Yamaha CS-50 Instruction Manual page 47

HOW
THE SYNTHESIZER
WORKS
i
i
The
synthesizer consists of
sound producing and
sound modifying
circuits,
all
related
by a
number
of
signal
paths
and
control
circuits.
Oscillators
and Noise
Generators produce
the
raw ingredients for
sounds.
Wave Shape
Converters.
Filters,
Amplifiers,
a
Ring
Modulator,
and
Sub
Oscillators
further
modify
the
sound
(the
audio signals). These
circuits,
plus the
distinction between
audio and
control functions, are
detailed
below. While voltages
are
discussed,
it
is
not
really
necessary to understand
how
voltages work;
when you move
the controls and knobs,
you
are
adjusting voltages inside
the synthesizer.
Audio Signals
&
Control Voltages
Electric
currents that flow through synthesizers
can
be
thought of
in
two
categories:
audio
signals
and
control
voltages.
The audio
signals
constitute the actual
sound
as
it
is
generated, modified,
and
ultimately fed to
the output.
The
control voltages themselves are never
heard,
but
are instead used to adjust the circuits
which
process the audio.
Audio
signals are alternating currents
(AC) with
frequencies
in
the audible range
which,
as
you
probably know,
covers
about 10
octaves
from 20
cycles per
second
(Hz)
to
20,000
cycles per second
(Hz).
Audio
signal
voltages vary
at
different points
in
the synthesizer,
but they average about 0.775
volts
at the
output
when
the
rear panel
HIGH/LOW
switch
is
at
HIGH
position
(OdBm
into
600 ohms).
Control voltages are usually
10
volts or less,
and
may
be
DC
(direct
current) or
AC
(alternating current).
AC
control voltages vary
in
frequency
from
very low.
sub-audio
frequencies
(1/2Hz)
up
to the
audio
fre-
quency
range
(as
high as
500Hz
or
more).
The
effect
produced
by a
voltage controlled
circuit will
vary
in
proportion
to the control voltage applied. For example,
a
VCA
(voltage
controlled amplifier) will
cause the
audio
signal to
be
higher
in
volume
when
the control
voltage
is
higher in
level.
If
a steady
DC
control
voltage
is
applied to the
VCA,
the
volume of
sound
coming
out of the
VCA
will increase
by a proportion-
ate
amount and
will
remain
at
that
level.
If
an
AC
control voltage
is
applied to
the
same VCA, then the
volume
will
vary up and down,
corresponding
to the
variations of the
AC
voltage; this
is
AM,
or
amplitude modulation.
When
a
DC
voltage
is
applied
to
a
VCO
(voltage
controlled oscillator), the oscillator increases
its
frequency.
When
an
AC
control voltage
is
applied to
a
VCO.
the
frequency
varies
up and down,
producing an
effect
known
as vibrato or
FM
(frequency modulation).
Similarly,
when
AC
or
DC
voltages are applied to
VCF's
(voltage
controlled
filters)
the
filter characteris-
tics
change; the cutoff points
move
up
or
down.
Refer to the
programming block diagram on
which
represents the
programmable
panel's func-
tions.
This
is
the
same
diagram appearing
on
the
synthesizer front
panel,
and
is
often
helpful as a
reminder
of
how
the panel functions
are related to
one
another.
A
key
to
the block diagram symbols
is
shown below the diagram.
Audio
signal
paths run
from
left
to
right, as
shown
by the horizontal
lines that join
the
blocks (colored lines). All vertical lines that
point
to the
blocks represent control voltage
paths.
The
block
diagram
is
divided into three sections
which
correspond
to the
VCO. VCF
and
VCA
sections of
the panel.
A
more complete
block diagram
of the
full
synthesizer
is
shown on
the following
page.
Like the
programming block diagram, audio
signal
flows
from
left
to right.
However, unlike the programming block
diagram, vertical
and
horizontal lines
do not
distinguish
control
and audio
signals;
audio signals are
still
shown
by the colored
lines
and control
signals
are
shown
by
the black
lines.
Oscillators
An
oscillator
is
a circuit
that produces
AC
voltage,
generating voltages
which go
up and
down
in level
according to
some
regular,
defined pattern
(waveform)
and
at
some defined
rate
(frequency). There are
many
types of oscillators,
some
for
very low frequencies
and
others for
audio frequencies. Oscillators that operate
in the
audio frequency range are generally used
as
sources of sound.
The
VCO
is a
voltage controlled
oscillator.
The
CS-60 has
eight
main
VCO's,
one
VCO
for
each of
8
notes (the
CS-50 has four
main
VCO's).
Any
main
VCO
is
capable of producing
all
the notes,
but only one
note
at a
time.
When
you
play the keyboard, special
digital
circuitry assigns
different control voltages to
the available
VCO's
so that the desired notes are
produced.
Wave Shape
Converters
The
synthesizer's
main VCO's produce sawtooth
waves.
These waves
may
be used, unaltered,
as
the
sound
source,
but they can also be processed by the
wave shape converter (WSC)
to
form
square waves or
sine
waves, as desired.
The WSC's
are
considered to be
part of the VCO's.
Noise Generator
A
noise
generator
is
like
an
oscillator
that
is
con-
stantly
and rapidly changing
its
frequency and
its
waveform so that the output appears
to
be a
random
mixture of
all
sounds simultaneously.
White
noise
is
a
type of noise that has
equal level,
on
the average,
across the
full
audio spectrum.
The
noise generator
is
not voltage controlled, but
is
included
in
the
VCO
section
of the programmable
panels
because
it
intro-
duces
noise at the
same point in
the
circuit as the
VCO's:
just
before the
filters.
Filters
A
filter
is
a circuit that allows
some
frequencies
to pass
through
it.
but
eliminates other frequencies.
In
the
CS-50 and
CS-60, there are
two
types of
audio
filters,
high pass
(HPF) and low
pass
(LPF).
(Many
synthesizers have only
a
low
pass
filter.)
A
low
pass
filter
blocks
all
audio signals above
its
cutoff
frequency
(cutoff point).
When
the
LPF
cutoff
point
is
set at
a high frequency,
it is said to be
"wide
open"
because
the fundamental note and
all its
harmonics (overtones) are below
the
cutoff point and
will
pass
through the
filter.
As
the
LPF
cutoff
point
is
lowered,
more and more
of the
harmonics and then
the
fundamental
are eliminated,
and
the
filter
is
said
to
be "closed
down."
A
high pass
filter
blocks
all
audio
signals
below
its
cutoff frequency.
When
the
HPF
cutoff point
is
set at
a
low frequency,
it is
said to be
"wide open" because
the
fundamental note and
all its
harmonics
are
above
the cutoff point
and
will
pass through the
fitter.
As
the
HPF
cutoff point
is
raised,
the fundamental
is
blocked, then
the
lower harmonics,
and eventually
all
harmonics, so the
filter is
said to
be "closed
down."
43
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