Yamaha CS-50 Instruction Manual page 49

I
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-50 and CS-60
are
VCA's
(voltage
controlled
amplifiers),
and
they
gener-
ally 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
complex
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
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
generator,
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-50 and
CS-60
both have an
overall
SUB OSCILLATOR
[81
and
several
other sub
oscillators.
For example,
the
PULSE WIDTH MODULATION
(PWM)
|16| available
on
the programmable
panel (and CS-60 memory}
is
produced by sub
oscillators.
The RING
MODULATOR
[12) also includes
a
sub
oscillator.
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
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 [8]
.
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
[36).
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
ONI. and
a
narrow
pulse
width square
wave
might have
a
90%
duty
cycle (which
sounds the same
as
10%
duty
cycle)-ON 90%
of the
time.
The
PW
control [17) 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 [161 applies
an
AC
control voltage to the same
point
in
the
WSC
(wave shape
control) circuit, thereby varying
(modulating)
the pulse
width. That
PWM
signal
is
created by
a
sub
oscillator,
and
the
SPEED
[15! of
pulse
width modulation
is
actually
changed by
ad-
justing
the frequency
of the
PWM
sub
oscillator.
The sub
oscillator in
the
RING MODULATOR
section functions
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
it-
self
is
a changing dc voltage which
rises
from
zero (no
voltage) to
some
maximum
point,
and
eventually
falls
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.
In
the CS-50, there
are actually
4
envelope generators
for
the
VCF's
and
another 4
for the
VCA's
(8
each
in
the CS-60).
Envelope
generators
(EG) which
control VCF's
are
known
as
filter envelope generators.
In
the
CS-series
synthesizers,
the
filter EG's are unique envelope
genera-
tors,
having 5
sliders:
Initial
Level (ID, 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 VCA's
are
known
as
amplitude envelope
generators.
The CS-50
and CS-60
amplitude EG'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-50
or
CS-60
by
setting the
VCA
Decay Time
(D)
and
Sustain Level
(S) sliders at
maximum,
and using only the
A
and R
sliders.
The Keyboard &
Related Circuits-General
As
suggested
in
the preceeding paragraphs, the
CS-50
has four
sets
of note-generating
circuit
com-
ponents and the CS-60
eight
sets,
each
set
consisting
of a
VCO. WSC,
VCF
and VCA, and two
EG's.
When
you
move
any
of the panel programming
controls,
it
actually affects
all
4
or
8
sets
of
note-
generating components. While
the CS-50 has 4 note
simultaneous
capability,
its
keyboard
has
49
keys,
and
while
the CS-60
has 8
note simultaneous
capability,
its
keyboard
has 61
keys.
Thus,
there
must
be
a special
way
of assigning the
specific
keys you
play to those
4
to
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.
on
the
CS-50, a
fifth
key
is
depressed while 4 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
4
keys are released. (Similarly,
the
CS-60
cannot do
anything about
a
ninth
key
until
one
of the
first
eight keys
are released.)
If
you
play
only
one
key,
and
play
it several
times
in succession, the key assigner
will
successively
feed the
"word"
for that
note to each of the note-generating
45