Distortion-cancellation circuit for audio peak limiting
First Claim
1. In an audio peak limiting apparatus, an improvement for reducing distortion comprising:
- a clipper coupled to receive an input audio signal;
a differencing means coupled to receive said input audio signal and the output of said clipper, for providing a signal representative of the clippings from said input audio signal;
a band-pass filter coupled to said output of said clipper;
a voltage-controlled amplifier for amplifying low-frequencies in said input audio signal;
control signal generation means coupled to said differencing means for generating a control signal for said amplifier, said control signal representing a direct current level and low-frequency spectral energy in said signal representative of said clippings, said control signal coupled to said amplifier such that less amplification occurs as said low-frequency spectral energy increases;
combining means for combining the outputs of said band-pass filter and said amplifier to provide an output signal.
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Abstract
Apparatus for reducing distortion in a peak limited audio siganl. The amount of low-frequency spectral energy in the input audio signal is determined for a peak limited signal and used to control low-frequencies in the output audio signal. Identical low-pass filters with cut-off frequencies of 2.2 kHz are used; one passing a peak limited signal, the other a non-peak limited signal. When no noticeable low-frequency spectral energy is present in the peak limited (clipped) signal, the peak level is controlled below 2.2 kHz, without distortion, by a VCA and above 2.2 kHz by a clipper. When low-frequency spectral energy is present, the VCA reduces gain on each low-frequency wave to prevent uncontrolled safety clipping.
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Citations
12 Claims
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1. In an audio peak limiting apparatus, an improvement for reducing distortion comprising:
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a clipper coupled to receive an input audio signal; a differencing means coupled to receive said input audio signal and the output of said clipper, for providing a signal representative of the clippings from said input audio signal; a band-pass filter coupled to said output of said clipper; a voltage-controlled amplifier for amplifying low-frequencies in said input audio signal; control signal generation means coupled to said differencing means for generating a control signal for said amplifier, said control signal representing a direct current level and low-frequency spectral energy in said signal representative of said clippings, said control signal coupled to said amplifier such that less amplification occurs as said low-frequency spectral energy increases; combining means for combining the outputs of said band-pass filter and said amplifier to provide an output signal. - View Dependent Claims (2, 3, 4, 5, 6, 8, 9)
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7. An improvement for reducing distortion in an audio peak limiting apparatus comprising:
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a clipper coupled to receive an input audio signal; a first low-pass filter having a cut-off frequency of f1, coupled to receive the output of said clipper; a second low-pass filter having a cut-off frequency of f2 coupled to receive said output of said clipper, where f1 is a higher frequency than f2; combining means for combining audio signals, the outputs of said first low-pass filter and second low-pass filter being coupled to said combining means such that said output of said second low-pass filter is subtracted from said output of said first low-pass filter; a third low-pass filter having a cut-off frequency of approximately f2, the input of said third low-pass filter being coupled to receive said input audio signal; a voltage-controlled amplifier coupled to receive the output of said third low-pass filter, the output of said amplifier being coupled to said combining means so as to be combined wtih said output of said first low-pass filter, the output of said combining means providing the output signal of said audio peak limiting apparatus; a differencing means coupled to receive said input audio signal and said output of said clipper, for providing a signal representative of the clippings from said audio signal; a rectifier coupled to said differencing means for rectifying the output of said differencing means; a fourth low-pass filter coupled to receive the output of said rectifier; the output of said fourth low-pass filter being coupled to control the gain of said amplifier such that the gain of said amplifier is reduced as the output of said fourth low-pass filter increases.
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10. In an audio peak limiting apparatus, an improvement for reducing distortion comprising:
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a clipper coupled to receive an input audio signal; a first low-pass filter having a cut-off frequency equal to f1, coupled to receive the output of said clipper; a second low-pass filter having a cut-off frequency approximately equal to f2, coupled to receive said output of said clipper where f1 is a higher frequency than f2; a combining means for combining signals coupled to receive the outputs of said first and second low-pass filters such that said output of said second low-pass filter is subtracted from the output of said first low-pass filter; a third low-pass filter having a cut-off frequency approximately equal to f2, the input to said third low-pass filter being coupled to receive said input audio signal; a voltage-controlled amplifier coupled to receive the output of said third low-pass filter, the output of said amplifier being coupled to said combining means such that the output of said amplifier is combined with said output of said first low-pass filter in said combining means, the output of said combining means providing an output signal for said peak limiting apparatus; a rectifier having only a single input and a single output coupled to receive and rectify said input audio signal; a thresholding means for providing a signal which exceeds a predetermined threshold, said thresholding means coupled to the output of said rectifier; a fourth low-pass filter coupled to receive the output of said thresholding means, the output of said fourth low-pass filter being coupled to control the gain of said amplifier such that as the output of said fourth low-pass increases the gain of said amplifier decreases. - View Dependent Claims (11, 12)
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Specification