Integrated vehicle voice enhancement system and hands-free cellular telephone system

• US 6,505,057 B1
• Filed: 01/23/1998
• Issued: 01/07/2003
• Est. Priority Date: 01/23/1998
• Status: Expired due to Term

• Current Assignee: Digisonix Incorporated
• Original Assignee: Digisonix Incorporated
• Inventors: Nowak, Michael P., Finn, Brian M.
• Primary Examiner(s): Urban, Edward F.
• Assistant Examiner(s): Gesesse, Tilahun
• Application Number:
  US09/012,529
• Time in Patent Office: 1,810 Days
• Field of Search: 455/569, 455/557, 455/556, 455/575, 455/90, 455/566, 379/410, 379/391, 379/406, 379/388, 381/71, 381/94, 381/66
• US Class Current: 455/569.2
• CPC Class Codes: G10L 2021/02166 Microphone arrays; Beamforming
• CPC Class Codes: G10L 21/0208 Noise filtering

Abstract

An integrated vehicle voice enhancement system and hands-free cellular telephone system implements microphone steering techniques and noise reduction filtering to improve the intelligibility and clarity of transmitted signals. A microphone steering switch is provided for the cellular telephone interface which allows only one of the microphones to be switched in to an “on” state at any given time. The microphone steering switch generates a raw telephone input switch that is a combination of 100% of the designated primary microphone signal and approximately 20% of the microphone signals from microphones in the “off” state. In this manner, the telephone line does not appear dead to a listener on the other end of the telephone line when speech is not present in the telephone input signal. A noise reduction filter filters the raw telephone signal in the time domain in real time to improve the clarity of the telephone input signal when speech is present in the telephone input signal. A microphone steering switch for the voice enhancement system is also provided to implement switching between acoustically coupled microphones located within the vehicle.

Claims

1. An integrated vehicle voice enhancement system and hands-free cellular telephone system comprising:

a near-end acoustic zone;

a far-end acoustic zone;

a near-end microphone that sense sound in the near-end zone and generates a near-end voice signal;

a far-end microphone that sense sound in the far-end zone and generates a far-end voice signal;

a near-end loudspeaker that inputs a near-end input signal and outputs sound into the near-end zone;

a far-end loudspeaker that inputs a far-end input signal and outputs sound into the far-end zone;

a near-end adaptive acoustic echo canceler that receives the near-end input signal and generates a near-end echo cancellation signal;

a near-end echo cancellation summer that inputs the near-end voice signal and the near-end echo cancellation signal and outputs an echo-cancelled, near-end voice signal;

a far-end adaptive acoustic echo canceler that receives the far-end input signal and generates a far-end echo cancellation signal;

a far-end echo cancellation summer that inputs the far-end voice signal and the far-end echo cancellation signal and outputs an echo-cancelled, far-end voice signal;

a microphone steering switch that inputs the echo-cancelled, near-end voice signal and the echo-cancelled, far-end voice signal and outputs a telephone input signal; and

a cellular telephone that inputs the telephone input signal;

wherein at least one noise reduction filter is used to improve the clarity of the telephone input signal inputting the cellular telephone;

wherein the noise reduction filter is a recursive implementation of a discrete cosine transform modified to stabilize its performance in a digital signal processor, each of the plurality of fixed filters is a finite impulse response filter, and

the finite impulse response filters are represented by the following expression;

$z_m<br><br>\left(k\right)=\underset{n=o}{\overset{m-1}{\sum <br><br>}}<br><br>\left[\frac{\mathrm{Gm}}{M}<br><br>{\mathrm{\gamma <br><br>}}^N<br><br>\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(2<br><br>n+1\right)<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{2<br><br>M}\right)\right]\times <br><br>\left(k-n\right)$

where M is the number of fixed filters, x(k-n) is a time-shifted version of the raw input signal, n=0,1 . . . M-1, z_m(k) is the filtered input signal for the m^th filter, m=0,1, . . . M-1, γ

is a stability factor, and G_m=1 for m=0, and G_m=2 for m≠

0.

