Noise reduction systems
阅读：696发布：2022-04-10
专利汇可以提供Noise reduction systems专利检索，专利查询，专利分析的服务。并且A noise reduction system comprising a compressor feeding an information channel and a complementary expander treating the output of the channel. The system is applicable to audio and visual signals using compressors and expanders with appropriately scaled frequency selective circuits which narrow the band in which compression and expansion take place as the signal level rises. Distortion and tracking accuracy problems are reduced by the use of compressor and expander configurations embodying a main signal circuit and a further signal circuit, the main circuit providing a first signal which has dynamic range linearity with respect to the input signal and the further circuit providing a second signal which is restricted to a small part of the dynamic range of the signal in the main circuit. The restriction may be effected by one or more variable filter means having a pass band which narrows to exclude large signal components from the compression or expansion action. The second signal is combined additively with the first signal for compressor operation and subtractively for expander operation. True complementarity is attainable by the use of a compressor and expander together to provide an overall noise reduction action without introducing defects into the signal being processed.，下面是Noise reduction systems专利的具体信息内容。
1. 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.
2. 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.
3. 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.
4. 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.
5. A system according to claim 4 wherein there is a plurality of further signal circuits.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
11. A system according to claim 7 wherein at least one signal is derived From the output of said further signal circuit.
12. 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.
13. A signal processing system according to claim 1 wherein said main and further signal circuits are parallel signal paths.
14. 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.
15. 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.
16. 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.
17. 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.
18. 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.
19. A method of processing an input signal to produce an output signal with reduced dynamic range comprising the steps of combining additively first and second signal components to produce said output signal, providing said first signal component in the form of a signal which is substantially proportional to said input signal; providing said second signal component by filtering and limiting a signal derived from said input signal by one or more filters and by one or more variable gain elements which contribute negligible non-linear distortion to the signals handled thereby and which have the characteristics that below predetermined low levels there is no gain reduction of signals handled thereby and above said low levels there is substantial gain reduction, said second signal component produced thereby being restricted to a part of the frequency band occupied by said first signal component and being limited to a small fractional part of the maximum amplitude of said first signal component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.
20. A method of processing an input signal to produce an output signal with increased dynamic range comprising the steps of combining subtractively first and second signal components to produce said output signal, providing said first signal component in the form of a signal which is substantially proportional to said input signal; providing said second signal component by filtering and limiting a signal derived from said output signal by one or more filters and by one or more variable gain elements which contribute negligible non-linear distortion to the signals handled thereby and which have the characteristic that below predetermined low levels there is no gain reduction of signals handled thereby and above said low levels there is substantial gain reduction, said second signal component produced thereby being restricted to a part of the frequency band occupied by said first signal component and being limited to a small fractional part of the maximum amplitude of said first signal component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.
21. 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.
22. 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.
23. 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.
24. A system according to claim 21 wherein there is a plurality of said further signal circuits.
25. 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.
26. 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.
27. 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.
28. 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.
29. 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.
30. 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.
31. 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.
32. 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.
33. 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.
34. 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.
35. 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.
36. A method of processing an input signal to produce an output signal with reduced dynamic range comprising the steps of combining linearly first and second signal components so that said second component boosts said first component to produce said output signal, providing said first signal component in the form of a signal which has dynamic range linearity relative to said input signal; providing said second signal component by filtering a signal which is derived from said input signal and by automatically narrowing the band pass of said filtering action to restrict, above a low-level threshold, said second signal component produced thereby to a part of the frequency band occupied by said first signal component and to a small fractional part of the maximum amplitude of said first signal component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.
37. A method of processing an input signal to produce an output signal with increased dynamic range comprising the steps of combining linearly first and second signal components so that said second component bucks said first component to produce said output signal, providing said first signal component in the form of a signal which has dynamic range linearity relative to said input signal; providing said second signal component by filtering a signal which is derived from said output signal and by automatically narrowing the band pass of said filtering action to restrict, above a low-level threshold, said second signal component produced thereby to a part of the frequency band occupied by said first signal component and to a small fractional part of the maximum amplitude of said first signal component, where said small fractional part is in the region of an order of magnitude less than unity, or smaller.
