VCO
-
VOLTAGE
CONTROLLED
OSCILLATOR
6.
PULSE
WIDTH AND
PULSE WIDTH MODULATION
Puise width refers to the ratio of the widths
of the top and
bottom
portions
of
the
pulse
wave.
Puise width is measured in per-
centages
as
shown in
the
drawing
at
the
right. 10% pulse width produces a sound
very
rich in
harmonics and
is
often used for
synthesizing sounds
such as the oboe,
bassoon,
and human voice. Note that a
50% puise width is nothing more than a
square wave.
In addition to using fixed pulse widths, it
is
possible
to modulate the puise wave so
that
the
width
of the pulses varies
con-
tinually, as shown in the drawing at the
right.
Using the LFO
to
modulate the
pulse
width
produces
chorus-like
sounds.
The
output of the envelope generator can be
used to modulate the pulse width to
produce sounds very much like those
of
pizzicato strings.
With the
PWM
MODE switch at
MANUAL,
this control allows manual adjustment of
the pulse width. Note that the
lowest
position,
50%,
will produce
a
square
wave.
At the
top
is MIN (minimum)
which
produces
a pulse
width
of about 10%.
With the
PWM MODE
switch set at
LFO or
ENV-1
, this slider
controls
the depth
of
the
LFO or
ENV-1 modulation.
7.
PWM MODE SWITCH
With this switch
at
MANUAL, the pulse
widths
may
be set
manually
by
means of
the PULSE WIDTH MOD slider. In
LFO,
the
pulse width will be modulated
by
the
LFO
output,
and
in
ENV-1,
by the
output
of
ENVELOPE
GENERATOR
1.
(1)
PULSE
WIDTH
WAVEFORM
10%
25%
50%
,
—
PULSE
WIDTH MOD
PULSE WIDTH
MIN
1
i to
Imanual
ENV-1
o
AX
(2)
PWM
BY LFO
—
PULSE WIDTH MOD
—
^
PULSE
WIDTH
MSN
A
.
AA
LFO
OUTPUT
WAVEFORM
(3)
PWM
BY ENV-1
ENV-1
SIGNAL
OUTPUT
WAVEFORM
r
ENV-1
—
PULSE
WIDTH
MOD
—
^
PULSE WIDTH
min
n
. .
AA
LFO
y
ENV-1
WAVEFORMS AND
HARMONICS
All
waveforms except the
sine wave
can be
generated
by
a
combination
of a
fundamental
sine wave of the
pitch
frequency and a group
of
sine
waves which are
multiples of the
fundamental; therefore,
any waveform
can
be
synthesized by combining a
fundamental sine
waveform with a
proper ratio of
harmonics.
The drawing
below
shows
a
funda-
mental
with its harmonics.
The
white
notes are even numbered
harmonics
and the black
notes are odd
num-
bered harmonics. (True
harmonics
are
actually
slightly different
from the
pitches produced by
the
equally
tempered
scale.) Ail waveforms,
except
the
sine wave,
contain harmonics.
It is
this harmonic
content that
gives
each
sound its particular
tone color.
~e~
8v*V
-e-
i
FUNDAMENTAL
2ND 3RD
4TH 5TH 6TH 7TH
8TH
9TH
10TH
HARMONIC
8' 4*
2-2/3'
2'
1-2/3'
1-1/3"
-
r 8/9'
4/5'
11
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