Signal compressors and expanders

Claim 2

A circuit for modifying, within a predetermined input dynamic range, the dynamic range of an input signal comprising an input terminal and a first terminal, a first circuit path connected between the input terminal and the first terminal, the first path passing said input signal to said first terminal with dynamic range linearity, a second circuit path connected to said input terminal and having a second terminal, and an output terminal, a variable coupling means including at least one variable impedance providing a variable degree of coupling between the first and second terminals, the second circuit path including at least one impedance element which is connected to the second terminal, the frequency response of the said circuit being determined at least in part by the interacting combination of said impedance element and said variable impedance and varying as the degree of coupling between the first and second terminals provided by the variable coupling means varies, the variable coupling means being arranged so to vary in dependence upon the level of a signal existing in the circuit as to assume such a first extreme value of the coupling between the first and second terminals that the signal level and frequency response at the output terminal are determined substantially by fixed elements when the input signal level is in a first extreme part of the predetermined dynamic range and to assume such an opposite extreme value of the coupling when the input signal level is in the opposite extreme part of the predetermined dynamic range, the sense of said coupling being to narrow the frequency band in which dynamic range modification takes place when the input signal level increases.

Claim 3

A circuit according to claim 1, wherein the first extreme part is a low part, the opposite extreme part is a high part, the first extreme value is a high value, and the opposite extreme value is a low value.

Claim 4

A circuit according to claim 1, wherein the first extreme part is a low part, the opposite extreme part is a high part, the first extreme value is a low value, and the opposite extreme value is a high value.

Claim 5

A circuit according to claim 1, wherein the output signal has dynamic range linearity relative to the signal at the second terminal.

Claim 6

A circuit according to claim 1, wherein the first circuit path includes a filter.

Claim 7

A circuit according to claim 1, wherein the circuit path gains are so proportioned that, in at least part of the frequency band, the circuit provides a boosted signal at the output when the input signal level is in the low part of the predetermined dynamic range, whereby the circuit acts as a compressor.

Claim 8

A circuit according to claim 1, wherein the circuit path gains are so proportioned that, in at least part of the frequency band, the circuit provides an attenuated signal at the output when the input signal level is in the low part of the predetermined dynamic range, whereby the circuit acts as an expander.

Claim 9

A circuit according to claim 1, wherein the said network comprises a tuned circuit for dealing with a carrier frequency.

Claim 10

A circuit according to claim 1, wherein the variable coupling means is constituted by a variable impedance.

Claim 11

A circuit according to claim 9, wherein the variable impedance comprises a variable resistance.

Claim 12

A circuit according to claim 9, wherein the variable impedance comprises a variable reactance.

Claim 13

A circuit according to claim 1, wherein said signal existing in the circuit is a signal which exists at said variable coupling means and which is limited at high input signal levels when the circuit operates to narrow the said frequency band.

Claim 14

A circuit according to claim 1, wherein the circuit path gains are such that, at low input signal levels and outside the said frequency band the signal levels at said first and second terminals are substantially the same.

Claim 15

A noise reductiOn system wherein a compressor compresses the dynamic range of an input signal before the signal is transmitted or recorded and wherein a complementary expander expands the dynamic range of an input signal thereto which is the received signal or the signal played back from the recording to restore the linearity of the dynamic range relative to the input signal, characterized in that: A. the compressor comprises an input terminal and a first terminal, a first circuit path connected between the input terminal and the first terminal, the first circuit path passing said input signal to the compressor with dynamic range linearity, a second circuit path connected to said input terminal and having a second terminal, and an output terminal, a variable coupling means including at least one variable impedance providing a variable degree of coupling between the first and second terminals, the second circuit path including at least one impedance element which is connected to the second terminal, the frequency response of the compressor being determined at least in part by the interacting combination of said impedance element and said variable impedance and varying as the degree of coupling between the first and second terminals provided by the varible coupling means varies, the variable coupling means being arranged so to vary in dependance upon the level of a signal existing in the compressor as to assume such a first extreme value of the coupling between the first and second terminals that the signal level and frequency response at the output terminal are determined substantially by fixed elements when the input signal level is in a first extreme part of its dynamic range and to assume such an opposite extreme value of the coupling when the input signal level is in the opposite extreme part of its dynamic range, the sense of said coupling being to narrow the frequency range in which compression takes place when the input signal level increases, and the circuit path gains being so proportioned that, in at least a part of the frequency band, the compressor provides a boosted signal at the output when the input signal level is in the low part of its dynamic range, B. the expander comprises an expander input terminal and an expander first terminal, an expander first circuit path connected between the expander input terminal and the expander first terminal, the expander first circuit path passing said input signal to the expander with dynamic range linearity, an expander second circuit path connected to said expander input terminal and having an expander second terminal, and an expander output terminal, an expander variable coupling means including at least one expander variable impedance providing a variable degree of coupling between the expander first and second terminals, the expander second circuit path including at least one expander impedance element which is connected to the expander second terminal, the frequency response of the expander being determined at least in part by the interacting combination of said expander impedance element and said expander variable impedance and varying as the degree of coupling between the expander first and second terminals provided by the expander variable coupling means varies, the expander variable coupling means being arranged so to vary in dependence upon the level of a signal existing in the expander as to assume such a first extreme value of the coupling between the expander first and second terminals that the signal level and frequency response at the expander output terminal are determined substantially by fixed elements when the expander input signal level is in a first extreme part of its dynamic range and to assume such an opposite extreme value of the coupling when the expander input signal level is in the opposite extreme part of its dynamic range, the sense of said coupling being to narrow the frequency range in which expansion takes place when the expander input signal level increases, and the expander circuit path gains beinG so proportioned that, in at least a part of the frequency band, the expander provides an attenuated signal at the expander output when the expander input signal level is in the low part of its dynamic range.

