Yamaha TX 802 Owner's Manual page 26

WHAT'S FM?
WHA
T'S
FM?
A
complete
discussion of
FM
synthesis
is
beyond
the scope
of
this
manual. Here
we
will
cover only the basic
ideas of
FM.
For a
more
detailed study,
refer
to
one
of the
many
guidebooks on
the
Yamaha DX
series
and
FM
synthesis that
have been
published.
FREQUENCY MODULATION
FM
stands
for
Frequency
Modulation.
In
FM, one
waveform
modulates another waveform,
creating a
new,
complex waveform,
as
shown
below.
So
to
do
FM
synthesis,
we
need two
oscillators.
The
upper one
is
called
the
"Modulator" and
the
lower one {which goes
directly to
the output
of the synthesizer}
is
called
the "Car-
rier".
The
complexity
or
"brightness"
of the
resulting
waveform
will
depend
of the output
level
of the
Modula-
tor.
Modulator
Modulator
Output
Level
Carrier.
'V
*\
Resulting
Sound
'V
'V
^v
EG
(Envelope Generator)
By
varying the output
level of
the modulator,
we
can produce
all
kinds of sounds.
However, sounds
in
the
real
world
usually
change as time goes
by.
This
"shape
in
time"
is
called
the Envelope.
If
the output
level
of the
modulator changes,
the tone
will
change.
If
the
output
level
of
the
carrier
changes,
the
volume
will
change.
Piano envelope
0rgan
enve|Qpe
Key
On
Key
Off
Key On
T"
Key
Off
OPERATOR
In
Yamaha
FM
synthesizers,
each
oscillator
has
its
own
Envelope Generator
(EG) to
vary
its
output
level
over
time.
This "package"
is
called
an Operator.
An
operator can be
either a
modulator
or
carrier.
The
only
difference
is
how
it
is
used.
ALGORITHM
To do
simple FM, you need
only
two
operators, but the
TX802
has
six,
and
these
six
operators can be
arranged
in
32
different
ways. Each arrangement
is
called
an Algorithm.
The 32
algorithms
are
shown
on
the
pull-out operation guide
under
the
LCD
display.
Algorithm 3 has
two
carriers,
each
with
two
modulators
"stacked" on top
of
it.
In
algorithm 22, three
carriers are
modulated
by
a single
modulator, and another
carrier
has
its
own
modulator. Algorithm
32
has
six
carriers
and no modulators.
FEEDBACK
In
the algorithm
chart,
you
will
also notice that
one
of the operators has
its
output "looped". This
means
that
it
can modulate
itself;
a
bit like
having an
additional operator.
RATIO
FREQUENCY
The
basic
frequency
of
each
operator
can be
set
independently.
When
an operator
is in
Ratio
mode,
it
will
pro-
duce
a
frequency corresponding
to the
key
that
is
pressed.
It
the
frequency
ratio of
the carrier:modulator
is
1:1,
the
resulting
sound
will
contain
all
the
harmonic
partials.
For example,
if
the
fundamental
(the
"basic
pitch")
is
100hz, harmonics
of
200hz, 300hz,
400hz
...
will
be
generated.
If
the frequency
ratio
is
1:2,
the
odd-numbered harmonic
partials will
be generated
OOOhz,
300hz,
500hz
will
generate
non-harmonic
spectra
typical
of clashing, metallic sounds.
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