Electrical audio signal processing systems and devices – Noise or distortion suppression – Interpolation
Patent
1993-07-09
1995-03-14
Isen, Forester W.
Electrical audio signal processing systems and devices
Noise or distortion suppression
Interpolation
H04B 1500
Patent
active
053982860
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates generally to a system for enhancing analog signals in real time.
BACKGROUND OF THE INVENTION
In many environments, it is desirable, in fact necessary, to reduce the amplitude of noise, vibrations, and/or other interfering signals. The prior art has attempted to accomplish this reduction using a variety of techniques, both passive and active.
Passive reduction or attenuation is generally accomplished by disposing one or more layers of barrier, absorbing, and/or damping materials between the source of the noise or vibration and the area where a reduced or attenuated noise level is desired. While effective in some situations, passive attenuation systems are often unsuitable for applications where size, weight, and/or cost considerations prevent the use of attenuating materials.
Other prior art techniques have focussed on active signal reduction techniques such as Active Noise Cancellation (ANC). Active Noise Cancellation has received a considerable amount of interest in a variety of signal cancellation applications, e.g., air ducts, exhaust fans, zonal quieting, head phones, vibration cancellation in structures, and echo cancellation in electronic signal communications. The active reduction of sound waves in the audible range is performed by processing the electrical cancellation signals at a rate greater than the rate of propagation of those sound waves in a particular propagation medium. In the time it takes for a sound wave to propagate from a location where the sound is measured to a second location where it may be cancelled, there is time to sample the sound wave signal, process that information in a processing circuit, and produce a signal to drive an actuator to introduce a cancelling signal 180.degree. out-of-phase and equal in amplitude to the propagating sound wave.
The function block diagram shown in FIG. 1(a) is useful in explaining basic principles of active noise cancellation systems or vibration cancellation systems. A noise or vibrational disturbance 10 is detected by a suitable sensor 12. The sensor 12 converts the noise or vibration 10 into an electrical signal which is processed in some fashion in a controller 14. The controller 14 determines a cancellation signal, typically the inverse of the sensed noise or vibration signal, and uses this cancellation signal to drive an actuator 16. In the case of acoustic noise, the actuator 16 is simply a speaker. If the cancellation is appropriately timed, the original noise signal is cancelled by an output signal generated by the actuator 16. This cancellation is represented mathematically as a summation of the sensed and cancellation signals at a summer 18.
A graphic depiction of the cancellation process is provided in FIG. 1(b). The noise signal represented by a waveform signal "a" is essentially cancelled by another waveform signal "b" of equal amplitude but having a phase difference of 180.degree.. The sum of these two waveforms leaves only a residual signal "c".
Systems for actively cancelling repetitive noise and vibration have been proposed for example in Chaplin, U.S. Pat. Nos. 4,153,815; 4,490,841 and 4,654,871; as well as Warnaka et al, U.S. Pat. No. 4,562,589; and Ziegler, Jr., U.S Pat. No. 4,878,188.
In U.S. Pat. Nos. 4,153,815 and 4,654,871, Chaplin describes the use of a synchronizing timing generator to provide cancellation of a repetitive noise. Initially, a noise or vibration signal is detected and analyzed so that a cancelling signal waveform can be generated. Once the cancelling waveform has been determined, a controller and pulse generators attempt to synchronize the timing of the cancellation signal so that the noise or vibration is cancelled. Any remaining noise is fed back to the controller as an error signal. The noise signal is divided into multiple intervals, and the amplitude of the cancelling signal is adjusted in each interval in response to the sign or amplitude of the error signal.
In U.S. Pat. No. 4,490,841, Chaplin describes the use of Fourier transforms to process s
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Balestri Robert
Eng Stephen M. L.
Lezec Robert B.
Pagliuca Joseph P.
Rich W. Foster
Booz-Allen & Hamilton Inc.
Isen Forester W.
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