Jyri Pakarinen

Discrete-time modelling of musical instruments

This article describes physical modelling techniques that can be used for simulating musical instruments. The methods are closely related to digital signal processing. They discretize the system with respect to time, because the aim is to run the simulation using a computer. The physics-based modelling methods can be classified as mass–spring, modal, wave digital, finite difference, digital waveguide and source–filter models. We present the basic theory and a discussion on possible extensions for each modelling technique. For some methods, a simple model example is chosen from the existing literature demonstrating a typical use of the method. For instance, in the case of the digital waveguide modelling technique a vibrating string model is discussed, and in the case of the wave digital filter technique we present a classical piano hammer model. We tackle some nonlinear and time-varying models and include new results on the digital waveguide modelling of a nonlinear string. Current trends and future directions in physical modelling of musical instruments are discussed.

A review of digital techniques for modeling vacuum-tube guitar amplifiers

Although semiconductor technologies have displaced vacuum-tube devices in nearly all fields of electronics, vacuum tubes are still widely used in professional guitar amplifiers. A major reason for this is that electric-guitar amplifiers are typically overdriven, that is, operated in such a way that the output saturates. Vacuum tubes distort the signal in a different manner compared to solid-state electronics, and human listeners tend to prefer this. This might be because the distinctive tone of tube amplifiers was popularized in the 1950s and 1960s by early rock and roll bands, so musicians and listeners have become accustomed to tube distortion. Some studies on the perceptual aspects of vacuum-tube and solid-state distortion have been published (e.g., Hamm 1973; Bussey and Haigler 1981; Santo 1994). Despite their acclaimed tone, vacuum-tube amplifiers have certain shortcomings: large size and weight, poor durability, high power consumption, high price, and often poor availability of spare parts. Thus, it is not surprising that many attempts have been made to emulate guitar tube amplifiers using smaller and cheaper solid-state analog circuits (e.g., Todokoro 1976; Sondermeyer 1984). The next step in the evolution of tube-amplifier emulation has been to simulate the amplifiers using computers and digital signal processors (DSP). A primary advantage of digital emulation is that the same hardware can be used for modeling many different tube amplifiers and effects. When a new model is to be added, new parameter values or program code are simply uploaded to the device. Furthermore, amplifier models can be implemented

Wave Digital Simulation of a Vacuum-Tube Amplifier

Virtual analog modeling is needed when simulating classic analog circuitry by DSP, for example when emulating analog sound synthesis and sound reproduction systems. In this paper we show how wave digital filters (WDFs) can be applied to efficient real-time simulation of vacuum-tube amplifier stages, typical in professional guitar amplifiers, which pose nonlinear behavior for desired distortion effects

Model-based sound synthesis of the guqin

This paper presents a model-based sound synthesis algorithm for the Chinese plucked string instrument called the guqin. The instrument is fretless, which enables smooth pitch glides from one note to another. A version of the digital waveguide synthesis approach is used, where the string length is time-varying and its energy is scaled properly. A body model filter is placed in cascade with the string model. Flageolet tones are synthesized with the so-called ripple filter structure, which is an FIR comb filter in the delay line of a digital waveguide model. In addition, signal analysis of recorded guqin tones is presented. Friction noise produced by gliding the finger across the soundboard has a harmonic structure and is proportional to the gliding speed. For pressed tones, one end of a vibrating string is terminated either by the nail of the thumb or a fingertip. The tones terminated with a fingertip decay faster than those terminated with a thumb. Guqin tones are slightly inharmonic and they exhibit phantom partials. The synthesis model takes into account these characteristic features of the instrument and is able to reproduce them. The synthesis model will be used for rule based synthesis of guqin music.

Enhanced Wave Digital Triode Model for Real-Time Tube Amplifier Emulation

Electric circuits containing vacuum tubes form an integral part of various audio equipment, such as guitar amplifiers, certain equalizers, microphone preamplifiers, and dynamic range compressors. Although most audio signal processing operations are straightforward to implement with modern computers, real-time digital simulation of vacuum tubes poses a significant challenge due to the dynamic nonlinearities of the tube circuits. Most of the current vacuum-tube emulators model the unidirectional signal path of the circuit using linear filters and nonlinear waveshapers, possibly with signal-dependent parameters. This paper introduces a physical model of a tube stage circuit using wave digital filters. In contrast to previous unidirectional signal models, the wave digital model implements bidirectional signal propagation. This allows realistic simulation of interesting dynamical nonlinearities, such as the bias variation under reactive load. The new model is an extension of the wave digital tube presented in ¿Wave digital simulation of a vacuum-tube amplifier¿ (M. Karjalainen and J. Pakarinen Proc. Int. Conf. Acoustics, Speech, Signal Processing, 2006, vol. V, pp. 153-156). In particular, the enhanced model for the tube grid-to-cathode connection enables the simulation of interstage coupling and blocking distortion.

