The Synth Engine

The C15's first synthesis engine is named "Phase 22". The following block diagramm shows its components and signal flow. Please note that two levels of detail are available.

Block Diagramm
Blockdiagramm

Click to enlarge.

Design Concepts

The design goal for Phase 22 is to take advantage of digital synthesis algorithms in order to build an expressively playable instrument that has a strong individual character. At the same time the set of parameters should be small enough to be easily accessible.
The core structure is a phase modulation synth that is based on two sine oscillators and two sine shapers only. Their signals can be passed through a comb filter, a state-variable filter, and a chain of five effects.
The Comb Filter is a tool for complex spectral shaping and also works as a resonator. The State Variable Filter can be applied flexibly for subtractive filtering.
The signal routing is determined by the settings of the Output Mixer and the Feedback Mixer. The feedback bus can be used to create feedback loops between different function blocks.
The filters and effects have an important influence on the feedback behavior: their amplitude and phase responses determine the frequencies where the feedback can result in self oscillation.

For more background on the synthesis engine, please read here.

The Components in Detail

Oscillator Shaper

Oscillator/Shaper groups in the graphical UI
(click to enlarge)

Oscillator Oscillator

Oscillator Group

Oscillator Group

Osc-Dia Osc-Dia

Oscillator signal without modulation, with a small amount of self modulation, and with stronger self modulation

Shaper Shaper

Shaper Group

Shaper-Drive-Dia Shaper-Drive-Dia

The influence of the Drive parameter of the Shaper.

Shaper-Fold-Dia Shaper-Fold-Dia

The influence of the Fold parameter of the Shaper.

Shaper-Asymetry-Dia Shaper-Asymetry-Dia

The influence of the Asymetry parameter of the Shaper.

Oscillator A, B

The two sine wave oscillators are the only signal sources. With a random frequency fluctuation they can also produce tunable noise. They are equipped with three phase modulation inputs for:

  • self modulation
  • modulation from the other oscillator
  • modulation from the feedback (FB) signal

Here you see the oscillator signal for different amounts of self modulation.

Shaper A, B

The signal of each oscillator is processed by a wave shaper. The shaping curve is a sine function with adjustable foldback and asymmetry. The mix amounts of the shaper signal can be adjusted individually for each phase modulation branch and for the output.
The Shaper block also contains mixing points for the feedback signal and the result of a ring modulation between the output signals of the branches A and B.

These diagrams show the output signal depending on Drive, Fold and Asymetry.

Envelopes Envelopes

Envelope groups in the graphical UI
(click to enlarge)

Oscillator Oscillator

Envelope B Group

(the same as Envelope A Group)

Envelope AB Envelope AB

The four segments of the envelopes.

Envelope C Envelope C

Envelope C Group

Envelope C Dia Envelope C Dia

Examples of Envelope C shapes with different polarities of the Breakpoint and Sustain levels.

Envelope A, B

These two ADBDSR envelopes are the control sources for:

  • the output amplitude of the Oscillator-Shaper branch
  • the Oscillators' phase modulation depths
  • the Drive (input gain) of the corresponding Shaper

There is detailed control of how the velocities and the key position influence levels and times.

This diagram shows the segments of the envelopes.

Envelope C

The third ADBDSR envelope has bipolar Breakpoint and Sustain levels. It can modulate:

  • the pitches and fluctuations of Oscillator A and B
  • the feedback signal
  • parameters of the Comb Filter and the State Variable Filter

This diagram shows possible shapes of Envelope C.

Comb Filter

Comb Filter group in the graphical UI
(click to enlarge)

Comb Filter Comb Filter

Comb Filter Group

Comb Filter Dia 1 Comb Filter Dia 1

Signal flow of the Comb Filter

Comb Filter Dia 2 Comb Filter Dia 2

Basic frequency responses of the Comb Filter.

Setting the Decay to negative values has similar results like the inverted mix.

Comb Filter

The Comb Filter contains:

  • a precisely tunable delay (Pitch)
  • a second-order Allpass filter
  • a control for the Decay time of the impulse response
  • a lowpass (Hi Cut) in the feedback loop
  • a delay modulation by the Oscillator signals, similar to phase modulation (PM)

Here you see a block diagram of the Comb Filter and these graphs show two basic frequency responses.

