Key & harmony
Semitone
A semitone is the smallest step in Western music — one fret, one piano key including the black ones. Twelve of them make an octave, and everything a pitch fader does is measured in fractions of one.
In practice
Twelve semitones to an octave, and roughly 6% of tempo change to one. That is the tempo-to-key exchange rate.
Keep the two consequences straight. With key lock off, a 6% tempo move is a semitone of key change — sometimes useful, sometimes the reason a planned harmonic mix stopped working. With key lock on, the key holds and you pay in artefacts instead. And when you deliberately shift a record to fit, one semitone is safe almost anywhere, two is usually fine on an instrumental, and three is a decision you should make on purpose.
12
Semitones in an octave
~6%
Tempo change per semitone
1–2
Safe shift, in semitones
Where these come from: The equal-temperament ratio and standard pitch-shift tolerances in DJ use — arithmetic and practice, not catalog figures. For what we can measure, see twelve of these.
Why it matters
It is the unit that connects tempo to key, which is the connection most DJs never make explicitly. Speeding a record up raises its pitch continuously, and once you have moved it about six per cent you have moved it a full semitone — which is a different key, and a different set of records it will mix with.
It also sets the honest limit on pitch shifting. One semitone of shift is inaudible on almost anything; two is fine on instrumentals and starts to show on voices; beyond that you are hearing the processing rather than the music.
Frequently asked
How much tempo change is one semitone?+
About six per cent, because equal temperament divides an octave — a doubling — into twelve equal ratios. It is why a big pitch move changes which records will mix with yours.
How many semitones can I shift a track?+
One is safe on almost anything, two is usually fine on instrumentals, and past that vocals start to sound processed. Make bigger moves deliberately rather than by accident.
Is a semitone the same as a half step?+
Yes — two names for the same interval. "Half step" is the more common term in American teaching and "semitone" in British.
Related terms
Octave
An octave is a doubling of frequency — twelve semitones, and the interval at which a note repeats itself with the same name. It is the one interval every listener hears as "the same note, higher".
Scale
A scale is the ordered set of notes a piece of music draws on. Major and minor are the two that dominate club music, and which one a track uses is half of what its key name tells you.
Key lock
Key lock — master tempo on a CDJ — holds a track at its original pitch while you change its tempo. Without it, speeding a record up raises its key and quietly undoes whatever harmonic mixing you had planned.
Pitch vs tempo
Tempo is how fast a track plays; pitch is how high it sounds. On a turntable they are one control, because speeding a record up raises its pitch. Digitally they can be separated, which is what key lock does.
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