Centerline

Most tuner apps bury a simple job under ads, sign-ins, and subscriptions. Centerline had to open quickly, stay readable in a dim room, work offline with no ads or account, and follow one note at a time across a 62–800 Hz range.

Centerline is live on Android and iPhone, with private offline chromatic single-note pitch analysis from 62–800 Hz, no ads or accounts, and a one-time Complete purchase.

• Android + iPhone

  public releases

  The product is available now on Android and iPhone.

• 62–800 Hz

  single-note analysis

  Chromatic pitch analysis follows one note at a time.

• One-time

  upgrade, no subscription

  Free covers everyday tuning. Complete is a single purchase.

• Focused

  product scope

  The app stays centered on fast, legible tuning.

I took the product from the first interaction through its public Android and iPhone releases:

• Product designDefined the tuning flow, visual hierarchy, display modes, and paid boundary around how a guitarist actually uses the app.
• Native Android buildBuilt the Android app in Kotlin and Jetpack Compose, including microphone capture, pitch detection, state handling, and Play Billing.
• Native iPhone buildBuilt the iPhone app in Swift and SwiftUI, with the same tuning behavior and StoreKit purchases.
• Release workPrepared the store listings, privacy material, purchase flow, and production releases.
• Ongoing ownershipMaintain the product, respond to platform changes, and keep the public claims aligned with what the current release does.

A few decisions kept the tuning flow simple and the pitch feedback clear:

• Keep audio localMicrophone input goes through the pitch detector on the phone. The core task works offline with no ads, account, or internet connection.
• Show direction before detailThe interface makes flat, sharp, and in-tune states legible at a glance, with cents and frequency available as supporting information.
• Make the free mode usefulFree includes Chromatic and all display modes, and a single purchase unlocks the rest. Complete adds alternate and custom six-string tunings, capo, A4 432–446 Hz calibration, and reference tones.
• Build natively on each platformKotlin and Jetpack Compose on Android, Swift and SwiftUI on iPhone, so microphone access, lifecycle, accessibility, and billing follow each platform directly.

• The main interaction should need no explanationA utility earns trust when its primary state is obvious before someone reads a label or opens settings.
• Privacy can simplify the architectureKeeping audio on-device removes an entire class of server, account, consent, and reliability concerns.
• A release includes more than the main featureBilling, privacy disclosures, store assets, release checks, and updates are part of maintaining the app.
• Two native apps, one productKeeping the Kotlin and Swift builds in step took shared specs and a device test matrix, not shared code.