Noise reduction apparatus using spectral subtraction or scaling and signal attenuation between formant regions

Claim 2

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into spectral component signals representing the magnitudes of spectral components of the input signals; processing means for applying to said spectral component signals a spectral subtraction process; reconversion means for converting said spectral component signals into a time-varying signal; means for identifying formant regions of the speech spectrum; and means for effecting, at a predetermined point after said application of said subtraction process, further attenuation of those frequency components lying outside the formant regions.

Claim 3

A noise reduction apparatus according to claim 1 in which the conversion means is operable to perform a discrete Fourier transform on segments of the input signal.

Claim 4

A noise reduction apparatus according to claim 1 further comprising means for recognizing periods during which speech is absent from the input signal and storing signals representing the power spectrum of the input signal during such periods to represent an estimated noise spectrum of the input signal, and wherein the processing means performing the spectral subtraction process subtracts from signals representing the power spectrum of the input signal, the signals representing an estimated noise spectrum of the input signal.

Claim 5

A noise reduction apparatus according to claim 1 in which the means to identify formant regions is responsive to the input signal or a derivative of said input signal to produce frequency response signals, and in which the attenuation means is operable to multiply the power spectrum of the signal by the frequency response signals.

Claim 6

A noise reduction apparatus according to claim 4 in which the means to identify formant regions includes Linear Predictive Analysis means to produce a linear predictive (LP) spectrum.

Claim 7

A noise reduction appartus according to claim 5 in which the means to identify formant regions includes thresholding means such that the frequency response signals are unity wherever the LP spectrum is above a threshold value and otherwise are a function of the LP spectrum.

Claim 8

A noise reduction apparatus according to claim 4, in which the means to identify formant regions is responsive to the output of the processing means.

Claim 9

A noise reduction apparatus according to claim 4 in which the means to identify the formant regions is responsive to the spectral component signals following processing by auxiliary processing means operable to apply the spectral subtraction process to said spectral component signals.

Claim 10

A noise reduction apparatus according to claim 4 further comprising auxiliary conversion means for converting the time-varying input signal into further spectral component signals representing the magnitudes of spectral components of the input signals and auxiliary processing means operable to apply the spectral substraction process to said further spectral component signals; and in which the means to identify the formant regions is responsive to the output of the auxiliary processing means.

Claim 11

A noise reduction apparatus according to claim 9 in which the conversion means is operable to produce said spectral component signals for each of successive fixed time periods of the input signal and the auxiliary conversion means is operable to produce said further spectral component signals for each successive time period of speech, those periods having durations differing from the said fixed time periods.

Claim 12

A noise reduction apparatus according to claim 10 further comprising means for monitoring the stationarity of the input speech signal and to control the duration of the time periods employed by the auxiliary conversion means.

Claim 13

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into signals representing the magnitudes of spectral components of the input signals; processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; reconversion means to convert the said spectral component signals into a time-varying signal; means to identify format regions of the speech spectrum; means to attenuate those frequency components lying outside the formant regions; the means to identify formant regions being responsive to the input signal or a derivative of the input signal to produce frequency response signals, and the attenuation means being operable to multiply the power spectrum of the signal by the frequency response signals; the means to identify formant regions including Linear Predictive Analysis means to produce a linear predictive (LP) spectrum; and thresholding means such that the frequency response signals are unity wherever the LP spectrum is above a threshold value and otherwise are a function of the LP spectrum.

Claim 14

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into signals representing the magnitudes of spectral components of the input signals; processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; reconversion means to convert the said spectral component signals into a time-varying signal; means to identify format regions of the speech spectrum; means to attenuate those frequency components lying outside the formant regions; the means to identify formant regions being responsive to the input signal or a derivative of the input signal to produce frequency response signals, and the attenuation means being operable to multiply the power spectrum of the signal by the frequency response signals; the means to identify formant regions including Linear Predictive Analysis means to produce a linear predictive (LP) spectrum; and thresholding means such that the frequency response signals are unity wherever the LP spectrum is above a threshold value and otherwise are a function of the LP spectrum.

Claim 15

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into signals representing the magnitudes of spectral components of the input signals; processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; reconversion means to convert the said spectral component signals into a time-varying signal; means to identify format regions of the speech spectrum; means to attenuate those frequency components lying outside the formant regions; the means to identify formant regions being responsive to the input signal or a derivative of the input signal to produce frequency response signals, and the attenuation means being operable to multiply the power spectrum of the signal by the frequency response signals; the means to identify the formant regions being further responsive to the spectral signals following processing by auxiliary processing means operable to effect a reduction in the magnitude of low magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals.