2. An integrated vehicle voice enhancement system and hands-free cellular telephone system comprising:

a near-end acoustic zone;

a far-end acoustic zone;

a near-end microphone that senses sound in the near-end zone and generates a near-end voice signal;

a far-end microphone that sense sound in the far-end zone and generates a far-end voice signal;

a near-end loudspeaker that inputs a near-end input signal and outputs sound into the near-end zone;

a far-end loudspeaker that inputs a far-end input signal and outputs sound into the far-end zone;

a near-end adaptive acoustic echo canceler that receives the near-end input signal and generates a near-end echo cancellation signal;

a near-end echo cancellation summer that inputs the near-end voice signal and the near-end echo cancellation signal and outputs an echo-cancelled, near-end voice signal;

a far-end adaptive acoustic echo canceler that receives the far-end input signal and generates a far-end echo cancellation signal;

a far-end echo cancellation summer that inputs the far-end voice signal and the far-end echo cancellation signal and outputs an echo-cancelled, far-end voice signal;

a microphone steering switch that inputs the echo-cancelled, near-end voice signal and the echo-cancelled, far-end voice signal and outputs a telephone input signal; and

a cellular telephone that inputs the telephone input signal;

wherein at least one noise reduction filter is used to improve the clarity of the telephone input signal inputting the cellular telephone,

wherein the noise reduction filter is a recursive implementation of a discrete cosine transform modified to stabilize its performance in a digital signal processor, and the plurality of fixed filters are infinite impulse response filters.

3. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 2 wherein the infinite impulse response filters are represented by the following expressions:

$z_0<br><br>\left(k\right)=\left[\frac{1}{M}\right]<br><br>\text{\hspace{1em}<br><br>}\left[x<br><br>\left(k\right)-{\mathrm{\gamma <br><br>}}^M<br><br>x<br><br>\left(k-M\right)\right]+\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>z_0<br><br>\left(k-1\right)$

for fixed filter m=0, and $z_m<br><br>\left(k\right)=\left[\frac{2}{M}<br><br>{\cos }^2<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{2<br><br>\text{\hspace{1em}<br><br>}<br><br>M}\right)\right]<br><br>\text{\hspace{1em}<br><br>}[\text{\hspace{1em}<br><br>}<br><br>(x<br><br>\left(k\right)-\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-1\right)+<br><br>{\left(-1\right)}^m<br><br>{\mathrm{\gamma <br><br>}}^{M+1}<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-\left[M+1\right]\right)-{\left(-1\right)}^m<br><br>{\mathrm{\gamma <br><br>}}^M<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-M\right)]+2<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{M}\right)<br><br>z_m<br><br>\left(k-1\right)-{\mathrm{\gamma <br><br>}}^2<br><br>\text{\hspace{1em}<br><br>}<br><br>z_m<br><br>\left(k-2\right)$

for fixed filter m=1,2 . . . M-1,

where γ

is a stability parameter, x(k) is the raw input signal for sampling period k, M is the number of fixed filters, and z_m(k) is the filtered input signal for the m^th filter, m=0,1 . . . M-1.

4. An integrated vehicle voice enhancement system and hands-free cellular telephone system comprising:

a near-end acoustic zone;

a far-end acoustic zone;

a near-end microphone that senses sound in the near-end zone and generates a near-end voice signal;

a far-end microphone that senses sound in the far-end zone and generates a far-end voice signal;

a near-end loudspeaker that inputs a near-end input signal and outputs sound into the near-end zone;

a far-end loudspeaker that inputs a far-end input signal and outputs sound into the far-end zone;

a near-end adaptive acoustic echo canceler that receives the near-end input signal and generates a near-end echo cancellation signal;

a near-end echo cancellation summer that inputs the near-end voice signal and the near-end echo cancellation signal and outputs an echo-cancelled, near-end voice signal;

a far-end adaptive acoustic echo canceler that receives the far-end input signal and generates a far-end echo cancellation signal;

a far-end echo cancellation summer that inputs the far-end voice signal and the far-end echo cancellation signal and outputs an echo-cancelled, far-end voice signal;

a microphone steering switch that inputs the echo-cancelled, near-end voice signal and the echo-cancelled, far-end voice signal and outputs a telephone input signal; and

a cellular telephone that inputs the telephone input signal;

wherein at least one noise reduction filter is used to improve the clarity of the telephone input signal inputting the cellular telephone

wherein the noise reduction filter comprises;

a plurality of fixed filters, each fixed filter inputting a raw input signal derived from at least one of the systems microphone signals and outputting a respective filtered signal;

a time-varying filter gain element corresponding to each fixed filter that inputs the respective filtered signal and outputs a weighted and filtered signal, each time-varying filter gain element having a value that varies over time in proportion to a signal strength level for the respective filtered signal; and

a summer that inputs the weighted and filtered input signals and outputs a noise reduced signal, and

wherein the value of each time-varying filter gain element is determined in accordance with the following expression;

