Noise reduction systems

Claim 2

A signal processing system which responds to an input signal to produce an output signal with modified dynamic range, said signal processing system comprising a main signal circuit responsive to the input signal to provide in a specified frequency band a first signal with dynamic range linearity relative to the input signal; means in the main signal circuit for combining linearly a second signal in said specified frequency band with the first signal to provide the output signal; and a further signal circuit responsive to a signal from the main signal circuit for providing the second signal such as to affect the level of the output signal significantly at very low input signal levels, the further signal circuit including one or more variable gain elements with outputs thereof which are substantially free of non-linear distortion at all frequencies in said specified frequency band, the outputs of said variable gain elements being coupled to the output of the further signal circuit to provide the second signal, said variable gain elements having the characteristics that below predetermined low levels there is no gain reduction of signals in said further signal circuit and above said low levels there is substantial gain reduction, the outputs of said variable gain elements and the second signal produced thereby being limited to levels corresponding to small fractional parts of the maximum signal level applied to the further signal circuit, where said small fractional parts are in the region of an order of magnitude less than unity, or smaller.

Claim 3

A signal processing system according to claim 1 wherein said further signal circuit is responsive to a signal derived from said input signal to provide said second signal, and said combining means so combine said first and second signals that said second signal boosts said first signal, whereby said system operates as a dynamic range compressor.

Claim 4

A signal processing system according to claim 1 wherein said further signal circuit is responsive to a signal derived from said output signal to provide said second signal, and said combining means so combine said first and second signals that said second signal bucks said first signal, whereby said system operates as a dynamic range expander.

Claim 5

A system according to claim 1 wherein said further signal circuit includes filter means for restricting the frequencies passed thereby to a part of said specified frequency band.

Claim 6

A system according to claim 4 wherein there is a plurality of further signal circuits.

Claim 7

A system according to claim 1 wherein said further signal circuit further includes means for restricting the amplitudes of transient overshoots in said second signal.

Claim 8

A system according to claim 1 wherein said further signal circuit includes control means for rectifying and smoothing at least one signal derived from said system to produce a control signal or controlling the limiting action of said variable gain elements.

Claim 9

A system according to claim 7 wherein said smoothing means comprise a first smoothing circuit; and a second smoothing circuit coupled to said first smoothing circuit; said first smoothing circuit having a relatively fast response time; and said second smoothing circuit having a relatively slow response time under substantially uniform signal level conditions and a relatively fast response time under dynamic signal conditions.

Claim 10

A system according to claim 8 wherein said second smoothing circuit is coupled to said first smoothing circuit through an amplifier having a high input impedance and a low output impedance.

Claim 11

A system according to claim 8 wherein said second smoothing circuit includes a series resistor; and a shunt capacitor; and at least one diode means in parallel with said resistor, said diode means having a polarity arranged to permit rapid charging of said capacitor.

Claim 12

A system according to claim 7 wherein at least one signal is derived From the output of said further signal circuit.

Claim 13

A signal processing system according to claim 1 wherein said main signal circuit is such that said first signal is instantaneously proportional to said input signal.

Claim 14

A signal processing system according to claim 1 wherein said main and further signal circuits are parallel signal paths.

Claim 15

A signal processing system according to claim 1 wherein said substantial gain reduction results in the amplitude of said second signal rising, peaking, and then falling as a function of signal level applied to the further signal circuit.

Claim 16

A system according to claim 2 wherein there is a plurality of further signal circuits which operate in a plurality of frequency bands covering the audio spectrum and in which the said boosting action is approximately 10 dB.

Claim 17

A system according to claim 3 wherein there is a plurality of further signal circuits which operate in a plurality of frequency bands covering the audio spectrum and in which the said bucking action is approximately 10 dB.

