Yamaha CS-80 Instruction Manual page 45

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
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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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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)
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Overall
Synthesizer Block Diagram