Claim 16

A noise reductiOn system wherein a compressor compresses the dynamic range of an input signal before the signal is transmitted or recorded and wherein a complementary expander expands the dynamic range of an input signal thereto which is the received signal or the signal played back from the recording to restore the linearity of the dynamic range relative to the input signal, characterized in that: A. the compressor comprises an input terminal and a first terminal, a first circuit path connected between the input terminal and the first terminal, the first circuit path passing said input signal to the compressor with dynamic range linearity, a second circuit path connected to said input terminal and having a second terminal, and an output terminal, a variable coupling means including at least one variable impedance providing a variable degree of coupling between the first and second terminals, the second circuit path including at least one impedance element which is connected to the second terminal, the frequency response of the compressor being determined at least in part by the interacting combination of said impedance element and said variable impedance and varying as the degree of coupling between the first and second terminals provided by the varible coupling means varies, the variable coupling means being arranged so to vary in dependance upon the level of a signal existing in the compressor as to assume such a first extreme value of the coupling between the first and second terminals that the signal level and frequency response at the output terminal are determined substantially by fixed elements when the input signal level is in a first extreme part of its dynamic range and to assume such an opposite extreme value of the coupling when the input signal level is in the opposite extreme part of its dynamic range, the sense of said coupling being to narrow the frequency range in which compression takes place when the input signal level increases, and the circuit path gains being so proportioned that, in at least a part of the frequency band, the compressor provides a boosted signal at the output when the input signal level is in the low part of its dynamic range, B. the expander comprises an expander input terminal and an expander first terminal, an expander first circuit path connected between the expander input terminal and the expander first terminal, the expander first circuit path passing said input signal to the expander with dynamic range linearity, an expander second circuit path connected to said expander input terminal and having an expander second terminal, and an expander output terminal, an expander variable coupling means including at least one expander variable impedance providing a variable degree of coupling between the expander first and second terminals, the expander second circuit path including at least one expander impedance element which is connected to the expander second terminal, the frequency response of the expander being determined at least in part by the interacting combination of said expander impedance element and said expander variable impedance and varying as the degree of coupling between the expander first and second terminals provided by the expander variable coupling means varies, the expander variable coupling means being arranged so to vary in dependence upon the level of a signal existing in the expander as to assume such a first extreme value of the coupling between the expander first and second terminals that the signal level and frequency response at the expander output terminal are determined substantially by fixed elements when the expander input signal level is in a first extreme part of its dynamic range and to assume such an opposite extreme value of the coupling when the expander input signal level is in the opposite extreme part of its dynamic range, the sense of said coupling being to narrow the frequency range in which expansion takes place when the expander input signal level increases, and the expander circuit path gains beinG so proportioned that, in at least a part of the frequency band, the expander provides an attenuated signal at the expander output when the expander input signal level is in the low part of its dynamic range.