${\mathrm{\beta <br><br>}}_m<br><br>\left(k\right)={\left[1-\frac{1}{{\mathrm{SSL}}_m<br><br>\left(k\right)+\mathrm{\alpha <br><br>}}\right]}^{\mathrm{\mu <br><br>}}$

where β

_m(k) is the value of the time-varying filter gain element for the m^th fixed filter at sampling period k, m=0,1 . . . M-1, SSL_m(k) is the speech strength level for the respective filtered telephone input signal at sampling period k, and μ and

α

are preselected performance parameters having values greater than 0.

5. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 4 wherein time-varying filter gain elements β

_m(k) for the m^th fixed filter is set equal to zero if noise power for the respective frequency band is greater than a preselected threshold value.

6. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 4 wherein the performance parameter μ

is approximately equal to 4 and the performance parameter α

is approximately equal to 2.

7. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 4 wherein the speech strength level for the respective filtered input signal at sample period k is determine in accordance with the following expression:

${\mathrm{SSL}}_m<br><br>\left(k\right)=\frac{{\mathrm{s\_pwr}}_m<br><br>\left(k\right)}{{\mathrm{n\_pwr}}_m<br><br>\left(k\right)}$

where s_pwr_m(k) is an estimate of combined speech and noise power in the m^th filtered input signal at sample period k and n_pwr_m(k) is an estimate of noise power in the m^th filtered input signal used for sample period k.

8. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 7 wherein the noise power level estimate n_pwr_m(k), m=0,1 . . . M-1 for sample period k for each of the filtered input signals is accomplished in accordance with the following expression:

9. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 8 wherein time constant λ

_o is set to a small value, thereby providing a long averaging window for estimating the noise power level.

10. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 7 wherein the combined speech and noise power level s_pwr_m(k), m=0,1 . . . M-1 for sample period k for each of the filtered input signals is estimated in accordance with the following expression:

11. An integrated vehicle voice enhancement system and hands-free cellular telephone system comprising:

a near-end acoustic zone;

a far-end acoustic zone;

a plurality of near-end microphones that each sense sound in the near-end zone and each generate a near-end voice signal;

a plurality of far-end microphones that each sense sound in the far-end zone and each generate a far-end voice signal;

at least one near-end loudspeaker that inputs a near-end input signal and outputs sound into the near-end zone;

at least one far-end loudspeaker that inputs a far-end input signal and outputs sound into the far-end zone;

one or more near-end adaptive echo cancellation channels, each receiving a respective near-end input signal and outputting a near-end cancellation signal for an associated near-end microphone;

a near-end echo cancellation summer of each near-end microphone that inputs the respective near-end voice signal from the respective near-end microphone and any near-end echo cancellation signal form the associated one or more near-end adaptive echo cancellation channels, and outputs a respective echo-cancelled, near-end voice signal;

one or more far-end adaptive echo cancellation channels, each receiving a respective far-end input signal and outputting a far-end echo cancellation signal for an associated far-end microphone;

a far-end echo cancellation summer for each far-end microphone that inputs the far-end voice signal from the respective far-end microphone and any far-end echo cancellation signal from the associated one or more far-end adaptive echo cancellation channels, and output a respective echo-cancelled, far-end voice signal;

a microphone steering switch that inputs the echo-cancelled, near-end voice signals and the echo-cancelled far-end voice signals and outputs a telephone input signal;

a cellular telephone that inputs the telephone input signal;

wherein at least one noise reduction filter is used to improve the clarity of the telephone input signal inputting the cellular telephone, p1 wherein the noise reduction filter is a recursive implementation of a discrete cosine transform modified to stabilize its performance on a digital signal processor, each of the plurality of fixed filters is a finite impulse response filter, and the finite impulse response filters are represented by the following expression;

$z_m<br><br>\left(k\right)=\underset{n=o}{\overset{m-1}{\sum <br><br>}}<br><br>\left[\frac{\mathrm{Gm}}{M}<br><br>{\mathrm{\gamma <br><br>}}^N<br><br>\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(2<br><br>n+1\right)<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{2<br><br>M}\right)\right]\times <br><br>\left(k-n\right))$

where M is the number of fixed filters, x(k-n) is a time-shifter version of the raw telephone input signal, n=0,1 . . . M-1, z_m(k) is the filtered telephone input signal for the m^th filter, m=0,1, . . . M-1, γ

is a stability factor, and G_m=1 for m=0, and G_m=2 for m≠

0.

