Drop Forge is a drum synthesizer that generates every sound in real-time from mathematical models of analog percussion circuits. There are no samples. The synthesis engine combines four exciter types with three filter types, producing voices that range from tight 808 kicks to chaotic FM metalwork to resonant bell tones.
The instrument is built with the Apple Pencil treated as a first class input device. Many of its features are optimized for pencil movement. When you strike a pad with the Pencil, six continuous dimensions are captured at 240Hz: force, tilt, azimuth, barrel roll, and position within the pad (X and Y). Each dimension maps to a different aspect of the sound, so no two hits are identical unless you want them to be. Finger multi-touch works simultaneously for players without a Pencil, though with fixed expression values.
Drop Forge runs as a standalone app on iPad (iPadOS 18 or later) and as an AUv3 instrument plugin inside hosts such as AUM, Logic Pro, and Ableton Live on Apple Silicon Macs. A single purchase covers both platforms.
The screen is divided into three vertical regions in a fixed landscape layout. The arrangement does not change or reconfigure; every module is always visible and accessible.
KIT displays the loaded kit name, the selected pad, and a preset browser showing all 12 voices in the current kit. Tap any voice to reassign it to the selected pad. The KITS dropdown loads factory or custom kits. The MEMORY button opens the scene management overlay.
DISPATCH shows MIDI routing status: connected destinations, active channel, and the inter-app / hardware MIDI indicators.
MIXER provides 12 vertical faders with phosphor-green level meters, one per pad. A RESET button returns all faders to unity.
The upper portion is the pad grid — a 4-column by 3-row array of touch-sensitive pads rendered by the Metal shader. Each pad shows its label, mute (M) and flam (F) toggles, and a settings dot that opens a per-pad popover for DECAY and LEVEL adjustments.
The lower portion is the PATTERN section containing the step sequencer, transport controls (PLAY, REC, CLR, FILL), beat division pills (TIME, DIV, pattern slots A–D), and the step grid.
FX contains reverb and delay controls with per-pad send amounts, a tunable comb filter section, and the global FX on/off toggle.
CONTOUR is the envelope editor for the selected pad. It displays a parametric amplitude curve (DECAY, SHAPE) or a drawn contour if one exists. Controls include DRAW, CLEAR, pitch sweep (DEPTH, TIME), ATTACK, and mode toggles for CYCLE and HOLD.
PULSE shows the current BPM with increment/decrement buttons. When synced to a host DAW, the display switches to HOST and tracks the host's tempo.
Tap or strike any pad to trigger its voice. The instrument responds to both Apple Pencil and finger input simultaneously. With a Pencil, the following dimensions shape each hit in real-time:
| Force | Controls velocity and harmonic intensity. Light touches produce soft, thin hits. Firm strikes produce loud, rich ones. |
| Tilt (altitude) | Modulates decay time. A steep pencil angle shortens the envelope. A flat angle lengthens it. Each hit can have a different decay without touching any controls. |
| Azimuth | Affects tonal color and FX send routing. Rotating the pencil around its vertical axis shifts the character of the sound. |
| Roll | Available on Pencil Pro. Barrel roll controls noise color and filter texture, adding grit or smoothness depending on the rotation angle. |
| Position X | Tunes pitch within the pad, spanning one octave above and below the base frequency. The left edge is one octave down; the right edge is one octave up. |
| Position Y | Sweeps the filter cutoff. Moving vertically within the pad opens or closes the filter in real-time as you play. |
After striking a pad, a quick swipe before lifting the pencil triggers a post-hit modulation. The direction of the swipe determines the effect:
| Flick up | Bandpass filter sweep from 2kHz to 8kHz, rising through the spectrum. |
| Flick down | Instant choke. The voice is killed within 5 to 35 milliseconds. |
| Flick left | Pitch bend down by half an octave, then snap back. |
| Flick right | Pitch bend up by half an octave, then snap back. |
A high-speed flick in any direction also fires a ghost retrigger at 25% velocity — a faint echo of the original hit.
