Julian Parker

Fifty Years of Artificial Reverberation

The first artificial reverberation algorithms were proposed in the early 1960s, and new, improved algorithms are published regularly. These algorithms have been widely used in music production since the 1970s, and now find applications in new fields, such as game audio. This overview article provides a unified review of the various approaches to digital artificial reverberation. The three main categories have been delay networks, convolution-based algorithms, and physical room models. Delay-network and convolution techniques have been competing in popularity in the music technology field, and are often employed to produce a desired perceptual or artistic effect. In applications including virtual reality, predictive acoustic modeling, and computer-aided design of acoustic spaces, accuracy is desired, and physical models have been mainly used, although, due to their computational complexity, they are currently mainly used for simplified geometries or to generate reverberation impulse responses for use with a convolution method. With the increase of computing power, all these approaches will be available in real time. A recent trend in audio technology is the emulation of analog artificial reverberation units, such as spring reverberators, using signal processing algorithms. As a case study we present an improved parametric model for a spring reverberation unit.

A Virtual Model of Spring Reverberation

The digital emulation of analog audio effects and synthesis components, through the simulation of lumped circuit components has seen a large amount of activity in recent years; electromechanical effects have seen rather less, primarily because they employ distributed mechanical components, which are not easily dealt with in a rigorous manner using typical audio processing constructs such as delay lines and digital filters. Spring reverberation is an example of such a system-a spring exhibits complex, highly dispersive behavior, including coupling between different types of wave propagation (longitudinal and transverse). Standard numerical techniques, such as finite difference schemes are a good match to such a problem, but require specialized design and analysis techniques in the context of audio processing. A model of helical spring vibration is introduced, along with a family of finite difference schemes suitable for time domain simulation. Various topics are covered, including numerical stability conditions, tuning of the scheme to the response of the model system, numerical boundary conditions and connection to an excitation and readout, implementation details, as well as computational requirements. Simulation results are presented, and full energy-based stability analysis appears in an ????? .

Linear Dynamic Range Reduction of Musical Audio Using an Allpass Filter Chain

The reduction of signal dynamic range through limiting of peak amplitude is an important process in modern audio signal processing, mainly for loudness maximisation. Traditional processes are non-linear, and can produce significant distortion of the processed signal. In this paper we present a new linear technique that reduces the peak amplitude of transient signals using golden ratio allpass filters. The system is applied to test signals consisting of both isolated musical sounds and mixed musical audio. The average reduction of the peak amplitude of the musical passages considered is 2.5 dB. The system can be applied alongside non-linear methods, to reduce the distortion associated with a particular reduction in peak amplitude.

A directional diffuse reverberation model for excavated tunnels in rock

Acoustic impulse responses of an excavated tunnel were measured. Analysis of the impulse responses shows that they are very diffuse from the start. A reverberator suitable for reproducing this type of response is proposed. The input signal is first comb-filtered and then convolved with a sparse noise sequence of the same length as the filters delay line. An IIR loop filter inside the comb filter determines the decay rate of the response and is derived from the Yule-Walker approximation of the measured frequency-dependent reverberation time. The particular sparse noise sequence proposed in this work combines three velvet noise sequences, two of which have time-varying weights. To simulate the directional soundfield in a tunnel, the use of multiple such reverberators, each associated with a virtual source distributed evenly around the listener, is suggested. The proposed tunnel acoustics simulation can be employed in gaming, in film sound, or in working machine simulators.

Efficient Dispersion Generation Structures for Spring Reverb Emulation

Spring reverberation is a sonically unique form of artificial reverberation, desirable as an effect distinct from that of more conventional reverberation. Recent work has introduced a parametric model of spring reverberation based on long chains of all-pass filters. Such chains can be computationally expensive. In this paper, we propose a number of modifications to these structures, via the application of multirate and multiband methods. These changes reduce the computational complexity of the structure to one third of its original cost and make the effect more suitable for real-time applications.