FM4 // manual



FM4 is a four operator FM synthesizer for iPad. Modelled on the popular DX series from the 1980s FM4 provides the same gritty sound, with a modern interface, and without being bound by the limitations of the original units. Every aspect of the hardware — from envelope curves to sine-table sizes and quantisation errors to digital-to-analogue converters — has been meticulously analysed, in order to recreate the raw and dirty sound of the original units. The interface has been carefully thought-out and is simple and intuitive, yet powerful — with every parameter being available on a single page. Soundwise FM4 is capable of producing a multitude of sounds, ranging from punchy basses, crystal-clear bells, and lush pads, to organic textures, striking drums, and piercing leads.

We hope that you will enjoy using FM4 as much as we have creating it!


We assume basic knowledge of synthesizer specific terminology such as waveforms, envelopes and LFOs. The scope of this manual is not to explain synthesizer theory, so if these words sounds unfamiliar to you, you might wanna check out some of the many great resources that are freely available on the internet. Here are a few places to get started:

FM4 uses frequency modulation (FM) synthesis as implemented by Yamaha in their DX series of keyboards. Basically in FM synthesis the timbre of a simple waveform (such as a sine wave) is changed by modulating its frequency with a modulator frequency that is also in the audio range, resulting in a more complex waveform and a different-sounding tone. FM synthesis can create both harmonic and inharmonic sounds. For generating harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of frequency modulation increases, the sound grows progressively more complex. Through the use of modulators with frequencies that are non-integer multiples of the carrier signal (i.e. non harmonic), atonal and tonal bell-like and percussive sounds can easily be created. For a detailed description of FM synthesis we recommend the following resources:


In the case of bugs, freezes, and/or cashes DO NOT open FM4 an attempt to fix it yourself. Instead we recommend that you contact your local Primal Audio dealer or write to us directly at ;)


All product names used in this manual are trademarks of their respective owners, and in no way constitutes an association or affiliation with Primal Audio. All trademarks are solely used to identify the products whose sound was studied during the development of FM4.


FM4 is divided into five main parts:

FM4 overview

  1. the top bar
  2. the left panel
  3. the operator section
  4. the right panel
  5. keyboard

// User-Interface Elements

There are just a few different types of user-interface elements. Some may change their behaviour depending on overlying modes that may be activated.


There are two types of buttons, on/off buttons and selector buttons. On/off buttons are used for activating a mode, e.g. such as key-hold, or for enabling/disabling sections of FM4, e.g. such as the four operator sections. Selector buttons are used to select a setting in an array of settings, e.g. such as the octave.


Knobs and faders controls continous parameters on FM4 such as levels, rates, and amounts. Drag the knob or the fader up/down to increase/decrease the value of the associated parameter. If FINE-MODE is activated the resistance of the knob/fader will be increased which enables the associated parameter to be fine tuned. Use a double-tap to reset the parameter to its default value.


Dials and switches controls discrete parameters such as waveforms, modes, etc. Drag the dial or the switch up/down to increase/decrease the value of the associated parameter. Use a single tap to step through the possible values.

All titlebars that have a small triangle ▼ to the right contains a drop-down popover that can be accessed by clicking on the titlebar.

// Top Bar

The top bar contains the following elements:

FM4 top bar
  1. Routing
  2. Tempo button
  3. Tuning button
  4. Midi button
  5. Main display
  6. Previous/next preset buttons
  7. Save preset button


FM4 routing

A routing is a scheme that defines how the four operators are interconnected and is depicted with a diagram on top of the operator on/off buttons. Each operator can either function as a modulator or as a carrier. A modulator is an operator that moduletes the frequency of other operators and a carrier is an operator that outputs directly to the audio output. In the below routing operator 4 is a modulator (modulating operator 3) and operator 1-3 are carriers.

Tempo button

Opens the tempo pop over which is covered in the TEMPO POP OVER section.

Global Tuning button

Opens the global tuning menu which is covered in the GLOBAL TUNING section.