Drawn Contours. Tap the DRAW button in the CONTOUR module or the pad's settings popover, then draw a shape on the pad with the pencil. The stroke becomes the pad's amplitude envelope, replacing the parametric decay curve. CLEAR removes the drawn contour and restores the parametric envelope. Drawn contours are saved with scenes and custom kits.
Finger touches trigger pads with fixed expression values: 0.75 normalized force, neutral tilt, and center position. Multi-touch is supported, so you can play chords or rolls with your fingers while the pencil handles expressive leads. On iPad Pro models, finger force data is available and maps to velocity.
Each pad has an M (mute) button and an F (flam) button visible at the bottom of the pad cell. Muting a pad silences it from all trigger sources — pencil, finger, sequencer, and MIDI. Flam inserts a grace note 20 milliseconds before the main hit at 35% velocity, creating a ruff or drag effect.
Drop Forge ships with 20 factory kits containing 240 voice presets. Each kit assigns 12 voices across the pad grid. Factory kits load clean every time and cannot be overwritten.
After reassigning voices to pads using the preset browser, you can save the configuration as a named custom kit through the MEMORY overlay. Custom kits appear in the KITS dropdown alongside factory kits and can be loaded, renamed, or deleted. They persist across app launches.
Each voice combines one exciter with one filter, along with 25 or more adjustable parameters. The exciter generates the initial energy; the filter shapes its spectral character and decay.
| Pulse | A single-sample impulse. The classic analog drum trigger used in 808 kicks and toms. |
| Noise | White or pink noise. The basis for snares, hi-hats, shakers, and brushes. |
| FM Burst | A short burst of frequency modulation. Produces tonal, metallic, or bell-like attacks depending on the ratio and index. |
| Tri-Sine | A triangle-wave oscillator with tanh soft-clipping, modeling the 909's back-to-back diode clipper circuit. Generates continuous quasi-sine tones. |
| Comb | A tuned delay-line resonator modeled on the 808's bridged-T network. Produces pitched, ringing tones from impulse or noise inputs. |
| Bandpass | A resonant state-variable filter that passes a band of frequencies. Used for shaping noise into hats, claps, and tuned percussion. |
| Lowpass | A resonant state-variable filter that attenuates high frequencies. Controls brightness, boom, and tonal warmth. |
The sequencer occupies the lower center of the screen. It provides 12 tracks (one per pad) across a variable number of steps, displayed as a grid of colored dots. Active steps are bright; inactive steps are dim. The current playback position is highlighted.
PLAY / STOP starts or stops playback. REC arms real-time recording — hits you play while the sequencer runs are quantized and written into the active pattern. CLR clears all steps in the active pattern slot. FILL is a hold-to-engage performance tool: while pressed, the sequencer switches to a fill pattern (snare build, tom cascade, or full kit), then reverts to the original pattern when released at the next bar boundary.
The TIME pills select a time signature (4/4, 3/4, or 6/8). The DIV pills set the step resolution (1/8, 1/16, or 1/32). Changing the division remaps existing step positions rather than truncating, so a pattern created at 1/16 retains its musical content when switched to 1/32.
Four slots labeled A through D store independent patterns. Tap a slot letter to switch. The BEATS dropdown loads one of 20 factory patterns into the active slot.
With REC armed and the sequencer playing, striking a pad writes a step at the current quantized position. The full pencil snapshot — all six dimensions plus velocity — is stored per step and reproduced on playback. Flick gestures performed during recording are also captured and replayed.
Parameter Locks. Each step can override voice parameters independently. When a step fires, its locked values temporarily replace the pad's base settings, then revert before the next step. Lockable parameters include frequency, decay time, damping cutoff, reverb send, delay send, pitch envelope depth, pitch envelope time, attack time, and output level. This is the same concept as Elektron's parameter locks: per-step sound variation without changing the underlying voice.