12. An integrated vehicle voice enhancement system and hands-free cellular telephone system comprising:

a near-end acoustic zone;

a far-end acoustic zone;

a plurality of near-end microphones that each sense sound in the near-end zone and each generate a near-end voice signal;

a plurality of far-end microphones that each sense sound in the far-end zone and each generate a far-end voice signal;

at least one near-end loudspeaker that inputs a near-end input signal and outputs sound into the near-end zone;

at least one far-end loudspeaker that inputs a far-end input signal and outputs sound into the far-end zone;

one or more near-end adaptive echo cancellation channels, each receiving a respective near-end input signal and outputting a near-end cancellation signal for an associated near-end microphone;

a near-end echo cancellation summer for each near-end microphone that inputs the respective near-end voice signal from the respective near-end microphone and any near-end echo cancellation signal from the associated one or more near-end adaptive echo cancellation channels, and outputs a respective echo-cancelled, near-end voice signal;

one or more far-end adaptive echo cancellation channels, each receiving a respective far-end input signal and outputting a far-end echo cancellation signal for an associated far-end microphone;

a far-end echo cancellation summer for each far-end microphone that inputs the far-end voice signal from the respective far-end microphone and any far-end echo cancellation signal from the associated one or more far-end adaptive echo cancellation channels, and outputs a respective echo-cancelled, far-end voice signal;

a microphone steering switch that inputs the echo-cancelled, near-end voice signals and the echo-cancelled far-end voice signals and outputs a telephone input signal;

a cellular telephone that inputs the telephone input signal;

wherein at least one noise reduction filter is used to improve the clarity of the telephone input signal inputting the cellular telephone,

wherein the noise reduction filter is a recursive implementation of a discrete cosine transform modified to stabilize its performance on a digital signal processor, the plurality of fixed filters are infinite impulse response filters, and the infinite impulse response filters are represented by the following expressions;

$z_0<br><br>\left(k\right)=\left[\frac{1}{M}\right]<br><br>\text{\hspace{1em}<br><br>}\left[x<br><br>\left(k\right)-{\mathrm{\gamma <br><br>}}^M<br><br>x<br><br>\left(k-M\right)\right]+\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>z_0<br><br>\left(k-1\right)$

for fixed filter m=0, and $z_m<br><br>\left(k\right)=\left[\frac{2}{M}<br><br>{\cos }^2<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{2<br><br>\text{\hspace{1em}<br><br>}<br><br>M}\right)\right]<br><br>\text{\hspace{1em}<br><br>}[\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k\right)-\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-1\right)+<br><br><br><br>{\left(-1\right)}^m<br><br>{\mathrm{\gamma <br><br>}}^{M+1}<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-\left[M+1\right]\right)-{\left(-1\right)}^m<br><br>{\mathrm{\gamma <br><br>}}^M<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-M\right)]+2<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{M}\right)<br><br>z_m<br><br>\left(k-1\right)-{\mathrm{\gamma <br><br>}}^2<br><br>\text{\hspace{1em}<br><br>}<br><br>z_m<br><br>\left(k-2\right)$

for fixed filter m=1,2 . . . M-1,

where γ

is a stability parameter, x(k) is the raw telephone input signal for sampling period k, M is the number of fixed filters, and z_m is the filtered telephone input signal for the m^th filter, m=0,1 . . . M-1.