Claim 18

A signal compressor which responds to an input signal to produce an output signal with reduced dynamic range, said signal compressor comprising a main signal path responsive to the input signal to provide in a specified frequency band a first signal which is instantaneously proportional to the input signal; means in the main signal path for combining additively a second signal within said specified frequency band with the first signal to provide the output signal; and a further signal path responsive to the input signal to provide the second signal such as to increase the level of the output signal significantly at very low input signal levels, the further signal path including filter means for restricting the second signal to part of said specified frequency band; and one or more variable gain elements with outputs thereof which are substantially free of non-linear distortion at all frequencies in said specified frequency band, the outputs of said variable gain elements being coupled to the output of the further signal path to provide the second signal, said variable gain elements having the characteristics that below predetermined low levels there is no gain reduction of signals in said further signal path and above said low levels there is substantial gain reduction, the outputs of said variable gain elements and the second signal produced thereby being limited to levels corresponding to small fractional parts of the maximum signal level applied to the further signal path, where said small fractional parts are in the region of an order of magnitude less than unity, or smaller, the second signal increasing the level of the output signal insignificantly at maximum input signal level.

Claim 19

A signal compressor which responds to an input signal to produce an output signal with reduced dynamic range, said signal compressor comprising a main signal path responsive to the input signal to provide in a specified frequency band a first signal which is instantaneously proportional to the input signal; means in the main signal path for combining additively a second signal within said specified frequency band with the first signal to provide the output signal; and a further signal path responsive to the input signal to provide the second signal such as to increase the level of the output signal significantly at very low input signal levels, the further signal path including filter means for restricting the second signal to part of said specified frequency band; and one or more variable gain elements with outputs thereof which are substantially free of non-linear distortion at all frequencies in said specified frequency band, the outputs of said variable gain elements being coupled to the output of the further signal path to provide the second signal, said variable gain elements having the characteristics that below predetermined low levels there is no gain reduction of signals in said further signal path and above said low levels there is substantial gain reduction, the outputs of said variable gain elements and the second signal produced thereby being limited to levels corresponding to small fractional parts of the maximum signal level applied to the further signal path, where said small fractional parts are in the region of an order of magnitude less than unity, or smaller, the second signal increasing the level of the output signal insignificantly at maximum input signal level.

Claim 20

A signal expander which responds to an input signal to produce an output signal with increased dynamic range, said signal expander comprising a main signal path responsive to the input signal to provide in a specified frequency band a first signal which is instantaneously proportional to the input signal; means in the main signal path for combining subtractively a second signal within said specified frequency band with the first signal to provide the output signal; and a further signal path responsive to the output signal to provide the second signal such as to decrease the level of the output signal significantly at very low input signal levels, the further signal path including filter means for restricting the second signal to part of said specified frequency band; and one or more variable gain elements with outputs thereof which are substantially free of non-linear distortion at all frequencies in said specified frequency band, the outputs of said variable gain elements being coupled to the output of the further signal path to provide the second signal, said variable gain elements having the characteristic that below predetermined low levels there is no gain reduction of signals In said further signal path and above said low levels there is substantial gain reduction, the outputs of said variable gain elements and the second signal produced thereby being limited to levels corresponding to small fractional parts of the maximum signal level applied to the further signal path, where said small fractional parts are in the region of an order of magnitude less than unity, or smaller, the second signal decreasing the level of the output signal insignificantly at maximum input signal level.

Claim 21

A signal expander which responds to an input signal to produce an output signal with increased dynamic range, said signal expander comprising a main signal path responsive to the input signal to provide in a specified frequency band a first signal which is instantaneously proportional to the input signal; means in the main signal path for combining subtractively a second signal within said specified frequency band with the first signal to provide the output signal; and a further signal path responsive to the output signal to provide the second signal such as to decrease the level of the output signal significantly at very low input signal levels, the further signal path including filter means for restricting the second signal to part of said specified frequency band; and one or more variable gain elements with outputs thereof which are substantially free of non-linear distortion at all frequencies in said specified frequency band, the outputs of said variable gain elements being coupled to the output of the further signal path to provide the second signal, said variable gain elements having the characteristic that below predetermined low levels there is no gain reduction of signals In said further signal path and above said low levels there is substantial gain reduction, the outputs of said variable gain elements and the second signal produced thereby being limited to levels corresponding to small fractional parts of the maximum signal level applied to the further signal path, where said small fractional parts are in the region of an order of magnitude less than unity, or smaller, the second signal decreasing the level of the output signal insignificantly at maximum input signal level.