Midi button

Opens the MIDI menu which is covered in the MIDI section.


The main display shows the currently selected preset, the value of the most recently modified parameter, and the global tempo (in beats-per-minute). Press the display to open the PRESET NAVIGATOR.

Previous/next preset buttons

Use the preset buttons to toggle through the presets in the currently selected bank. The bank can be changed in the PRESET NAVIGATOR. If KEY-HOLD is activated the currently sustained note/chord will be triggered each time a new preset is selected with the preset buttons. You can use this to quickly preview presets while browsing through them.


Opens the the save-preset popover which is described in the PATCHES section.

// Left Panel

The left panel is divided into five sections

FM4 left panel
  1. master section
  2. feedback section
  3. utility section
  4. octave section


The master level section contains a single knob that sets the master level.


The feedback section contains a single knob that sets the feedback amount for operator 4.


The utility section contains three on/off buttons:

The FINE button toggles fine mode on and off. Activate fine mode to fine tune knobs, faders, and operator pitches — as described in the USER INTERFACE section.

The HOLD button toggles hold mode on and off. Activate hold mode to hold chords…

The VELO button switches the velocity on and off.


Transposes the global pitch in whole octaves.

// Operators

FM4 has four operator sections, one for each operator.

FM4 operators

An operator section contains the following controls:


The operator on/off button activates/deactivates the operator.


Sets the output level of the operator.


When an acoustic musical instrument produces sound, the loudness and spectral content of the sound change over time in ways that vary from instrument to instrument. The "attack" and "decay" of a sound have a great effect on the instrument's sonic character. Sound synthesis techniques often employ an envelope generator that controls a sound's parameters at any point in its duration. Most often this is an "ADSR" (Attack Decay Sustain Release) envelope, which may be applied to overall amplitude control, filter frequency, etc. The envelope may be a discrete circuit or module, or implemented in software.

The contour of an ADSR envelope is specified using four parameters:

  1. Attack time is the time taken for initial run-up of level from nil to peak, beginning when the key is first pressed.
  2. Decay time is the time taken for the subsequent run down from the attack level to the designated sustain level.
  3. Sustain level is the level during the main sequence of the sound's duration, until the key is released.
  4. Release time is the time taken for the level to decay from the sustain level to zero after the key is released.
FM4 envelope


Sets the frequency ratio for the operator. Activate FINE-MODE to fine-tune the frequency ratio.


Sets the waveform for the operator. FM4 features eight different waveforms derived from classic FM hardware.

FM4 waveforms


Sets the depth of the lfo amplitude modulation for the operator. When at zero no amplitude modulation will occur.


Sets the velocity sensitivity for the operator. When at zero the operator will not respond to velocity.


Sets the keyboard tracking for the operator. When at zero no keyboard tracking will occur.

// Right Panel

The right panel is divided into five sections:

FM4 right panel
  1. voice section
  2. model section
  3. lfo section
  4. pitch envelope section
  5. arpeggiator section


The voice section has tree parameter. Unison, which stacks and detunes voices for a wider and bigger sound. Glide sets the speed of the glide / portamento. The top bar drop down controls the FM4 voice mode. FM4 offers POLYPHONIC, MONOPHONIC and LEGATO.


The model sections contains a single toggle which changes the model used to generate the sound.

  • Model I is the cleanest but still has notisable noise
  • Model II adds more noise but also some filtering in the top end
  • Model III adds even more noise but the DAC modelling also make this model the darkest sounding


The LFO section is a classic LFO with a WAVE selector knob, RATE knob and a PITCH knob.

The WAVE selector knobs lets you choose beetween five waveforms: TRIANGLE, SAW, RAMP, SQUARE and RANDOM.

The RATE knob controls the rate/speed of the LFO.

The PITCH knob controls the amount the LFO affects the global pitch.

Note: The LFO amount works in conjunction with the MOD WHEEL, meaning that when the modewheel is set to zero, the LFO is not modulating anything.

The LFO can work in three different ways FREE, SYNCED and TRIGGERED accessed by pressing the ▼ in the title bar.

