Sytrus - Basics of FM Synthesis and the Modulation Matrix

Basics of FM Synthesis

Most of today's software synthesizers use a process known as 'subtractive synthesis' - a spectrum-rich oscillator (saw, square, triangle, etc.) is processed with a low-pass, band-pass, or high-pass resonant filter to produce the final sound. FM (Frequency Modulation) uses a different approach - pure tones (sine waves) processed in such a way that additional harmonics are created (one sine wave modulates the frequency of another) and added to the signal to produce the final sound. Unlike sub-synths, the basic module of the FM synth is called an 'operator', which includes a pure tone oscillator (sine wave) and an articulation section. At the basic level the articulation section is at least a simple ADSR volume envelope.

FM synthesizers contain two or more operators (Sytrus supports up to six operators). When an operator is connected to the input of another, a pitch (frequency) modulation occurs (see diagram, above). The modulating operator is known as the 'modulator', while the modulated operator is called a 'carrier' (in Sytrus a single operator can act both as a carrier and modulator).

Sytrus Implementation

Sytrus offers everything found in classic FM synthesizers and supports up to six operators and a modulation matrix, in which you can define the synthesis algorithm. Sytrus also includes a set of advanced features which allow you to create many unique sounds:

NOTE: If you want to create your own Sytrus patches or modify existing ones, it is recommended to check the Sytrus processing diagram. It describes in detail how the Sytrus modules are processed and mixed:

The same diagram is quickly accessible from a button on the Sytrus interface:


Modulation Matrix

The modulation matrix in Sytrus allows you to set up the FM synthesis algorithm and to adjust the operator send levels for effects and filter modules, and panning and 'dry' output levels.

Each knob controls a specific function or mapping (as explained below). The neutral position for each knob is the middle Use Alt + Left-click to reset a knob to its neutral position. You can also Right-click a knob to quickly mute/unmute, while preserving the knob value (this feature is useful when testing and tuning a patch)

The modulation matrix comprises several discrete parts. Below we will take a more detailed look at each section and its applications:

Modulation Setup

The FM/RM section sets up the modulation algorithm of Sytrus. Each row represents an operator and determines which operators will modulate it and by how much.

The knobs determine the amount of modulation - if the value is negative (turn the wheel at left) the modulation phase will be inverted. At the neutral level (Alt + Left-click a knob), there will be no modulation.

It is not necessary for an operator to be modulated in order for it to produce output.

Note - If you're unsure of the purpose of any knob in the matrix, simply hover over it with your mouse and check the FL Studio Hint Bar.

Here are few very basic algorithms and their representation in the Sytrus matrix:

In these examples, Operator 1 is used as a carrier while the rest of the active operators are modulators. You can modulate an operator by itself (in example 3 - row 2, column 2 - modulating operator 2 by itself), thus creating a feedback effect.

Note that the carrier must be assigned an output, either directly (as in the examples), or via the filter modules. For more information on filters and output assignment, see the other two matrix sections covered below.

Sytrus also supports RM (ring modulation) interaction between operators. To see and adjust the RM setup, click the FM/RM switch at the bottom of the matrix:

Please keep in mind the switch only affects the modulation setup section of the matrix, as the rest of the settings (pan, FX send, filter send, etc.) are shared among the FM/RM setups.

Filter Send Levels

This section adjusts the amount of signal sent from each operator to the filter modules. Negative values will send an inverted signal to the filter modules.

Sytrus includes three filter modules. Each row in the section represents one module. If you want to send 50% of the output from operators 3 and 4 to filter section 2, adjust the knobs as follows: in row F2 (filter section 2), adjust the knob in column 3 (operator 3) to 50%; in the same row adjust the knob in column 4 (operator 4) to 50%.

To reset a knob to a neutral position, use Alt + Left-click.

Note that to hear the output of the filter sections, they need to be assigned an output. To learn more, see the Pan, FX Send and Output section below.

Pan, FX Send and Output

In this section you can define the panning, effects send and output amounts for each of the operators and the three filter sections.

The first column sets the panning of its corresponding module (operator or filter). The default position is center.

The second column defines the amount of signal sent to the effects module. If you set this knob to a negative value, an inverted signal will be sent to the effects module. In the default neutral position (middle), no signal is sent to the effects module.

The third column defines the output amount for its corresponding module (operator or filter). A negative value will send an inverted signal to the output.

IMPORTANT: Neither the operators or filter modules in Sytrus produce audio automatically. You'll need to use the matrix to rout them to the output stages as follows:

1. Assign an output level for the module from the matrix section.

2. Assign an effects send level for the module (the effects module is automatically sent to the output).

3. (operators only) Assign the operator a filter send level via the Filter Send Levels matrix section (see above). The filter module you send to needs to 'reach' an output by itself.

4. (Filters only) Use the Send to Next knob (see the filter module page for more info).