Hearing aid device incorporating signal processing techniques

• US 6,885,752 B1
• Filed: 11/22/1999
• Issued: 04/26/2005
• Est. Priority Date: 07/08/1994
• Status: Expired due to Fees

• Current Assignee: Brigham Young University (The Church of Jesus Christ of Latter-Day Saints)
• Original Assignee: Brigham Young University (The Church of Jesus Christ of Latter-Day Saints)
• Inventors: Christiansen, Richard Wesley, Borough, William Charles, Chabries, Douglas Melvin, Hammond, Aaron Michael
• Primary Examiner(s): Le, Huyen
• Assistant Examiner(s): PENDLETON, DIONNE
• Application Number:
  US09/444,972
• Time in Patent Office: 1,982 Days
• Field of Search: 381/23.1, 381/106, 381/98, 381/99, 381/312, 381/320, 381/321, 381/107, 455/232.1, 455/234.1, 455/234.2, 455/235.1, 455/247.1, 455/251.1, 455/303, 455/306
• US Class Current: 381/321
• CPC Class Codes: H03G 9/005 of digital or coded signals
• CPC Class Codes: H03G 9/025 frequency-dependent volume compression or expansion, e.g. multiple-band systems H03G9/10, H03G9/18 take precedence
• CPC Class Codes: H04R 2430/03 Synergistic effects of band splitting and sub-band processing
• CPC Class Codes: H04R 25/356 Amplitude, e.g. amplitude shift or compression
• CPC Class Codes: H04R 25/453 electronically
• CPC Class Codes: H04R 25/502 using analog signal processing

Abstract

A hearing compensation system for the hearing impaired comprises a plurality of bandpass filters having an input connected to an input transducer and each bandpass filter having an output connected to the input of one of a plurality of multiplicative AGC circuits whose outputs are summed together and connected to the input of an output transducer. The multiplicative AGC circuits attenuate acoustic signals having a constant background level without the loss of speech intelligibility. The identification of the background noise portion of the acoustic signal is made by the constancy of the envelope of the input signal in each of the several frequency bands. The background noise that will be suppressed includes multi-talker speech babble, fan noise, feedback whistle, florescent light hum, and white noise.

Claims

1. A multiplicative automatic gain control (AGC) circuit for an apparatus for processing audio signals, said apparatus comprising an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy, a plurality of audio bandpass filters coupled to the output of said input transducer, a plurality of said multiplicative AGC circuits comprising noise suppression circuitry, wherein each of said multiplicative AGC circuits is coupled to the output of one of said audio bandpass filters, a first summing junction coupled to the output of said multiplicative AGC circuits, a first amplifier coupled to the output of said first summing junction, and an output transducer for converting electrical energy into acoustical energy, said multiplicative AGC circuit comprising:

an absolute value circuit having one input coupled to the output of one of said audio bandpass filters;

a first lowpass filter having an input coupled to the output of said absolute value circuit;

a logarithmic element having an input coupled to the output of said lowpass filter;

a filter element having an input coupled to the output of said logarithmic element;

an exponential element having an input coupled to the output of said filter element;

a delay element having an input coupled to the input of said absolute value circuit, wherein said delay element compensates for the delay through said filter element; and

a multiplier having one input coupled to the output of said exponential element and a second input coupled to the output of said delay element.

2. The multiplicative AGC circuit as defined in claim 1, wherein said filter element further comprises:

a high pass filter having an input coupled to the output of said logarithmic element;

a second amplifier having an input coupled to the output of said high pass filter, wherein said second amplifier has a gain equal to (G−

1);

a second lowpass filter having an input also coupled to the output of said logarithmic element;

a second summing junction having a first output coupled to the output of said second lowpass filter and a second input equal to −

log[e_max], where e_max is the maximum allowable value of the audio envelope for which AGC gain is applied;

a third amplifier having an input coupled to the output of said first summing junction, wherein said third amplifier has a gain equal to (K−

1); and

a third summing junction having a first input coupled to the output of said second amplifier, a second input coupled to the output of said third amplifier and a third input equal to log[atten], wherein atten is a linear attenuation factor.

