1. A method of operating a processing apparatus to determine a measure of harmonicity of an audio signal, the method comprising: accepting an audio input signal, the accepting providing digitized samples of the audio input signal, including in the case the audio input signal is an analog signal, using a digitizer on the analog input signal; determining a spectrum of an amplitude measure for a set of frequencies, including time-to-frequency transforming the accepted audio input signal to form a set of frequency components of the audio signal at the set of frequencies; and determining as a measure of harmonicity a quantity indicative of the normalized difference of (i) the total spectral content based on the amplitude measure in a first subset of frequencies, said first subset corresponding to harmonic components of the audio signal, and (ii) the total spectral content based on the amplitude measure in a second subset of frequencies, said second subset corresponding to inharmonic components of the audio signal, said normalized difference being the difference normalized by the total spectral content in the first and second subsets, all total spectral contents being up to a maximum frequency.