The MiMi-d

Filter page of the MiMi-d Virtual Analog Plugin, running on the Zynthian platform

For a while I've been checking out various, mostly free, synthesizers in plugin form. However, after a while I get itchy fingers, wanting to change things or fix deficiencies. However, that would create a similar but incompatible instrument, so I've had the thought of designing my own virtual analog synthesizer for some time.

Queue the OB-Xd, which is a widely acclaimed Open Source virtual analog rendering of the Oberheim OB-X polyphonic analog synthesizer.

As people who know me will attest, Oberheim synths are not my favorites, but the OB-Xd has a number of interesting traits:

• Open Source
• Very good sounding Zero Delay Feedback filter
• Low alias oscillators
• Envelopes and filter coefficients are updated at the sample rate, meaning there is none of the characteristic "zipper noise" that plagues most digital synthesizer renditions, and even most digitally controlled analog synthesizers. (In fact, this specific issue was addressed in the MiMi-a by implementing 'ramp generators' to generate the digitally controlled envelopes and LFOs).

So, I looked into using the OB-Xd as a starting point to create a digital synthesizer plugin. Basically, the idea was to keep the DSP code with a few modifications, and some typical Pollux features added (see below).

The first goal is to create a plugin for the Zynthian platform, using the native Zynthian GUI with four knobs per parameter page.

The instrument implements a fairly basic two oscillator synthesizer, with the idea of keeping the number of parameters low, so that it's possible to grasp the instrument easily, thus being more inviting to create sounds on. In essence, it is a combination of the best ideas from the Delta Seven and MiMi-a synthesizers, in digital form.

• Two oscillators, with selectable standard waveforms (saw, triangle, pulse).
• 24 dB/octave low pass filter, with additional 18, 12 and 6 dB/octave modes.
• Rudimentary settable 6 dB/octave high pass filter.
• VCA with overdrive function.
• Cross modulation from one oscillator to the other.
• Audio modulation from one oscillator to the filter.
• Variable level sync between oscillators.
• Two five stage ADSSR envelopes (for the filter and VCA), i.e. your standard ADSR envelope with an additional sustain time parameter so that the sustain phase can function as a second decay.
• Two per-voice LFOs, with optional key sync.
• No modulation matrix! Instead, the output from each modulator is controlled by a dedicated mod level and routing controls.
• Bidirectional filter envelope modulation - no more running out of the low end of the cutoff control when adding a lot of envelope modulation.
• After touch and velocity sensitivity.

The manual can be found in the github repository (see below), or here.

Nitty gritty details

• The OB-Xd envelopes are excellent, but the asymptote level for the attack phase is about 10% over the peak, in contrast to the original CEM 3310 envelope chip which like most analog ADSR envelope generators have an asymptote that is 1.3 times the peak envelope value. This makes for a more linear attack and snappier attack response which is usually more desirable.
• Also, the decay asymptote in the OB-Xd is zero rather than the sustain level, which tends to cause the decay phase to cut off rather abruptly.
• The OB-Xd GUI has a filter tracking parameter which is either on or off, but the underlying parameter is actually continuously variable between 0 and 1, and this will be taken advantage of in the MiMi-d.
• While there is a small advantage of having separate on-off switches for the oscillator and LFO waveforms, it eats up precious space in the 4-parameter-per-page Zynthian UI, and so more traditional waveform selector parameters will be used. It also saves a bit on DSP usage not to have to generate more than one waveform at a time.

New! MiMi-d gets a GUI!

Note that there is no preset/patch management in the GUI, which instead relies on the plugin host to handle this. When running in Jalv (above) Jalv adds preset management in the form of a Presets menu. VST3 and AU hosts generally manage presets themselves as well.

Releases for downloading