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.
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 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.
These diagrams show the output signal depending on Drive, Fold and Asymetry.
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.
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.
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
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
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.
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.
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.
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.
A 4-pole lowpass and a 4-pole highpass in a parallel or serial structure, creating a flexible band-rejection or band-pass filter.
A stereo delay effect.
This diagram shows the signal flow of the Echo.
A room and hall simulation.
The four Macro Control sliders A, B, C, D can work as master controls for freely assignable target parameters with individually adjustable amounts.
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.