Yamaha CS-80 - Ring Modulator

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
43