3. An apparatus for processing audio signals comprising:

an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of audio bandpass filters coupled to the output of said input transducer;

a plurality of multiplicative automatic gain control (AGC) circuits comprising noise suppression circuitry, each of said multiplicative AGC circuits coupled to an output of one of said audio bandpass filters, one of said plurality of AGC circuits comprising;

an absolute value circuit having one input coupled to the output of one of said audio bandpass filters;

a first lowpass filter having an input coupled to the output of said absolute value circuit;

a logarithmic element having an input coupled to the output of said lowpass filter;

a filter element having an input coupled to the output of said logarithmic element;

an exponential element having an input coupled to the output of said filter element;

a delay element having an input coupled to the input of said absolute value circuit, wherein said delay element compensates for the delay through said filter element; and

a multiplier having one input coupled to the output of said exponential element and a second input coupled to the output of said delay element;

a first summing junction coupled to the output of said multiplicative AGC circuits;

a first amplifier coupled to the output of said first summing junction; and

an output transducer for converting electrical energy into acoustical energy.

4. An apparatus for processing audio signals comprising:

a means for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of means for audio bandpass filtering coupled to the output of said means for converting acoustical energy;

a plurality of means for multiplicative automatic gain control (AGC), each of said means for multiplicative AGC coupled to an output of one of said means for audio bandpass filtering, one of said plurality of AGC circuits comprising;

an means for providing an absloute value having one input coupled to the output of one of said means for audio bandpass filtering;

a first means for lowpass filtering having an input coupled to the output of said means for providing an absloute value;

a means for providing a logarithmic value having an input coupled to the output of said first means for lowpass filtering;

a means for filtering having an input coupled to the output of said means for providing a logarithmic value;

an means for providing an exponential value having an input coupled to the output of said means for filtering;

a means for delaying having an input coupled to the input of said means for providing an absloute value, wherein said means for delaying compensates for the delay through said means for filtering; and

a means for multiplying having one input coupled to the output of said means for providing an exponential value and a second input coupled to the output of said means for delaying;

a first means for summing coupled to the output of said means for multiplicative AGC;

a first means for amplifying coupled to the output of said first means for summing; and

a means for converting electrical energy into acoustical energy.

5. A multiplicative automatic gain control (AGC) circuit for an apparatus for processing audio signals, said apparatus comprising an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy, a plurality of audio bandpass filters coupled to the output of said input transducer, a plurality of said multiplicative AGC circuits comprising noise suppression circuitry, wherein each of said multiplicative AGC circuits is coupled to the output of one of said audio bandpass filters, a first summing junction coupled to the output of said multiplicative AGC circuits, a first amplifier coupled to the output of said first summing junction, and an output transducer for converting electrical energy into acoustical energy, said multiplicative AGC circuit comprising:

a logarithmic element having an input coupled to the output of one of said audio bandpass filters, said logarithmic element having a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input coupled to said second output of said logarithmic element, said filter element comprising;

a lowpass filter having an input coupled to said second output of said logarithmic element;

a band pass filter having an input also coupled to said second output of said logarithmic element;

a high pass filter having an input also coupled to said second output of said logarithmic element;

a second summing junction having a first input coupled to the output of said lowpass filter and having a second input equal to −

log[e_max], where e_max is the maximum allowable value of the audio envelope for which AGC gain is applied;

a second amplifier having an input coupled to the output of said second summing junction;

a third amplifier having an input coupled to the output of said band pass filter; and

a third summing junction having a first input coupled to the output of said high pass filter, a second input coupled to the output of said second amplifier, a third input coupled to the output of said third amplifier and a fourth input equal to log[atten], wherein said third summing junction provides an output equal to the sum of its four inputs, and wherein atten is a linear attenuation factor;

a delay element having an input coupled to first output of said logarithmic element, wherein said delay element compensates for the delay through said filter element;

an exponential element having a first input coupled to the output of said delay element and a second input coupled to the output of said filter element; and

a fourth amplifier element coupled to the output of said exponential element, wherein said fourth amplifier has a gain equal to (e_max), where e_max is the maximum allowable value of the audio envelope for which AGC gain is applied.

