Korg iM1
Remake of Korg M1 Digital Synth from the 1980s. Huge preset collection, and powerful synthesis engine.
App type: App and AUv3
App store link: 
Developer: korg_im1
Developer AB Forum Handle:
Additional Info
Yes, it is a synth. Actually 16 synths, grouped in 8 pairs. Each synth is a typical subtractive synth including Oscillator, VDF (low-pass filter), and VDA (amplifier). The Osc is the ROMpler part, because you can select from 900 different waveforms. Some are traditional synth waves (saw, square, pulse); some are generated (DWGS); some are characteristic PCM instrument sound samples; and some are multisampled (for different note ranges). Each Osc, VDF, and VDA has an Envelope Generator (EG). Each pair of Oscs, VDFs, and VDAs has an LFO (MG) with a wide range of options. Finally, the output of each pair of synths is sent through two IFX (insert effects).
You can either play a single synth pair (a Program), or create a Combi with up to eight Timbres (Programs) playing simultaneously. Each Timbre can be limited to a specific range of notes and velocities. If you tap the Combi button and open the browser, it offers Combi presets like FilmScore.
The envelopes are interesting, because they are more than the typical 4-parameter ADSR. Each of the four stages has both a time and an end level, so they are capable of generating a wide variety of effects. You adjust the envelopes graphically, by dragging the endpoints. The osc pitch and filter cutoff envelopes are bipolar; they can go above and below the nominal value. Both envelope amplitude and evolution speed can be modulated by velocity or note value. Note that the "Time" modulation is actually speed; positive modulation is faster. You'd likely never use all these features at once, but they all have their uses. It is somewhat inconvenient that every envelope is independent; to have the same envelope for both filter and amp, you'd have to set them up the same. And there are no handy editing modes (copy, paste). Apparently, the VST plugin has some useful editing features, but iM1 is very basic.
Filter keyboard tracking needs a bit of explanation. When tracking is set to zero, tracking is perfect; cutoff varies with each note (this differs from most synths). Setting tracking to -50 cancels this effect, making the cutoff remain constant for all notes. Tracking between zero and -50 is likely the choice for most conventional instrument sounds. The filter cutoff control is uncalibrated, but a setting of 19 puts cutoff at the note pitch. Each change of about 9 units changes cutoff by an octave. If the tracking setting is not zero, this description applies at the configured Center Key. I tested these features using a saw wave, with filter resonance set to 99, so there's a sharp peak at cutoff.
External control capabilities are limited. Each Program can respond to CCs 1, 2, and 92, to alter amplitude and frequency of the MG on Osc, VDF, and VDA respectively. The AUv3 parameters include eight Performance Parameters that can alter the sound of each Timbre. They are pre-defined and include Osc Balance, Filter Cutoff, etc. These are bias amounts that offset the preset values of those parameters. The Performance Parameters can also be accessed directly by built-in MIDI CCs.
References:
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