synth-engine_head

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 background on the synthesis engine, please read here.

The Components in Detail

Oscillator
Shaper

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Oscillator Oscillator

Oscillator

Shaper Shaper

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
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.

Envelopes Envelopes
Oscillator Oscillator

Envelope B

Envelope C Envelope C

Envelope C

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.

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
Comb Filter
Comb Filter Comb Filter

Comb Filter

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)
State Variable Filter
State Variable Filter State Variable Filter
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
Output Mixer
Output Mixer Output Mixer

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.

Feedback Mixer
Feedback Mixer Feedback Mixer

Feedback 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.

Cabinet
Cabinet Cabinet

Cabinet

Cabinet

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

Gap Filter
Gap Filter Gap Filter

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.

Flanger
Flanger Flanger

Flanger

Flanger

A versatile modulation delay effect.

Echo
Echo Echo

Echo

Echo

A stereo delay effect.

Reverb
Reverb Reverb

Reverb

Reverb

A room and hall simulation.

Macro Controls
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 Hardware Sources

Hardware Sources

Hardware Sources

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

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