Analysis and evaluation of irregularity in pitch vibrato for string-instrument tones

In describing musical performances, the use of the term vibrato can imply any periodic (or quasi-periodic) fluctuation in pitch, amplitude, or timbre of a sustained musical tone. The current study focused only upon analysis and evaluation of ‘pitch vibrato’ observed in recorded performances on a number of string instruments (three violins, a viola, a cello, and a bass violin). In order to gain a better understanding of the identifiable auditory attributes associated with the perception of pitch vibrato as performed, a multi-parameter vibrato synthesis algorithm was employed in the creation of a set of stimuli for evaluation by human listeners. Besides rate and depth of pitch modulation, a third parameter was included in the synthesis that allowed for a manipulation of the quasi-periodic nature of simulated vibrato intended to mimic performed vibrato. Control for this third parameter, effectively capturing the amount of irregularity in pitch modulation, was enabled via adjustment of the Q value of a resonant low-pass filter that was used to either spread or concentrate a modulation signal’s energy around the nominal pitch modulation frequency (vibrato rate). A high Q value was associated with pitch modulation that sounded very regular, practically sinusoidal at the sub-audio vibrato rate (when the Q value exceeded around 30). Lower Q values were associated with irregular sounding pitch modulation that was heard as more rough, and could become very rough at the lowest Q values (below Q=3). Performances recorded without substantial pitch vibrato were processed via a delay-modulation algorithm that employed a collection of the synthesized modulation signals in an attempt to match the character and quality of vibrato performances recorded on the same instruments. A group of fifteen listeners was employed to determine how detectably different the synthetic vibrato was as the Q value was varied, and to what extent changes in Q value influenced the perceived ‘fluctuation strength’ of the synthetic vibrato.