This contribution provides an overview of optical techniques for sound recording, sound synthesis, and sound processing. Several applications of optical devices, examples, and references describing the history and current trends in this field will be given. For sound recording, optical devices can be used to capture vibrations and to transform mechanical sound waves into an electrical signal. Optical storage of audio has a long history beginning with analog sound tracks on film up to recent optical media as for example the Blue-ray Disc. For sound synthesis based on optical systems we will give examples based on graphical sound and the photo-sonic disc for time-domain synthesis. Frequency-domain synthesis techniques which are controlled by a time-frequency (image domain) signal representation will be explained. For analog sound processing the use of a light emitting diode coupled to a photo-electric cell (light dependent resistor) constitutes a nonlinear two-port electrical network or element. These optocouplers have still a wide range of applications for switching, gating, and volume control of audio signals due to their special nonlinear behavior. Hence, there exists a broad interest for high-quality discrete-time models of these devices. An overview of several applications with these optical nonlinearities will be given.
KEYWORDS: Analog electronics, Process modeling, Light emitting diodes, Instrument modeling, Optical isolators, Analog electronics, Optical isolators, Resistance, Optimization (mathematics), Linear filtering, Amplifiers, Systems modeling
Virtual analog modeling is the process of creating a digital model of an analog system. In this work a virtual analog model of a dynamic range compression circuit for electrical guitars is constructed by analyzing and measuring the analog reference system. The particular property of the chosen compression system is the use of an analog optical isolator, also called optocoupler. It is a two-port circuit element used to electrically isolate different parts of the audio system while maintaining one-directional coupling via the optical channel. The used analog optical isolator was a Perkin Elmer VTL5C2, consisting of a light dependent resistor and a light emitting diode in an opaque enclosure. All the characteristics of the nonlinear elements were measured, especially the VTL5C2, then the circuit was analyzed to determine its static behavior. In the digital model the output signal is multiplied with a timevariant gain factor, which is dependent on the input signal. Several processing blocks are used to calculate the gain factor, emulating the static and dynamic behavior of the analog reference system. An iterative error minimization procedure is used to refine parameters for the digital model. Finally the output of digital model and analog reference are compared to show how well the model has been adapted to the reference device.
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