Claim 16

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into signals representing the magnitudes of spectral components of the input signals; processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; reconversion means to convert the said spectral component signals into a time-varying signal; means to identify format regions of the speech spectrum; means to attenuate those frequency components lying outside the formant regions; the means to identify formant regions being responsive to the input signal or a derivative of the input signal to produce frequency response signals, and the attenuation means being operable to multiply the power spectrum of the signal by the frequency response signals; the means to identify the formant regions being further responsive to the spectral signals following processing by auxiliary processing means operable to effect a reduction in the magnitude of low magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals.

Claim 17

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into signals representing the magnitudes of spectral components of the input signals; processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; reconversion means to convert the said spectral component signals into a time-varying signal; means to identify format regions of the speech spectrum; means to attenuate those frequency components lying outside the formant regions; the means to identify formant regions being responsive to the input signal or a derivative of the input signal to produce frequency response signals, and the attenuation means being operable to multiply the power spectrum of the signal by the frequency response signals; auxiliary conversion means for converting the time-varying input signal into signals representing the magnitudes of spectral components of the input signals and auxiliary processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; and in which the means to identify the formant regions is responsive to the output of the auxiliary processing means.

Claim 18

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into signals representing the magnitudes of spectral components of the input signals; processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; reconversion means to convert the said spectral component signals into a time-varying signal; means to identify format regions of the speech spectrum; means to attenuate those frequency components lying outside the formant regions; the means to identify formant regions being responsive to the input signal or a derivative of the input signal to produce frequency response signals, and the attenuation means being operable to multiply the power spectrum of the signal by the frequency response signals; auxiliary conversion means for converting the time-varying input signal into signals representing the magnitudes of spectral components of the input signals and auxiliary processing means operable to effect a reduction in the magnitude of low-magnitude ones of the said spectral component signals relative to that of higher magnitude ones of the said spectral component signals; and in which the means to identify the formant regions is responsive to the output of the auxiliary processing means.

Claim 19

A noise reduction apparatus according to claim 14 in which the conversion means is operable to produce said spectral component signals for each of successive fixed time periods of the input signal and the auxiliary conversion means is operable to produce said further spectral component signals for each successive time period of speech, those periods having durations differing from the said fixed time periods.

Claim 20

A noise reduction apparatus according to claim 15 including means for monitoring the stationarity of the input speech signal and to control the duration of the time periods employed by the auxiliary conversion means.

Claim 21

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into spectral component signals representing the magnitudes of spectral components of the input signals; processing means for applying to said spectral component signals a spectral scaling process; reconversion means for converting said spectral component signals into a time-varying signal; means for identifying formant regions of the speech spectrum; means for effecting, at a predetermined point after said application of said scaling process, further attenuation of those frequency components lying outside the formant regions.

Claim 22

A noise reduction apparatus comprising: conversion means for converting a time-varying input signal into spectral component signals representing the magnitudes of spectral components of the input signals; processing means for applying to said spectral component signals a spectral scaling process; reconversion means for converting said spectral component signals into a time-varying signal; means for identifying formant regions of the speech spectrum; means for effecting, at a predetermined point after said application of said scaling process, further attenuation of those frequency components lying outside the formant regions.

Claim 23

A noise reduction apparatus according to claim 17 in which the conversion means is operable to performs a discrete Fourier transform on segments of the input signal.

Claim 24

A noise reduction apparatus according to claim 17 in which the processing means performing the spectral scaling process applies to said spectral component signals a nonlinear transfer characteristic such as to attenuate low magnitude spectral component signals relative to high magnitude ones.

Claim 25

A noise reduction apparatus according to claim 17 in which the means for identifying formant regions is responsive to the input signal or a derivative of said input signal to produce frequency response signals, and the attenuation means is operable to multiply the power spectrum of the signal by the frequency response signals.

Claim 26

A noise reduction apparatus according to claim 20 in which the means to identify formant regions includes Linear Predictive Analysis means to produce a linear predictive (LP) spectrum.

Claim 27

A noise reduction apparatus according to claim 21 in which the means for identifying formant regions includes thresholding means such that the frequency response signals are unity wherever the LP spectrum is above a threshold value and otherwise are a function of the LP spectrum.

Claim 28

A noise reduction apparatus according to claim 20 in which the means to identify formant regions is responsive to the output of the processing means.

Claim 29

A noise reduction apparatus according to claim 20 in which the means to identify the formant regions is responsive to the spectral component signals following processing by auxiliary processing means operable to apply the spectral scaling process to said spectral component signals.

Claim 30

A noise reduction apparatus according to claim 20 further comprising auxiliary conversion means for converting the time-varying input signal into further spectral component signals representing the magnitudes of spectral components of the input signals and auxiliary processing means operable to apply the spectral scaling process to said further spectral component signals; and in which the means to identify the formant regions is responsive to the output of the auxiliary processing means.

Claim 31

A noise reduction apparatus according to claim 25 in which the conversion means is operable to produce said spectral component signals for each of successive fixed time periods of the input signal and the auxiliary conversion means is operable to produce said further spectral component signals for each successive time period of speech, those periods having durations differing from the said fixed time periods.

