Download Block-oriented modeling of distortion audio effects using iterative minimization Virtual analog modeling is the process of digitally recreating an analog device. This study focuses on analog distortion pedals for guitarists, which are categorized as stompboxes, because the musician turns them on and off by stepping on the switch. While some of the current digital models of distortion effects are circuit-based, this study uses a signal-based approach to identify the device under test (DUT). An algorithm to identify any distortion effect pedal in any given setting by input-output (I/O) measurements is proposed. A parametric block-oriented Wiener-Hammerstein model for distortion effects and the corresponding iterative error minimization procedure are introduced. The algorithm is implemented in Matlab and uses the Levenberg-Marquardt minimization procedure with boundaries for the parameters.
Download Wave Digital Filter Adaptors for Arbitrary Topologies and Multiport Linear Elements We present a Modified-Nodal-Analysis-derived method for developing Wave Digital Filter (WDF) adaptors corresponding to complicated (non-series/parallel) topologies that may include multiport linear elements (e.g. controlled sources and transformers). A second method resolves noncomputable (non-tree-like) arrangements of series/parallel adaptors. As with the familiar 3-port series and parallel adaptors, one port of each derived adaptor may be rendered reflection-free, making it acceptable for inclusion in a standard WDF tree. With these techniques, the class of acceptable reference circuits for WDF modeling is greatly expanded. This is demonstrated by case studies on circuits which were previously intractable with WDF methods: the Bassman tone stack and Tube Screamer tone/volume stage.
Download Resolving Wave Digital Filters with Multiple/Multiport Nonlinearities We present a novel framework for developing Wave Digital Filter (WDF) models from reference circuits with multiple/multiport nonlinearities. Collecting all nonlinearities into a vector at the root of a WDF tree bypasses the traditional WDF limitation to a single nonlinearity. The resulting system has a complicated scattering relationship between the nonlinearity ports and the ports of the rest of the (linear) circuit, which can be solved by a Modified-NodalAnalysis-derived method. For computability reasons, the scattering and vector nonlinearity must be solved jointly; we suggest a derivative of the K-method. This novel framework significantly expands the class of appropriate WDF reference circuits. A case study on a clipping stage from the Big Muff Pi distortion pedal involves both a transistor and a diode pair. Since it is intractable with standard WDF methods, its successful simulation demonstrates the usefulness of the novel framework.
Download Feature design for the classification of audio effect units by input/output measurements Virtual analog modeling is an important field of digital audio signal processing. It allows to recreate the tonal characteristics of real-world sound sources or to impress the specific sound of a certain analog device upon a digital signal on a software basis. Automatic virtual analog modeling using black-box system identification based on input/output (I/O) measurements is an emerging approach, which can be greatly enhanced by specific pre-processing methods suggesting the best-fitting model to be optimized in the actual identification process. In this work, several features based on specific test signals are presented allowing to categorize instrument effect units into classes of effects, like distortion, compression, modulations and similar categories. The categorization of analog effect units is especially challenging due to the wide variety of these effects. For each device, I/O measurements are performed and a set of features is calculated to allow the classification. The features are computed for several effect units to evaluate their applicability using a basic classifier based on pattern matching.
Download Improving the robustness of the iterative solver in state-space modelling of guitar distortion circuitry Iterative solvers are required for the discrete-time simulation of nonlinear behaviour in analogue distortion circuits. Unfortunately, these methods are often computationally too expensive for realtime simulation. Two methods are presented which attempt to reduce the expense of iterative solvers. This is achieved by applying information that is derived from the specific form of the nonlinearity. The approach is first explained through the modelling of an asymmetrical diode clipper, and further exemplified by application to the Dallas Rangemaster Treble Booster guitar pedal, which provides an initial perspective of the performance on systems with multiple nonlinearities.
Download Approximating non-linear inductors using time-variant linear filters In this paper we present an approach to modeling the non-linearities of analog electronic components using time-variant digital linear filters. The filter coefficients are computed at every sample depending on the current state of the system. With this technique we are able to accurately model an analog filter including a nonlinear inductor with a saturating core. The value of the magnetic permeability of a magnetic core changes according to its magnetic flux and this, in turn, affects the inductance value. The cutoff frequency of the filter can thus be seen as if it is being modulated by the magnetic flux of the core. In comparison to a reference nonlinear model, the proposed approach has a lower computational cost while providing a reasonably small error.