VARIABLE BANDPASS DYNAMIC NOISE FILTER

• US 3,753,159 A
• Filed: 07/26/1971
• Issued: 08/14/1973
• Est. Priority Date: 11/03/1970
• 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:
  US05/166,197
• Time in Patent Office: 750 Days
• Field of Search: 333/17,28T,7R 325/477 179/1D,1P,1VC 330/107,109 328/167
• 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 variable bandpass filter for a dynamic noise filtering effect that reduces the perceptible noise in an audio reproduction system. The variable bandpass filter responds to peak signal levels in relatively high and relatively low frequency portions of the audio spectrum to automatically and independently vary high and low frequency cutoff points for the filter in correspondence with the level of signals at those frequencies. Low distortion and wide dynamic range is achieved in a filter configuration which comprises a forward signal path and a reverse signal path, each having a variable integration response provided by temperature compensated and linearized field-effect transistor circuits. The integration response of the two paths imparts a high and low frequency filtering effect. A further constant gain feedback path establishes a uniform middle frequency amplification for the variable bandpass filter.

Claims

1. A low distortion, variable frequency response filter operative to vary a cutoff frequency over a wide dynamic range in response to a control signal including:

filter means having a gain characteristic varying unidirectionally with frequency;

means for reducing the gain variation of said filter means to provide an upper limit to the gain of said filter means whereby said filter means has a frequency response curve substantially flat over a portion and varying unidirectionally with frequency over a further portion, said cutoff frequency being between said portions;

variable impedance means for determining the gain of said filter means as a function of its impedance with said cutoff point being correspondingly determined by the impedance of said variable impedance means;

means for causing the impedance of said variable impedance means to vary in response to said control signal; and

said reducing means including feedback means for controlling said upper gain limit of said filter means during variation in said cutoff point.

2. The variable frequency response filter of claim 1 further including in said means for causing said impedance variation in response to said control signal:

means for filtering the signal applied to said variable frequency response filter and responsive primarily to the range of frequencies over which said cutoff point varies;

means for peak rectifying the filtered signal; and

means for nonlinearly filtering the peaked rectified signal to provide, in association with said peak rectifying means, said control signal with a smoothed, fast attack and slow decay characteristic in response to signal levels predominantly in the corresponding range of frequencies.

3. The variable frequency response filter of claim 1 further including means for reducing DC transients from said filter means in response to variations in said variable impedance means from said control signal.

4. The low distortion, variable frequency response filter of claim 1 wherein said means for controlling said upper gain limit includes means for reducing the variation therein during variation in said cutoff point.

5. A low distortion, variable frequency response filter operative to vary a cutoff frequency over a wide dynamic range in response to a control signal, said filter including:

filter means having a gain characteristic varying unidirectionally with frequency and including;

an integrator providing said variable gain characteristic and having a first variable impedance in an input thereto;

a second variable impedance;

a feedback control circuit operative to provide a bias signal which varies the impedance of said first and second variable impedances in response to said control signal;

means for associating said second variable impedance with said control circuit to cause a variation in said bias signal in response to a variation in the impedance of said second variable impedance and to cause compensation in the variation of said first variable impedance induced by changes in operating conditions of said first and second variable impedances;

means for reducing the gain variation of said filter means to provide an upper limit to the gain of said filter means whereby said filter means has a frequency response curve substantially flat over a portion and varying unidirectionally with frequency over a further portion, said cutoff frequency being between said portions;

means for associating said first variable impedance with said filter means for determining the gain of said filter means according to its impedance, said cutoff point being correspondingly determined by the impedance of said first variable impedance.

6. The variable frequency response filter of claim 5 wherein:

said first and second variable impedance means are field-effect transistors; and

means are provided to cause the signal level of the gate of said first variable impedance means to be substantially midway between the signal levels of the controlled terminals thereof to reduce distortion.

