Ring Modulator - Yamaha CS-80 Instruction Manual

i
fed by several
sources of
AC
and dc voltages.
The
level
can be varied
up and
down
for
a
tremolo
effect
by applying an
AC
control voltage
which
is
produced
by the
SUB OSCILLATOR
section [11].
The
depth
of the tremolo effect
would
be adjusted
by applying
more
or less
of the
AC
voltage
produced by the sub
oscillator
to the
VCA. The
speed of the effect
would
be adjusted by changing the
sub
oscillator's
frequency.
The
average
volume around which
the
tremolo
is
centered
is
adjusted by changing the dc control
voltage, using
the
LEVEL
slider [41
]
Pulse
width
refers
to the
amount
of
time
a
square
wave
is
OFF,
and
is
also
known
as
"duty
cycle."
A
perfectly
symmetrical square
wave would
have
a
50%
duty cycle
(OFF
as
much
as
ON), and
a
narrow
pulse
width square
wave might
have
a
90%
duty
cycle
(which sounds the
same
as a
10% duty cycle—
ON
10%
of the time).
The
PW
control [22] applies
a
dc
control
voltage to the
WSC
circuit
which
sets
the
basic
pulse
width (duty cycle) of the square
wave
at
any point
between
50% and 90%. The
PWM
control [21] applies
an
AC
control voltage to the
same
point
in
the
WSC
(wave shape converter) circuit,
thereby varying
(modu-
lating)
the pulse width. That
PWM
signal
is
created
by
a
sub
oscillator,
and the
SPEED
[20] of pulse
width
modulation
is
actually
changed by adjusting the
frequency
of the
PWM
sub
oscillator.
The
sub oscillators
in
the
RING
MODULATOR
and
TREMOLO/CHORUS
sections
function
similarly to
the
main
SUB OSCILLATOR
and
the
PWM
sub
oscillators
described above. Changing the
amount
of
AC
voltage applied varies the
depth
of
the
effect,
and
changing the
frequency
of the
sub
oscillator varies
the speed
of
the
effect.
Envelope Generators
An
envelope generator
is
a circuit
which produces
a single,
carefully defined
waveform
—
a
one-shot
voltage pattern
—
when
the generator
is
stimulated by
a
pulse
(trigger
impulse)
from
the keyboard.
The
envelope
itself
is
a
changing dc
voltage
which
rises
from
zero (no voltage) to
some maximum
point,
and
eventually
fails
back
to zero
in a
pattern
which
is
varied
by using the envelope generator's controls.
No
sound
goes
through
the
envelope generator
itself.
Instead, the envelope generator's
output
is
fed
to the control input of
a
VCF
or
a
VCA.
There
are
actually 16 envelope generators for the
VCF's and
another 16 for the VCA's.
Envelope generators
(EG) which
control
VCF's
are
known
as
filter
envelope
generators. In
the CS-80, the
filter
EC's
are
unique envelope generators, having 5
sliders: Initial
Level (IL),
Attack
Level
(AL), Attack
Time
(A),
Decay
Time
(D)
and
Release
Time
(R).
These
sliders
all
change the "shape" of the envelope,
which
in
turn creates
changes
in
HPF
and
LPF
filter
cutoff points
each time
a
note
is
played.
When
all
the
filter
EG
sliders
are set
at
minimum,
there
is
no output
from the
EG, hence no change
in filter
characteristics.
Envelope generators
which control the
VGA's
are
known
as
amplitude envelope generators.
In
the
CS-80, the amplitude EC's have 4
sliders:
Attack
Time
(A),
Decay
Time
(D),
Sustain Level
(S)
and
Release
Time
(R).
These
sliders
change the "shape" of
the envelope,
which
in
turn creates
changes
in
the
volume (amplitude) of the
sound
when
you play
a
note.
When
all
amplitude
EG
sliders are set at
minimum,
there
is
only
a
very
brief
pulse of
output
voltage
from
the
EG, hence only
a brief
"blip" of
sound
can be heard.
Conventional synthesizers
sometimes have
simplified EG's,
with only Attack
Time
(A)
and
Release
Time
(R)
sliders;
the
same A-R
effect
can be
achieved on the
CS-80 by
setting
the
VCA
Decay
Time
(D)
and
Sustain Level (S) sliders at
maximum,
and
using
only the
A
and R
siiders.
