# Understanding Pitch Detection with Autocorrelation Methods

I've been reading through A Smarter Way to Find Pitch, describing its pitch detection algorithm using autocorrelation.

I've been having trouble understanding the accuracy claims. It says:

MPM runs in real time with a standard 44.1 kHz sampling rate. It operates without using low-pass filtering so it can work on sound with high harmonic frequencies such as a violin and it can display pitch changes of one cent reliably.

I cannot fathom how detecting changes of one cent is possible. Considering a note played at 441Hz, sampled at 44.1kHz, one would measure a first peak at tao=100 samples (delay space).

The next closest note which could be measured, by my understanding, would have a peak at the 101st sample. This would be a frequency of 437Hz, or about 16 cents lower than the first note. What am I missing here?

to add to hot's answer, when using autocorrelation or an equivalent method like AMDF or ASDF, although the autocorrelation $$R_x[k]$$ is evaluated only at integer values of lag $$k$$, which means the peak value and initial period length will be an integer number of samples, you can use quadratic interpolation using the point at the peak and the two neighboring values, to get the location of the "true" peak. That will give you a precision (not the same as "accuracy") of better than an integer number of samples lag.