SARC Research Presented at DAFx 2017
The 20th edition of the International Conference on Digital Audio Effects (DAFx) took place at the University of Edinburgh from 5–9 September 2017 and was hosted by SARC alumnus Prof Stefan Bilbao and his Acoustics and Audio Group. This year’s DAFx featured a significant SARC contingent, including contributions from two Faculties—Dr Maarten van Walstijn (Lecturer in Music Technology, EEECS) and Dr Kurt James Werner (Lecturer in Audio, AEL)—as well as three of Dr van Walstijn’s PhD supervisees: Jamie Bridges, Ben Holmes and Sandor Mehes. All five contributed to the peer review process and both Drs van Walstijn and Werner also served on the conference’s program committee. Collectively, the SARC group presented four first-authored conference papers and were involved in an additional four as co-authors. Five papers involved international collaborations with academic institutions including CIRMMT / McGill University in Canada, Helmut Schmidt Universität in Germany, and CCRMA at Stanford University in the United States. Two papers involved collaboration with Berlin-based industry researchers from Native Instruments and E-RM Erfindungsbüro.
This year, submissions on physical modelling and virtual acoustic and analogue models were especially encouraged. The SARC papers aligned closely with this theme, contributing theoretical advances, demonstrating case studies, and presenting artistic positions on topics related to modal string simulation, percussion synthesis, and virtual analogue modelling frameworks and transistor models.
Modal Approaches to Acoustic Instrument Simulation
A first group of three papers dealt with modal approaches to the simulation of acoustic instruments, including string instruments and bells. ‘An explorative string-bridge model with tunable parameters’ by Maarten van Walstijn and Sandor Mehes  reports on a computer algorithm that simulates a generalised string-bridge-plate musical instrument using modal representations of the string and plate, coupled nonlinearly by a bridge. This enables real-time controlled synthesis of a range of new mechano-acoustic sound effects, including bridge rattling (sound examples available on http://www.socasites.qub.ac.uk/mvanwalstijn/dafx17a/). This work expands on an earlier bridge-less model ; a description of a preliminary version and associated user study are given in ‘Virtual-acoustic instrument design: Exploring the parameter space of a string-plate model’ (Mehes, van Walstijn and Stapleton ), which was awarded Best Poster Award at NIME 2017 in Copenhagen, Denmark.
‘Modal based tanpura simulation: Combining tension modulation and distributed bridge interaction’ by Jamie Bridges and Maarten van Walstijn  adds tension modulation to a physics-based model of the Indian tanpura plucked string instrument in a modal context. The main contribution of the paper is a new numerical formulation to combine tension modulation with collisions while preserving both numerical stability and computational robustness. In addition, the model allows investigating the effects of tension modulation and the tanpura’s distributed bridge shape (for an aural assessment, see http://www.socasites.qub.ac.uk/mvanwalstijn/dafx17c/). Based on strong reviewer scores and interest from the journal editors, this work has been invited for expansion and submission to a special issue on ‘Sound and Music Computing’ in Applied Sciences. This work continues a tanpura modelling theme at SARC, including ‘A real-time synthesis oriented tanpura model’ (van Walstijn, Bridges and Mehes ), which was awarded 4th Prize in the Best Paper Awards at DAFx 2016.
The first two papers use modal descriptions in the context of numerical simulation of the equations of motion of musical instruments. A third paper, ‘Modal audio effects: A carillon case study’ by Elliot Kermit Canfield-Dafilou (CCRMA) and Kurt James Werner  rather fits modal representations to audio recordings of real carillon bells, using techniques from the recent modal approach to artificial reverberation . This sidesteps the complicated equations of motion for vibrating bodies and yields a simplified and pliable representation that can be used for abstract bell-derived sound synthesis. In particular, the proposed techniques are applied to a corpus of carillon recordings from the University of Michigan Lurie Carillon. Sound examples and modal data are available here: https://ccrma.stanford.edu/~kermit/website/bells.html.
Virtual Analogue Modelling of Audio Circuits
A second group of five papers expanded on the theory and application of virtual analogue modelling of audio circuits. The first four deal with the wave digital filter formalism. ‘Generalizing root variable choice in wave digital filters with grouped nonlinearities’ by Kurt James Werner, Michael Jørgen Olsen (CCRMA), Maximilian Rest (E-RM) and Julian D. Parker (N.I.)  leverages graph theory to expand Dr Werner's grouped nonlinearity approach to wave digital filter modelling of audio circuits – to encompass a more universal class of circuits involving nonlinearities.
Wave digital filter modelling usually assumes a fixed simulation time step; ‘Network variable preserving step-size control in wave digital filters’ by Michael Jørgen Olsen (CCRMA), Kurt James Werner and François G. Germain (CCRMA)  proposes a new technique for using variable step sizes without distorting the simulation voltage and current, applying this technique along with a novel anti-aliasing application to the simulation of a relaxation square-wave oscillator. The third author François Germain will join the School of Arts, English and Languages as a Visiting Research Associate from September 2017 through April 2018 to continue research on discretisation of audio circuits for virtual analogue.
‘Modeling circuits with operational transconductance amplifiers using wave digital filters’ by Ólafur Bogason (McGill) and Kurt James Werner  applies the techniques of  to the simulation of audio circuits involving OTAs and a case study on a classic, extra-funky auto-wah guitar pedal. Sound examples are available here: https://soundcloud.com/obblong/sets/funk-guitar-simulation. ‘WDF modeling of a Korg MS-50 based non-linear diode bridge VCF’ by Maximilian Rest (E-RM), Julian D. Parker (N.I.) and Kurt James Werner  is a case study applying the techniques of  to the simulation of a vintage Korg filter based on the embedding of a unique arrangement of six diodes into a Sallen-Key topology, along with an analysis of that ‘diode bridge’.
Virtual analogue approaches often rely on simplified models of device physics. A final paper ‘Comparison of germanium bipolar junction transistor models for real-time circuit simulation’ by Ben Holmes, Martin Holters (Helmut Schmidt Universität) and Maarten van Walstijn  uses simulations of two vintage Dallas guitar effect pedals—the Rangemaster Treble Booster and the Arbiter Fuzz-Face (a favourite of Jimi Hendrix)—to compare a simplified (Ebers-Moll) model of germanium BJT transistors to a more advanced models and assesses the differences in a virtual analogue modelling context. Sound examples are available here: https://github.com/bholmesqub/DAFx17.