38. 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.
39. 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.
40. A method of effecting noise reduction for a signal storage or transmission channel wherein an input signal is converted to an output signal, comprising the steps of: providing a first signal component which, in a specified frequency band, has dynamic range linearity relative to said input signal, providing a second signal component within said band by restricting, above a low-level threshold, a signal derived from said input signal to a small fractional part of the maximum amplitude of said first signal component, combining linearly said first and second components so that said second component boosts said first component to produce a third signal, said second component increasing the level of said third signal by a given proportion at very low input signal levels, feeding said third signal to said channel and recovering a fourth signal from said channel, providing a fifth signal component which, within said band, has dynamic range linearity relative to said fourth signal, providing a sixth signal component within said band by restricting, above a low-level threshold, a signal derived from said output signal to a small fractional part of the maximum amplitude of said fifth signal component, combining linearly said fifth and sixth component decreasing the level of said output signal By a given proportion at very low fourth signal levels, the two said small fractional parts being in the region of an order of magnitude less than unity, or smaller, whereby said second component increases the level of said third signal by a small fraction of said given proportion at high input signal levels at which high levels said third signal is approximately equal to said first component, whereby said sixth component decreases the level of said output signal by a small fraction of said given proportion at high fourth signal levels at which high levels said output signal is approximately equal to said fifth component, the two said restricting actions being substantially comparable and the said boosting and bucking actions being in reciprocal proportions such that, within said band, said output signal is substantially proportional to said input signal, except for the effects within said band of channel non-proportionalities between said third and fourth signals and whereby low level channel noise within said band appears at reduced level in said output signal when said output signal level is low.
41. A system according to claim 27 wherein said at least one signal is derived from the output of said further signal circuit.
42. 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.
43. A method of effecting noise reduction for a signal storage or transmission channel wherein an input signal is converted to an output signal, comprising the steps of: providing a first signal component which, in a specified frequency band, has dynamic range linearity relative to said input signal, providing a second signal component within said band by reducing the dynamic range of a signal derived from said input signal, combining linearly said first and second components so that said second component boosts said first component to produce a third signal, feeding said third signal to said channel and recovering a fourth signal from said channel, providing a fifth signal component which, within said band, has dynamic range linearity relative to said fourth signal, providing a sixth signal component within said band by reducing the dynamic range of a signal derived from said output signal, combining linearly said fifth and sixth components so that said sixth component bucks said fifth component to produce said output signal, the two said dynamic range reducing actions being substantially comparable and the said boosting and bucking actions being in reciprocal proportions.
44. A method of treating a signal for a storage or transmission channel wherein an input signal is converted to an output signal, comprising the steps of providing a first signal component which, in a specified frequency band, has dynamic range linearity relative to said input signal, providing a second signal component within said band by processing a signal derived from said input signal, combining linearly said first and second components to produce a third signal, feeding said third signal to said channel and recovering a fourth signal from said channel, providing a fifth signal component which, within said band, has dynamic range linearity relative to said fourth signal, providing a sixth signal component within said band by processing a signal derived from said output signal, combining linearly said fifth and sixth components to produce said output signal, the two said processing actions being substantially comparable and the said linear combining actions being in opposite senses and reciprocal proportions.
45. A method of compressing an input signal in a specified frequency band to produce an output signal having a decreased dynamic range within at least one restricted part of and specified frequency band, comprising the steps of: providing a circuit having variable frequency response characteRistics over said restricted part of said specified frequency band; supplying said input signal to said circuit; deriving a control signal in response to signals in said circuit; and varying said variable frequency response characteristics of said circuit in response to increasing amplitudes of said control signal so as to narrow said restricted part of said specified frequency band.
46. 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.
47. A method in accordance with claim 45 and further including the steps of rectifying and smoothing said control signal.
48. 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.
49. 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.
50. 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.