6. A multiplicative automatic gain control (AGC) circuit for an apparatus for processing audio signals, said apparatus comprising an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy, a plurality of audio bandpass filters coupled to the output of said input transducer, a plurality of said multiplicative AGC circuits comprising noise suppression circuitry, wherein each of said multiplicative AGC circuits is coupled to the output of one of said audio bandpass filters, a first summing junction coupled to the output of said multiplicative AGC circuits, a first amplifier coupled to the output of said first summing junction, and an output transducer for converting electrical energy into acoustical energy, said multiplicative AGC circuit comprising:

an absolute value circuit having an input coupled to the output of one of said audio bandpass filters;

a lowpass filter having an input coupled to the output of said absolute value circuit;

a logarithmic element having an input coupled to the output of said lowpass filter;

a second summing junction having a first input coupled to the output of said logarithmic element and a second input equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit;

a noise estimator having an input coupled to the output of said second summing junction;

a third summing junction having a first input coupled to the output of said second summing junction and a second input coupled to the inverted output of said noise estimator;

a second amplifier having an input coupled to the output of said third summing junction, wherein said second amplifier has a gain of G′

;

a fourth summing junction having a first input coupled to the output of said second amplifier;

a fifth summing junction having an input coupled to the output of said noise estimator and having a second input equal to log[atten], wherein atten is a linear attenuation factor;

a third amplifier having an input coupled to the output of said fourth summing junction;

a sixth summing junction having a first input coupled to the output of said third amplifier, a second input coupled to the output of said logarithmic element and a third input equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit;

an exponential element having an input coupled to the output of said sixth summing junction; and

a multiplier having a first input coupled to the output of said exponential element and having a second input coupled to the output of one of said audio bandpass filters.

7. A multiplicative automatic gain control (AGC) circuit for an apparatus for processing audio signals, said apparatus comprising an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy, a plurality of audio bandpass filters coupled to the output of said input transducer, a plurality of said multiplicative AGC circuits comprising noise suppression circuitry, wherein each of said multiplicative AGC circuits is coupled to the output of one of said audio bandpass filters, a first summing junction coupled to the output of said multiplicative AGC circuits, a first amplifier coupled to the output of said first summing junction, and an output transducer for converting electrical energy into acoustical energy, said multiplicative AGC circuit comprising:

a logarithmic element having an input coupled to the output of one of said audio bandpass filters, said logarithmic element having a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input coupled to said second output of said logarithmic element, said filter element comprising;

a high pass filter having an input coupled to the output of said logarithmic element;

a lowpass filter having an input also coupled to the output of said logarithmic element;

a second summing junction having a first input coupled to the output of said lowpass filter and having a second input coupled to a value equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit;

a noise estimator having an input coupled to the output of said second summing junction;

a third summing junction having a first input coupled to the output of said second summing junction and a second input coupled to the inverted output of said noise estimator;

a second amplifier having an input coupled to the output of said third summing junction;

a fourth summing junction having an input coupled to the output of said noise estimator and a second input equal to log[atten], wherein atten is a linear attenuation factor;

a fifth summing junction having a first input coupled to the output of said second amplifier and a second input coupled to the output of said fourth summing junction;

a third amplifier having an input coupled to the output of said fifth summing junction; and

a sixth summing junction having a first input coupled to the output of said third amplifier, a second input coupled to the output of said highpass filter, and a third input equal to equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit, wherein said sixth summing junction provides an output equal to the sum of its three inputs;

a delay element having an input coupled to first output of said logarithmic element, wherein said delay element compensates for the delay through said filter element; and

an exponential element having a first input coupled to the output of said delay element and a second input coupled to the output of said filter element.

