Defining a vibrotactile toolkit for digital musical instruments: characterizing voice coil actuators, effects of loading, and equalization of the frequency response

The integration of vibrotactile feedback in digital music instruments (DMIs) is thought to improve the instrument’s response and make it more suitable for expert musical interactions. However, given the extreme requirements of musical performances, there is a need for solutions allowing for independent control of frequency and amplitude over a wide frequency bandwidth (40–1000 Hz) and low harmonic distortion, so that flexible and high-quality vibrotactile feedback can be displayed. In this paper, we evaluate cost-effective and portable solutions that meet these requirements. We first measure the magnitude–frequency and harmonic distortion characteristics of two vibrotactile actuators, where the harmonic distortion is quantified in the form of total harmonic distortion (THD). The magnitude–frequency and THD characteristics in two unloaded cases (actuator suspended freely or placed on a sandbag) are observed to be largely identical, with minor attenuation for actuators placed on the sandbag. Loading the actuator (when placed in a DMI) brings resonant features to its magnitude–frequency characteristics, increasing the output THD and imposing a dampening effect. To equalize the system’s frequency response, an autoregressive method that automatically estimates minimum-phase filter parameters is introduced, which by design, remains stable upon inversion A practical use of this method is demonstrated by implementing vibrotactile feedback in the poly vinyl chloride chassis of an unfinished DMI, the t-Stick . We finally compare the result of equalization by performing sinesweep measurements on the implementation and discuss the degree of equalization achieved using it.