In FREE mode the LFO is monophonic and runs free in the background.

In SYNCED mode the speed of the LFO is synchronized to the GLOBAL TEMPO. In this mode the RATE knob set the time signature.

In TRIGGERED mode the LFO is polyphonic and it's phase is retriggered everytime a key is triggered.


The Pitch Envelope is a simple decay stage with a curve control and a bipolar amount knob.

The DECAY knob changes the speed of the decay

The CURVE knob alters the decay curve from logarithmic to linear.

The AMOUNT knob is bipolar which means that when positioned at 12 o'clock there is zero amount of pitch modulation. Moving it clockwise adds positive amount of pitch modulation, making the pitch drop. Moving it anticlockwise adds negative modulation resulting in a pitch rise.


The MODE knob changes the order of the notes the arpeggiator plays.

  • O = the arpeggiator is off.
  • UP = notes are played from low to high.
  • DOWN = notes are played from high to low.
  • UP DOWN = notes are played from low to high and back to low again.
  • RANDOM = notes are played in a random order.

The RATE knob sets the speed of the arpeggiator.

The RANGE knob sets how many octaves the notes of the arpeggiator will span across.

// Keyboard Section

The keyboard part consists of three components:

FM4 keyboard
  1. pitch wheel
  2. mod wheel
  3. the keys


The pitch wheel bends the pitches of triggered notes up or down. FM4 responds to Pitch Bend MIDI data from MIDI devices or from Inter-app Audio hosts. The pitch bend range can be set in the global tuning popover as described in the GLOBAL TUNING section.


The mod wheel controls the LFO modulation depth of the pitch and/or operator amplitudes. When set to zero the LFO will be disabled. Set the desired LFO amplitude modulation depth individually for each operator with the operator lfo fader as described in the OPERATORS section.


The keyboard contains 25 velocity sensitive keys. Note velocity is determined by the y-axis of keys — the higher the key is triggered the higher the velocity and vice versa. The octave can be set with the octave buttons as described in the LEFT PANEL section.

// Patches


Pressing the MAIN DISPLAY opens up the preset browser.

FM4 preset browser


Pressing the SAVE PRESET button in the TOP BAR opens up the SAVE PRESET POPOVER, where you can name you preset. Alternatively you can also save your preset directly from the SAVE PRESET POPOVER.


If you want to make a new sound from scratch you can load the INIT PRESET in the top right of the preset browser.


In the tempo popover you can either set the tempo manually, tap the tempo or sync FM4 to an external clock signal like a MIDI clock or an Inter-App Audio host like Garageband.

FM4 tempo



FM4 global tuning

The TEMPERAMENT knob sets the global tuning temperament for the FM4. These setting provide very subtle changes in the way the FM4 is tuned. You can read more about tuning temperaments here »

The TUNE knob allows you to set the global tune of the FM4 from -7 to +7 semitones.

The BEND RANGE knob sets the range of the PITCH WHEEL in semitones.


Pressing the MIDI BUTTON opens the MIDI POPOVER:

FM4 midi

Under Devices you can select devices for controlling the FM4. A device can be a midi keyboard, a midi controller, a Wi-Fi MIDI session etc.

Under Channel you select the midi channel you wan't FM4 to recieve on. By default FM4 receives midi on all 16 channels (OMNI).

// INTER-APP Audio

You can load the FM4 from an IAA (Inter App Audio) host e.g. Garageband. When the FM4 is connected to an IAA host, the MAIN DISPLAY shows the icon of the host app and the PRESET BUTTONS in the TOPBAR now works as transport control for the host.

FM4 Inter App Audio

// Changing polyphony

You can change the number of max voices the FM4 uses in the iOS settings app, under FM4. Here you can choose between 4, 8, 12, 16, 20 and 24. The default is 8 voices. All current iPads should be able to run 8 voices, but if you are running many apps simultaneously on an older iPad and you experience audio dropout or clicks, you might want to lower the polyhphony to 4 voices.

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