Entire Engine Overview
Orca has a simple subtractive synthesis structure, being comprised of an oscillator, filter, fixed amp envelope, velocity sensitive filter envelope and an amp module. The monophonic voice engine in Orca runs at twice the hosts sample rate to reduce the aliasing caused by the non-linear analog modeling of the filter and amp modules. A voice priority of newest note is used, and every new note re-triggers the amp and filter envelopes.
Asymmetric Non-linear Analog Modeling
The key component to Orca's incredible sound comes from not only the high quality anti-aliased unison oscillator, but the asymmetric non-linear models of circuit components. For example, the diode limiter in the resonance feedback loop of the filter contains two diodes in a limiting structure as shown below:
When each diode is manufactured there are slight differences in them which is called component mismatch. These small variations mean that circuits like the above are asymmetric, so when you run audio signals through them you get second order harmonics which are very pleasing to the ear. These harmonics are typically labelled "warm" and "rich".
Sometimes mismatch is not a good thing, sometimes you need matched components to achieve a particular task, but other times it's critical to obtaining a great sound. Care has been taken to accurately reproduce the main asymmetries in the signal path to give the filters and amp section in orca its great sound.
The oscillator is a fixed Saw (75% volume) plus Sub Sqr (25% volume) that has continuously smooth varying unison from 1 to 9 voices, and a detune parameter from 0 to 12 semitones. Oscillator sync is available from the mod wheel of up to 48 semi-tones.
The BLEP technique is used to bandlimit steps generated by the analog model of the oscillator. Since the oscillator is passed into several non-linear stages care is taken to have a fairly smooth time domain transition on waveform jumps, which results in less aliasing.
Modeled on two cascaded 2 pole Oberheim SEM low pass filters, it provides 4 poles of attenuation, and has resonance with self oscillation up to 130%. The filter contains 6 non-linearities in total, 2 amplifier non-linearities and 1 diode clipper per 2 pole filter section. The asymmetry of components here is critical and gives Orca most of its tone.
The final section of Orca is the Amp which contains one non-linearity to further shape the sound.
Velocity Sensitive Filter Envelope
The depth of the envelope is changed via velocity with the attack rate remaining constant, so as you play harder it takes longer for the envelope to reach its peak and start decaying.