Yamaha CS-80 Instruction Manual page 45

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HOW
THE SYNTHESIZER
WORKS
i
The
synthesizer
consists of
sound producing and
sound modifying
circuits,
all
related
by
a
nunnber
of
signal
paths
and
control
circuits.
Oscillators
and
Noise
Generators
produce
the
raw
ingredients for
sounds.
Wave
Shape
Converters,
Filters,
Amplifiers,
a
Ring
Modulator,
a
Tremolo, 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
fre-
quencies
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 synthe-
sizer,
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
proportionate
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
modula-
tion).
Similarly,
when
AC
or
dc
voltages are applied to
VCF's
(voltage
controlled
filters),
the
filter
character-
istics
change; the cutoff
points
move
up
or
down.
Refer
to the
programming
block diagram
on
this
page.
which
represents the
functions
of
one
of the
program-
mable
panels.
This
is
the
same
diagram appearing on
the
memory
pane! cover,
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; the
TOUCH
RESPONSE
section
is
diagrammed
as
being
part of
the
VCF
and
VCA
sections, since
it
actually
effects
both
of
these functions.
A
more
complete
block diagram
of the
full
synthesizer
is
shown
below
Like the
simplified
block diagram, audio
signal
flows
from
left
to
right.
However,
unlike the simplified
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.
Programmable
Panel
Block Diagram
41
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TOUCH
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m
A
C
S
R
A
=
Attack
Time
AL
=
Attack
Level
AMP
=
Amplifier
D
=
Decay
Time
EG
=
Envelope Generator
EXT
IN = External Input
HPF
= High
Pass
Filter
IL
=
Initial
Level
LPF
=
Low
Pass
Filter
PW
=
Pulse
Width
R
= Release
Time
S
= Sustain Level
TUNE
= Pitch Controls
R
3
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MTIAL-.
AFTEfl'
SLBVC
TOUCH
RESPOKSF
AB.PB'i
1
*
y
AFTER
= After
Touch
Response
(Pressure)
INITIAL
=
Initial
Touch
Response
(Velocity)
LFO
-
Low
Frequency
Oscillator
M1-M8
=
Main
Sound
Generating
Circuit
Boards
MOD
=
Modulation
or
Modulated
Signal
PWM
= Pulse
Width Modulation
PWM
OSC
= Pulse
Width Modulation Sub
Oscillator
RESh
= High
Pass
Filter
Resonance
RESl
=
Low
Pass
Filter
Resonance
VCA
= Voltage Controlled Amplifier
VCF
=
Voltage Controlled
Filter
VCO
= Voltage Controlled
Oscillator
WSC
=
Wave Shape
Converter
(Part
of
VCO)
FOOT SW FOOT CONTROl^LR
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DEP
Overall
Synthesizer
Block Diagram

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