Shock impact reliability characterization of a handheld product in accelerated tests and use environment

Abstract The effect of mechanical shock impacts is a key factor in the reliability of modern handheld products. Due to differences in product enclosures, impact orientations, strike surfaces and mountings of component boards, the loading conditions induced in a true product drop differ from those encountered in standardized board-level tests. In order to better understand the correlation between board-level drop testing and actual drops of a complete device, series of board and product-level drop tests were conducted using specialized test boards. The mechanical shock impact response of the commercial handheld device component board was characterized with the help of acoustic excitation laser vibrometry and finite element analysis. The results were used to design the mechanically compatible specialized test board for both 4-point supported board-level and unsupported product-level drop tests. Special care was taken to ensure that the vibration behavior of the test board accurately represented the vibration behavior of the commercial component board. Additional board-level drop tests were conducted using a JEDEC JESD22-B111 compliant component board for comparison. The drop test results showed that, even though the test board design and supporting method have a marked influence on the strain conditions and lifetime of solder interconnections, the primary failure mode and mechanism under the product-level drop tests is comparable to that typically encountered in the standard JEDEC JESD22-B111 board-level drop tests. More detailed analyses suggest that the comparability of the shock impact loading conditions affecting solder interconnections can be characterized using three metrics: (1) the maximum component board strain rate, (2) the maximum board strain amplitude and (3) the damping of the component board.

Real-Time Audio Transformer Emulation for Virtual Tube Amplifiers

An audio transformer is used in a guitar amplifier to match the impedances of the power amplifier and a loudspeaker. It is important to understand the influence of the audio transformer on the overall sound quality for realistic tube amplifier emulation. This paper proposes to simulate the audio transformer using a wave digital filter model, which is based on the gyrator-capacitor analogy. The proposed model is two-directional in the sense that it outputs the loudspeaker current, but it also connects backward to the power amplifier thus affecting its behavior in a nonlinear manner. A practical parameter estimation procedure is introduced, which requires only the measurement of basic electrical quantities but no knowledge of material properties. Measurements of a Fender NSC041318 and a Hammond T1750V transformer are presented as case studies, as well as parameter fitting and simulation for the Fender transformer. The results show that these practical transformer designs introduce distortion at low frequencies only, below about 100 Hz for the Fender and 30 Hz for the Hammond transformer, and that the proposed model faithfully reproduces this effect. The proposed audio transformer model is implemented in real time using the BlockCompiler software. Parametric control allows varying and also exaggerating the model nonlinearities.

Virtual slide guitar

This article describes a system for virtual slide guitar playing. From the control point of view, it can be seen as a successor of the virtual air guitar (VAG) developed at Helsinki University of Technology (TKK) a few years ago (Karjalainen et al. 2006). The original VAG allows the user to control an electric guitar synthesizer by mimicking guitar-playing gestures. In the current work, the same gesture-control approach that was found successful in the VAG is used: a computer-visionbased system, in which a camera detects the players hands and a computer tracks the hand movements and converts them into control data, such as pluck events and string length. Sound synthesis in the virtual slide guitar application is based on an energy-compensated time-varying digital waveguide model of a guitar with new extensions to generate contact sounds caused by the slide tube touching the strings. Video files showing the virtual slide guitar in action can be found on the accompanying Web page (www.acoustics.hut.fi/publications/papers/vsg/) and the forthcoming 2008 Computer Music Journal Sound and Video Anthology DVD. Although the current implementation uses a camera-based user interface, it can also be controlled by other humanmachine interfaces or computer programs. Excellent reviews on gestural control of music synthesis have been written by Paradiso (1997) and by Wanderley and Depalle (2004). Camera-based gesture analysis has become very sophisticated owing to increased computing power and new methodologies provided by research, as exemplified by the EyesWeb platform (Camurri et al. 2000, 2005; Gorman et al. 2007). Digital waveguide modeling is mature technology, which can be applied to high-quality synthesis of various musical instruments (Smith 1992; Valimaki et al. 2006). For more information on waveguide synthesis of string instruments, see Valimaki et al. (1996), Karjalainen, Valimaki, and Tolonen (1998), and Karjalainen et al. (2006). The roots of this study are in an early prototype of the TKK virtual air guitar, which here is called the rubber-band virtual air guitar. It was a simplified gesture-control system, in which the distance between the hands was detected and di-

New Family of Wave-Digital Triode Models

A new family of wave-digital vacuum tube triode models is presented. These models are inspired by the triode model by Cardarilli , which provides realistic simulation of the triodes transconductance behavior, and hence high accuracy in saturation conditions. The triode is modeled as a single memoryless nonlinear three-port wave digital filter element in which the outgoing wave variables are computed by locally applying the monodimensional secant method to one or two port voltages, depending on whether the grid current effect is taken into account. The proposed algorithms were found to produce a richer static harmonic response, introducing comparable or less aliasing and requiring approximately 50% less CPU time than previous models. The proposed models are suitable for real-time virtual analog circuit simulation.

Recent Advances in Real-Time Musical Effects, Synthesis, and Virtual Analog Models

This paper reviews some of the recent advances in real-time musical effects processing and synthesis. The main emphasis is on virtual analog modeling, specifically digital emulation of vintage delay and reverberation effects, tube amplifiers, and voltage-controlled filters. Additionally, adaptive effects algorithms and sound synthesis and processing languages are discussed.