7. A variable bandpass audio frequency filter comprising:

a forward signal path having a gain characteristic varying unidirectionally with frequency;

a reverse signal path having a similar gain characteristic varying unidirectionally with frequency and providing signal feedback around said forward signal path;

first means for providing wide range adjustment in the position of the gain characteristic of said forward signal path in response to signal content in a first portion of the audio frequency range;

second means for providing wide range adjustment in the position of the gain characteristic of said reverse signal path in response to signal content in a second portion of the audio frequency range;

said second adjustment providing means including means for adjusting the gain characteristic oppositely to said first adjustment providing means in response to signal content; and

means for limiting the gain of said forward signal path to provide a substantially constant center frequency gain to said variable bandpass filter.

8. The variable bandpass filter of claim 7 wherein:

said forward and reverse signal paths comprise variable rate integration circuits; and

said first and second adjustment providing means include field-effect transistor inputs to said integration circuits and means for controlling the resistance of said field-effect transistors in response to the respective signal contents thereby to control the rate of integration and signal filtering characteristics of said forward and reverse signal paths.

9. A variable bandpass filter having an input terminal and an output terminal, said filter comprising:

a forward signal path for conduction between said input and output terminals;

a parallel reverse signal path for conduction between said input and output terminals;

a feedback circuit operative to limit the gain of said forward path;

said forward signal path including;

first means having a gain characteristic varying with frequency;

first variable resistance means operative to determine the position of the gain characteristic of said first variable gain means and operative to vary its resistance in response to a first bias signal applied to a control input thereof;

first means for developing said first bias signal to represent the difference in signal level between a first reference and a first control signal;

second variable resistance means operative to vary the resistance between said first developing means and one of said first reference and first control signals;

feedback means associated with said first developing means for causing the resistance of said second variable resistance means to vary in response to said first bias signal;

said reverse signal path including;

second means having a gain characteristic varying unidirectionally with frequency;

third variable resistance means operative to determine the position of the gain characteristic of said second variable gain means and operative to vary its resistance in response to a second bias signal applied to a control input thereof;

second means for developing said second bias signal representative of the difference in signal level between a second reference and a second control signal;

fourth variable resistance means connected to vary the resistance between said second Developing means and one of said second reference and second control signals; and

feedback means associated with said second developing means for causing the resistance of said fourth variable resistance means to vary in response to variation in said second bias signal.

10. The variable bandpass filter of claim 9 wherein said first and second variable resistance means are maintained in substantially the same temperature environment and said third and fourth variable resistance means are maintained in substantially the same temperature environment whereby temperature changes effecting said first and third variable resistance means are compensated by changes in said first and second bias signals induced by corresponding changes in said second and fourth variable resistance means.

11. The variable bandpass filter of claim 9 wherein:

means are provided for maintaining the AC signal levels on the control inputs of said first and third variable resistance means approximately midway between the AC signal levels of controlled terminals thereof; and

said variable resistance means are field-effect transistors.

12. The variable bandpass filter of claim 9 wherein:

said forward and feedback signal paths are connected in a closed loop; and

said closed loop includes means for providing a substantially flat gain characteristic in the response of said variable bandpass filter at maximum pass band widths.

13. The variable bandpass filter of claim 9 further including:

means for preemphasizing high frequencies applied to said variable bandpass filter; and

means for deemphasizing high frequencies in the signal at the output of said variable bandpass filter.

14. The variable bandpass filter of claim 9 further including as means for developing said first and second control signals:

means for filtering the signal applied to said variable bandpass filter and responsive primarily to the range of frequencies over which a cutoff point of said variable bandpass filter varies in response to one of said first and second control signals;

means for peak rectifying the filtered signal; and

means for nonlinearly filtering the peaked rectified signal to provide, in association with said peak rectifying means, said one of said first and second control signals with a smoothed, fast attack and slow decay characteristic in response to signal levels predominantly in the corresponding range of frequencies.

15. The variable bandpass filter of claim 14 further including means for limiting the range of variation in said control signals.

16. The variable bandpass filter of claim 14 wherein said nonlinear filtering means operates to provide smoothing of said peak rectified signal whenever said peak rectified signal is within a predetermined percentage of said nonlinearly filtered signal and to provide fast attack response to said peak rectified signal whenever said peak rectified signal exceeds said predetermined percentage of said nonlinearly filtered signal.

17. The variable bandpass filter of claim 14 operative as a multichannel system wherein:

each channel comprises one of said variable bandpass filters; and

one control signal developing means is provided for controlling the bandpass of two or more channels.