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What Key?

What Key determines the key of a song in real time. Combining the spectrum analyzer from Pitch to Note with music theory algorithms, What Key matches the notes being played to major, minor, blues, bebop (jazz) and other scales in all keys, picking the scale and key with the best match. What Key is a universal app that runs on iPod Touch, iPhone, and iPad.


The bottom toolbar toggles between LIVE and iTUNES. LIVE mode operates on notes detected with the device's built-in or attached microphone. iTUNES lets you to select any song from your iPod music library. After a brief conversion delay (which depends on the length of the song), you may tap the play button to start the song.

A bank of filters detects strong and persistent notes from C3 to C6, where A4 = 440 Hz. The row of lights just above the keyboard shows the output of the filter bank collapsed into a single octave. The top toolbar controls the operation of the app.

Note - Shows a time history of notes ("waterfall") above the keyboard. The displayed notes follow the melody line, subject to a short processing delay. In most music where multiple instruments are playing What Key will detect the strongest pitch, and so will often not exactly follow the melody unless it is much louder than the accompaniment. The top right slider controls the speed of the waterfall. Tap the play button on the bottom control bar to stop and start playback. This freezes the waterfall to give you time to work out that part of a song.

Key - Displays a graph where the largest value is the current key. MUSYS matches different scales to the notes that have recently been played to determine the key. Twelve scales are defined:

  • Major
  • Harmonic minor
  • Melodic minor
  • Major pentatonic
  • Minor pentatonic
  • Blues
  • Bebop dominant
  • Bebop major
  • Double harmonic
  • Whole tone
  • Major Locrian
  • Diminished

For example major and minor scales are usually sufficient to determine the key of classical music. Add pentatonic for rock, blues for blues, and bebop scales for jazz. The other scales are useful for alternative and experimental music. Tap the scales you wish to use in "Settings" mode. The top right slider controls key smoothing. Move it to the left for frequent changes (less accurate), or to the right for more smoothing and greater accuracy. (The time interval over which the key is determined is controlled by the slider in "Note" mode.)

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