Claim 32

A noise reduction apparatus according to claim 26 further comprising means for monitoring the stationarity of the input speech signal and to control the duration of the time periods employed by the auxiliary conversion means.

Claim 35

A method for reducing noise according to claim 28 in which: the step of converting a time-varying input signal into spectral component signals is performed using a discrete Fourier transform on segments of the input signal.

Claim 36

A method for reducing noise according to claim 28 further comprising the steps of: recognizing periods during which speech is absent from the input signal and storing signals representing the power spectrum of the input signal during such periods to represent an estimated noise spectrum of the input signal, and performing the spectral subtraction process subtraction from signals representing the power spectrum of the input signal, the signals representing an estimated noise spectrum of the input signal.

Claim 37

A method for reducing noise according to claim 28 in which: the step of identifying formant regions further comprises producing frequency response signals in response to the input signal or a derivative of said input signal, and the step of effecting further attenuation further comprises multiplying the power spectrum of the signal by the frequency response signals.

Claim 38

A method for reducing noise according to claim 31 in which the step of identifying formant regions includes using Linear Predictive Analysis to produce a linear predictive (LP) spectrum.

Claim 39

A method for reducing noise according to claim 32 in which the step of identifying formant regions further comprises: setting the frequency response signals to be unity wherever the LP spectrum is above a predetermined threshold value and otherwise to be a function of the LP spectrum.

Claim 40

A method for reducing noise according to claim 31 in which: the step of identifying formant regions is responsive to applying said spectral subtraction process to said spectral component signals.

Claim 41

A method for reducing noise according to claim 31 in which: the step of identifying the formant regions is responsive to said spectral component signals following application of the spectral subtraction process to said component signals.

Claim 42

A method for reducing noise according to claim 31 further comprising the steps of: converting the time-varying input signal into further spectral component signals representing the magnitudes of spectral components of the input signals; and applying the spectral subtraction process to said further spectral component signals; and in which the step of identifying the formant regions is responsive to the output of converting the time-varying input signal into said further spectral component signals.

Claim 43

A method for reducing noise according to claim 36 in which the step of converting the time-varying input signal into spectral component signals further includes: producing said spectral component signals for each of successive fixed time periods of the input signal, and in which the step of converting the time-varying input signal into further spectral component signals further includes producing said further spectral component signals for each successive time period of speech, those periods having durations differing from the said fixed time periods.

Claim 44

A method for reducing noise according to claim 37 further comprising: monitoring the stationarity of the input speech signal and controlling the duration of the time periods employed in the step of producing said spectral component signals for each of successive fixed time periods of the input signal.

Claim 47

A method for reducing noise according to claim 39 in which: the step of converting a time-varying input signal into spectral component signals is performed using a discrete Fourier transform on segments of the input signal.

Claim 48

A method for reducing noise according to claim 39 in which the step of performing the spectral scaling process further comprises: applying to said spectral component signals a nonlinear transfer characteristic to attenuate low magnitude spectral component signals relative to high magnitude ones.

Claim 49

A method for reducing noise according to claim 39 in which the step of identifying formant regions further comprises: producing frequency response signals in response to the input signal or a derivative of said input signal, and the step of effecting further attenuation further comprises multiplying the power spectrum of the signal by the frequency response signals.

Claim 50

A method for reducing noise according to claim 42 in which: the step of identifying formant regions includes using Linear Predictive Analysis to produce a linear predictive (LP) spectrum.

Claim 51

A method for reducing noise according to claim 43 in which the step of identifying formant regions further comprises: setting the frequency response signals to be unity wherever the LP spectrum is above a predetermined threshold value and otherwise to be a function of the LP spectrum.

Claim 52

A method for reducing noise according to claim 42 in which: the step of identifying formant regions is responsive to applying said spectral scaling process to said spectral component signals.

Claim 53

A method for reducing noise according to claim 42 in which: the step of identifying the formant regions is responsive to said spectral component signals following application of the spectral scaling process to said component signals.

Claim 54

A method for reducing noise according to claim 42 further comprising the steps of: converting the time-varying input signal into further spectral component signals representing the magnitudes of spectral components of the input signals; and applying the spectral scaling process to said further spectral component signals; and in which the step of identifying the formant regions is responsive to said converting the time-varying input signal into said further spectral component signals.

Claim 55

A method for reducing noise according to claim 47 in which: the step of converting the time-varying input signal into spectral component signals further includes producing said spectral component signals for each of successive fixed time periods of the input signal, and the step of converting the time-varying input signal into further spectral component signals further includes producing said further spectral component signals for each successive time period of speech, those periods having durations differing from said fixed time periods.

Claim 56

A method for reducing noise according to claim 48 further comprising: monitoring the stationarity of the input speech signal and controlling the duration of the time periods employed in the step of producing said spectral component signals for each of successive fixed time periods of the input signal.