Keybeat

How Keybeat works

The short version for curious musicians. No math, no code. Just what the app listens for and why it sometimes hesitates.

Where the sound comes from

Keybeat can listen in two ways. You pick one under Listen to in the menu.

  1. System Audio. The app taps the sound your Mac is sending to the speakers or headphones. Every app is included: streaming, browser tabs, DJ software, your DAW. Nothing is changed on the way out; you hear exactly what you heard before.
  2. Microphone. The app listens to the room. Use this for a record player, a rehearsal, or a song on someone else's phone.

When you plug in headphones or switch speakers, the app follows the new output on its own.

Finding the tempo

Tempo is about when things happen. The app looks for the pulse.

  1. It listens for hits. One hundred times a second, it checks how much louder the sound just got. Drum hits, plucked notes and chord changes all make a spike. The result is a curve that jumps on every beat.
  2. It looks for the spacing that repeats. If the spikes line up every half second, that is 120 beats per minute. The app tries every spacing between 60 and 200 BPM and scores how well each one fits.
  3. It considers the multiples. A good tempo also fits at twice and four times the spacing, so each candidate is scored together with its multiples. Candidates close to 120 BPM get a little extra weight, because most music lives there.
  4. It settles. The number in the menu bar is the middle value of the last ten readings, so one odd second does not make it jump. The BPM is shown with one decimal.

The Tempo Range menu lets you limit step 2. Auto tries 60 to 200 BPM. The narrower presets rule out the half-speed or double-speed answer, and 40 to 80 BPM is there for very slow music.

Finding the key

Key is about which notes are there, and how much of each. This needs a much longer look at the sound than tempo does, because low notes sit very close together in pitch.

  1. It picks out the pitched sounds. Only the clear peaks in the sound count. The noise of drums and the hiss between notes is ignored.
  2. It checks the tuning. Not every track is tuned to A = 440 Hz. The app measures how far the notes sit from standard tuning and corrects for it. When the offset is more than 8 cents, the menu shows it.
  3. It sorts every sound into twelve notes. A C is a C whether it is high or low. Each sound is also counted as a possible overtone of a lower note, because real instruments ring at several pitches at once.
  4. It weighs the notes. Every moment of music gets the same vote, so a loud chorus does not drown out a quiet verse. The last 20 seconds decide.
  5. It compares with the 24 keys. Each major and minor key has a typical pattern of note strengths. The app finds the closest match. The bars under Show Note Strengths are exactly the pattern it matched against.

The relative-key problem

Here is the honest part. C major and A minor contain exactly the same seven notes. What separates them is which chord the music comes home to, and that is a matter of phrasing, not of which notes are present.

So when the best match and the second-best match are relative keys, and the app is less than 75% sure, it names both instead of pretending. The menu then reads something like:

C major (8B) 55%, or A minor (8A)

For DJs this is less of a problem than it sounds. On the Camelot wheel a key and its relative minor sit right next to each other (8B and 8A), so a mix between them still works.

What the confidence means

Both readings come with a percentage, and the ring in the menu bar shows the weaker of the two.

  • Tempo confidence is high when one spacing clearly wins and the recent readings agree with each other. Music with a steady beat scores close to 100%. Noise scores much lower.
  • Key confidence is how much better the winning key fits than the others. A drum loop with no pitched content scores almost nothing, as it should.

Below 35% key confidence the menu bar shows a dash instead of a key. The menu still shows the guess with its percentage, so you can decide for yourself.

What it cannot do

We would rather you know this before you buy than after.

  • Major or relative minor. When the notes are the same, the app cannot always tell the two apart. It names both when it is unsure. Some minor progressions that avoid the leading tone will read as the relative major first.
  • Half or double tempo on very even loops. A click track or a loop with perfectly even hi-hats fits equally well at half and double speed. Real tracks with accents are easier. Use Tempo Range when it happens.
  • Auto starts at 60 BPM. A 40 BPM piece reads as 80 in Auto. Pick the 40 to 80 range for very slow material.
  • It needs about 20 seconds. The key is built from the last 20 seconds of music. After a track change or a section change, give it a moment to settle.
  • It hears the mix, not the parts. A track that changes key halfway will show the key of what is playing now. A long ambient pad with no beat gives a low tempo confidence.

We have tested the detection on synthetic material so far. We are building a test set of real tracks next. We will not publish accuracy numbers until we have them.

Ready to try it? Read the install guide or see the price.