51. A method of compressing an input signal in a specified frequency band to produce an output signal having a decreased dynamic range within each of a plurality of restricted parts of said specified frequency band, comprising the steps of: providing a circuit having variable frequency response characteristics over each of said plurality of restricted parts of said specified frequency band; supplying said input signal to said circuit; deriving a plurality of control signals each of which is representative of the level of said input signal in a corresponding restricted part of said specified frequency band; and varying the variable frequency response characteristics of said circuit in each of said plurality of restricted parts of said specified frequency band so as to narrow said restricted parts.
52. A method of expanding an input signal in a specified frequency band to produce an output signal having an increased dynamic range within at least one restricted part of said specified frequency band, comprising the steps of: providing a circuit having variable frequency response characteristics over said restricted part of said specified frequency band; supplying said input signal to said circuit; deriving a control signal from a signal appearing in said restricted part of said frequency band; varying said variable frequency response characteristics of said circuit in response to increasing amplitudes of said control signal so as to narrow said restricted part of said specified frequency band, whereby said signal appearing in said restricted part of said frequency band is excluded from said narrowed, restricted part of said specified frequency band.
53. A method in accordance with claim 52 and further including the steps of rectifying and smoothing said control signal.
54. 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.
55. A method of expanding an input signal in a specified frequency band to produce an output signal having an increased dynamic range within each of a plurality of restricted parts of said specified frequency band, comprising the steps of: providing a circuit having variable frequency response characteristics over each of said plurality of restricted parts of said specified frequency band; supplying said input signal to said circuit; deriving a plurality of control signals each of which is representative of the level of said input siGnal in a corresponding restricted part of said specified frequency band; and varying the variable frequency response characteristics of said circuit in each of said plurality of restricted parts of said specified frequency band.
56. A method of effecting noise reduction by compressing a first input signal in a specified frequency band to produce a first output signal having a decreased dynamic range within a least one restricted part of said specified frequency band, transmitting and receiving or recording and playing back said first output signal to provide a second input signal, and expanding said second input signal in said specified frequency band to produce a second output signal having an increased dynamic range within at least one restricted part of said specified frequency band, said compressing comprising the steps of: providing a circuit having variable frequency response characteristics over said restricted part of said specified frequency band; supplying said input signal to said circuit; deriving a control signal in response to signals in said circuit; and varying said variable frequency response characteristics of said circuit in response to increasing amplitudes of said control signal so as to narrow said restricted part of said specified frequency band, and said expanding likewise comprising the steps of: providing a circuit having variable frequency response characteristics over said restricted part of said specified frequency band; supplying said input signal to said circuit; deriving a control signal in response to signals in said circuit; and varying said variable frequency response characteristics of said circuit in response to increasing amplitudes of said control signal so as to narrow said restricted part of said specified frequency band.
57. 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.
58. 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.
59. 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.
60. 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.
61. 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.
62. 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.
63. 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.
64. A method of treating an input signal x to provide an output signal z comprising the steps of: operating linearly upon said input signal to provide a first signal component proportional to x, operating non-linearly upon said input signal to provide a second signal component F1x where F1 is a non-linear operator decreasing as x increases, combining said first and second signal components additively to provide an intermediate signal y (1 + F1) x, transmitting and receiving or recording and playing back said intermediate signal, operating linearly upon said intermediate signal as received or played back to provide a third signal component proportional to y, operating non-linearly upon said output signal to provide a fourth signal component F2z where F2 is a non-linear operator at least substantially the same as F1, and combining said third and fourth signal components subtractively to provide said output signal z y - F2z, whereby z (1 + F1) x/(1 + F2).
65. 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.
66. A signal processing system according to claim 21 wherein the said main and further signal circuits are parallel signal paths.
67. A signal processing system according to claim 21 wherein said first signal is instantaneously proportional to said input signal.
68. A signal processing system according to claim 21 wherein said input signal comprises high frequency audio signals.
69. A signal processing system according to claim 21 wherein said input signal comprises a low frequency audio signal.
70. A signal processing system according to claim 21 wherein said input signal comprises both low frequency and high frequency audio signals.
71. A signal processing system according to claim 21 wherein said input signal comprises high frequency video signals.
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