Pulse Width - Yamaha CS-80 Instruction Manual

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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