Resonant Audio Filter for Stable Through-Zero Frequency Modulation
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Solution Overview
Problem
Existing electronic music filters lack the capability for linear frequency modulation through zero frequency, which is necessary for creating pleasing timbral modulation, and state variable filters are prone to instability when attempting negative frequency modulation.
Innovation Solution
A stable electronic music resonant filter with through-zero linearly variable resonant frequency is implemented, using a state variable filter configuration that allows linear frequency modulation through zero while maintaining stability, and incorporates exponential times linear multiplication to produce resonant frequencies responsive to control voltages for both musical octave/semitone control and timbre modulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If state variable filter is used for negative frequency modulation, then frequency modulation capability is improved, but system stability deteriorates
Solution Approach 1:
The patent introduces an intermediary mechanism (absolute value detection and sign control circuit) between the frequency modulation input and the state variable filter. This intermediary detects the sign of the modulation frequency and controls the filter's resonant frequency accordingly, preventing direct negative frequency input that causes instability while preserving the desired timbral modulation effect.
Solution Approach 2:
The patent changes the parameter control method by separating frequency magnitude and sign control. Instead of directly applying negative frequency values to the state variable filter (which causes instability), the system maintains positive resonant frequency values while using sign information to control waveform inversion, thus achieving through-zero modulation without compromising stability.
2Ease of operation
If exponential frequency control is used, then musical pitch control is improved, but timbral modulation capability deteriorates
Solution Approach 1:
The patent segments the frequency control function into two independent control paths: one for musical pitch (exponential control) and one for timbral modulation (linear control). This segmentation allows each control type to operate optimally without interfering with the other, enabling both precise pitch control and rich timbral variation simultaneously.
Solution Approach 2:
The filter is designed with multi-functional control inputs that can handle both exponential pitch control and linear timbral modulation through the same resonant frequency mechanism. The system universally accepts different control types and routes them appropriately to achieve both musical pitch accuracy and timbral expression.
3Ease of manufacture
If linear frequency modulation through zero is implemented, then timbral modulation quality is improved, but filter stability deteriorates
Solution Approach 1:
The patent introduces an intermediary mechanism (absolute value detection and sign control circuit) between the frequency modulation input and the state variable filter. This intermediary detects the sign of the modulation frequency and controls the filter's resonant frequency accordingly, preventing direct negative frequency input that causes instability while preserving the desired timbral modulation effect.
Solution Approach 2:
The patent converts the potentially harmful effect of negative frequency input (which causes instability) into a beneficial control signal for waveform inversion. By detecting the sign of the modulation frequency and using it to control the filter's operation mode, the system transforms what would be a destabilizing factor into a useful timbral control parameter.
Data Source
AI summary
An audio filter and corresponding method with through-zero linearly variable resonant frequency are described. An example method includes receiving an input signal; receiving one or more control input signals; and providing a resonant filter for electronic music for processing the input signal. The resonant filter can include an exponential times linear multiplication to produce the resonant frequency control responsive to control voltages exponentially for musical octave/semitone control, and linearly for timbre modulation, the waveform being invariant over corresponding exponential changes in pitch of both input signal and filter resonance. The linear modulation can include inverting the sign of a bandpass feedback based on the sign of the resonant frequency variable so as to maintain stability. In the example method, to prevent a glitch on the output when frequency changes sign, a separate highpass output signal may be generated.