B.1 OVERVIEW

APPENDIX B: THE FM TONE SYNTHESIS > B.1 OVERVIEW · PDF p. 106 · OS 1.10D

# B.1 OVERVIEW

APPENDIX B: THE FM TONE SYNTHESIS
APPENDIX B: THE FM TONE SYNTHESIS
B.1 OVERVIEW
The Digitone II's FM TONE machine is a four operator Frequency Modulation (FM) synth in the style of the
classic 80s implementations. However, unlike the early FM synths, the Digitone II use its FM engine more
like a complex tone generator than a complete synthesizer voice (although it does have this capability too).
The Digitone II signal path is more similar to a typical subtractive synth than a classic FM voice.
FM
Overdrive
Multimode
Filter
Base-width
Filter
Amp
The idea of this design is to harness the raw, and often complex, soundscapes of FM synthesis and use a
more well-known and approachable subtractive method for the overall sound shaping.
FM is a synthesis method where you add harmonics to the timbre by using modulation or layering to shape
the sound. You add harmonics by modulating one oscillator’s pitch with the output of another oscillator. FM
works similarly to how you would apply vibrato via an LFO. At slower modulation rates, it is simply vibrato,
but when the modulating frequency reaches audio rate, the vibrato effects becomes a part of the sound and
becomes a timbral effect instead.
B.2 OPERATORS
In FM synthesis the oscillators are called operators. Unlike the analog oscillator, the operator also contains
an envelope and specific input and outputs, making it a sort of macro oscillator. FM combines two or more
operators to generate a more harmonically rich output. An operator that is used to modulate another opera
tor is called a modulator. The operator that generates, or carries, the resulting tone is called a carrier.
Feedback is used to increase the sharpness of a sound and is usually only applied to modulators. The
output of the operator is fed back into itself, resulting in added harmonics. For example, have a look at the
Sounds B001 and B002 in the Digitone II to see how feedback is used to create a basic square and saw
waveform. For more information, please see “A.2.1 FM TONE” on page 89.
Operator
Waveform
Envelope
Feedback
Modulation
input
Output
Amplitude
Wavelength
(frequency)
Output
Modulator
(ratio: 8)
Carrier
(ratio: 1)

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