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This is a real-time emulation of the 'Moog Ladder' Resonant lowpass filter (by solving the equations of the original electronic circuits using the Volterra Series). The success of this analog filter is its non-linearity (the patch changes with the signal level which provides a "special sound signature" and "grain"), its high order (4), its easily controllable resonance (the control parameters are directly related to the quality factor and cutoff frequency). The benefit of using the Volterra series for this model is: reduced computational costs, the ability to make rapid changes in the parameters without causing artifacts, to prevent aliasing often unavoidable in nonlinear processes, and artificially separate the linear and the distorted components. This last point may be of interest for sound design and creates interesting effects, e.g impulse sounds and percussion. Several versions exist codes: A VST-Mac developed in C++ by T. Helie, a version developed by J. Max Lochard, and then an optimized version in C++ and archived (svn repository MoogFilter) by T. Carpentier.


Function 1

The first function performed is to take an input and provide a sound output corresponding to the filtered version. The filter is controlled very simply by two parameters (cutoff frequency, quality factor of resonance). The level of the input signal has an effect on the timbre.

Function 2

The linear portion (part of the stamp not sensitive to signal level, as a "standard filter") and further the distorted part can be isolated and directed to two separate outputs.

Associated Software: IrcamFilter
Associated People: Thomas Hélie