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Chapter 5: Oscillators and Filters
ANDROMEDA A6 REFERENCE MANUAL 109
OSC 2 FM Parameters
Using a VCO as a Mod Source
The frequency of
OSC 1 can be modulated by OSC 2, hence the term “OSC 2 FM”, or
“frequency modulation by Oscillator 2”. Using this modulation is a bit more
involved than simply turning the
OSC 2 FM knob clockwise to increase the amount of
OSC 2 modulating OSC 1 although amount is the main parameter of FM.
What is “FM”?
The abbreviation for frequency modulation, FM can be easily confused with other
forms of frequency modulation such as an LFO providing vibrato or trills by
modulating the frequency of the VCOs. “FM”, however, has become synonymous
with modulating an oscillator by another using one of several technologies that
emerged during the 1930s.
As a modulation source, a VCO oscillates at a rate that puts its frequency in the
audible range, as opposed to an LFO which typically oscillates at a slower rate that is
below the range of pitched audio. Since a VCO can cover the entire audible
frequency range (and then some: 20Hz to 22Khz), its effect as a modulation source
for another oscillator is much different than of an LFO.
When you modulate a VCO by another VCO, the modulation rate is high enough to
produce an entirely new set of harmonics called sidebands. The result of this type of
modulation is often described as “buzzing” or “metallic”. Also significant in the
modulating process is VCO tracking by the keyboard. When tracked by the
keyboard, the frequency of the source VCO changes with each new key played,
changing the characteristics of the sound.
There are two types of oscillator-generated FM in the A6, linear and exponential.
OSC
1
features both linear and exponential FM, OSC 2 features exponential FM only.
DISPLAY
PAGE PANEL LABEL
PARAMETER
DISPLAY
OPTIONS
or RANGE
DESCRIPTION
OSC2FM
OSC 2 FM —
This knob controls the amount of the FM type
currently selected. There are three type of FM:
Linear, Exponential and Pulse Width
Modulation, described next.
— LINFM 0 ··· 100
Linear FM causes the source VCO to modulate
the destination VCO such that harmonics are
added without shifting the pitch of the notes
except at extreme amount settings.
— EXPFM 0 ··· 100
Exponential FM, the type found on most
synthesizers that have FM, causes
OSC 1 to
modulate
OSC 2 such that harmonics are
added by shifting the pitch of the notes. The
audible result is clangorous, inharmonic tones.
— PWM 0 ··· 100
PW Mod by FM causes the pulse width of
OSC 1 to be modulated by OSC 2.
— ASSIGN LINFM
EXPFM
PWID
Turning this soft knob selects the type of FM
to be controlled by the
OSC 2 FM knob: Linear,
Exponential, Pulse Width or combinations:
Linear + Expo, Linear + PW, Expo + PW, all
three together or none.