Claim 26

A signal processing system which responds to an input signal to produce an output signal with a modified dynamic range, said signal processing system comprising a main signal circuit responsive to the input signal to provide in a specified frequency band a first signal having dynamic range linearity relative to the input signal; and means in the main signal circuit for combining linearly a second signal within said specified frequency band with the first signal to provide the output signal; and a further signal circuit responsive to a signal derived from the main signal path for providing the second signal such as to effect the level of the output signal significantly at very low input signal levels, the further signal circuit including variable filter means, the band pass of which narrows automatically to restrict, above a low-level threshold, said second signal to a level corresponding to a small fractional part of the maximum signal level applied to the further signal circuit, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.

Claim 27

A signal processing system which responds to an input signal to produce an output signal with a modified dynamic range, said signal processing system comprising a main signal circuit responsive to the input signal to provide in a specified frequency band a first signal having dynamic range linearity relative to the input signal; and means in the main signal circuit for combining linearly a second signal within said specified frequency band with the first signal to provide the output signal; and a further signal circuit responsive to a signal derived from the main signal path for providing the second signal such as to effect the level of the output signal significantly at very low input signal levels, the further signal circuit including variable filter means, the band pass of which narrows automatically to restrict, above a low-level threshold, said second signal to a level corresponding to a small fractional part of the maximum signal level applied to the further signal circuit, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.

Claim 28

A signal processing system according to claim 21 wherein said further signal circuit is responsive to a signal derived from saId input signal; and said signal combining means so combines said first and said second signals that said second signal boosts said first signal, whereby said system operates as a dynamic range compressor.

Claim 29

A signal processing system according to claim 21 wherein said further signal circuit is responsive to a signal derived from said output signal; and said signal combining means so combines said first and said second signals that said second signal bucks said first signal, whereby said system operates as a dynamic range expander.

Claim 30

A system according to claim 21 wherein there is a plurality of said further signal circuits.

Claim 31

A signal processing system according to claim 21 wherein said variable filter means has the characteristic that said second signal is substantially free of non-linear distortion.

Claim 32

A system according to claim 25 wherein said further signal circuit further includes means for restricting the amplitudes of transient overshoots in said second signal.

Claim 33

A system according to claim 25 wherein said variable filter means further includes control means for rectifying and smoothing at least one signal derived from said system to produce a control signal; and means responsive to said control signal to control said restricting action.

Claim 34

A system according to claim 27 wherein said smoothing means comprise a first smoothing circuit having a relatively fast response time; and a second smoothing circuit coupled to said first smoothing circuit, said second smoothing circuit having a relatively slow response time under substantially uniform signal level conditions and a relatively fast response time under dynamic signal conditions.

Claim 35

A system according to claim 21 wherein said variable filter means includes capacitance means the first end of which is coupled to the input of said filter means and the second end of which is coupled to the output of said filter means; and diode means connected between reference points and said output and arranged to conduct when the signal at said output exceeds a predetermined level whereby the frequency response of said filter means is controlled by the dynamic impedance of said diode means, thereby to provide said restriction of said second signal.

Claim 36

A system according to claim 29 and further including an amplifier responsive to the output of said filter means for producing an output voltage for controlling the voltage of said reference points.

Claim 37

A system according to claim 21 wherein said input signal is a carrier signal having one or more sideband signals, wherein said variable filter means includes at least one rejection filter circuit for rejecting said carrier signal and for attenuating said sideband signals.

Claim 38

A system according to claim 21 wherein said input signal is a broad band signal in combination with a carrier signal having one or more sideband signals, said further signal circuit further including a fixed rejection for rejecting said carrier signal and for attenuating said sideband signals.

