Oral Session 6: Virtual Analog and Spatial Audio

15:40-17:00 on Wednesday, 4th September

P350 Lecture Theatre, Parkside

Chiar: Kurt Werner
Digital Grey Box Model of the Uni-Vibe Effects Pedal
Champ Darabundit, Russell Wedelich and Pete Bischoff

This paper presents a digital grey box model of a late 1960s era Shin-ei Uni-Vibe(r) 1 analog effects foot pedal. As an early phase shifter, it achieved wide success in popular music as a unique musical effect, noteworthy for its pulsating and throbbing modulation sounds. The Uni-Vibe is an early series all-pass phaser effect, where each first-order section is a discrete component phase splitter (no operational amplifiers). The dynamic sweeping movement of the effect arises from a single LFO-driven incandescent lamp opto-coupled to the light dependent resistors (LDRs) of each stage. The proposed method combines digital circuit models with measured LDR characteristics for the four phase shift stages of an original Uni-Vibe unit, resulting in an efficient emulation that preserves the character of the Uni-Vibe. In modeling this iconic effect, we also aim to offer some historical and technical insight into the exact nature of its unique sound.

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Sound Source Separation in the Higher Order Ambisonics Domain
Mohammed Hafsati, Nicolas Epain, Rémi Gribonval and Nancy Bertin

In this article we investigate how the local Gaussian model (LGM) can be applied to separate sound sources in the higher-order ambisonics (HOA) domain. First, we show that in the HOA domain, the mathematical formalism of the local Gaussian model remains the same as in the microphone domain. Second, using an off-the shelf source separation toolbox (FASST) based on the local Gaussian model, we validate the efficiency of the approach in the HOA domain by comparing the performance of toolbox in the HOA domain with its performance in the microphone domain. To do this we discuss and run some simulations to ensure a fair comparison. Third, we check the efficiency of the local Gaussian model compared to other available source separation techniques in the HOA domain. Simulation results show that separating sources in the HOA domain results in a 1 to 12 dB increase in signal-to-distortion ratio, compared to the microphone domain. Multichannel source separation, local Gaussian model, Wiener filtering, 3D audio, Higher Order Ambisonics (HOA).

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Directional Source Simulation in FDTD Room Acoustic Modeling via Weighted First-Order Reflections
Stephen Oxnard

This paper proposes a means of simulating directional sources in Finite Difference Time Domain (FDTD) based acoustic models for impulse response (IR) calculation which, unlike other currently available methods, is able to accommodate highly irregular directivity patterns. Weighted numerical free-field IRs are applied to the simulation of first-order reflections at the boundaries of a modeled acoustic domain such that the continuing interior wave field is that which would be produced by a directional sound source. Numerical results demonstrate the application of this approach for a 2-Dimensional wave field, implementing both simple and irregular (e.g. highly directional, discontinuous) sound source directivity patterns.

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First-Order Ambisonic Coding with PCA Matrixing and Quaternion-Based Interpolation
Pierre Mahe, Stephane Ragot and Sylvain Marchand

We present a spatial audio coding method which can extend existing speech/audio codecs, such as EVS or Opus, to represent first-order ambisonic (FOA) signals at low bit rates. The proposed method is based on principal component analysis (PCA) to decorrelate ambisonic components prior to multi-mono coding. The PCA rotation matrices are quantized in the generalized Euler angle domain; they are interpolated in quaternion domain to avoid discontinuities between successive signal blocks. We also describe an adaptive bit allocation algorithm for an optimized multi-mono coding of principal components. A subjective evaluation using the MUSHRA methodology is presented to compare the performance of the proposed method with naive multi-mono coding using a fixed bit allocation. Results show significant quality improvements at bit rates in the range of 52.8 kbit/s (4 × 13.2) to 97.6 kbit/s (4 × 24.4) using the EVS codec.

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