Harmonic Tuner app: #Oxford Academy / @paulgoodwin123 recorded Johann Sebastian Bach’s St Matthew Passion on baroque tuning pitch of ca. 417Hz on period instruments. Reference pitch meaurement was done playing back the the e-minor opening chorus “Kommt, ihr Töchter, helft mir klagen” to the mobile phone & app. The key detection feature lets you conclude that reference pitch is not at standard 440 Hz but one semitone below, at baroque tuning pitch.
Demo clip of Harmonic Tuner app’s reference pitch and key detection features: