Virtual analog synthesis and audio effects

Virtual analog synthesis refers to the real-time simulation of electronic music synthesis, which has been traditionally implemented using analog electronics. Around 1995, music technology companies started to develop digital musical instruments based on the virtual analog approach. In some products, samples are used to play back old sounds. We are developing discrete-time algorithms to simulate the analog filters and the waveform generation of analog music synthesizers. This work can be seen parallel to our physical modeling research, where we design computational simulations of acoustic systems - in virtual analog synthesis, computational simulations of electronic systems are designed. A related topic is the simulation of tube amplifiers used by electric guitar, bass, and electric organ players. This problem is difficult due to nonlinearities in the tube behavior. Our goal is to obtain physically valid digital models for tube amplifiers. This implies accurate imitations of all electrical parts of the amplifier. A further topic of interest is the modeling of analog effects processing boxes, such as phasers and flangers.

The Minimoog was one of the most successful analog music synthesizers (Photo by Laurent de Soras).

The following table lists our virtual analog synthesis and audio effects - related research in inverse chronological order. For some publications, the PDF-file and a companion web-page containing sound examples are provided. These can be found in the leftmost column.

Publication Short description V. Välimäki and A. Huovilainen, "Antialiasing oscillators in subtractive synthesis," IEEE Signal Processing Magazine, vol. 24, no. 2, pp. 116–125, March 2007. Signal processing techniques for implementing digital waveform oscillators with reduced aliasing are reviewed. Currently known methods are classified, briefly reviewed, and compared. A new method based on polynomial bandlimited step functions (PolyBLEP) is introduced. An auditory error measure, the noise-to-mask ratio (NMR), is used for evaluating the sound quality. M. Karjalainen and J. Pakarinen, "Wave digital simulation of a vacuum-tube amplifier", in the IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP'06), Toulouse, France, May 15-19, 2006. This paper discusses wave digital simulation of tube amplifier stages, typical e.g. in guitar amplifiers. A new real-time model is introduced and different implementation techniques are presented. N/A V. Välimäki and A. Huovilainen, "Oscillator and Filter Algorithms for Virtual Analog Synthesis", in Computer Music Journal, vol. 30, no. 2, 2006. New algorithms for the generation of alias-free digital versions of classical analog waveforms (the sawtooth, pulse, and triangular wave) are introduced. Modifications and simplifications to the digital nonlinear model of the Moog ladder filter proposed previously by Huovilainen are given. A. Huovilainen, "Enhanced digital models for analog modulation effects", in Proc. 8th Int. Conf. on Digital Audio Effects (DAFx'05), pp. 155-160, Madrid, Spain, Sept. 2005. The operation and non-idealities of analog phaser and flanger / chorus effects are discussed. Two new digital models for phaser and one for flanger / chorus are presented that more closely match the operation of the analog implementations. R. Trausmuth and A. Huovilainen, "Powerwave - A high performance single chip interpolating wavetable synthesizer", in Proc. 8th Int. Conf. on Digital Audio Effects (DAFx'05), pp. 293-296, Madrid, Spain, Sept. 2005. Design for a high quality wavetable synthesizer is presented. The synthesizer is implemented with a single Xilinx FPGA chip containing both audio generation and control functions. N/A A. Huovilainen and V. Välimäki, "New Approaches to Digital Subtractive Synthesis", in Proc. Int. Computer Music Conf. (ICMC'05), Barcelona, Spain, pp. 399-402, Sept. 2005. This paper presents new methods for digital subtractive synthesis that have both low computational cost and relatively high quality. The DPW algorithm for producing reduced alias sawtooth waveform is reviewed. A new variant of the digital Moog filter with multiple responses and low cost non-linearity is also shown. V. Välimäki and A. Huovilainen, "Virtuaalista nostalgiaa - digitaalinen vähentävä äänisynteesi", Musiikki, vol. 35, no. 1-2, pp. 78-98, 2005. This is a review of oscillators and resonant filters for digital subtractive music synthesis in the Finnish language. The DPW sawtooth algorithm, the simplified nonlinear Moog filter, and two examples of digital synthesizer patches are briefly explained. N/A V. Välimäki, "Discrete-Time Synthesis of the Sawtooth Waveform with Reduced Aliasing", IEEE Signal Processing Letters, vol. 12, no. 3, pp. 214-217, March 2005. This paper introduces a new efficient signal processing technique, called the DPW, to generate an alias-free sawtooth waveform for virtual analog synthesis. The algorithm differentiates a piecewise parabolic waveform, and it is easy to implement with a computer. Two variants of the algorithm are given, one of which employes multirate signal processing. N/A M. Karjalainen, T. Mäki-Patola, A. Kanerva, A. Huovilainen, and P. Jänis, ''Virtual Air Guitar,'' in Proceedings of AES 117th Convention, preprint no. 6203, San Francisco, CA, USA, October 28-31, 2004. Signal processing and gesture-control methods are discussed that enable playing of an electric guitar synthesizer by waving your hands in the air. Highly entertaining real-time demonstrations have been implemented using these techniques. For more info, see http://airguitar.tml.hut.fi/. A. Huovilainen, "Non-linear Digital Implementation of the Moog Ladder Filter", in Proc. 7th Int. Conf. Digital Audio Effects (DAFx'04), pp. 61-64, Naples, Italy, October 5-8, 2004. This paper analyzes the structure and behaviour of the Moog ladder filter. A large-signal model is presented and discretized to yield a digital model that correctly simulates the non-linear behaviour. An alternative method of tuning correction is also presented.