FD vs In Theory
Price, ratings, monetisation and update history for both apps, side by side — with what reviewers say about each.
FD
This app is a semi-modular audio effect plugin featuring dedicated filter, delay, and modulation sections. It allows for the creation of rhythmic filter sequences and a wide range of complex audio effects through its flexible modulation matrix.
- Semi-modular effect plugin
- Modulated filter section
- Delay stage
- Step sequencer
- Low Frequency Oscillator
- Mod matrix
- Tempo sync
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FD-1 is a Semi-Modular Effect Plugin including dedicated Filter, Delay and Modulation sections. At it's core, FD-1 is designed to create filter rythms and grooves by first sending the signal through a modulated filter section before feeding it to a delay stage but it goes much further than that. Built-in Step Sequencer and Low Frequency Oscillator serve as modulation sources and are ideal for creating rythmic filter sequences either free running or tempo-synced to a host application. However, due it's semi modular nature, not just filter cutoff but every control within FD-1 can be a modulation target allowing for an increased range effects and combinations ranging from Chorus and Flanger type effects over Analog and Tape Style echos to complex Stutter and Repeat effects. Overview: - VA Multi Mode Filter with 2 & 4 Pole options - Digital Stereo Delay with Tempo Sync and Time Range Control - Analog Style Step Sequencer - Low Frequency Oscillator - Easy to use Mod Matrix - Input Low-Cut and Drive Controls - Individual Return Levels for Filter and Delay Connectivity: - Standalone - AUv3 - IAA - AudioBus (Filter) MIDI: - Tempo Sync - CC Control Requirements: - iOS9+ - iPad 4th generation or newer
In Theory
This app provides a unique interval-based MIDI keyboard with over 110 scales. It allows for flexible chord creation by toggling additional intervals and notes from different octaves. It integrates with external MIDI keyboards and sequencers for advanced music production.
- Interval-based MIDI keyboard
- Over 110 built-in scales
- Customizable chord voicing
- MIDI keyboard input support
- MIDI CC message control
- AUv3 plugin compatibility
- Visual scale and note display
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In Theory is an interval based MIDI keyboard app with more than 110 scales. It has a keypad at the bottom with the interval buttons like +2, +1, 0, -1, -2 which would change the current note. Zero means the current note, +1 means the note after the current note, +2 means two notes after the current note, -2 means two notes before the current note in the scale and so on. Above the keypad, you can toggle additional intervals in the scale for creating chord voices. You can also select the notes from an octave above and an octave below for creating rich, extended chords. You can use your MIDI keyboard to play In Theory as well. The middle C4 key on your MIDI keyboard triggers the 0 (zero) button on the app's keypad. The white notes after the C4 note would trigger the positive buttons, the white notes before the C4 note would trigger the negative notes on the keypad. You can also toggle the chord intervals with MIDI CC messages. You can set a MIDI CC number and a MIDI Channel for toggling the chord intervals from the settings menu. If you send a MIDI CC value between 0-62, you can toggle the chord intervals respectively. With the MIDI capabilities, you can also sequence the app with a MIDI sequencer app like PolyBud, ShiftBud, MelodyBud etc. In Theory has more than 110 scales built-in. You can also use the custom scales you made in ScaleBud 2 app as well. The key and scale could be changed externally from a MIDI controller in AUv3 plugin with the AU parameters. The circular display on the UI will show you where you are in the scale. Also, when you play a note, you will see the every playing note on the screen with the root note and the chord intervals. Please note that In Theory is a MIDI app and does not produce any kind of sound on its own. You need to connect the MIDI output of the app to an audio app's MIDI input. For the AUv3 plugin you'll need an AUv3 host app such as AUM, Cubase, Nanostudio, apeMatrix etc.
Screenshots
Verdict
The clearest difference is review sentiment: In Theory at 100% against FD's 67%. In Theory also leads on rating (5.0 vs 4.6) and update cadence (every 3 weeks vs every 4 months). FD's advantage is ratings volume (32 vs 4) and iOS requirement (9.0 vs 13.0). On ads, in-app purchases, monetization model and device support there is nothing between them.
Scored on Price · Rating · Positive reviews · Number of ratings · Update frequency · Ads · In-app purchases · Monetization · Best chart rank · Devices · Requires iOS
FD costs $9.99 and In Theory $3.99 up front. Neither carries in-app purchases, so what you see is what you pay.
In Theory holds the better App Store score, 5.0 against 4.6. That said, the two scores are not equally well evidenced: FD has 32 ratings behind it against 4, so its average is the more settled of the two. Written reviews point the same way: 67% of FD's written reviews are positive against 100% of In Theory's.
FD ships an update every 4 months, In Theory every 3 weeks. The most recent releases landed on September 12, 2026 and September 21, 2026 respectively.
| Parameter | FD | In Theory |
|---|---|---|
| Price | $9.99 | $3.99 — better |
| Rating | 4.6 (32 ratings) | 5.0 (4 ratings) — better |
| Positive reviews | 66.7% of reviews | 100.0% of reviews — better |
| Number of ratings | 32 — better | 4 |
| Update frequency | Every 4 months | Every 3 weeks — better |
| Ads | No | No |
| In-app purchases | No | No |
| Monetization | Paid | Paid |
| Devices | iPhone, iPad, iPod | iPhone, iPad, iPod |
| Requires iOS | 9.0 — better | 13.0 |
| Further details — not scored | ||
| Size | 61 MB | 7 MB |
| Age rating | 4+ | 4+ |
| Developer | Kai Aras | Cem Olcay |
Customer experience
FD
In Theory
In-app purchases
FD
No in-app purchases
In Theory
No in-app purchases








