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

• Current Assignee: Kyocera Corporation
• Original Assignee: Richard S. Burwen
• Inventors: Burwen, Richard S.
• Primary Examiner(s): Gensler, Paul L.
• Application Number:
  US05/086,398
• Time in Patent Office: 623 Days
• Field of Search: 333/17,7R,28T 325/477 179/1D,1P,1VC
• US Class Current: 333/17.1
• CPC Class Codes: G11B 5/00 Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor G11B11/00 and G11B13/00 take precedence
• CPC Class Codes: H03G 11/08 Limiting rate of change of amplitude H03G11/008 takes precedence
• CPC Class Codes: H03G 3/345 Muting during a short period of time when noise pulses are detected, i.e. blanking H03G3/348 takes precedence
• CPC Class Codes: H03G 5/16 Automatic control
• CPC Class Codes: H03G 5/18 in untuned amplifiers
• CPC Class Codes: H03G 5/24 in frequency-selective amplifiers
• CPC Class Codes: H03G 7/08 incorporating negative feedback
• CPC Class Codes: H03G 9/025 frequency-dependent volume compression or expansion, e.g. multiple-band systems H03G9/10, H03G9/18 take precedence
• CPC Class Codes: H03H 11/0405 Non-linear filters
• CPC Class Codes: H03H 11/1291 Current or voltage controlled filters

Abstract

A dynamic noise filter especially adapted for use in high fidelity audio reproduction systems to eliminate perceived noise. Controllable high-pass and low-pass filters are employed which respectively filter the low and high frequency portions of the audio spectrum. A very wide dynamic range of controllable cutoff points is achieved by each filter with different selectable rates of attenuation with frequency. The same attenuation characteristics are maintained throughout the range of cutoff points for each filter. In the presence of masking audio signals a fast attack, slow decay control is developed for each filter to cause the amount of filtering to decrease with increasing masking signals. The novel noise filter further features high frequency click impulse reduction, hum noise attenuation, and corner peaking and attenuation rate control.

Claims

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.

2. A dynamic noise filter for reducing the effect of noise within a predetermined frequency range of a signal comprising:

means for receiving said signal over at least its predetermined frequency range;

at least one means for filtering said received signal and including means for providing a uni-directional variation in gain with frequency;

said filter means having gain limiting means to limit the gain of said uni-directional varying characteristic over a portion of said predetermined frequency range to provide a low distortion corner characteristic between said portions of unidirectionally varying gain and limited gain;

said filter means further having means for varying the cutoff point associated with said corner characteristic in response to a control signal without significant variation in the shape of said corner and variable gain characteristics; and

means for generating said control signal in response to said received signal and representative of the frequency content of said received signal within the portion of the predetermined frequency range over which said cutoff point is variable.

3. The dynamic noise filter of claim 2 wherein said means for providing uni-directional variation in gain comprises active amplification elements and said control signal is operative to vary the amplification thereof to produce a wide range variation in said cutoff point of said filtering means.

4. The dynamic noise filter of claim 2 wherein saiD means for generating said control signal further includes:

means for filtering said received signal to substantially eliminate all signal frequencies outside the range of variation of said cutoff point; and

means for peak rectifying the filtered signal and adapted to develop said control signal with a controlled fast attack and slow decay characteristic in responding to increases in the magnitude peaks of the filtered signal.

5. The dynamic noise filter of claim 2 further including means for high frequency pre-emphasis and de-emphasis placed respectively before and after said filtering means thereby to augment the response of said filtering means to the higher frequency portions of said predetermined frequency range.

6. The dynamic noise filter of claim 2 further including narrow-band pre-filtering means for hum noise elimination thereby to prevent response by said control signal generating means to said hum noise.

7. The dynamic noise filter of claim 2 wherein said means for generating said control signal comprises:

means for filtering said received signal to substantially eliminate all signal frequencies outside the range of variation of said cutoff point;

means for magnitude limiting the filtered signal;

means for peak rectifying the limited signal; and

means for non-linearly filtering the peak rectified signal and adapted to low-pass filter small amplitude variations and decreases in said peak rectified signal and to pass with significantly less filtering all increases in said peak rectified signal which exceed a predetermined percentage of said peak rectified signal prior to said increase.

8. The dynamic noise filter of claim 7 wherein said means for generating said control signal further includes means for magnitude compressing said peak rectified signal before said non-linear filtering.

9. The dynamic noise filter of claim 7 wherein said means for generating said control signal further includes means for inverting said non-linearly filtered signal.

10. The dynamic noise filter of claim 2 further including pre-filtering means for limiting short duration click noise impulses in said received signal thereby to prevent activation of said control signal generating means in response to said short duration click noise impulses and to attenuate signals representative of said click noise impulses before application to said filtering means.

11. The dynamic noise filter of claim 10 wherein said prefiltering means includes:

means for detecting a click noise impulse;

switchable means having first and second selectable states for providing in said first state an output signal which is a substantial reproduction of an input signal applied to said pre-filtering means, and for providing in said second state an output signal approximately equal to the average low frequency signal content of said pre-filter input signal during said click noise impulse; and

means operative in response to each detected click noise impulse for switching said switchable means to said second state during the occurrence of each detected click noise impulse and to said first state in the absence of detection of click noise impulses.

