Spectrum synthesizer for iOS

Downloading

You can download Spectrum from the iOS App Store.

Support

Please use the Spectrum forum thread on the Audiobus forum for support issues.

Usage

Spectrum is a series of Audio Unit extensions. To use them, load them in your favorite AudioUnit host or DAW, such as Audiobus, AUM or Garageband. The Spectrum app itself does not make sound.

General

This section describes functionality and layout common to all the Spectrum audio units.

Accessibility

Spectrum has been designed to work in both landscape and portrait mode on iPhone and iPad devices. As well, the UI responds to Apple’s Dynamic Text so the font size will increase if your device is configured for larger sizes.

Navigation and Interaction

At the bottom of the UI there is a tab bar. Tapping on the word will navigate the main view to that page of controls. Depending on your UI size, some pages will be scrollable.

If your screen is narrow (or phone is in portrait orientation), pages will be reoriented to flow from top to bottom and the XY Pads will not show up.

To reset the value of a control, tap its label.

Common Sections

This section describes the pages common to all Spectrum audio units.

Modulation Page

Spectrum synthesizer modulation page

The modulation page controls the modulation LFO and envelope. As well, there are dedicated modulation targets for the LFO and Envelope allowing for quick modulation without taking up a mod matrix rule.

LFO

LFO Rate controls the speed of the oscillation of the LFO.

LFO Shape controls the main shape of the LFO. Five shapes are available: Sine, Triangle, Pulse, Stepped Triangle, and Clocked Noise.

LFO ShapeMod modifies the shape of the LFO depending on the main shape of the LFO.

LFO ShapeShape Modification
Sine wavefolding
Triangle angle (from downward slope to upward ramp)
Pulse Pulse width
Stepped Triangle Number of steps in LFO
Clocked Noise Noise interpolation (S&H to sharp to smooth)

Envelope

Envelope is a typical ADSR envelope, triggered by an incoming MIDI note. Effect Audio Units can also be routed MIDI notes, which will trigger the envelope.

A, D, S, R, sliders control the envelope attack time, decay time, sustain level, and release time.

Dedicated Mod Targets

By default the modulation sources do nothing. Use either the dedicated mod targets on the Modulation page, or Mod Matrix rules to route modulations sources to destinations.

The Modulation page has 2 dedicated modulation targets for the LFO, and 2 for the envelope. Use the arrow buttons to select a modulation destination, and use the knob to set the modulation depth (positive or negative).

Mod Matrix

Spectrum synthesizer modulation matrix

The Mod Matrix page allows for more complex modulation. Each rule will modulate the modulation target by (modulation input 1) x (modulation input 2) x (depth).

Note that for Audio Units that have XY Touchpads, by default the touchpad does not do anything. You must build modulation rules using the Pad X, Pad Y, and Pad Gate modulation sources.

Instruments

This section will define the functionality and operation of each Audio Unit.

Spectrum

Spectrum synthesizer for iOS

Spectrum is a 8-voice polyphonic synthesizer built around a macro-oscillator with 14 different oscillator models. The Harmonics, Timbre, and Morph knobs control different parameters depending on oscillator model. You can see the entire list on SynthModes.

Main

Pitch adjusts the note of all voices by semitones. Detune adjusts the frequency of all voices by +/- 1 semitone.

Slop simulates the inaccuracies of vintage synthesizers by randomizing the pitch of each voice randomly, which can thicken the sound.

Portamento “slides” voices from their previous pitch to their new pitch. This will affect all voices all the time. Note that portamento time is a modulation target, and MIDI Gate is a mod source, so you can use a mod matrix rule where MIDI Gate positively targets Portamento to make it slide notes only when playing legato.

Bend Range sets the range, in semitones, of the pitch bend wheel.

The reset of this page hosts an XY touch pad. Assign Pad X, Y and Gate to modulation targets in the Modulation Matrix.

Amp Page

Spectrum synthesizer: amp page

Spectrum’s oscillator is passed through a digital simulation of a low pass gate (LPG).

Volume controls the overall amplitude of the synthesizer.

LPG Colour controls how much low-pass filtering is applied to the oscillators when the LPG is closed. At fully clockwise, no filtering is applied making it operate as a VCA.

Attack/Decay/Sustain/Release control the dedicated amp envelope which directly controls the level of the low pass gate. Note that Release time will affect the LPG decay directly, which can make results a little unexpected for those expecting a typical ADSR controlling amplitude.

Pan controls the overall balance between left and right audio outputs.

Pan Spread places makes each voice alternate between left and right outputs.

