Programmable tone control filters for electric guitar

A method for programming programmable digital computation means, such as a programmable digital signal processor or a microprocessor, to produce a digital filter having response characteristics equivalent to those of a desired analog tone control circuit, said tone control circuit having at least one adjustable tone control for controlling treble, middle or bass frequencies, said method comprising the following steps:

• (a) simulating the analog tone control circuit by an equivalent circuit comprising low-pass and high-pass filters having outputs summed, and each filter having adjustable gain, zeroes, and poles;
• (b) selecting initial settings of the treble, middle and bass controls of the simulated analog tone control circuit;
• (c) using simulation software, generating magnitude and phase frequency responses for the desired analog tone control circuit circuits at the selected settings;
• (d) choosing gain, zero, and pole values for the equivalent circuit such that the gain and phase responses of the equivalent circuit match as exactly as possible the gain and phase responses provided by the circuit simulation computer program;
• (e) performing a bilinear transformation to produce a digital filter prototype
• (f):
• designing update equations for the digital filter gains, poles and zeroes, such that changing the values for the simulated treble, middle and bass controls produces corresponding changes in the values of the digital filter gains, poles and zeroes.