13. An integrated vehicle voice enhancement system and hands-free cellular telephone system comprising:

a near-end acoustic zone;

a far-end acoustic zone;

a plurality of near-end microphones that each sense sound in the near-end zone and each generate a near-end voice signal;

a plurality of far-end microphones that each sense sound in the far-end zone and each generate a far-end voice signal;

at least one near-end loudspeaker that inputs a near-end input signal and outputs sound into the near-end zone;

at least one far-end loudspeaker that inputs a far-end input signal and outputs sound into the far-end zone;

one or more near-end adaptive echo cancellation channels, each receiving respective near-end input signal and outputting a near-end echo cancellation signal for an associated near-end microphone;

a near-end cancellation summer for each near-end microphone that inputs the respective near-end voice signal from the respective near-end microphone and any near-end echo cancellation signal from the associated one or more near-end adaptive echo cancellation channels, and outputs a respective echo-cancelled, near-end voice signal;

one or more far-end adaptive echo cancellation channels, each receiving a respective far-end input signal and outputting a far-end echo cancellation signal for an associated far-end microphone;

a far-end echo cancellation summer for each far-end microphone that inputs the far-end voice signal from the respective far-end microphone and any far-end echo cancellation signal from the associated one or more far-end adaptive echo cancellation channels, and outputs a respective echo-cancelled, far-end voice signal;

a microphone steering switch that inputs the echo-cancelled, near-end voice signals and the echo-cancelled far-end voice signals and outputs a telephone input signal;

a cellular telephone that inputs the telephone input signal;

wherein at least one noise reduction filter is used to improve the clarity of the telephone input signal inputting the cellular telephone;

wherein the noise reduction filter comprises;

a plurality of fixed filters, each fixed filter inputting a raw input signal derived from at least one of the systems microphone signals and outputting a respective filtered signal;

a time-varying filter gain element corresponding to each fixed filter that inputs the respective filter signal and outputs a weighted and filtered signal, each time-varying filter gain element having a value that varies over time in proportion to a signal strength level for the respective filtered signal; and

a summer that inputs the weighted and filtered input signals and outputs a noise reduced signal, and

wherein the value of each time-varying filter gain element is determined in accordance with the following expression;

${\mathrm{\beta <br><br>}}_m<br><br>\left(k\right)={\left[1-\frac{1}{{\mathrm{SSL}}_m<br><br>\left(k\right)+\mathrm{\alpha <br><br>}}\right]}^{\mathrm{\mu <br><br>}}$

where β

_m(k) is the value of the time-varying filter gain element for the m^th fixed filter at sampling period k, m=0,1 . . . M-1, SSL_m(k) is the speech strength level for the respective filtered telephone input signal at sampling period k, and μ and

α

are preselected performance parameters having values greater than 0.

14. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 13 wherein time-varying filter gain elements β

_m(k) for the m^th fixed filter is set equal to zero if noise power for the respective frequency band is greater than a preselected threshold value.

15. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 13 wherein the performance parameter μ

is approximately equal to 4 and the performance parameter α

is approximately equal to 2.

16. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 13 wherein the speech strength level for the respective filtered input signal at sample period k is determine in accordance with the following expression:

${\mathrm{SSL}}_m<br><br>\left(k\right)=\frac{{\mathrm{s\_pwr}}_m<br><br>\left(k\right)}{{\mathrm{n\_pwr}}_m<br><br>\left(k\right)}$

where s_pwr_m(k) is an estimate of combined speech and noise power in the m^th filtered input signal at sample period k and n_pwr_m(k) is an estimate of noise power in the m^th filtered input signal used for sample period k.

17. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 16 wherein the noise power level estimate n_pwr_m(k), m=0,1 . . . M-1 for sample period k for each of the filtered input signals is accomplished in accordance with the following expression:

18. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 17 wherein time constant λ

_o is set to a small value, thereby providing a long averaging window for estimating the noise power level.

19. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 16 wherein the combined speech and noise power level s_pwr_m(k), m=0,1 . . . M-1 for sample period k for each of the filtered input signals is estimated in accordance with the following expression:

20. A method of generating a noise-reduced telephone input signal in a hands-free telephone system for a vehicle, the method comprising the steps of:

sensing background noise within the vehicle and driver and passenger speech within the vehicle using at least one microphone located within the vehicle, and generating an input signal in response thereto;

filtering the input signal through a plurality of M fixed filters to generate a plurality of M filtered input signals, the fixed filters being a recursive implementation of a discrete cosine transform modified to stabilize its performance on a digital signal processor;

estimating a noise power level for each of the M filtered input signals;

estimating a combined speech and noise power level of each of the M filtered input signals;

weighting each of the plurality of M filtered input signals by a respective time-varying filter gain β