Claim 39

A system according to claim 21 wherein said variable filter means includes a parallel resonant network means the first end of which is coupled to the input of said filter means and the second end of which is coupled to the output of said filter means; and diode means connected between reference points and said output and arranged to conduct when the signal at said output exceeds a predetermined level, whereby the frequency response of said filter means is controlled by the dynamic impedance of said diode means, thereby to provide said restriction of said second signal.

Claim 40

A signal compressor which responds to an input signal to produce an output signal with reduced dynamic range, comprising means responsive to said input signal to produce a first signal component which has dynamic range linearity relative to said input signal, means for so combining linearly said first component and a second sigNal component that said second component boosts said first component to produce said output signal, said second component increasing the level of said output significantly at low input signal levels, and variable filter means operative to produce said second component by filtering a signal derived from said input signal, the band pass of said filter means narrowing automatically, to restrict, above a low-level threshold, said second component to part of the frequency band occupied by said first component and to a small fractional part of the maximum amplitude of said first component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.

Claim 41

A signal compressor which responds to an input signal to produce an output signal with reduced dynamic range, comprising means responsive to said input signal to produce a first signal component which has dynamic range linearity relative to said input signal, means for so combining linearly said first component and a second sigNal component that said second component boosts said first component to produce said output signal, said second component increasing the level of said output significantly at low input signal levels, and variable filter means operative to produce said second component by filtering a signal derived from said input signal, the band pass of said filter means narrowing automatically, to restrict, above a low-level threshold, said second component to part of the frequency band occupied by said first component and to a small fractional part of the maximum amplitude of said first component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.

Claim 42

A signal expander which responds to an input signal to produce an output signal with increased dynamic range, comrpising means responsive to said input signal to produce a first signal component which has dynamic range linearity relative to said input signal, means for so combining linearly said first component and a second signal that said second component bucks said first component to produce said output signal, said second component reducing the level of said output signal significantly at low input signal levels, and variable filter means operative to produce said second component by filtering a signal derived from said output signal, the band pass of said filter means narrowing automatically, to restrict, above a low-level threshold, said second component to part of the frequency band occupied by said first component and to a small fractional part of the maximum amplitude of said first component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.

Claim 43

A signal expander which responds to an input signal to produce an output signal with increased dynamic range, comrpising means responsive to said input signal to produce a first signal component which has dynamic range linearity relative to said input signal, means for so combining linearly said first component and a second signal that said second component bucks said first component to produce said output signal, said second component reducing the level of said output signal significantly at low input signal levels, and variable filter means operative to produce said second component by filtering a signal derived from said output signal, the band pass of said filter means narrowing automatically, to restrict, above a low-level threshold, said second component to part of the frequency band occupied by said first component and to a small fractional part of the maximum amplitude of said first component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.

Claim 48

A signal processing system for processing a system input signal to and an information signal from an information channel to produce a system output signal substantially identical to said system input signal, said signal processing system comprising: a signal compressor for processing said system input signal to produce an intermediate signal for said information channel, said signal compressor including a first mAin signal circuit responsive to said system input signal and including means for providing in a specified frequency band a first signal having dynamic range linearity relative to said system input signal; first combining means for linearly combining at least two signals to produce said intermediate signal, one signal of which is said first signal; a first further signal circuit responsive to a signal derived from said first main signal circuit for producing a second signal such as to increase the level of said intermediate signal significantly at very low input signal levels, said first further signal circuit including first limiting means for limiting, above a low-level threshold, said second signal to an amplitude corresponding to a fractional part of the maximum amplitude of the signal applied to said first limiting means; and means for applying said second signal to said first combining means; and a signal expander for processing said information signal from said information channel to produce said system output signal, said signal expander including a second main signal circuit responsive to said information signal and including means for providing in a specified frequency band a third signal dynamic range linearity relative to said information signal; second combining means for linearly combining at least two signals to produce said system output signal, one signal of which is said third signal; a second further signal circuit coupled to said second main signal path and responsive to a signal derived from said second main signal circuit for producing a fourth signal such as to decrease the level of the system output signal significantly at very low information signal levels, said second further signal circuit including second limiting means for limiting, above a low-level threshold, said fourth signal to an amplitude corresponding to a fractional part of the maximum amplitude of the signal applied to said second limiting means; and means for applying said fourth signal to said second combining means, wherein the characteristics of said first further signal circuit are substantially identical to those of said second further signal circuit whereby the operation of said signal compressor and said signal expander are complementary, and said system output signal is substantially identical to said system input signal.