Left Source and Right Source are quite powerful, but need a bit of an explanation. The oscillator model in Spectrum emits two separate signals, main and aux. These do not always make sense as a Left and Right stereo field. For example, sometimes the aux output is a suboscillator. The Source parameters control what mix of main and aux outputs are sent to the Left and Right output, respectively. Setting these to the same value will create a monophonic result, but setting them differently by a bit or by a lot can create large stereo depth. Note that Left and Right source are also modulation targets, so you can create moving stereo fields by routing to them in the modulation matrix.

Unison will make all active voices play the same note. Used in conjunction with Slop and Pan Spread and you can create typical thick unison sounds.

Polyphony controls how many active voices are available to play notes (or to stack in Unison). Reducing this will reduce CPU load.

Spectrum MIDI CC Map

MIDI CC # Parameter Name
2Pad X
3Pad Y
4Pad Gate
6Algorithm
7Pitch
8Detune
9LPG Colour
10Timbre
11Harmonics
12Morph
13Volume
14Left Source
15Right Source
16Pan
17Pan Spread
18LFO Rate
19LFO Shape
20ShapeMod
22Attack
23Decay
24Sustain
25Release
26Bend Range
30Attack
31Decay
34Sustain
35Release
36Portamento
37Unison
38Polyphony
39Slop

Spectrum: Modal synthesizer

Modal combines an exciter synthesis section generating raw, noisy sounds characteristic of bowing (filtered friction noise), blowing (pitch-controlled granular noise), or striking (stick, mallet, hammer or brush sample playback… or bursts of synthetic impulsions).

These sources, or external audio signals, are processed by a modal filter bank – an ensemble of 64 tuned band-pass filters simulating the response of various resonant structures (plates, strings, tubes…) with adjustable brightness and damping. A stereo ambience reverberator adds depth and presence to the sound.

Modal has three alternative modes: non-linear string, chord tuning for the resonator, and a 2-operator FM voice.

When Modal is loaded into an effect slot, input audio is processed depending on the Input Dest parameter. When Input Dest is set to Env, input is gated by the Bow/Blow envelope. When Input Dest is set to Res, input audio is passed directly to the resonator. As well, an Input Gain control is added when using as an effect that controls amplification of the input signal prior to processing.

Modal MIDI CC Map

MIDI CC # Parameter Name
2Env Shape
3Bow Level
4Bow Timbre
5Blow Level
6Blow
7Blow Timbre
8Strike Level
9Strike
10Strike Timbre
11Geometry
12Brightness
13Damping
14Position
15Space
16Volume
17Mode
18Pitch
19Detune
20LFO Rate
21LFO Shape
22ShapeMod
24Attack
25Decay
26Sustain
27Release
28Input Gain (applicable when effect)
29Input Dest (applicable when effect)

Resonator

Spectrum: Resonator

Resonator employs physical modelling synthesis, sending bursts of noise to a resonator filterbank to generate string-like sounds. When loaded as an instrument, MIDI notes generate bursts of noise sent to the resonator, tuned to the MIDI note.

Resonator can also be loaded into an effect slot, in which case input audio is sent directly to the resonator.

Main

Resonator’s main parameters control different parameters depending on the current synthesis mode. See SynthModes for a full description of parameters under each mode.

The reset of this page hosts an XY touch pad. Assign Pad X, Y and Gate to modulation targets in the Modulation Matrix.

Resonator MIDI CC Map

MIDI CC # Parameter Name
2Pad X
3Pad Y
4Pad Gate
6Structure
7Brightness
8Damping
9Position
10Volume
11Mode
13Pitch
14Detune
15LFO Rate
16LFO Shape
17ShapeMod
18Attack
19Decay
20Sustain
21Release
22Input Gain
23Stereo Spread

Granular

Spectrum: Granular Effect

Granular is a multi-mode granular synthesis effect/delay/sampler/reverb powerhouse.

Main

Main controls Position, Size, Texture, Density do different things depending on the current processing algorithm (which is chosen from the Blend page). See SynthModes: Clouds for a description of the parameters under each mode.

The reset of this page hosts an XY touch pad. Assign Pad X, Y and Gate to modulation targets in the Modulation Matrix.

Blend

Spectrum: Granular Blend page

Blend page controls the current processing algorithm, as well as some effect routing parameters.

The second half of this screen is intentionally left blank for future expansion.

Granular MIDI CC Map

MIDI CC # Parameter Name
2Pad X
3Pad Y
4Pad Gate
5Texture
6Feedback
7Dry/Wet
8Reverb
9Stereo
10Input Gain
11Trigger
12Freeze
13Mode
14Position
15Size
16Density
18Pitch
19Detune
20LFO Rate
21LFO Shape
22ShapeMod
24Attack
25Decay
26Sustain
27Release
28Volume

Credits

Spectrum is based on MIT-licensed open source eurorack modules created by Mutable Instruments.

Special thanks to Trevor Llewellyn for beta testing.