Virtual devices


Virtual counterparts of the devices of the Studio were developed through the following steps.

  1. Analysis of the devices through project schemes and direct inspection;
  2. Validation of the analysis through simulations with ad hoc tools;
  3. Development of physical models of the analogue devices, which allow efficient and accurate numerical simulations of their functioning;

Some specific electrophone instruments of the Studio, namely the early sinusoidal oscillators and the so-called amplitude selector, were considered as relevant case studies, both from a technical and from a historical and musicological viewpoints. The oscillators are particularly interesting because they no longer exist, and only the original project schemes are still available. On the other hand, the amplitude modulator is of particular interest both for its design (it is a highly non-linear filter based on an original project by Lietti), and for its historical role in the musical production of the Studio. The original project schemes are generally well documented in terms of characteristics of the basic components and of circuitry. In some cases where the documentation is not detailed enough, it was necessary to inspect directly the original instruments through ad-hoc measurements on some components.

By analysing the original schemes drawn by Lietti, one can see that the oscillators’ project is based on a well known bridge circuit: the “Wien bridge oscillator”, originally developed by German physicist Max Wien in 1891 and commonly used for audio sinusoidal oscillators. The modern circuit is derived from the formulation proposed by William Hewlett in his master’s degree thesis at Stanford University (1939) and later commercialized by new-born company Hewlett-Packard (co-founded by Hewlett and David Packard).

The amplitude selector still exists, although the control panel is not the one original one, and is instead the one which was incorporated in the post-1968 equipment. A direct inspection showed that the original project schemes provide an accurate documentation of the actual circuitry. It was found that one of the valves currently installed in the instrument does not correspond to the one indicated in the original scheme. This inconsistency is probably to be attributed to a later replacement due to failure of the original component. Other details cannot be inferred from the scheme. Pictures of the internal circuitry of the amplitude selector reveal a in the metal case at the bottom of the figure, which shows that the second (post-1968) control panel was simply superimposed on the original one.

All circuit schemes were numerically simulated using SPICE (a software especially designed to analyse and simulate analogue electronic circuits), to provide benchmarks of the functioning of the devices. Starting from these simulations, simplified versions of the circuits were developed (e.g., some of the non-linear elements were substituted by their linearized counterparts since after analysis of benchmark simulations). Finally, real-time models were developed (first through prototyping in Matlab, then in C language and within the Pure-Data platform), in which the computational load was highly optimized using numerical techniques developed by DREAM researchers.

 Posted by at 16:21