Fundamental Frequency Estimation
As mentioned in §6.11.2 above, it is advisable to estimate the fundamental frequency of vibration (often called ``F0'') in order that the partial overtones are well resolved while maintaining maximum time resolution for estimating the decay time-constant.
Below is a summary of the F0 estimation method used in calibrating loop filters with good results :
- Take an FFT of the middle third of a recorded plucked string tone.
- Find the frequencies and amplitudes of the largest peaks, where is chosen so that the retained peaks all have a reasonable signal-to-noise ratio.
- Form a histogram of peak spacing
- The pitch estimate is defined as the most common spacing in the histogram.
Approximate Maximum Likelihood F0 Estimation
In applications for which the fundamental frequency F0 must be 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 :
Note that freely vibrating strings are not exactly periodic due to exponenential decay, coupling effects, and stiffness (which stretches harmonics into quasiharmonic overtones, as explained in §6.9). However, non-stiff strings can often be analyzed as having approximately harmonic spectra ( periodic time waveform) over a limited time frame.
Since string spectra typically exhibit harmonically spaced nulls associated with the excitation and/or observation points, as well as from other phenomena such as recording multipath and/or reverberation, it is advisable to restrict to a range that does not include any spectral nulls (or simply omit index when is too close to a spectral null), since even one spectral null can push the product of peak amplitudes to a very small value. As a practical matter, it is important to inspect the magnitude spectra of the data manually to ensure that a robust row of peaks is being matched by the harmonic comb. For example, a display of the frame magnitude spectrum overlaid with vertical lines at the optimized harmonic-comb frequencies yields an effective picture of the F0 estimate in which typical problems (such as octave errors) are readily seen.
References on F0 Estimation
An often-cited book on classical methods for pitch detection, particularly for voice, is that by Hess . The harmonic comb method can be considered an approximate maximum-likelihood pitch estimator, and more accurate maximum-likelihood methods have been worked out [114,547,376,377]. More recently, Klapuri has been developing some promising methods for multiple pitch estimation [254,253,252].7.12A comparison of real-time pitch-tracking algorithms applied to guitar is given in , with consideration of latency (time delay).
Extension to Stiff Strings
An advantage of the harmonic-comb method, as well as other frequency-domain maximum-likelihood pitch-estimation methods, is that it is easily extended to accommodate stiff strings. For this, the stretch-factor in the spectral-peak center-frequencies can be estimated--the so-called coefficient of inharmonicity, and then the harmonic-comb (or other maximum-likelihood spectral-matching template) can be stretched by the same amount, so that when set to the correct pitch, the template matches the data spectrum more accurately than if harmonicity is assumed.
EDR-Based Loop-Filter Design
Dispersion Filter Design