Download Simulation of the Diode Limiter in Guitar Distortion Circuits by Numerical Solution of Ordinary Differential Equations The diode clipper circuit with an embedded low-pass filter lies at the heart of both diode clipping “Distortion” and “Overdrive” or “Tube Screamer” effects pedals. An accurate simulation of this circuit requires the solution of a nonlinear ordinary differential equation (ODE). Numerical methods with stiff stability – Backward Euler, Trapezoidal Rule, and second-order Backward Difference Formula – allow the use of relatively low sampling rates at the cost of accuracy and aliasing. However, these methods require iteration at each time step to solve a nonlinear equation, and the tradeoff for this complexity must be evaluated against simple explicit methods such as Forward Euler and fourth order Runge-Kutta, which require very high sampling rates for stability. This paper surveys and compares the basic ODE solvers as they apply to simulating circuits for audio processing. These methods are compared to a static nonlinearity with a pre-filter. It is found that implicit or semiimplicit solvers are preferred and that the filter/static nonlinearity approximation is often perceptually adequate.
Download Real-time Auralisation System for Virtual Microphone Positioning A computer application was developed to simulate the process of microphone positioning in sound recording applications. A dense, regular grid of impulse responses pre-recorded on the region of the room under study allowed the sound captured by a virtual microphone to be auralised through real-time convolution with an anechoic stream representing the sound source. Convolution was performed using a block-based variation on the overlap-add method where the summation of many small subconvolutions produced each block of output data samples. As the applied RIR filter varied on successive audio output blocks, a short cross fade was applied to avoid glitches in the audio. The maximum possible length of impulse response applied was governed by the size of audio processing block (hence latency) employed by the program. Larger blocks allowed a lower processing time per sample. At 23.2ms latency (1024 samples at 44.1kHz), it was possible to apply 9 second impulse responses on a standard laptop computer.
Download Residual-Driven Adaptive Multi-Rate Quadratic Programming Framework for Nonlinear Analog Audio Circuit Emulation This work extends our previously proposed Quadratic Programming (QP) approach for the emulation of nonlinear analog audio circuits by formalizing its main numerical ingredients and introducing a residual-driven adaptive multi-rate scheme. Starting from a state-space Differential Algebraic System of Equations (DAE) formulation, the nonlinear algebraic circuit device relations are replaced inside the QP by a first-order surrogate linear constraint, and the post-step nonlinear residual is shown to act as a valid defect indicator for adaptive step-size control. This yields a single-step simulation procedure that avoids the usual combination of nonlinear iterative solves and separate integration updates. The method is evaluated on a diode clipper, a BJT common-emitter amplifier, and a Colpitts oscillator, using SPICE as a baseline reference. The results show that adaptive step sizing considerably improves agreement with the reference solution, that the pseudo-inverse implementation is essentially equivalent to the full equality-constrained QP in the tested cases, and that the proposed formulation remains effective beyond the baseline clipper example, including for a self-oscillating circuit. These results position the proposed method as a promising bridge between SPICE-like interpretability and the efficiency demands of virtual analog (VA) audio applications.
Download CONMOD: Controllable Neural Frame-Based Modulation Effects Deep learning models have seen widespread use in modelling LFOdriven audio effects, such as phaser and flanger. Although existing neural architectures exhibit high-quality emulation of individual effects, they do not possess the capability to manipulate the output via control parameters. To address this issue, we introduce Controllable Neural Frame-based Modulation Effects (CONMOD), a single black-box model which emulates various LFOdriven effects in a frame-wise manner, offering control over LFO frequency and feedback parameters. Additionally, the model is capable of learning the continuous embedding space of two distinct phaser effects, enabling us to steer between effects and achieve creative outputs. Our model outperforms previous work while possessing both controllability and universality, presenting opportunities to enhance creativity in modern LFO-driven audio effects. Additional demo of our model is available in the accompanying website.1
Download Harmonize-Decompose Audio Signals with Global Amplitude and Frequency Modulations A key building block in music transcription and indexing operations is the decomposition of music signals into notes. We model a note signal as a periodic signal with slow (frequency-selective) amplitude modulation and global frequency-warping. Global frequency-warping allows for an inharmonic frequency modulation, while the global amplitude modulation allows the various harmonics of the periodic signal to decay at different speeds. The global frequency-warping is achieved by a Laguerre transform (that has shown to fit stiffed strings inharmonic behavior). Assuming additive noise, the estimation of the model parameters and the optimization is performed in a Harmonize-Extract fashion. Simulations illustrate that the extraction technique oversteps the limitation of the global AM-FM representation and analysis techniques and allows the processing of inharmonic string instruments (e.g. piano).
Download Real-Time Virtual Analog Modelling of Diode-Based VCAs Some early analog voltage-controlled amplifiers (VCAs) utilized
semiconductor diodes as a variable-gain element. Diode-based
VCAs exhibit a unique sound quality, with distortion dependent
both on signal level and gain control. In this work, we examine the
behavior of a simplified circuit for a diode-based VCA and propose
a nonlinear, explicit, stateless digital model. This approach avoids
traditional iterative algorithms, which can be computationally intensive. The resulting digital model retains the sonic characteristics
of the analog model and is suitable for real-time simulation. We
present an analysis of the gain characteristics and harmonic distortion produced by this model, as well as practical guidance for
implementation. We apply this approach to a set of alternative
analog topologies and introduce a family of digital VCA models
based on fixed nonlinearities with variable operating points.
Download Effect of Latency on Playing Accuracy of Two Gesture Controlled Continuous Sound Instruments Without Tactile Feedback The paper reports results from an experimental study quantifying how latency affects the playing accuracy of two continuous sound instruments. 11 subjects played a conventional Theremin and a virtual reality Theremin. Both instruments provided the user only audio feedback. The subjects performed two tasks under different instrument latencies. They attempted to match the pitch of the instrument to a sample pitch and they played along a short sample melody and a metronome. Both the sample sound and the instrument’s sound were recorded on different channels of a sound file. Later the pitch of the sounds was extracted and user performance analyzed. The results show that the time required to match a given pitch degrades about five times the introduced latency suggesting that the feedback latency cumulates over the whole task. Errors while playing along a sample melody increased 80% by average on the highest latency of 240ms. Latencies until 120ms increased the errors only slightly.
Download More Acoustic Sounding Timbre From Guitar Pickups Amplified guitars with pickups tend to sound ’dry’ and electric, whether the instrument is acoustic or electric. Vibration or pressure sensing pickups for acoustic guitars do not capture the body vibrations with fidelity and in the electric guitar with magnetic pickups there often is no resonating body at all. Especially with an acoustic guitar there is a need to reinforce the sound by retaining the natural acoustic timbre. In this study we have explored the use of DSP equalization to make the signal from the pickup sound more acoustic. Both acoustic and electric guitar pickups are studied. Different digital filters to simulate acoustic sound are compared, and related estimation techniques for filter parameters are discussed.