Chromatic Guitar Tuner

Play an open string and this names the note, shows how many cents sharp or flat it is, and tells you which way to turn the peg. It handles eight tunings, plays reference tones if you would rather tune by ear, and the A4 pitch is adjustable for anything that is not at concert pitch.

How to use it

1. Press Start and allow microphone access. Nothing is recorded.
2. Choose your tuning. The string list shows each target with its frequency.
3. Mute the strings you are not tuning, then pluck one and let it ring.
4. Read the verdict: flat means tighten, sharp means loosen. The needle centres when you are within five cents.
5. Tap any string in the list to hear its reference tone if you prefer tuning by ear.

Why this uses autocorrelation and not an FFT

Most browser tuners take a Fast Fourier Transform of the incoming audio and pick the loudest frequency bin. It is the obvious approach and it is the wrong one for a guitar, for a reason that is easy to quantify.

An FFT divides the frequency range into equally sized bins. The width of each bin is the sample rate divided by the window size. At the usual 44,100 Hz with a 2048-sample window, that is about 21.5 Hz per bin.

Now consider the low E string at 82.41 Hz. One semitone above it is F at 87.31 Hz — a gap of under 5 Hz, which is less than a quarter of a single bin. The tuner physically cannot tell E from F down there, let alone resolve the two or three cents that tuning requires. You could widen the window to 16,384 samples to get 2.7 Hz bins, but that is a third of a second of audio, and the pitch of a plucked string drifts measurably in that time as the tension settles.

Autocorrelation sidesteps the problem by working in the time domain. Instead of asking which frequencies are present, it slides the waveform against itself and finds the delay at which it best matches — the period. Frequency is then simply the sample rate divided by that period. Because the period is measured in samples rather than in frequency bins, resolution at low frequencies is excellent: the 82 Hz E has a period of about 535 samples, and getting that to within one sample is 0.2% accuracy, or roughly three cents. Interpolating a parabola through the correlation peak, which this tool does, pushes it under one cent.

Two safeguards matter as much as the algorithm. A root-mean-square gate rejects windows that are simply too quiet, and a correlation-confidence gate rejects windows that are loud but not periodic — room hum, a chair scraping, someone talking. A tuner that confidently displays a note for background noise is worse than one that displays nothing, because you will start turning a peg based on it. When this tool has nothing trustworthy to report, it says so.

Tuning in the right order

Tuning strings changes the tension on the neck, which very slightly retunes the strings you already did. On a guitar with a floating bridge the effect is pronounced. Two habits deal with it: work from the lowest string upward, then go back around a second time. And always tune up to the target rather than down — bringing a string up to pitch leaves the winding under tension and holds better than dropping onto pitch from above, which leaves slack in the nut slot that slips later.

What it does not do

It detects one pitch at a time, so it cannot check a chord or tune two strings at once. It has no strobe display for the sub-cent work a luthier does when setting intonation. Very heavily distorted or effect-laden signals confuse the periodicity detection, so tune from a clean pickup or acoustically. In a noisy room the confidence gate will refuse readings rather than guess, which is the correct behaviour but can look like the tool has stopped working.

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Standard tuning frequencies at A4 = 440 Hz

String	Note	Frequency
6th (lowest)	E2	82.41 Hz
5th	A2	110.00 Hz
4th	D3	146.83 Hz
3rd	G3	196.00 Hz
2nd	B3	246.94 Hz
1st (highest)	E4	329.63 Hz

Alternate tunings included

Tuning	Strings, low to high
Standard	E A D G B E
Drop D	D A D G B E
Open G	D G D G B D
Open D	D A D F# A D
DADGAD	D A D G A D
Half step down	E♭ A♭ D♭ G♭ B♭ E♭
Ukulele	G C E A
Bass	E A D G