_m which is determined in accordance with the respective estimate of the combined speech and noise power level and the estimate of the noise power level; and

combining the M weighted and filtered input signals to form a noise-reduced input signal,

wherein the noise power level estimate for sample period k for each of the M filtered input signals n_pwr_n(k), m=0,1 . . . M-1, is accomplished in accordance with the following expression;

21. An integrated vehicle voice enhancement system and hands-free cellular telephone system as recited in claim 20 wherein time-varying filter again elements β

_m(k) for the m^th fixed filter is set equal to zero if noise power for the respective frequency band is greater than a preselected threshold value.

22. A method as recited in claim 20 wherein the time constant λ

_o is set to a small value, thereby providing a long averaging window for estimating the noise power level n_pwr_m(k).

23. A method as recited in claim 20 wherein the combined speech and noise power level for sample period k for each of the M filtered input signals, s_—

pwr_m(k), m=0,1 . . . M-1, is accomplished in accordance with the following expression;

24. A method as recited in claim 23 wherein the M time-varying filter gains β

_m(k) are determined in accordance with the following expressions;

${\mathrm{\beta <br><br>}}_m<br><br>\left(k\right)={\left[1-\frac{1}{{\mathrm{SSL}}_m<br><br>\left(k\right)+\mathrm{\alpha <br><br>}}\right]}^{\mathrm{\mu <br><br>}}$${\mathrm{SSL}}_m<br><br>\left(k\right)=\frac{{\mathrm{s\_pwr}}_m<br><br>\left(k\right)}{{\mathrm{n\_pwr}}_m<br><br>\left(k\right)}$

where α

, μ

≧

0 are performance parameters, and SSL_m(k) is the speech strength level for the m^th filtered input signal at sample period (k).

25. A method of generating a noise-reduced telephone input signal in a hands-free telephone system for a vehicle, the method comprising the steps of:

sensing background noise within the vehicle and driver and passenger speech within the vehicle using at least one microphone located within the vehicle, and generating an input signal in response thereto;

filtering the input signal through a plurality of M fixed filters to generate a plurality of M filtered input signals, the fixed filters being a recursive implementation of a discrete cosine transform modified to stabilize its performance on a digital signal processor;

estimating a noise power level for each of the M filtered input signals;

estimating a combined speech and noise power level of each of the M filtered input signals;

weighting each of the plurality of M filtered input signals by a respective time-varying filter gain β

_m which is determined in accordance with the respective estimate of the combined speech and noise power level and the estimate of the noise power level; and

combining the M weighted and filtered input signals to form a noise-reduced input signal;

wherein the plurality of fixed filters are infinite impulse response filters represented by the following expressions;

$z_0<br><br>\left(k\right)=\left[\frac{1}{M}\right]<br><br>\text{\hspace{1em}<br><br>}[\left(x<br><br>\left(k\right)-{\mathrm{\gamma <br><br>}}^m<br><br>x<br><br>\left(k-M\right)\right]+\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>z_0<br><br>\left(k-1\right)$

for m=0$z_m<br><br>\left(k\right)=\left[\frac{2}{M}<br><br>{\cos }^2<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{2<br><br>\text{\hspace{1em}<br><br>}<br><br>M}\right)\right]<br><br>\text{\hspace{1em}<br><br>}[\text{\hspace{1em}<br><br>}<br><br>(x<br><br>\left(k\right)-\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-1\right)+<br><br>{\left(-1\right)}^m<br><br>{\mathrm{\gamma <br><br>}}^{M+1}<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-\left[M+1\right]\right)-{\left(-1\right)}^m<br><br>{\mathrm{\gamma <br><br>}}^M<br><br>\text{\hspace{1em}<br><br>}<br><br>x<br><br>\left(k-M\right)]+2<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\gamma <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>m}{M}\right)<br><br>z_m<br><br>\left(k-1\right)-{\mathrm{\gamma <br><br>}}^2<br><br>\text{\hspace{1em}<br><br>}<br><br>z_m<br><br>\left(k-2\right)$

for m=1,2 . . . M-1

where γ

is a preselected stability parameter, x(k) is the raw input signal for sample period k, and z_m is the filtered input signal for the m^th fixed filter m=0,1 . . . M-1.