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Amplifier Feedback

A more extreme effect used with distorted electric guitars is amplifier feedback. In this case, the amplified guitar waveforms couple back to the strings with some gain and delay, as depicted schematically in Fig.9.4 [489].

Figure 9.4: Simulation of a basic distorted electric guitar with amplifier feedback.
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The Amplifier Feedback Delay in the figure can be adjusted to emphasize certain partial overtones over others. If the loop gain, controllable via the Amplifier Feedback Gain, is greater than 1 at any frequency, a sustained ``feedback howl'' will be produced. Note that in commercial devices, the Pre-distortion gain and Post-distortion gain are frequency-dependent, i.e., they are implemented as pre- and post-equalizers (typically only a few bands, such as three). Another simple choice is an integrator $ g/(1-rz^{-1})$ for the pre-distortion gain, and a differentiator $ (1-rz^{-1})$ for the post-distortion gain.

Faust software implementing electric-guitar amplifier feedback may be found in [454].


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About the Author: Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.


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