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xMod Polyphonic FM Synthesizer

xMod is an easy to use full-function synth designed for iOS devices. The sound engine is a polyphonic 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. Two sound banks contain 32 preset patches (programs), with space for 32 of your own. Copy and paste any of our preset sounds into your sound bank and modify the sound easily with a envelope, modulation, and frequency controls. xMod automatically saves your changes.

N.B. Tap "PLAY" to expose the buttons to turn Core MIDI and Audiobus on/off.

Synthesizer Controls

The "<" and ">" buttons cycle through 32 preset and 32 custom (user programmable) sounds. BANK toggles between the two sound banks. C/P copies settings of one patch location that you can paste into another. All settings are saved whenever you paste a sound. N.B. SOUNDS IN THE PRESET SOUND BANK CANNOT BE CHANGED.

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.


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

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

ATTACK and RELEASE are standard envelope generator control parameters.

The envelope generator is triggered when a key is depressed. ATTACK controls the rise time of the sound; RELEASE 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. Slide from one icon to another to mix the two sounds. N.B. ONLY PRESET SOUNDS ARE DISPLAYED IN THE SOUND SPACE.

Top Controls

VOLUME, 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.

SOUND switches between the sound map and the synthesizer control panel. PLAY turns the piano keyboard on/off, revealing the MIDI player and other controls underneath. The current patch (sound) is displayed below these buttons.


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. The keys change when you play a MIDI file. Move the slider to turn off "stuck" notes if and when they occur.

MIDI Player

Tapping the PLAY button reveals a multitrack MIDI player. Choose from one of the many demo files that come with the app, or open a MIDI file downloaded from a website or sent in an email. Files are stored in the app's documents directory, which can be edited via iTunes file sharing. The player supports multiple MIDI tracks; however all tracks are played with the same sound (patch). Percussion sounds are not provided. Depending on the complexity of the song some tracks may have to muted (unchecked from the track list). The sound will skip if there are too many tracks playing.

There are a number of other control knobs/buttons to the left of the track/channel lists.

  • TEMPO lets you change the tempo of the MIDI file. You must tap PLAY to restart if you change the tempo.
  • TRANSPOSE changes the pitch of a MIDI file by up to ± 6 semitones.
  • ARPEGGIO plays depressed keys one at a time in either ascending (knob to the right) or descending (knob to the left) order. The amount left/right controls the rate.
  • MIDI IN/OUT turns on/off the sending and receiving of MIDI control signals.
  • AUDIOBUS OUT turns on/off the synth output to Audiobus. When it is off the synth is routed to the speaker/headphone output.

Final comments:

This app is designed to run on newer iPads. It will however operate at a reduced sample rate and performance level on older devices.

The synth can also be played using an external keyboard like M-Audio's Keystation Mini 32. Using external accessories like iRig MIDI you can play xMod from any external MIDI control device. xMod supports MIDIRTP for wireless control.

If you use this with other Midi apps, Midi Bridge or a similar app is required to route Midi signals between apps.

Turn off multitasking gestures in iPad settings when playing the keyboard for best results.

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