Full Bucket Music: Nabla ∇

German programmer and software engineer Björn Arlt began building VST plugins under the name Full Bucket Effects in 2010. Being a musician, and a synth enthusiast from a young age, he was drawn to analysing and building software synths. He now offers both synth emulations and effects as freeware through his website.

The Nabla is a software instrument based on the KORG Delta DL-50 Strings Synthesizer from 1979. At the heart of Nabla is a polyphonic oscillator followed by a filter and an amplifier. An ADSR envelope to control the filter cutoff and/or the amplitude, plus a LFO for vibrato or cutoff modulation.

The Synthesizer section is equipped with four Signal Generators providing square waves in four different octaves (16′, 8′, 4′, 2′) plus an additional Noise generator. The level of each signal is set by a dedicated fader. Note that the Signal Generators are realised by using only one Master oscillator and a ‘Top-Octave Synthesizer’, followed by a cascade of frequency dividers. This means that the four square wave signals are totally correlated. For example the rising edge of the 16′ square wave always falls together with the rising edge of the 8′ square wave. The same is true for the 4′ and 2′ waves as well as for keys (i.e. C to B) of different octaves.

Using a Zero-Delay Feedback design for the filter, the whole polyphonic signal pathway now becomes paraphonic, unless you switch to ‘real’ polyphony in ‘GOD Mode’. The VCF can be run in two modes: Lowpass and Bandpass. It features a Resonance parameter (like in the Delta without self-oscillation), a control for positive or negative modulation by the ADSR envelope, and a Key Follower switch, tracking the highest key that is currently pressed. And you can set the Cutoff Frequency, too. 

Features:

• Paraphonic Synthesizer and Strings sections
• Up to 64 voices polyphony
• Synthesizer section:
• Four band-limited frequency divider-driven signal generators
• Additional noise generator
• 4-pole zero-delay feedback bandpass/lowpass filter
• Strings section:
• Two band-limited frequency divider-driven signal generators
• 2-band equaliser
• Ensemble effect
• Flexible pitch/filter modulation by LFO or noise
• Tweaks (not to be found in the original Delta):
• ‘GOD Mode’ to provide true polyphony
• Panning for Synthesizer/Strings sections
• Built-in phaser and delay effects
• Double precision audio processing
• All parameters can be controlled by MIDI controllers
• Plugin supports Windows and macOS (32 bit and 64 bit)