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xMod Polyphonic FM Synthesizer

xMod is an full-function polyphonic synthesizer designed for iOS devices. The sound engine is a triple-oscillator cross-modulated FM synthesizer. FM is widely regarded as the most efficient way of generating complex "organic" musical sounds. The complexity of programming FM synthesizers is solved in xMod. In fact almost any setting works in xMod.

xMod runs on both iPhone and iPad.

Synthesizer Controls

The "<" and ">" buttons cycle through 32 preset and 32 custom (user programmable) sounds. C/P copies settings of one patch location that you can paste into another. N.B. Sounds in the preset sound bank (the first 32 sounds) cannot be changed. There are currently 2 voices per key, which can be changed using the V1/V2 button, and blended with the rotary control knob.

At start up for the first time, a file called "patches.xml" is created that contains all of the preset patches. To return to the "factory settings" remove this file from the app's documents directory using iTunes File sharing. When you restart a new copy will be created. Each time your settings are saved this file is automatically updated. You can also edit this file manually if you like. (Make sure the app is not running so your changes are not over written.)

xMod uses FM (frequency modulation) synthesis to generate complex sounds from three cross-connected oscillators. Oscillator A sets the fundamental frequency. The refeence frequency is A=440 Hz with the TUNE knob straight up. This is the default value. The tuning range for A is 415-465 Hz.

N.B. Double tap any control knob to return to the default setting for that control.

FREQ A shifts the pitch of oscillator A in octave increments up (knob right) or down (knob left). Oscillators B and C run at frequency ratios B:A and C:A relative to A, which are set by the RATIO B:A and RATIO C:A knobs. Although the knobs seem to operate in a continuous manner the frequency ratios are actually quantized to the following values {0, 1/3, 1/2, 2/3, 1, 4/3, 5/3, 2}. This insures the oscillators produce harmonic overtones, which makes it easier to program the synthesizer. Oscillators B and C also have DE-TUNE knobs that add up to ± 0.05 to the frequency ratios to create vibrato, warmth, and non-harmonic overtones.

ATTACK, DECAY, ENVELOPE and GAIN/MOD are controls common to all three oscillators.

GAIN/MOD is the constant output level of the oscillator.

ATTACK and DECAY (really RELEASE in ADSR) are standard envelope generator control parameters.

The envelope generator is triggered when a key is depressed. ATTACK controls the rise time of the sound; DECAY is the rate at which the sound decays after the key is released. ENVELOPE controls the amplitude of the envelope signal, which is added to the output level of the oscillator. When it is straight up there is no change in the output level when a key is pressed. With the knob to the right, the output level increases when a key is depressed and decreases when it is released. With the knob to the left the control signal is inverted, decreasing when a key is depressed and increasing when it is released. Moving the knob further left/right increases the amplitude range.

Push buttons A, B, and C route the oscillators to the output. When a button is pressed the envelope is over riden so the sound amplitude/level decays to zero. MODULATION MATRIX is a 3x3 array of push buttons that cross-connect the three oscillators. For example, the button B~A connects the output of oscillator B with the input of oscillator A so that B modulates A. Oscillator B's MOD/GAIN controls the modulation index. A~A feeds the output of A back to itself, which is a simple way to generate triangle waveforms, or even noise if the modulation index is high enough. In all there are 2^9 = 512 unique ways to connect the three oscillators.

Sound Space

The sound space displays all of the preset sound patches in a 2-D "space" of icons. Sounds that are similar in timbre are near each other. Tap an icon to move to that sound.

Global Controls

REVERB DELAY, and REVERB DEPTH are global settings. BEND changes the pitch of a note when you move your finger left/right after the key is depressed. Try it with stringed instruments. TREMOLO changes the volume of a note when you move your finger up/down along the key. PORTAMENTO slides from note to note. ARPEG generates arpeggios. N.B. TREMOLO, BEND, and ARPEGGIO only work with the device keyboard. They cannot be controlled by an attached MIDI keyboard. The app is Audiobus MIDI compatible.


The bottom right slider changes the size of the keys; the bottom left slider shifts the physical keyboard up-down. The lowest note is A0 and the highest is B7. As mentioned above the keys are after-touch sensitive. Move the slider to turn off "stuck" notes if and when they occur.

Here are some classical music selections played on xMod (older screen layout).

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