Multi-band, digital audio noise filter
First Claim
1. A multi-band, digital audio noise filter that permits a user to convert a digital audio input signal into a digital audio output signal, the filter comprising:
- a signal processor that processes the digital audio input signal to obtain the digital audio output signal by multiplying each of a plurality of filter coefficients with each of a plurality of time-based digital samples which compose a finite-length window of the digital audio input signal and by summing the results to obtain a digital value that is an output value of the digital audio output signal;
a user-interface that permits the user to vary contribution-thresholds for at least several of a plurality of frequency bins that correspond to composition frequencies of the digital audio input signal;
a filter generator that repeatedly updates said filter coefficients in dependence upon current input values of each of said digital audio input signal and said contribution-threshold for said plurality of frequency bins, said filter generator includingFFT means for receiving the digital audio input signal and applying thereto a Fast Fourier Transform to produce, in response thereto, at least one FFT value for each of the plurality of frequency bins, said FFT values each representing the contribution of harmonics to the digital audio input signal from frequencies within said frequency bins, index generating means for comparing, for each of said plurality of frequency bins, said FFT values which correspond to each frequency bin with a corresponding user-set signal threshold level, and for generating an attenuation index for each of said plurality of frequency bins in response thereto, said attenuation index representing an attenuation of harmonics within the frequency bin if said corresponding FFT value is less than said corresponding user-set signal threshold level, and,IFFT means coupled to said index generating means for generating and updating, in response to said attenuation index for each of said plurality of frequency bins, said plurality of filter coefficients.
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Accused Products
Abstract
This disclosure provides a multi-band, digital audio noise filter that is especially useful in the restoration of motion picture film soundtracks. More particularly, the preferred embodiment presented herein utilizes a remote fader board having eight faders which permit a user to control thresholds for sixty-four frequency bins. These faders are monitored by a MOTOROLA 56000-series microprocessor, which accepts digitized audio input signals, performs a Fast Fourier Transform upon a 128 sample window to yield signal contribution for each of the sixty-four frequency bins, and derives FIR filter coefficients for noise attenuation. The digitized audio input signals, which have been stored in a circular input buffer, are convolved with the FIR filter and output as the digitized output signals of the restored motion picture soundtrack.
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Citations
31 Claims
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1. A multi-band, digital audio noise filter that permits a user to convert a digital audio input signal into a digital audio output signal, the filter comprising:
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a signal processor that processes the digital audio input signal to obtain the digital audio output signal by multiplying each of a plurality of filter coefficients with each of a plurality of time-based digital samples which compose a finite-length window of the digital audio input signal and by summing the results to obtain a digital value that is an output value of the digital audio output signal; a user-interface that permits the user to vary contribution-thresholds for at least several of a plurality of frequency bins that correspond to composition frequencies of the digital audio input signal; a filter generator that repeatedly updates said filter coefficients in dependence upon current input values of each of said digital audio input signal and said contribution-threshold for said plurality of frequency bins, said filter generator including FFT means for receiving the digital audio input signal and applying thereto a Fast Fourier Transform to produce, in response thereto, at least one FFT value for each of the plurality of frequency bins, said FFT values each representing the contribution of harmonics to the digital audio input signal from frequencies within said frequency bins, index generating means for comparing, for each of said plurality of frequency bins, said FFT values which correspond to each frequency bin with a corresponding user-set signal threshold level, and for generating an attenuation index for each of said plurality of frequency bins in response thereto, said attenuation index representing an attenuation of harmonics within the frequency bin if said corresponding FFT value is less than said corresponding user-set signal threshold level, and, IFFT means coupled to said index generating means for generating and updating, in response to said attenuation index for each of said plurality of frequency bins, said plurality of filter coefficients. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. A multi-band, digital audio noise filter that permits a user to convert an audio input signal into a digital audio output signal, the filter comprising:
