Show HN: I made a free app to calibrate your turntable by simply playing a song

Original Article ↗ Hacker News Discussion ↗

What the app does & who it’s for

  • iOS app to measure turntable speed error (e.g., 33⅓ vs 34.6 RPM) by listening to a song through the phone’s mic.
  • Intended mainly for home listeners with belt‑drive or mid‑range tables lacking good calibration tools, not DJs doing on‑the‑fly beatmatching.
  • Several users report discovering 1–3.5% speed errors they could then correct via hidden trim screws or service procedures.

How it likely works

  • Strong consensus that it uses Apple’s ShazamKit: recognize the track, then read a “frequency skew” value to infer percentage speed error.
  • Evidence: requires popular/“Shazamable” songs with clear beats, fails for obscure tracks, and stops working offline or in airplane mode for many users.
  • It measures average speed offset, not wow & flutter or short‑term variation.

Turntable behavior & traditional calibration

  • Many consumer decks are belt‑drive; belts and bearings wear, motors drift, and factory calibration can be off by a few percent.
  • Higher‑end or DJ tables often have:
    • Direct drive motors with pitch faders.
    • Strobe markings plus mains‑synced lights or quartz‑locked speed control.
    • Test/calibration records with sine tones for precise speed and wow/flutter measurements.
  • Some argue traditional methods and dedicated test records remain superior in precision.

Alternative approaches discussed

  • Place phone on the platter and use accelerometer/gyroscope apps to read RPM directly; more precise but physically risky/uncomfortable.
  • Use camera or computer vision on the label, or leverage periodic artifacts (scratches, wow) via signal processing; several people describe FFT/autocorrelation ideas but note practical difficulties.
  • DIY strobe lights and printable strobe discs as low‑tech, reliable tools.

Reception, UX, and concerns

  • Many praise the simplicity, aesthetics, and “old‑school useful app” spirit; some say it made calibration trivial.
  • Others criticize the website copy for burying the core explanation and leaning into meta‑commentary.
  • Some are uneasy that the implementation is opaque, especially around online dependence and Shazam‑style fingerprinting, though the author stresses no audio recording or analytics.
  • A few note that if a turntable has no speed adjustment, knowing it’s wrong may only cause annoyance.