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THE ABC OF
SYNTHESIZER
•
BASIC
BLOCKS MAKING UP
A
SYNTHESIZER
PITCH,
determined
by
TONE COLOR
determined by
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CONTROLLER
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MODULATION
SIGNAL
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SOUND
SIGNAL
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VOLTAGE
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
(Hz).
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
16,000Hz.
In
synthesizers, pitch
is
controlled
by
the
VCO
(Voltage Controlled
Oscillator).
An
oscillator
is
an
electronic
circuit
which
generates
a
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
colormuch
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
the
harmonic
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
ciiup
wave
harmonics
shown
°
c
*"
c
FIG. 3
-VOLUME
=
AMPLITUDE
BIG
SMALL
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