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B.3 ALGORITHMS

APPENDIX B: THE FM TONE SYNTHESIS > B.3 ALGORITHMS · PDF p. 107 · OS 1.10D

# B.3 ALGORITHMS

APPENDIX B: THE FM TONE SYNTHESIS
In some instances, an operator can be both carrier and modulator. This means that it outputs its result as a
tone while also using it to modulate another operator. We have divided the operators into three groups: C, A,
and B (B1 and B2) to lessen the complexity and make the Digitone II easier to use. Since group B consist of
two operators, the parameter controls for B are macro mapped to both operators.
B.3 ALGORITHMS
An algorithm is a set routing, or combination, of the operators. Routing the modulation in different ways
gives you several different methods of applying modulation and naturally different results.
Carrier
Envelope
Modulator
Feedback
Feedback
Modulator
Envelope output
Carrier
Direct output
Output
Algorithm
Modulation
Level
Pictured above is a 2 operator algorithm, which is FM synthesis in its most basic form. The Digitone II has
four operators available which can be routed in many different ways. Different algorithms dramatically
changes the characteristics of the output - for example stacking four operators can lead to very complex
timbres.
The Digitone II has eight different algorithms where the four operators are routed in different ways. For
more information, please see “A.2.1 FM TONE” on page 89. Each algorithm has two carrier outputs (X
and Y) that come from two different operators depending on what algorithm you chose. It is possible to
use the MIX parameter to mix between these outputs so that you can cross-fade between two separate
timbres. For more information, please see “A.2.1 FM TONE” on page 89.
The lines going to X and Y indicate the output from a carrier. There are two different ways a carrier output
is sent to the X/Y outputs: 1. Its amplitude is unaffected by the operator envelope (dotted line). 2. Its ampli­
tude is affected by the operator envelope (filled line). The sound from the X/Y outputs is then routed via the
overdrive to the filters.
B.4 FM RATIOS
Frequency modulation is applied in multipliers of the principal frequency to retain the sounds tonality. These
multipliers are known as ratios. Each operator group’s ratio is a multiplier of the input pitch (note value). The
higher the ratio, the higher the pitch frequency. For example, when applying modulation with a ratio of 1:2
(carrier:modulator), the carrier output resembles a square wave. A ratio of 1:1 sounds like a sawtooth, and
odd numbers can be used for various metallic or other “natural” sounds. In Digitone II, the FM ratios for the
different operator groups works like this:

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