The Keyboard
&
Related Circuits— General
As suggested
in
the preceeding paragraphs, each
channel of the
CS-80
has eight
sets
of
note-generating
circuit
components, each
set
consisting of
a
VCO,
WSC,
VCF
and
VCA,
and
two
EG's.
When
you
move
any one of the panel
programming
controls,
it
actually
affects
all
8
sets
of note-generating
components
on
the
corresponding channel. While there
is
8 note
simultaneous
capability,
there
are
61 keys
on
the
keyboard. Thus, there has to be
a
way
of assigning
the keys
you
play to those
8
different
note
generating
circuits.
This
is
the function of
the
Key
Coder and Key Assigner
circuits.
The Key Coder
&
Key
Assigner
The key coder and key assigner are
digital circuits,
a
sort of
micro-computer.
The
key coder produces
a
digital
"word"
that describes the
note
(or
notes)
played.
The
key assigner
"looks"
to see
which,
if
any,
of
the note-generating
circuits
Is
available and, at
the
same
time,
it
continuously monitors the
key coder
to
see
which
notes are
being played.
The
assigner then
feeds the
digital
word
for
each note to one of the
note-generating circuits.
If
a
ninth key
Is
depressed
while 8 other keys are
already being played, the
assigner
cannot do anything with
that additional
information, so
no
new
note
will
be heard
until
one
of the
first
8 keys
is
released.
If
you
play
only one
key,
and play
it
8 times
in
succession, the key assigner
will
successively feed the
"word"
for that note to each of the
8 note-generating
circuits.
Since each
circuit's
VCO,
VCF
and
VCA
will
differ slightly
from
the
next due
to
normal
component
tolerances,
the 8 notes
will
not be identical. This
is
how
the
CS-80 produces such natural sound, rather
than
a
mechanical,
"too perfect" sound.
D-to-A Converters
The
note-generating circuits
each have
a
D-to-A
Converter
(digital
to analog)
which changes the
digital
code
for
a
note
Into a
corresponding dc voltage.
That
dc voltage
level
Is
fed to the
VCO, which
reacts to set
the pitch (frequency) of the note.
The
voltage
Is
also
fed to the
VCF, which
reacts
by moving the
HPF
and
LPF
filter
cutoff frequencies so
they maintain the
desired relationship to the frequency of the note
(so
they track).
Trigger
Output
The
instant
a
key
is
depressed, the keyboard
produces
a trigger
output,
in
addition to the digital
word.
The
trigger
is a
brief
voltage pulse that occurs
once,
and
it is
routed to
two
envelope generators, the
filter
EG
in
the
VCF
section
and
the
amplitude
EG
in
the
VCA
section.
The amplitude
EG
reacts to
the
trigger
and generates
a
one-shot
waveform
to
"shape"
the
volume
of
the note according to the preset or
programmed
A-D-S-R
characteristics.
The
fitter
EG
reacts to the trigger
and produces
a
one-shot
waveform
which
changes the
tone
of the
note
if
the
I
L-AL-A-D-R
controls are appropriately
programmed
(or
If
VCF
envelope
is
part of the preset patch).
Touch
Sensitivity
To
understand
how
the
touch
sensitivity
works,
it is
necessary to understand the
method
by which the
keyboard
itself
functions.
The CS-80 keyboard
has
a
proprietary, patented technique for switching
a
note
ON
when
you
strike a
key, plus
a
secondary system
for
adding
effects
by
pressing
harder
after
the key
hits
bottom.
Velocity
Sensitivity
At the
rear
of each key, there
is a
single
pole/
double throw
leaf
switch.
When
you
begin to
press
down
a
key, the first set
of
switch contacts open.
Then,
as
the
key nearly
hits
bottom, the
second
set of
switch contacts close.
The
closing of the
second
set of
contacts activates
the key coder, key assigner,
and
subsequent
circuits
to generate the note.
However,
the
time interval
between the opening of the
first
contact
pair
and the closing of the
second contact
pair
is
used
by another
circuit to
produce
a
control
signal.
The Key Timing
Circuit utilizes
sophisticated logic
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