Claim 49

A signal processing system for processing a system input signal to and an information signal from an information channel to produce a system output signal substantially identical to said system input signal, said signal processing system comprising: a signal compressor for processing said system input signal to produce an intermediate signal for said information channel, said signal compressor including a first mAin signal circuit responsive to said system input signal and including means for providing in a specified frequency band a first signal having dynamic range linearity relative to said system input signal; first combining means for linearly combining at least two signals to produce said intermediate signal, one signal of which is said first signal; a first further signal circuit responsive to a signal derived from said first main signal circuit for producing a second signal such as to increase the level of said intermediate signal significantly at very low input signal levels, said first further signal circuit including first limiting means for limiting, above a low-level threshold, said second signal to an amplitude corresponding to a fractional part of the maximum amplitude of the signal applied to said first limiting means; and means for applying said second signal to said first combining means; and a signal expander for processing said information signal from said information channel to produce said system output signal, said signal expander including a second main signal circuit responsive to said information signal and including means for providing in a specified frequency band a third signal dynamic range linearity relative to said information signal; second combining means for linearly combining at least two signals to produce said system output signal, one signal of which is said third signal; a second further signal circuit coupled to said second main signal path and responsive to a signal derived from said second main signal circuit for producing a fourth signal such as to decrease the level of the system output signal significantly at very low information signal levels, said second further signal circuit including second limiting means for limiting, above a low-level threshold, said fourth signal to an amplitude corresponding to a fractional part of the maximum amplitude of the signal applied to said second limiting means; and means for applying said fourth signal to said second combining means, wherein the characteristics of said first further signal circuit are substantially identical to those of said second further signal circuit whereby the operation of said signal compressor and said signal expander are complementary, and said system output signal is substantially identical to said system input signal.

Claim 50

A noise reduction system according to claim 38 wherein said first further signal circuit is responsive to a signal derived from said system input signal; and said second further signal circuit is responsive to a signal derived from said system output signal.

Claim 53

A system according to claim 27 wherein said at least one signal is derived from the output of said further signal circuit.

Claim 54

A signal processing system according to claim 23 wherein said main signal circuit is such that said first signal is instantaneously proportional to said input signal.

Claim 61

A method in accordance with claim 45 wherein said control signal is derived from a signal appearing in said restricted part of said frequency band, whereby said latter signal is excluded from said narrowed, restricted part of said specified frequency band.

Claim 62

A method in accordance with claim 45 and further including the steps of rectifying and smoothing said control signal.

Claim 63

A method in accordance with claim 45 wherein said input signal is a carrier signal having one or more sideband signals and further including the steps of: rejecting said carrier signal; and attenuating said sideband signals.

Claim 64

A method in accordance with claim 45 wherein said varying step is in response to increasing amplitudes of said signals in said circuit above a predetermined low level.

Claim 65

A method in accordance with claim 49 wherein the predetermined low-level is in the region of an order of magnitude lower, or less than the maximum amplitude of said signals in said circuit.

Claim 70

A method in accordance with claim 52 and further including the steps of rectifying and smoothing said control signal.

Claim 71

A method in accordance with claim 52 wherein said input signal is a carrier signal having one or more sideband signals and further including the steps of: rejecting said carrier signal; and attenuating said sideband signals.

Claim 76

A method according to claim 56 wherein the said first output signal is recorded and played back to provide said second input signal, and wherein the compressing and the expanding are effected by the same circuit switched to a compression configuration for compressing said first input signal and to an expansion configuration for expanding said second input signal.

