In applications for which the fundamental frequency
measured very accurately in a periodic signal, the estimate obtained
by the above algorithm can be refined using a gradient search
which matches a so-called harmonic comb to the magnitude
spectrum of an interpolated FFT
The purpose of is an insurance against multiplying the whole expression by zero due to a missing partial (e.g., due to a comb-filtering null). If in (10.1), it is advisable to omit indices for which is too close to a spectral null, since even one spectral null can push the product of peak amplitudes to a very small value. At the same time, the product should be penalized in some way to reflect the fact that it has fewer terms ( is one way to accomplish this).
As a practical matter, it is important to inspect the magnitude spectra of the data frame manually to ensure that a robust row of peaks is being matched by the harmonic comb. For example, it is typical to look at a display of the frame magnitude spectrum overlaid with vertical lines at the optimized harmonic-comb frequencies. This provides an effective picture of the estimate in which typical problems (such as octave errors) are readily seen.
References on Estimation