THE
ABC's OF
THE SYNTHESIZER
iBASIC
BLOCKS MAKING
UP
A
SYNTHESIZER
PITCH,
determined
by
TONE COLOR
determined
by
CONTROLLED
CONTROLLER
VCO
f\
C\
-y
VCF
/v
y^
Auto
Bend
Env-1
MODULATION
SIGNAL
OSCILLATOR
I
VOLUME
determined
by
VCA
_N^
Env-2
i
CUT
M-Vfj
V77Z
SOUND
SIGNAL
y
CONTROL
VOLTAGE
+.
GATE
PULSE
THE THREE QUALITIES
OF SOUND
The three qualities of sound
are:
pitch,
tone
color, and loudness.
Sound is the result
of physical vibration in
objects. The pitch
produced depends on the
vibration
rate.
Fig. 1
shows
that the
higher the
vibration
rate,
or the higher the frequency.
of
the vibrations, the higher the pitch is.
Frequency
is measured
in unit called the
Hertz (H2}. If a
sound
source vibrates
at a
rate of 100
vibrations
per second,
it is
said
to
have
a
frequency
of
100Hz. The normal human ear can hear
sounds
with frequencies from
about
20Hz to about
16,000 Hz.
In synthesizers, pitch is controlled by
the VCO
(Voltage Controlled Oscillator). An
oscillator is
an
electronic circuit which generates s
wave-
form, or
in this case, sound.
Voltage controlled
means that a voltage is used to
control the
frequency
(pitch,
in
this case)
of the oscillator;
the higher the voltage, the higher the frequency.
This, then, is the first quality of sound; pitch.
Fig.
1 shows what are
known
as
sine waves,
the simplest
mode
of vibration. The sound
of
a sine wave is very clean and
pure. Most sound
sources
vibrate at many
frequencies at the same
time, however.
The lowest
of the vibrations,
and usually the strongest,
is the one which
we
hear as the
musical pitch of the sound source.
The
presence of these
other frequencies
is what
gives a
sound source its
second quality:
tone
color.
The
frequencies
produced by
the complex
vibrations
of a
sound
source are called har-
monics.
Harmonics are
usually multiples of the
pitch frequency.
For
example, consider a sound
source
which vibrates so
as
to
produce a
pitch
with
a
frequency
of
200Hz. The first
harmonic
would be
200Hz {1 x
200
=
200). The first
harmonic is
also called the
fundamental because
this is the
frequency
which gives
the sound its
musical
pitch. The
second
harmonic would
be
2 x
200, or
400Hz. The third
harmonic would
be
3 x 200,
or
600Hz; the
fourth harmonic
4 x 200, or
800Hz, etc.
Fig.
2
shows a
square wave. Square waves
produce
a
tone
col or much like that of a clarinet.
Square
waves contain
only the
odd numbered
harmonics, or
in other
words, those frequencies
which are one,
three, five,
nine,
etc.
times the
frequency
of the
fundamental, or pitch frequen-
cy. The
even numbered
harmonics are missing.
Tone
color, then, is
determined by
theharrnonic
content of
the sound; the more
harmonics
there are,
the brighter the tone color. In the
synthesizer,
tone color is controlled by the
VCF
(Voltage
Controlled Filter). The
VCO
produces
a basic sound wave rich in harmonics.
The
VCF
is used
to
remove (or filter out)
some of
these harmonics
thus controlling
the
harmonic
content of
the
finished
sound.
The
amount of
the harmonics removed is control-
lable by a control
voltage.
The third
quality of sound, loudness, is de-
termined in
a synthesizer by the level or
ampli-
tude of the waveform,
as shown in
Fig.
3,
and
is
controlled
by the VCA (Voltage
Controlled
Amplifier).
The loudness
of a sound changes during
its
production.
Strike a piano key- and the
sound
jumps the maximum
loudness. Hold the
key
down and
the sound slowly dies away. This
pattern of
loudness in a sound is called
its
envelope. The
output of the envelope
generator
is
a control voltage.
This control voltage
is used
to control the VCA,
thus shaping the
loudness
pattern of the
output sound of
the synthesizer.
FIG.
1 -SOUND PITCH
-
FREQUENCY
HIGH LOW
FIG.
2
-TONE
COLOR
=
HARMONICS CONTENT
SQUARE WAVE
with three of
its
S
i
NE wave
harmonics shown
on,n
"HVC
FIG. 3 VOLUME
=
AMPLITUDE
BIG SMALL
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