State Variable Filter

State Variable Filter group in the graphical UI
(click to enlarge)

State Variable Filter State Variable Filter

State Variable Filter Group

State Variable Filter Dia 1 State Variable Filter Dia 1

The signal flow of the State Variable Filter.

State Variable Filter Dia 2-1 State Variable Filter Dia 2-1

Frequency responses of the State Variable Filter with Parallel set to 0 %

State Variable Filter Dia 2-2 State Variable Filter Dia 2-2

Frequency responses of the State Variable Filter with Parallel set to 100 %

State Variable Filter

The State Variable Filter has a variable 4-pole structure with:

  • two internal 2-pole filters with splittable cutoff frequencies (Spread)
  • crossfade between serial and parallel modes
  • crossfade between lowpass, bandpass, and highpass mode
  • cutoff frequency modulation (FM) by the Oscillator signals

Here you see a block diagram of the State Variable Filter. Additional graphs show the frequency responses in serial and parallel mode.

Output Mixer

Output Mixer group in the graphical UI
(click to enlarge)

Output Mixer Output Mixer

Output Mixer Group

Output Mixer Dia Output Mixer Dia

Signal Flow of the Output Mixer

Output Mixer

The Output Mixer creates a stereo sum of the signals from Oscillator/Shaper A and B, the Comb Filter and the State Variable Filter. It includes a sine shaper for the sum signal of each voice.

This diagram shows the signal flow of the Output Mixer.

Feedback Mixer

Feedback Mixer group in the graphical UI
(click to enlarge)

Feedback Mixer Feedback Mixer

Feedback Mixer Group

Feedback Mixer Dia Feedback Mixer Dia

Signal flow of the Output Mixer

Feedback Mixer

This mixer combines the following signals for the feedback bus: the outputs of the Comb Filter and the State Variable Filter and the output of the effects chain, with separately adjustable amount of Reverb. It includes a sine shaper for the sum signal.
The feedback bus signal (FB) can be used for the phase modulation of the Oscillators and it can be injected into the signal path of the Shapers for direct audio feedback.

This diagram shows the signal flow of the Feedback Mixer.

Flanger

Flanger group in the graphical UI
(click to enlarge)

Flanger Flanger

Flanger Group

Flanger Dia Flanger Dia

Block diagram of the Flanger

Flanger

A modulation delay and a 4-pole allpass creating a wide range of chorus, flanger and phaser effects .

This diagram shows the signal flow of the Flanger.

Cabinet

Cabinet group in the graphical UI
(click to enlarge)

Cabinet Cabinet

Cabinet Group

Cabinet Cabinet

Block diagram of the Cabinet

Cabinet

A distortion unit (sine shaper) with pre and post filtering that can sound simular to a guitar amp driving a speaker.

This diagram shows the signal flow of the Cabinet.

Gap Filter

Gap Filter group in the graphical UI
(click to enlarge)

Gap Filter Gap Filter

Gap Filter Group

Gap Filter Dia Gap Filter Dia

Block diagram of the Gap Filter

Gap Filter Dia Gap Filter Dia

Frequency responses of the Gap Filter

Gap Filter

A 4-pole lowpass and a 4-pole highpass in a parallel or serial structure, creating a flexible band-rejection or band-pass filter.

This diagram shows the signal flow of the Gap Filter and here you see some of its frequency responses.

Echo

Echo group in the graphical UI
(click to enlarge)

Echo Echo

Echo Group

Echo Dia Echo Dia

Block diagram of the Echo.

Echo

A stereo delay effect.

This diagram shows the signal flow of the Echo.

Reverb

Reverb group in the graphical UI
(click to enlarge)

Reverb Reverb

Reverb Group

Reverb

A room and hall simulation.

Macro Controls

Macro Controls group in the graphical UI
(click to enlarge)

Macro Controls Macro Controls

Macro Controls

Macro Controls

The four Macro Control sliders A, B, C, D can work as master controls for freely assignable target parameters with individually adjustable amounts.

Hardware Sources

Hardware Sources group in the graphical UI
(click to enlarge)

Hardware Sources Hardware Sources

Hardware Sources Group

Modulation Scheme Modulation Scheme

Modulation Scheme

Hardware Sources

Eight physical control elements can be mapped to the four Macro Controls:

  • four Pedals
  • two Ribbons
  • the Pitchbender
  • global Aftertouch

This illustration shows how the modulation works.