Online Tuner

How the Tuner works

A tuner has two jobs: work out what pitch it is hearing, and decide which note you were aiming for. The first is signal processing; the second depends on the instrument.

How the online tuner hears pitch

The tuner takes a 4096-sample window of the microphone signal — about 85 ms — and runs a normalised autocorrelation on it, comparing the window with delayed copies of itself to find the delay at which it repeats. That delay is the period of the fundamental, and its reciprocal is the frequency. This approach ignores which harmonic is loudest, which matters on a guitar: the second harmonic of a low E string is often stronger than the fundamental, and a tuner that picked the loudest spectral peak would read an octave high. A parabolic fit between neighbouring delays resolves the reading to a fraction of a hertz, and readings are lightly smoothed so the needle moves steadily rather than twitching.

Choosing the target. In Chromatic mode the target is simply the nearest note in equal temperament, so the tuner works for any instrument, a voice or a tuning fork. Choose an instrument and the target becomes the nearest open string in that instrument's standard tuning — for a guitar E2, A2, D3, G3, B3 and E4. That is what stops the display flipping to F when a string is a long way flat: the tuner keeps pointing at the string you are working on and the needle tells you how far there is to go.

Cents and the green band. A cent is one hundredth of a semitone. The needle spans half a semitone either side of the target, and turns green inside ±5 cents, which is about the limit of what most people can hear on a single sustained note. Negative cents mean the note is flat, so the string needs tightening; positive means sharp, so slacken it. Tune up to the note rather than down to it where you can — approaching from below leaves the string settled against the tension of the machine head and it holds pitch better.

Reference pitch. Every note is calculated from A4, which is 440 Hz by international standard. Some orchestras tune to 442 or 443 Hz, baroque ensembles often use 415 Hz, and a band playing along with an old recording may need to match whatever the tape ran at. Change the reference and every target moves with it.

Getting a clean reading. Play one string at a time and let it ring. Pluck firmly but not so hard that the pitch bends sharp at the attack, then read the needle once the note has settled a moment later. Mute the other strings, move away from fans and other noise, and hold a phone or laptop microphone within about 30 cm of the sound hole or pickup. If the reading says it has not found a clear note, the signal is too quiet or too noisy to trust — the Clarity line shows how periodic the sound is.

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