Modified transient harmonic interpolator for an electronic musical instrument

Abstract

A method and apparatus for interpolating between the harmonic structures of a waveform stored in memory during portions or the entire transient periods of said waveform. In an electronic musical instrument having a greater number of selectively actuable switches than note generators to cause the production of sounds corresponding to the respective notes of a musical scale, the present invention interpolates between the harmonic structures of a waveform stored in memory during portions or the entire transient periods of said waveform. This is accomplished through the use of memory units having a number of locations or zones within each memory where the number of zones is equivalent to the number of discrete harmonic structures. The first of the memory units contains a discrete fixed harmonic structure in each of its zones, and a second of the memory units contains a difference value in each of its zones where the difference value is equal to the difference between the discrete fixed harmonic structure in adjacent zones of the first memory. Each of the memory units is addressed by a means for controlling the length of time of the interpolation between harmonic structures of a waveform during the transient periods of said waveform for selectively causing the reading out from each said memory the respective values therein which are converted to an analog current and scaled to provide correlating magnitudes so that the converted and scaled voltages may be summed to form the interpolated harmonic structures of the waveform. These interpolated harmonic structures are then scaled by a waveform envelope and fed into an audio amplifier for reproducing the waveform as audible sound through a suitable sound transducing device. The different transient periods of the waveform are detected by this scheme and cause either an attack or a decay type waveform to be generated. The length of time of the interpolation between harmonic structures is specifically controlled and may occur as either a linear or non-linear sequence occupying either the entire transient period or a shortened portion thereof.