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Pulse Width - Yamaha CS-80 Instruction Manual

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4
42
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-80
has sixteen
main VCO's, one
VCO
for
each
of
8 notes times the
two
voices.
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
CS-80'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
introduces
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-80,
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
filter.
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."
A VCF
is
a
voltage controlled
filter.
It
can
be an
HPF
or
an
LPF.
In
fact,
the
CS-80's
VCF's
actually
consist of
two
filter
sections each,
an
HPF
and
an
LPF,
as
described above. Thus, the
CS-80
has
16
VCF's
(32
filter
sections),
one
for
each
of
the
16
main
VCO's.
The
cutoffs
can
be
changed
automatically by
the
Filter
envelope
(I
L-AL-A-D-R)
or
manually
by
moving
a
filter
slider
(HPF
or
LPF).
When
you
play
a
note on
the
CS-80, the
sound
generated by
a
VCO
goes
through
the
HPF
section
of
the
VCF,
then
through
the
LPF
section of the
VCF.
The
VCF
cutoff frequencies "track" the note played,
moving up
In
frequency
as
you
move
up
the
keyboard,
so that the tonal
spectrum
of the notes
remains con-
sistent.
Recall that the
LPF
is
wide open
when
its
cut-
off
point
Is
high,
yet the
HPF
Is
wide open
when
its
cutoff point
Is
low.
Together, the
HPF
and
LPF
sections of the
VCF
may
be
considered
to
be
a
bandpass
filter
because
a
defined
band
of frequencies
Is
allowed
to pass
between
the
two
filter
cutoff
points.
As
the
HPF
cutoff point
Is
raised
and/or
the
LPF
cutoff point
is
lowered, the
width
of
the
bandpass
Is
decreased
until
there
is
no
bandpass (no sound). Thus,
we
can speak
of
the
VCF
as
being
a
bandpass
filter,
even
though no such
label
appears
on
the
panels.
If
either of
the
two
filter
sec-
tions
is
completely
closed
down,
then
It
will
block
all
sound,
and
the position
of
the other
filter
section
makes no
difference
because
you won't
hear anything.
The
HPF
and
LPF
filters
each have
a
resonance
slider.
These
controls
only have
an
effect
if
their
corresponding
HPF
or
LPF
slider
is
partially
closed.
As
the
resonance
of
a
given
filter
Is
increased,
a
narrow
range of frequencies
Is
boosted
(increased
In
level)
the frequencies
centered
just at
the cutoff
point—
because
the
cutoff point
Is
resonating.
Resonance
has
no
effect
when
a filter
Is
wide open
because
the cutoff point
is
well
beyond
the
limit
of the
fundamental
or
overtones, so the boost
falls in
an area
where no
signal
is
present.
However,
as
a
filter
Is
closed
down,
the effect of
resonance
becomes more
notice-
able;
resonance
will
tend
to
emphasize
a
given
har-
monic
or
the
fundamental, depending on
the
filter
cutoff
(HPF
or
LPF
setting).
Resonance
also
causes
additional
phase
shift
which
can be heard
If
the
filter
cutoff point
is
changed
while
a
note
is
being played.
Amplifiers
An
amplifier
is
a
device that
Increases
the
volume
or
the
power
of
a
signal.
Some
amplifiers, especially
VCA's,
also
can
be
used
to decrease the
power
or
volume.
When
an amplifier decreases
the
volume
to
inaudibility.
It
is
turning the
sound
OFF;
conversely,
when
an
amplifier Increases
the
volume
to
audibility,
it Is
turning the
sound
ON.
Most
of
the amplifiers
in
the
CS-80
are
VCA's
(voltage
controlled
amplifiers),
and
they
generally
operate
at
medium
line levels.
Thus,
external
power
amplifiers,
such
as
a
PA
system
or guitar amplifier
head,
are
required to boost the
power
sufficiently to
drive
loudspeakers.
VCA's
offer several
advantages
for
synthesizers
In
addition to
their ability
to
attenuate
(lower) the
volume
as
well
as
increase
it.
With
conventional type
amplifiers,
audio
signals
must
be
routed through
com-
plex paths
and
It
may
be necessary to
have
a
separate
amplifier to achieve
each effect—
volume
control,
tremolo, note
definition
by
an envelope,
and
so
forth.
With
a
VCA,
on
the other
hand,
numerous
control
voltages
can
be
mixed
together
and
fed to
one
amplifier,
producing
all
the desired
effects
with
a
minimum
of amplifiers.
Thus,
VCA's
enable the
circuitry
to be simplified
while reducing
the potential
for noise
and
distortion.
There
are
two VCA's
for
each
of
the
16
main
VCO/VCF
sound
sources.
These
VCA's
are
used
to
"define"
notes—
to turn
them
on,
vary
their
volume,
and
turn
them
off—
as
each note
Is
played;
this
is
done
by
a
control
signal
from
the
amplitude envelope
gen-
erator,
as
described
in
subsequent
text.
The VCA's
will
also
vary the
volume
in a
regularly
modulated
fashion
when
they
are
provided with
an
AC
control
signal
from
the
sub
oscillator.
Sub
Oscillators
A
sub
oscillator
generates
AC
voltages
which
are
used
to
modify
existing
audio
signals.
The CS-80
has
an
overall
SUB OSCILLATOR
[11]
and
several
other
sub
oscillators.
For example,
the
PULSE
WIDTH
MODULATION
(PWM)
available
on
the panels
and
memory
Is
produced by sub
oscillators.
The
TREMOLO/CHORUS
effect also Includes
a
sub
oscillator, as
does
the
RING
MODULATOR.
To
understand
how
a
sub
oscillator
Is
used,
one
should recognize
that
AC
and dc
control voltages
are
often
mixed (summed)
for
combined
functions.
For
example,
the
VCA's
level
(volume)
control
Input
is

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