<
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
To Next Page
Loading...