Claim 17

A circuit for modifying, within a predetermined input dynamic range, the dynamic range of an input signal, comprising a first terminal, a first circuit means for providing a signal at said first terminal in response to said input signal, the first circuit means providing at said first terminal a signal which has dynamic range linearity relative to said input signal, a second circuit means which is responsive to said input signal and having a second terminal, and an output terminal, variable coupling means including at least one variable impedance providing a variable detree of coupling between the first and second terminals, the second circuit means including at least one impedance element which is coupled to the second terminal, the frequency response of the said circuit being determined at least in part by the interacting combination of said impedance element and said variable impedance and varying as the degree of coupling between the first and second terminals provided by the variable coupling means varies, the variable coupling means being arranged so as to vary in dependence upon the level of a signal existing in the circuit as to assume such a first extreme value of the coupling between the first and second terminals that the signal level and frequency response at the output terminal are determined substantially by fixed elements when the input signal level is in a first extreme part of the predetermined dynamic range and to assume an opposite extreme value of the coupling when the input signal level is in the opposite extreme part of the predetermined dynamic range, the sense of said coupling being to narrow the frequency band in which dynamic range modification takes place when the input signal level increases.

Claim 18

A circuit for modifying, within a predetermined input dynamic range, the dynamic range of an input signal, comprising a first terminal, a first circuit means for providing a signal at said first terminal in response to said input signal, the first circuit means providing at said first terminal a signal which has dynamic range linearity relative to said input signal, a second circuit means which is responsive to said input signal and having a second terminal, and an output terminal, variable coupling means including at least one variable impedance providing a variable detree of coupling between the first and second terminals, the second circuit means including at least one impedance element which is coupled to the second terminal, the frequency response of the said circuit being determined at least in part by the interacting combination of said impedance element and said variable impedance and varying as the degree of coupling between the first and second terminals provided by the variable coupling means varies, the variable coupling means being arranged so as to vary in dependence upon the level of a signal existing in the circuit as to assume such a first extreme value of the coupling between the first and second terminals that the signal level and frequency response at the output terminal are determined substantially by fixed elements when the input signal level is in a first extreme part of the predetermined dynamic range and to assume an opposite extreme value of the coupling when the input signal level is in the opposite extreme part of the predetermined dynamic range, the sense of said coupling being to narrow the frequency band in which dynamic range modification takes place when the input signal level increases.

Claim 19

A circuit for modifying the dynamic range of an input signal, comprising a first terminal, a second terminal, a first circuit means for providing a first signal at said first terminal in response to said input signal, said first signal having dynamic range linearity relative to said input signal, a second circuit means including impedance means and which is responsive to said input signal to provide a second signal which is coupled through said impedance means to said second terminal, means for deriving an output signal from said second terminal, and variable coupling means including at least one variable impedance providing a variable degree of coupling between the first and second terminals, the frequency response of the said circuit being determined at least in part by the interacting combination of said impedance means and said variable coupling means, the variable coupling means being arranged to vary in dependence upon the level of a signal existing in the circuit, the sense of variation of said degree of coupling being such as to narrow the frequency band in which dynamic range modification takes place when the input signal level increases.

Claim 20

A circuit for modifying the dynamic range of an input signal, comprising a first terminal, a second terminal, a first circuit means for providing a first signal at said first terminal in response to said input signal, said first signal having dynamic range linearity relative to said input signal, a second circuit means including impedance means and which is responsive to said input signal to provide a second signal which is coupled through said impedance means to said second terminal, means for deriving an output signal from said second terminal, and variable coupling means including at least one variable impedance providing a variable degree of coupling between the first and second terminals, the frequency response of the said circuit being determined at least in part by the interacting combination of said impedance means and said variable coupling means, the variable coupling means being arranged to vary in dependence upon the level of a signal existing in the circuit, the sense of variation of said degree of coupling being such as to narrow the frequency band in which dynamic range modification takes place when the input signal level increases.

Claim 21

A circuit according to claim 16 wherein the variable coupling means assume such a first extreme value of the coupling between the first and second terminals when the input signal is at one extreme of a predetermined input dynamic range that the signal level and frequency response at the second terminal are then determined substantially by fixed elements, and wherein the variable coupling means assume a second extreme value of said coupling when the input signal is at the other extreme of said predetermined dynamic range.

Claim 22

A circuit according to claim 16 wherein said narrowing of the frequency band in which dynamic range modification takes place excludes from said dynamic range modification input signals whose level exceeds a low level threshold, said threshold being about one order of magnitude less, or even smaller, than the nominal maximum operating level.

Claim 23

A circuit accordinG to claim 16 comprising a control circuit responsive to said signal existing in the circuit to effect the variation of said variable coupling means, said control circuit including rectifying and smoothing means operative to provide syllabic action.

Claim 26

A method according to claim 20 wherein said coupling is varied in dependence upon the signal existing between said first and second terminals.