# FD vs In Theory

The clearest difference is review sentiment: In Theory at 100% against FD's 67%. In Theory also leads on update cadence (weekly vs every 4 months) and rating (5.0 vs 4.6). FD's advantage is ratings volume (32 vs 4) and iOS requirement (9.0 vs 13.0). On price, ads, in-app purchases and monetization model there is nothing between them.

FD costs $4.99 and In Theory $4.99 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 weekly. The most recent releases landed on October 24, 2026 and October 15, 2026 respectively.

## The numbers

| Metric | FD | In Theory |
| --- | --- | --- |
| Price | $4.99 | $4.99 |
| Rating | [4.6 (32 ratings)](https://appshunter.io/ios/app/fd-1-filter-delay/id1448696965/reviews) | **[5.0 (4 ratings)](https://appshunter.io/ios/app/in-theory-interval-keyboard/id1667984658/reviews)** |
| Number of ratings | **32** | 4 |
| Positive reviews | 66.7% of reviews | **100.0% of reviews** |
| In-app purchases | No | No |
| Ads | No | No |
| Update frequency | Every 4 months | **Weekly** |
| Monetization | Paid | Paid |
| Size | 61 MB | 7 MB |
| Requires iOS | **9.0** | 13.0 |
| Devices | iPhone, iPad, iPod | iPhone, iPad, iPod |
| Age rating | 4+ | 4+ |
| Developer | [Kai Aras](https://appshunter.io/developer/370820899) | [Cem Olcay](https://appshunter.io/developer/523633853) |

## What reviewers say

### FD

[App page](https://appshunter.io/ios/app/fd-1-filter-delay/id1448696965)

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.

**Highlights:** Semi-modular effect plugin, Modulated filter section, Delay stage, Step sequencer, Low Frequency Oscillator, Mod matrix, Tempo sync

**Features:** Vision Pro support

**In-app purchases:** none


### In Theory

[App page](https://appshunter.io/ios/app/in-theory-interval-keyboard/id1667984658)

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.

**Highlights:** 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

**Features:** macOS Compatibility, Vision Pro support

**In-app purchases:** none


## Frequently asked questions

### Is FD free?

FD costs $4.99, with no in-app purchases.

### Is In Theory free?

In Theory costs $4.99, with no in-app purchases.

### Which has better reviews, FD or In Theory?

In Theory is rated higher. FD averages 4.6 out of 5 from 32 ratings, while In Theory averages 5.0 from 4 ratings.

### Do FD or In Theory have ads?

Neither FD nor In Theory shows ads.

### Which is updated more often, FD or In Theory?

FD ships an update every 4 months, and In Theory weekly. Most recently, FD was updated on October 24, 2026 and In Theory on October 15, 2026.

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*Compared by [AppsHunter](https://appshunter.io/) from daily US App Store data. Review summaries are generated from verbatim App Store reviews.*