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a user-interface whereby the user may vary contribution-thresholds for at least several of a plurality of frequency bins that correspond to frequencies which combine to form the digital audio input signal; a digitizing mechanism that is coupled to receive the audio input signal, to sample said audio input signal, and to produce, in response thereto, a digitized audio input signal; a microprocessor-based system having a first connector that couples said microprocessor-based system to said user-interface to receive therefrom said contribution-thresholds, convolution means for convolving a window of samples of said digitized audio input signal with a digital filter having filter coefficients and for producing therefrom the digital audio output signal, a second connector that couples said digitizing mechanism to said convolution means such that said convolution means receives said digitized audio input signal, and filter configuration software that directs said microprocessor-based system to periodically and repeatedly sample said user-interface so as to receive sampled, digitized threshold values representative of said user-set contribution-thresholds, apply a Fast Fourier Transform to said digitized audio input signal to produce therefrom estimates of signal contribution for each of said plurality of frequency bins, compare said estimates, for each of said plurality of frequency bins, with a corresponding one of said contribution-thresholds, derive attenuation indices for each of said plurality of frequency bins in response to the comparison, and perform an inverse Fast Fourier Transform in response to said attenuation indices so as to periodically and repeatedly update said filter coefficients of said digital filter, and convolution software that directs said convolution means to convolve said digitized audio input signal with said digital filter to produce said digital audio signal output. - View Dependent Claims (15, 16, 17, 18, 19, 20)
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21. A method of reducing noise in at least one audio channel without the need of a test signal dedicated to noise measurement or a period of silence on an audio input signal, the method utilizing an analog-to-digital converter, data registers (which may be defined in random access memory), a microprocessor-based system that convolves (in the time-domain) a digital filter with the audio input signal to produce a digital audio output signal, and a user-interface adapted to create at least one electronic signal that represents user-variations of contribution-thresholds for at least several of a plurality of frequency bins, wherein the microprocessor-based system has random access memory and is coupled to the user-interface so as to receive the electronic signals, and wherein the digital filter is alterable in response to the user-settings of the contribution-thresholds for each of the plurality of frequency bins, the method comprising the steps of:
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applying the analog-to-digital converter to the audio input signal to produce a sequence of digital samples that form a digital audio input signal representative of the audio input signal; using the microprocessor-based system to obtain the contribution-thresholds by monitoring the electronic signals that represent the user-settings, apply a Fast Fourier Transform to a window of digital samples of the digital audio input signal to produce at least one FFT value for each of the plurality of frequency bins, for each of the plurality of frequency bins, compare the corresponding contribution-threshold with the corresponding FFT values, generate attenuation indices for each of the plurality of frequency bins in response to the comparison, the attenuation indices representing attenuation to be applied to all harmonic components falling within corresponding ones of the plurality of frequency bins, use a inverse Fast Fourier Transform to derive, from the attenuation indices, coefficients of the digital filter, and repeat each of the above steps to thereby periodically update coefficients of the digital filter; and
,using the microprocessor-based system to apply the digital filter to the audio input signal to produce the audio output signal. - View Dependent Claims (22, 23, 24, 25, 26, 27, 28, 29, 30)
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31. A multi-band filter that attenuates noise in a prerecorded sound track in response to threshold settings interactively supplied from a user that concurrently listens to the audio output, comprising:
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a user-interface that permits the user to independently vary thresholds for at least several of a plurality of frequency bins that represent harmonics of the sound track; a signal processor that receives an input signal representing the sound track, applies a filter to it to provide attenuation to it, and generates therefrom the audio output; a filter generator that repeatedly determines coefficients of the filter applied by said signal processor, said filter generator including a Fast Fourier Transform stage that receives the input signal and applies to the input signal a Fast Fourier Transform to produce, in response thereto, at least one value for each of frequency bin, an index generating stage that receives the thresholds from the user-interface, compares said at least one value for each bin with a corresponding threshold, and derives an attenuation index that represents attenuation of harmonics within the frequency bin if said corresponding FFT value is less than said corresponding user-set threshold, and, an Inverse Fast Fourier Transform stage that processes said attenuation index for each of said plurality of frequency bins, to derive the coefficients of the filter; and an audio player that permits the user to concurrently listen to the audio output and interactively adjust the thresholds for the frequency bins.
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Specification