Electrical audio signal processing systems and devices – Sound effects
Reexamination Certificate
1998-08-19
2003-01-07
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Sound effects
C381S094100, C381S084000
Reexamination Certificate
active
06504935
ABSTRACT:
TECHNICAL FIELD
This invention relates to digital audio signal processing, specifically to an improved method for imparting a harmonic distortion effect to a digital audio signal.
BACKGROUND—DESCRIPTION OF PRIOR ART
Most electronic systems that process (amplify, transmit, etc.) analog signals are designed to minimize the amount of harmonic distortion added to the signal. When processing audio signals, however, some types of harmonic distortion are desirable. Audio professionals, musicians, and audiophiles widely pursue the sonic characteristics of vacuum tubes, analog recording tape, transducers, and a variety of solid-state distortion devices. Harmonic distortion due to non-linear transfer characteristics, either inherent or deliberately induced, is mainly responsible for imparting the desirable effect.
The progress of digital recording and signal processing technologies is at a point where most audio signal processing tasks can be performed satisfactorily in the digital domain. One notable exception is distortion effects. Digital distortion effects generally lack the accuracy needed to be convincing replacements for their analog counterparts and tend to possess a harsh quality that reveals their digital origins.
U.S. Pat. No. 4,995,084 to Pritchard (Feb. 19, 1991) relates to a variety of analog semiconductor circuits for emulating the general characteristics of a vacuum tube amplifier. It also discloses digital versions that approximate the operation of the analog circuits.
U.S. Pat. No. 5,570,424 to Araya et al. (Oct. 29, 1996) relates to fixed distortion algorithms based on higher degree expressions, mainly comprising a series of cubic expressions.
Extending the Karpus-Strong Algorithm to Synthesize Electric Guitar Timbres with Distortion and Feedback by Charles R. Sullivan in the Computer Music Journal (Fall 1990) relates to an algorithm for computer music synthesis that includes a simple distortion function for providing a soft clipping effect.
The distortion generation methods used by Pritchard, Araya et al., and Sullivan use single-purpose, fixed algorithms that offer no programmability or variation of the distortion effect other than setting the input signal level. They do not emulate the distortion characteristics of any actual analog distortion device but instead provide only some characteristics of a generalized class of distortion device. They also do not provide any method of suppressing the aliasing noise that can result when performing a non-linear operation in a digital system.
Cool Edit Software version 1.53 by Syntrillium Software Corporation (1992-1996) relates to a computer program that provides numerous utilities for modifying digital audio files. It includes a distortion utility for graphically building a distortion curve from straight line segments. The curve represents the graph of an arbitrary transfer function that is applied symmetrically to the positive and negative values of an audio file. The term transfer function refers to the static or direct-current (DC) transfer characteristics of a distortion device. Cool Edit's distortion is limited to generating only odd harmonics due to the symmetry of the transfer function. It does not provide any method of suppressing the aliasing noise that can result when performing a non-linear operation in a digital system. The segmentation of the distortion curve generates high harmonics that are usually undesirable and further add to the aliasing noise.
SPKitWaveShaper—User's Guide by Kai Lassfolk in Sound Processing Kit—User's Guide (1995) relates to a distortion routine included in a unified collection of computer software routines designed for modifying digital audio files. The distortion routine imports a user-supplied file containing look-up table values that it uses to non-linearly transform an audio file. The routine simply executes the distortion process and has no bearing on the nature and origin of the distortion's characteristics. It does not provide any method of suppressing the aliasing noise that can result when performing a non-linear operation in a digital system.
Digital Waveshaping Synthesis by Marc Le Brun and Digital Synthesis of Complex Spectra by Means of Multiplication of Nonlinear Distorted Sine Waves by Daniel Arfib, both in the Journal of the Audio Engineering Society (April 1979 and October 1979, respectively) relate to a digital music synthesis technique for generating a complex waveform by applying a non-linear waveshaping function to a sinusoidal waveform. The work of Le Brun and Arfib provides a good theoretical insight into the relationship of a waveshaping function and the harmonics it produces. However, the focus is on the direct synthesis of musical instrument sounds and, as such, offers no method for applying a distortion effect to digital audio signals.
Musical Applications of Microprocessors—Second Edition by Hal Chamberlin (1985) relates to music synthesis and sound modification, providing a comprehensive collection of techniques, both analog and digital. More specifically, pages 48-54 describe the application of non-linear waveshaping to instrumental signals using analog techniques. Pages 473-480 describe digital non-linear waveshaping and state the advantages of digital waveshaping over the analog techniques described in the earlier section. The digital waveshaping functions described are limited to the synthesis of musical instrument sounds when applied to a sinusoidal waveform. No method is offered for generating a waveshaping function that would emulate another distortion device when applied to a digital audio signal.
U.S. Pat. Nos. 4,868,869 and 4,991,218 to Kramer (Sep. 19, 1989 and Feb. 5, 1991) relate to the non-linear waveshaping of digital audio signals, specifically when applied with a look-up table and performed in real time. They describe various general methods for generating waveshaping functions that are suitable primarily for experimentation. None involve the analysis of another distortion device. As such, no method is offered for generating a waveshaping function that would emulate another distortion device. No method is provided for suppressing the aliasing noise that can result when performing a non-linear operation in a digital system.
U.S. Pat. No. 5,235,410 to Hurley (Aug. 10, 1993), U.S. Pat. No. 4,949,177 to Bannister et al. (Aug. 14, 1990), U.S. Pat. No. 5,253,043 to Gibson (Oct. 12, 1993), and U.S. Pat. No. 5,349,546 to Sharman (Sep. 20, 1994) all relate to digital video signal processing, specifically non-linear processing that includes methods for reducing alias frequencies in the output signal. Although they do apply a form of distortion to a digital signal, their non-linear processing is specifically for video and is irrelevant for audio.
Besides the limitations already stated, none of the prior art perform direct analyses of existing distortion devices for deriving methods or models that can be used to emulate the distortion devices.
SUMMARY OF THE INVENTION
Accordingly, several objects and advantages of my invention are to provide a distortion modeling and synthesis method whereby analog audio distortion devices are accurately emulated using digital techniques, to provide a distortion modeling and synthesis method capable of synthesizing a virtually unlimited variety of distortion effects, and to provide a distortion modeling and synthesis method capable of suppressing the aliasing noise that can result when performing a non-linear operation in a digital system.
It is an additional object of the invention to provide a distortion modeling and synthesis method that permits modification of the distortion in terms of the harmonics it produces.
It is an additional object of the invention to provide a distortion modeling and synthesis method that facilitates efficient storage of distortion models for later recall and usage.
To accomplish the above-mentioned objects, the harmonic distortion effect of an audio distortion device is modeled by determining the harmonic characteristics of the distortion imparted to a signal by the d
Isen Forester W.
McChesney Elizabeth
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