Noise suppressor having weighted gain smoothing

• US 6,317,709 B1
• Filed: 06/01/2000
• Issued: 11/13/2001
• Est. Priority Date: 06/22/1998
• Status: Expired due to Term

• Current Assignee: ST-Ericcson SA (Telefonaktiebolaget LM Ericsson)
• Original Assignee: DSPC Technologies Limited
• Inventors: Zack, Rafael
• Primary Examiner(s): Dorvil, Richemond
• Assistant Examiner(s): Lerner, Martin
• Application Number:
  US09/583,896
• Time in Patent Office: 530 Days
• Field of Search: 704/225, 704/226, 704/227, 381/94.2, 381/94.3, 381/94.1, 375/232, 375/347
• US Class Current: 704/225
• CPC Class Codes: G10L 21/0208 Noise filtering

Abstract

A noise suppressor is provided which includes a signal to noise ratio (SNR) determiner, a channel gain determiner, a gain smoother and a multiplier. The SNR determiner determines the SNR per channel of the input signal. The channel gain determiner determines a channel gain γ_ch(i) per the ith channel. The gain smoother produces a smoothed gain {overscore (γ_ch+L (i,m))} per the ith channel and the multiplier multiplies each channel of the input signal by its associated smoothed gain {overscore (γ_ch+L (i,m))}.

Claims

1. A noise suppressor comprising:

a signal to noise ratio (SNR) determiner adapted to determine the SNR per channel of an input signal; and

a gain smoother adapted to produce a smoothed gain {overscore (γ

_ch+L (i,m))} for the ith channel,

wherein said smoothed gain {overscore (γ

_ch+L (i,m))} is a function of a previous gain value {overscore (γ

_ch+L (i,m−

1+L ))} for an ith channel and a forgetting factor α

which is a function of the current level of said SNR for said ith channel, said forgetting factor α

ranges between MAX_ALFA and MIN_ALFA according to the function $1-\frac{\mathrm{\sigma <br><br>}<br><br>\left(i,m\right)}{\mathrm{SNR\_DR}}$

where σ

(i,m) is the SNR of the current frame m of the ith channel and SNR_DR is the allowed dynamic range of the SNR.

2. A noise suppressor according to claim 1 and wherein MAX_ALFA=1.0, MIN_ALFA=0.01 and SNR_DR=30 dB.

3. A noise suppressor according to claim 1 and wherein said forgetting factor α

is determined by;

$\mathrm{\alpha <br><br>}=\min <br><br>\text{\hspace{1em}<br><br>}<br><br>\left\{\mathrm{MAX\_ALFA},\max <br><br>\text{\hspace{1em}<br><br>}<br><br>\left\{\mathrm{MIN\_ALFA},1-\frac{\mathrm{\sigma <br><br>}<br><br>\left(i,m\right)}{\mathrm{SNR\_DR}}\right\}\right\}.$

4. A noise suppressor comprising:

a channel gain determiner adapted to determine a channel gain γ

_ch(i) per ith channel; and

a gain smoother adapted to produce a smoothed gain {overscore (γ

_ch+L (i,m))} for the ith channel,

wherein said smoothed gain {overscore (γ

_ch+L (i,m))} is set to be either the channel gain γ

_ch(i) or a new value, wherein said new value is provided only if the channel gain γ

_ch(i) for the current frame m is greater than the smoothed gain {overscore (γ

_ch+L (i,m−

1+L ))} for the previous frame m−

1.

5. A noise suppressor according to claim 4 and wherein said smoothed gain {overscore (γ

_ch+L (i,m))} is defined by;

$\stackrel{\_}{{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)}=\{\begin{array}{ccc}\mathrm{\alpha <br><br>}·<br><br>\stackrel{\_}{{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m-1\right)}+\left(1-\mathrm{\alpha <br><br>}\right)·<br><br>{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)& \mathrm{if}& {\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)\ge <br><br>\stackrel{\_}{{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m-1\right)}.\\ {\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)& \mathrm{Otherwise}& \text{\hspace{1em}<br><br>}\end{array}$

6. A noise suppressor comprising:

a selector adapted to select between a channel gain γ

_ch(i) and a smoothed gain {overscore (γ

_ch+L (i,m))}, said smoothed gain {overscore (γ

_ch+L (i,m))} is selected when said channel gain γ

_ch(i) of a received frame m is greater than the smoothed gain {overscore (γ

_ch+L (i,m−

1+L ))} for a previous frame m−

1.

7. A noise suppressor according to claim 6 and wherein said smoothed gain {overscore (γ

_ch+L (i,m))} is defined by;

$\stackrel{\_}{{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)}=\{\begin{array}{ccc}\mathrm{\alpha <br><br>}·<br><br>\stackrel{\_}{{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m-1\right)}+\left(1-\mathrm{\alpha <br><br>}\right)·<br><br>{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)& \mathrm{if}& {\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)\ge <br><br>\stackrel{\_}{{\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m-1\right)}.\\ {\mathrm{\gamma <br><br>}}_{\mathrm{ch}}<br><br>\left(i,m\right)& \mathrm{Otherwise}& \text{\hspace{1em}<br><br>}\end{array}$

8. A noise suppressor according to claim 7 and wherein said α

is determined by;

$\mathrm{\alpha <br><br>}=\min <br><br>\text{\hspace{1em}<br><br>}<br><br>\left\{\mathrm{MAX\_ALFA},\max <br><br>\text{\hspace{1em}<br><br>}<br><br>\left\{\mathrm{MIN\_ALFA},1-\frac{\mathrm{\sigma <br><br>}<br><br>\left(i,m\right)}{\mathrm{SNR\_DR}}\right\}\right\}.$