Twenty patterns ship with the instrument, designed to work across all time signatures and divisions. They use fractional bar positions internally, so a pattern that sounds correct at 1/16 in 4/4 also works at 1/8 or 1/32 without losing musical intent. The patterns span genres from four-on-the-floor to halftime, broken beat to ambient pulse.
The FX module sits in the upper portion of the right rail. A global on/off toggle (green for active, red for bypassed) controls whether effects processing is applied to the output.
A Dattorro plate reverb algorithm with three controls: DECAY (0.3 to 8 seconds), DAMP (high-frequency absorption), and MIX (dry/wet balance). Each pad has an independent REV SEND amount, routing a percentage of its output to the reverb bus.
A tape-style delay with TIME (10ms to 2 seconds), FDBK (feedback amount), and built-in damping and saturation that darken and warm the repeats over successive taps. Each pad has a DLY SEND amount.
A per-pad tunable comb filter with FREQ (resonant frequency), RESO (feedback amount), and DAMP (high-frequency damping in the feedback loop). This is separate from the synthesis engine's comb filter — it processes the voice output, adding pitched resonance or metallic coloring after the main synthesis.
Per-Step FX Trigs. Individual sequencer steps can have their FX sends muted. When an FX trig is disabled on a step, that hit plays dry regardless of the pad's send settings. This allows patterns where certain hits punch through without reverb or delay, creating rhythmic contrast in the effects tail.
The MEMORY overlay (accessed from the KIT module) manages persistent state. It contains two sections: custom kits and scenes.
Sixteen numbered slots store complete instrument snapshots. A scene captures the loaded kit, all pad voice configurations, pattern bank (all four slots), FX settings, BPM, pad mutes, pad flams, and drawn contours. Loading a scene restores the instrument to exactly the state it was in when saved.
Scenes are the primary mechanism for switching between complete performance configurations. Saving is explicit — tap an empty slot or overwrite an existing one. Deleting a scene clears the slot.
Custom kits and scenes are stored as JSON files in the app's documents directory. Factory kits are immutable and always load from their built-in definitions. When running as an AUv3 plugin, the extension maintains its own storage, separate from the standalone app.
Drop Forge responds to incoming MIDI notes using the General MIDI drum map: note 36 (C1) triggers pad 1, note 37 triggers pad 2, through note 47 for pad 12. Velocity scales the hit intensity. Notes from hardware controllers, DAW piano rolls, or external sequencers all work through this mapping.
When playing with the Apple Pencil, six continuous control change messages stream at 240Hz on the configured MIDI channel: force (CC 1), X position (CC 16), Y position (CC 17), altitude (CC 11), azimuth (CC 2), and roll (CC 3). A gate message (CC 64) sends 127 on contact and 0 on lift. These CCs can drive external synthesizers, effects, or DAW automation from the pencil's physical gestures.
In standalone mode, Drop Forge receives MIDI clock (24 PPQN) and transport messages (start, stop, continue) from external sources. The BPM display reflects the incoming tempo, and the internal sequencer follows external start/stop commands.
When loaded as an AUv3 plugin, the instrument reads the host's tempo and transport state directly through the Audio Unit API. Pressing play in Logic, Ableton, or AUM starts the internal sequencer if a pattern with active steps is loaded. Stopping the host stops and resets the sequencer. The PULSE module displays HOST when synced to a DAW.
Hot-Plug Detection. MIDI sources that connect or disconnect while the app is running are detected automatically. There is no need to restart the app when plugging in a USB controller or pairing a Bluetooth MIDI device.
Drop Forge loads as an AUv3 instrument in compatible hosts. The full interface — pad grid, Metal rendering, sequencer, FX, contour drawing — is presented in the plugin window. Tested hosts include AUM (iPad), Logic Pro (macOS), and Ableton Live (macOS, with limited window sizing). On Apple Silicon Macs, the plugin runs via the Designed for iPad compatibility layer with mouse and trackpad input substituting for touch.