Music Feature Identification

Analyzing Sound Signals

  • Some things to do:

    • Identify beats, tempo

    • Identify time signature

    • Identify notes, melody, bass line

    • Identify key

    • Identify chords, harmonies

  • All of these are hard!

  • Caveat: Will mostly talk about pop

Beat Id: Direct Methods

  • Convert signal to power by squaring each sample (maybe sqrt, maybe log)

  • Search for impulse "spikes" in power

    • Lowpass filter to smooth the signal?

    • Look for spikes above some reference amplitude. Autodetect amplitude?

    • Look for high second derivative (difference) of power

  • Select based on expectations about time

    • Beats are unlikely to occur less than a few milliseconds apart

    • Leading edge of beat is unlikely to be more than a millisecond long

  • Can distinguish drumbeat (noise) from note attack (spectral)

  • Limitations of all this are many:

    • Heuristic, so much tuning is required. ML?

    • Music signals are complex: beats buried in signal, irregular beats, drum rolls, etc

Tempo Id

  • Take input beats and find "appropriate" periodicity

  • Obviously multiple and partial beats exist, so looking for the "fundamental" beat frequency

    • Tempo likely in the range of 60-300 bpm

    • Use a "Phase-Locked Loop" architecture or an autocorrelator to detect dominant beat frequency

Time Signature Id

  • Look for accented beats: 4/4 !.x. 3/4 !.. 6/8 !..x.. etc

  • Look for chord changes: see below

Note Id

Melody, Bass Line Id

  • Look for top and bottom sounding notes

  • Rests are an issue

Key, Chord Id

  • See which major or minor scale notes in key signature are most consonant with found notes

  • See which chords that make sense for the key seem to be present

  • Keep in mind that borrowed chords are a real thing, so improve heuristic scores by considering them

  • Chord stacks are a pain

  • ML may be really helpful here?

State of the Art

  • Is not that great

  • Garage Band cheats and uses MIDI/keys rather than notes? But transcription is cool

  • Shazam, SoundHound etc identify songs by first few notes of melody (also by soft matching against signal, which is not really analysis)

Last modified: Thursday, 30 May 2019, 3:41 PM