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DYNAMIC NOISE FILTER HAVING MEANS FOR VARYING CUTOFF POINT

  • US 3,678,416 A
  • Filed: 11/03/1970
  • Issued: 07/18/1972
  • Est. Priority Date: 07/26/1971
  • Status: Expired due to Term
First Claim
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1. A dynamic noise filter for reducing perceived noise in an audio signal having a wide range of audio frequencies, said dynamic noise filter comprising:

  • at least one means for generating a control signal in response to said audio signal, said control signal being representative of the amplitude of said audio signal over a limited frequency range thereof including an end portion of said wide range of audio frequencies;

    said generating means being adapted to produce said control signal with a controlled fast attack characteristic in responding to increases in peak magnitudes of said audio signal within said limited frequency range and with a slow decay characteristic in responding to decreases in peak magnitudes of said audio signal within said limited frequency range; and

    means for variable cutoff point filtering of said audio signal to provide reduction in perceived noise in said audio signal and having said cutoff point variable over a substantial portion of said wide range of audio frequencies in response to said control signal, the range of variation of said cutoff point including said end portion of said wide range of audio frequencies;

    said variable cutoff point filtering means being adapted to provide a selectable constant attenuation rate and a minimum attenuation limit to define a low distortion corner characteristic without significant variation thereof over the range of variation of said cutoff point;

    said generating means being adapted to increase the frequency range passed by said variable cutoff point filtering means as the amplitude of said audio signal within said limited frequency range increases;

    the relationship between said limited frequency range amplitude and the degree of increase in the frequency range passed by said variable cutoff point filtering means being prescribed by a preselected noise masking requirement.

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