12. The dynamic noise filter of claim 10 wherein said prefiltering means further includes a signal blanking circuit comprising:

first and second parallel channels;

means for time delaying the signal through said first channel;

means for low-pass filtering the signal in said second channel;

means for sampling and holding the filtered signal level in said second channel;

means for summing the outputs of said first channel and said sample and hold means;

means for interrupting the signal from said first channel to said summing means;

means for interrupting the signal from said sample and hold means to said summing means;

said means for interrupting said first channel signal being normally in a non-interrupting condition and said means for interrupting said sample and hold sigNal being normally in an interrupting condition;

means for detecting a click noise impulse at the input of said first and second channels; and

means for generating a blanking signal having a duration representative of and at least as great as the duration of each click noise impulse;

said interrupting means for said first channel signal and said interrupting means for said sample and hold signal being responsive to said blanking signal to reverse their condition upon the generation of said blanking signal;

said sample and hold means being responsive to said blanking signal to sample and hold for the duration of said blanking signal the approximate average of the low-pass filtered signal level present in said second channel during an impulse.

13. The dynamic noise filter of claim 10 wherein said prefiltering means includes:

first and second series connected wideband amplifiers with the output signal of said second amplifier negatively fed back to the input of said first amplifier; and

an integrator in a signal conducting path between said amplifiers;

said first amplifier being operative to provide an amplitude limited output in the presence of click impulses, said second amplifier providing in the absence of said amplitude limited output an output signal which is a frequency insensitive amplification of the input of said first amplifier, and providing in the presence of said amplitude limited output a triangular wave signal of substantially lower magnitude than said click impulses.

14. The dynamic noise filter of claim 13 wherein said first amplifier provides an output limited during click noise impulses to a magnitude approximately equal to the output magnitude at the start of a click noise impulse.

15. The dynamic noise filter of claim 2 wherein said means for filtering said received signal further includes at least one signal amplifier having:

a gain limit determining resistive feedback path; and

at least one controllable rate integrator having its integration rate varied in response to said control signal.

16. The dynamic noise filter of claim 15 wherein said at least one controllable rate integrator further comprises:

an integrating operational amplifier; and

a variable gain element having first and second inputs receiving respectively the output of said signal amplifier and said control signal whereby the amplification of said first input is dependent upon the signal level of said second input;

said integrating operational amplifier and said variable gain element being serially connected to form said controllable rate integrator.

17. The dynamic noise filter of claim 2 wherein said means for filtering said received signal further comprises:

a series combination of at least one controllable rate integrator; and

a signal path of gain limit determining feedback around at least one of said integrators;

each controllable rate integrator having its rate of integration varied in response to said control signal.

18. The dynamic noise filter of claim 17 wherein each said controllable rate integrator further comprises:

an integrating operational amplifier; and

a variable gain element having a first and second input receiving respectively the signal to be filtered and said control signal whereby the amplification of said first input is dependent upon the signal level of said second input;

one integrating operational amplifier and one variable gain element being serially connected to form each said controllable rate integrator.

19. The dynamic noise filter of claim 2 wherein said means for filtering said received signal further comprises:

a first filter section having a first attenuation rate in its filter roll-off characteristic; and

a second filter section in series with said first filter section and having a second attenuation rate substantially higher than said first rate;

said second section having a significant degree of corner peaking;

said first section having at the frequency of said corner peaking an attenuation of substantially the same degree as the degree of said corner peaking.

20. The dynamic noise filter of claim 19 further comprising:

means for selectively by-passing said first and second sections; and

means for substantially reducing the corner peaking of said second section when said first section is by-passed.

21. A dynamic noise filter for minimizing perceived audio noise in a received signal comprising:

means for low frequency, high-pass filtering said received signal over the lower frequency portion of an audio frequency range and including first varying gain means for providing a gain increasing with frequency and means for limiting the gain of said first varying gain means at a predetermined level to define a low distortion corner having a cutoff point, said first varying gain means being adapted to adjust its gain in response to a low frequency control signal whereby said cutoff point is variable over a substantial portion of said audio frequency range without producing significant variation in said corner and varying gain characteristics;

means for high frequency, low-pass filtering said received signal over the higher frequency portion of said audio frequency range and including second varying gain means for providing a gain decreasing with frequency and means for limiting the gain of said second varying gain means to a predetermined value to define a low distortion corner having a cutoff point, said second varying gain means being adapted to adjust its gain in response to a high frequency control signal thereby to vary the cutoff point of said corner characteristic without significant variation in said corner and varying gain characteristics;

means for generating said low frequency control signal in response to said received signal and representative of the amplitude of said received signal in the range of variation of said low frequency cutoff point; and

means for generating said high frequency control signal in response to said received signal representative of the amplitude of said received signal over the range of frequencies within the range of variation of said high frequency cutoff point;

said low and high frequency control signals being operable to vary the passband for said received signal from a minimum bandwidth including primarily middle audio frequency ranges to a maximum bandwidth containing substantially all audio frequencies.

22. The dynamic noise filter of claim 21 wherein said low and high frequency control signal generating means are adapted to increase the passband for said received signal by an amount which increases the audio noise at frequencies which may be substantially masked by signal content from which said control signals are generated.

23. The dynamic noise filter of claim 21 wherein said means for low frequency filtering further comprises:

at least one amplifier;

at least one variable rate integrator forming a feedback path around each said amplifier and having an integration rate controlled in response to said low frequency control signal; and

a gain limit determining path of negative feedback around each said amplifier; and

said means for high frequency filtering further comprises;

at least one controllable rate integrator having its integration rate controlled in response to said high frequency control signal; and

at least one path of negative gain limit determining feedback around at least one of said controllable rate integrators.

24. The dynamic noise filter of claim 22 wherein each said controllable rate integrator further comprises:

an integrating operational amplifier; and

a low distortion, two quadrant multiplier in series with said integrating operational amplifier, said multiplier having its multiplicand input receiving the control signal corresponding to the filter of which the integrator is a part and having its multiplier input in the signal path.