Claim 77

A method according to claim 56 wherein said control signal for compressing and said control signal for expanding are each derived from a signal appearing in said restricted part of said frequency band, whereby said latter signal is excluded from said narrowed, restricted part of said specified frequency band.

Claim 78

A method in accordance with claim 56 wherein said first input signal is a carrier signal having one or more sideband signals and said compressing and expanding each further including the steps of: rejecting said carrier signal; and attenuating said sideband signals.

Claim 79

A method according to claim 56 wherein each of said circuits provided for compressing and expanding has variable frequency response characteristics over each of a plurality of restricted parts of said specified frequency band; and wherein, in each of said compressing and expanding a plurality of control signals are derived, each of which is representative of the level of said first input signal for compressing and said second input signal for expanding in a corresponding restricted part of said specified frequency band; and wherein the variable frequency response characteristics of each said circuit are varied in each of said plurality of restricted parts of said specified frequency band in response to increasing amplitudes of said plurality of control signals so as to narrow each of said restricted parts of said specified frequency band.

Claim 80

A method in accordance with claim 56 wherein said varying steps are in response to increasing amplitudes of said signals in said circuit above a predetermined low level.

Claim 81

A method in accordance with claim 61 wherein the predetermined low level is in the region of an order of magnitude lower, or less, than the maximum amplitude of said signals iN said circuit.

Claim 82

A signal processing system for processing an input signal to and an information signal from an information channel to produce a system output signal substantially identical to said system input signal, said signal processing system comprising: a main signal circuit for providing a first signal component which, in a specified frequency band, has dynamic range linearity relative to a signal applied to said main signal circuit, a further signal circuit for providing a second signal component within said band having a limited dynamic range relative to the said signal applied to said main signal circuit, means for combining linearly said first and second signal components, and switching means having compressor and expander modes, said switching means being operative in the compressor mode to apply said input signal to said main signal circuit and to cause said combining means to boost said first signal component by said second signal component and to output said boosted first signal component as said information signal, said switching means being operative in the expander mode to apply said information signal to said main signal circuit and to cause said combining means to buck said first signal component by said second signal component and to output said bucked first signal component as said system output signal.

Claim 83

A signal processing system for processing an input signal to and an information signal from an information channel to produce a system output signal substantially identical to said system input signal, said signal processing system comprising: a main signal circuit for providing a first signal component which, in a specified frequency band, has dynamic range linearity relative to a signal applied to said main signal circuit, a further signal circuit for providing a second signal component within said band having a limited dynamic range relative to the said signal applied to said main signal circuit, means for combining linearly said first and second signal components, and switching means having compressor and expander modes, said switching means being operative in the compressor mode to apply said input signal to said main signal circuit and to cause said combining means to boost said first signal component by said second signal component and to output said boosted first signal component as said information signal, said switching means being operative in the expander mode to apply said information signal to said main signal circuit and to cause said combining means to buck said first signal component by said second signal component and to output said bucked first signal component as said system output signal.

Claim 86

A method according to claim 64 wherein said intermediate signal is recorded and played back, said input signal is operated upon by a circuit having a transfer characteristic F1 to provide said second signal component, and said output signal is operated upon by the same said circuit to provide said fourth signal component, whereby F2 identically equals F1.

Claim 87

A signal processing system according to claim 21 wherein the said main and further signal circuits are parallel signal paths.

Claim 88

A signal processing system according to claim 21 wherein said first signal is instantaneously proportional to said input signal.

Claim 89

A signal processing system according to claim 21 wherein said input signal comprises high frequency audio signals.

Claim 90

A signal processing system according to claim 21 wherein said input signal comprises a low frequency audio signal.

Claim 91

A signal processing system according to claim 21 wherein said input signal comprises both low frequency and high frequency audio signals.

Claim 92

A signal processing system according to claim 21 wherein said input signal comprises high frequency video signals.