates in VC02, it is
reasonable to
expect that
changing
the behav-
ior
of that oscillator
will cause a
corresponding
change in the behavior
of
the
VCA.
A
little
fiddling with
the frequency
slider of VC02 will
demonstrate
that this is
indeed the
case. Call
the undulating gain of the
VCA a
tremolo
and
say
to
yourself that the
frequency of the tremolo
is
directly governed by
the
frequency
of
VC02.
And of course there
isn't
any reason in
the
world why a tremolo
shouldn't be able to hap-
pen not only at
relatively
slow rates, like
those produced
by VC02
in
its
low range,
but
also at
higher rates or
even at audio grequencies.
With this in mind,
you might
try switching
VC02 to
its audio
range and
begin sweeping
it
upward
from 10Hz. The
resulting output
from
the
VCA has
of course
no audible
similarity to anything you
would
ordinarily call
a
"tremolo"
at all;
it is in
fact one of a
whole
class of what we shall call
"amplitude
modulation effects" or simply
"AM
effects" for short.
In
general,
a
fluctuating
control voltage will
have entirely dif-
ferent audible
effects depending
on whether
it
fluctuates
predominant-
ly
at subsonic
or at audio
frequencies. And
the effects it
produces on
VCO, VCF,
or VCA
behavior when it
fluctuates primarily at
audio fre-
quencies
we shall call
specifically
"modulation effects"—although,
as
we
have tried to
make clear,
any
systematic
modification of
a
waveform
may
technically be
considered a
"modulation" of
that
waveform.
The
concept
of modulation
is sometimes
even made
so
general that any
change
whatsoever
in
a
waveshape
is called
a
modulation; the
context
in which
the word is used
must generally be
depended on to
define
the
scope
of its
meaning.