Harmonic formant filter for an electronic musical instrument2009-10-20AbstractA fixed
formant filter for a digital electronic instrument of the type in which musical tones are generated from waveshape data calculated independently of frequency of the tone from constituent generalized Fourier components. The Fourier components are individually scaled by scale factors selected from a set of stored scale factors which define the transfer characteristic of the fixed
formant filter. Each separately addressable stored scale factor corresponds to a different frequency value in the musical scale. A scale factor is selected for separately scaling each harmonic of the tone generated in response to the particular note of the musical scale keyed on the instrument. The selected scale factors may be modified by a frequency deviation signal to provide vibrato or other frequency modulation effects. The selected scale factors are also corrected for any frequency deviation between the actual frequency of the harmonic of the keyed note being scaled and the frequency value associated with the selected scale factor.ClaimsWhat is claimed is:
1. A tone generator having a fixed
formant filter characteristic for generating a tone having a pitch selected by operating a key on a keyboard comprising a harmonic counterfor counting the harmonic components of the tone being generated, means initiating a plurality of harmonic component calculation cycles, said means advancing the harmonic counter at the start of each calculation cycle, means storing a plurality ofharmonic coefficient values, means reading out a different coefficient value from the storing means with each advance of the harmonic counter, means storing a plurality of sinusoid values, means reading out a sequence of sinusoid values with each settingof the harmonic counter, adder-accumulator means, means multiplying the coefficient value for each harmonic calculation cycle with a sequence of sinusoid values and adding the product values to the adder-accumulator, addressable means storing a pluralityof scale factors corresponding to the relative amplitude values of the fixed
formant filter characteristic at predetermined frequencies corresponding to the musical notes of the keyboard, means for addressing the addressable storage means in response tothe selected key on the keyboard and the state of the harmonic counter, means multiplying each of said product values added to the adder-accumulator by the associated scale factor read out of the storage means, and means converting each of the valuesstored in the adder-accumulator at the completion of said calculation cycles to an analog voltage in sequence at a rate determined by the pitch of the tone being generated, whereby the set of values stored in the adder-accumulator is converted to anaudio voltage signal having a fundamental frequency corresponding to the desired pitch of tone being generated, means for modifying the scale factor read out of the addressable scale factor storing means including means for generating the incrementaldifference between successive scale factors read out of the scale factor storing means, means generating a frequency deviation value, means multiplying said incremental difference by said deviation value, and means adding the content of the multiplyingmeans to the scale factor from the scale factor storing means.
2. Apparatus of claim 1 wherein the means generating the frequency deviation value is varied as a function of time to pro...
