Warp Core is a dual-algorithm phase distortion oscillator with internal and external phase modulation with support for polyphony. This module started as a firmware prototype for a Eurorack DSP platform and is being released as a free VCV Rack module to test the idea and controls, while a hardware version is tentatively in development for a future release.
Inspired by the phase distortion techniques pioneered by Casio's CZ synthesizers, Warp Core starts with a simple carrier phasor - a ramp increasing from 0 to 1 over the course of one period at the fundamental frequency. This phasor is then warped using two sequential non-linear phase distortion algorithms WARP A and WARP B as indicated by the LEDs and with amounts dialed in by the corresponding knobs and CV inputs.
The phasor is also modulated either pre- or post-distortion by an internal sinusoidal modulation oscillator with a configurable fixed ratio to the carrier and a modulation index (amount) controlled by INT PM and PM CV. This modulation oscillator is also summed with the EXT PM input before being applied to the phasor. Applying phase modulation pre-phase-distortion tends to produce much harsher and more intense timbres than applying post-phase-distortion.
The phasor is used to index a sine wave, producing a complex oscillator. As a result of the distortion and/or modulation, the output produces complex harmonics and shapes that would be difficult to achieve with the same fluidity using different techniques.
Finally, the output is optionally "windowed" using a unipolar sawtooth or triangle shape at the fundamental frequency of the carrier phasor. This technique was used in the Casio CZ series to mitigate discontinuities at the start or end of the phase-distorted wave when using specific algorithms. It can be used similarly in Warp Core, but it is also an interesting way to add additional color to the sound, as a form of simple amplitude modulation.
A second output is provided which defaults to a 90-degree shifted (cosine) lookup using the same phasor and applying the same windowing. This does not produce a 90-degree shifted copy of the first output, but rather a similar-but-different complex output waveform, since the phasor wrapping applies differently to the cosine than it does to the sine. (Hint: this sounds pretty cool when used as a "right" output for stereo processing). The second output may also be optionally configured to produce a pure sine wave at the same fundamental frequency as the carrier, a sine sub-oscillator one octave below the carrier, or to direclty output the distorted and modulated phasor.
- TUNE (Knob) – Carrier frequency tuning. 5 Octave range from C1 (32.7 Hz) to C6 (2646.4 Hz)
- INT PM Amount (Knob) – Internal phase modulation amount. While in Edit Internal PM Ratio mode, selects internal PM oscillator ratio.
- WARP A Selection (Button) – Cycle phase distortion algorithm for WARP A
- WARP B Selection (Button) – Cycle phase distortion algorithm for WARP B
- WARP A Amount (Knob) – Phase distortion amount for WARP A algorithm
- WARP B Amount (Knob) – Phase distortion amount for WARP B algorithm
- Indicator LEDs – Indicates selected phase distortion algorithm for WARP A (Blue) and WARP B (Red). While in Edit Internal PM Ratio mode, indicates PM ratio.
- WARP A CV Attenuverter (Knob) – Attenuverter for WARP A CV
- WARP B CV Attenuverter (Knob) – Attenuverter for WARP B CV
- PM Pre/Post (Switch) – Switches between phase modulation being applied before (pre) or after (post) phase distortion.
- Windowing Selection (Switch) – Switches between windowing options for final waveform: OFF (no windowing), Saw, or Triangle.
- WARP A CV (Input) – CV input for WARP A amount, attenu-verted by WARP A CV Attenuverter. Acts as offset for WARP A Amount knob.
- WARP B CV (Input) – CV input for WARP B amount, attenu-verted by WARP B CV Attenuverter. Acts as offset for WARP B Amount knob.
- V/Oct (Input) – Standard volt-per-octave frequency input for the carrier phasor.
- PM CV (Input) – CV input for internal PM amount. Acts as offset for INT PM Amount knob.
- EXT PM (Input) – Audio-rate external phase modulation input. Sums scaled incoming signal directly with (attenuated) Internal PM oscillator before modulating the phasor.*
- Main Output 0° (Output) – Main oscillator audio output
- Aux Output 90° (Output) – Auxiliary audio output. Defaults to "90°" oscillator output as described in the overview. Can be configured to output a pure sine tracking the carrier, a sub-octave sine tracking the carrier, or the distorted and modulated phasor.
* For purposes of SIMD performance optimization, Warp Core processes its DSP in blocks of samples rather than one at a time. The External PM input is internally buffered to maintain true audio rate processing, however as a result of the buffering, there is slight latency on the output. Therefore PM feedback patches will not necessarily sound great. A low-latency mode may be added in a future update to enable improved feedback patching.
There are currently four selectable phase distortion algorithms, one of which may be chosen for each of WARP A and WARP B. The phase distortion is non-linear and applied in series, so using the same algorithm for both will produce interesting results, as will changing the order of the algorithms.
More algorithms may be added in future updates!
Applies a "tension" style curve to the phasor. Amount controls the level of tension. This generally results in a narrowing of the waveform to one side, which turns a sine wave to something a little closer to a narrow pulse wave.
Emulation of oscillator hard-sync by wrapping the phasor back to zero at a frequency greater than the fundamental. Try this by itself with sawtooth windowing for a sound resembling a sweeping resonant filter.
"Pinches" the phasor toward the center of its period, leaving the beginning and end at the minimum and maximum point. This has sort of a formant-shifting effect on the sound.
Wavefolds the phasor by progressively mirroring it when it reaches the max/min level. This is similar to sync but with a wholly different sonic character.
Right click the module panel for a few additional options:
The internal phase modulation ratio can be adjusted by enabling this option and turning the INT PM knob. While this option is enabled, the LEDs indicate the ratio numerator and denominator in binary format. The numbers on the right side of the LEDs are a guide: blue represents the numerator and red the denominator. Pink means that digit applies to both.
Above about 12-o-clock the ratios are integer multiples of the carrier. Below that are other small-integer ratios and divisions of the carrier.
Example:
- Blue LEDs 1 and 2 –
Numerator = 1 + 2 = 3 - Red LED 1 –
Denominator = 1 - Ratio =
3:1(third harmonic)
As this module is intended to be a prototype for a future hardware module, the UI for this setting is meant to be something feasible to implement in hardware , e.g. by holding one of the buttons and turning the INT PM knob.
Instead of using the buttons on the panel you can also directly select the algorithms used for each of the WARP A and WARP B from a context menu.
The Aux Output (labeled 90° on the panel) may be configured to a few alternate modes:
- 90° – Uses the same phasor as the main output to index a cosine instead of a sine, producing an alternate pseudo-phase-shifted output. Try this as the right side of a stereo pair, with the main output as left!
- Sine (Unison) – Outputs a pure sine wave at the same frequency as the carrier phasor. Useful for mixing in some additional oomph to thicken up timbres that reduce fundamental harmonic energy.
- Sine (Sub) – Outputs a pure sine wave one octave below the carrier phasor. Also useful for mixing with the main output to thicken up the sound.
- Phasor – Outputs the distorted and modulated phasor directly. Useful for visualizing and understanding the effect of each algorithm, and possibly for other creative patching!
As phase distortion is inherently a nonlinear technique, this module is internally oversampled to mitigate aliasing. The default is 4x oversampling but you may use the context menu to configure the level of oversampling from 1x (none) to 16x. The CPU usage will increase the higher you go, especially when using the module for polyphony.