Here’s some notes I compiled while learning about the ACD, FAT and VAP engines. Take this with a grain off salt, it might contain some misconceptions.
FAT Engine
A 3 oscillator synth with one LFO only, and no mod matrix.
All 3 oscillators share the same waveform, which is controlled by Timbre and Brightness. There is a sub oscillator for each oscillator.
Timbre morphs the waveform from a saw wave (0) to a square wave (50), going further towards 100 the pulse width changes to a ratio of maybe 95:5
Brightness blends in 3 sub oscillators with square waves. It’s a bipolar control, values < 0 mix square waves one octave below the main oscillators, values > 0 mix in square waves one octave above.
Fatness detunes the 3 oscillators and sub oscillators against each other.
FatnessLFO modulates Fatness with the LFO. However, this has only a noticeable effect when Fatness is turned up a fair bit.
FAT has no mod matrix, just 3 fixed modulations: Filter Env Amount, FatnessLFO and FilterLFO
If it had a proper mod matrix, you could kick the Fatness, Brightness, Pulse Width with an envelope.
Comparison of ACD, FAT and VAP
| Engine: | ACD | FAT | VAP |
|---|---|---|---|
| Oscillators | 1 | 3 | 2 |
| Waveforms | Saw + Pulse¹ | Saw↔Pulse | Tri↔Saw↔Pulse |
| Noise |  |
|
|
| PWM | Env/LFO | – | Env/LFO |
| Oscillator Tune | – | – | ± 36st |
| Sync | – | – | |
| Sub Oscillator(s) | 1 | 3 | – |
| Filters | Low Pass (SVF 12/24, RD3) | Low Pass (MG, OB 12/24) | all filter types except RD3 |
| Pitch Tracking | |
– | |
| Mods | 4 fixed modulations | 3 fixed modulations | 6 slots |
| Envs | 2 | 2 | 3 |
| LFOs | 1 | 1 | 2 |
| LFO Sync | – | – | |
| Strengths | Sh-101 & TB-303 emulations, also convincing Juno sounds | fat sounds, like Model D with 3 oscillators, or superset sounds | complex sounds with modulations and a wide variety of filter types |
¹ Saw and Pulse Wave can be mixed together, like in the Roland Sh-101, Juno-6, Juno-60 and ⍺Juno.
Filters
There are various filter types, and all except the MG and RD3 are available in a 12db and a 24db variant:
MG appears to be a Moog transistor ladder filter, it’s only available as a 24dB low pass filter.
RD3 sounds like an emulation of the filter found in the TB303, a 24dB diode ladder low pass filter (only available in the ACD engine).
OB is described as a model of an Oberheim filter, it comes as low pass, band pass and high pass, each one is available as 12dB and 24dB variant.
SVF is a “State Variable Filter”, with models for low pass, band pass, high pass, and notch filter, again with 12dB and 24dB each.
I’m not sure what the SVF and OB filters are modeling, so I’ll avoid speculation here.
Which filters are available in which engines?
- WAVS, VAP, WTFM and PHZ have all filter types except for RD3
- ACD: Low Pass SVF (12/24) and RD3
- FAT: Low Pass MG and OB(12/24)
- PMD: Resonator
- GRAIN: 12dB SVF (Low Pass, Band Pass, Hight Pass, Notch)