8. A multiplicative automatic gain control (AGC) circuit for an apparatus for processing audio signals, said apparatus comprising an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy, a plurality of audio bandpass filters coupled to the output of said input transducer, a plurality of said multiplicative AGC circuits comprising noise suppression circuitry, wherein each of said multiplicative AGC circuits is coupled to the output of one of said audio bandpass filters, a first summing junction coupled to the output of said multiplicative AGC circuits, a first amplifier coupled to the output of said first summing junction, and an output transducer for converting electrical energy into acoustical energy, said multiplicative AGC circuit comprising:

an absolute value circuit having an input coupled to the output of one of said audio bandpass filters;

a lowpass filter having an input coupled to the output of said absolute value circuit;

a logarithmic element having an input coupled to the output of said lowpass filter;

a first comparator circuit having an input coupled to the output of said logarithmic element;

a second comparator circuit having an input also coupled to said output of said logarithmic element;

a gain multiplexer having a first select input coupled to the output of said first comparator circuit and also having a second select input coupled to the output of said second comparator circuit;

a normalization multiplexer having a first select input coupled to the output of said first comparator circuit and also having a second select input coupled to the output of said second comparator circuit;

a second amplifier having a first input coupled to the output of said logarithmic element and having a gain equal to the output of said gain multiplexer;

a second summing junction having a first input coupled to the output of said second amplifier and having a second input coupled to the output of said normalization multiplexer;

an exponential element having an input coupled to the output of said second summing junction;

a delay element having an input coupled to the output of one of said audio bandpass filters; and

a multiplier having a first input coupled to the output of said exponential element and having a second input coupled to the output of said delay element.

9. An apparatus for processing audio signals comprising:

an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of audio bandpass filters coupled to the output of said input transducer;

a plurality of multiplicative automatic gain control (AGC) circuits, each of said multiplicative AGC circuits coupled to an output of one of said audio bandpass filters, one of said AGC circuits comprising;

a logarithmic element having an input coupled to the output of one of said audio bandpass filters, said logarithmic element having a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input coupled to said second output of said logarithmic element, said filter element comprising;

a lowpass filter having an input coupled to said second output of said logarithmic element;

a band pass filter having an input also coupled to said second output of said logarithmic element;

a high pass filter having an input also coupled to said second output of said logarithmic element;

a second summing junction having a first input coupled to the output of said lowpass filter and having a second input equal to −

log[e_max], where e_max is the maximum allowable value of the audio envelope for which AGC gain is applied;

a second amplifier having an input coupled to the output of said second summing junction;

a third amplifier having an input coupled to the output of said band pass filter; and

a third summing junction having a first input coupled to the output of said high pass filter, a second input coupled to the output of said second amplifier, a third input coupled to the output of said third amplifier and a fourth input equal to log[atten], wherein said third summing junction provides an output equal to the sum of its four inputs, and wherein atten is a linear attenuation factor;

a delay element having an input coupled to first output of said logarithmic element, wherein said delay element compensates for the delay through said filter element;

an exponential element having a first input coupled to the output of said delay element and a second input coupled to the output of said filter element; and

a fourth amplifier element coupled to the output of said exponential element, wherein said fourth amplifier has a gain equal to (e_max), where e_max is the maximum allowable value of the audio envelope for which AGC gain is applied;

a first summing junction coupled to an output of said multiplicative AGC circuits;

a first amplifier coupled to an output of said first summing junction; and

an output transducer for converting electrical energy into acoustical energy.

10. An apparatus for processing audio signals comprising:

an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of audio bandpass filters coupled to the output of said input transducer;

a plurality of multiplicative automatic gain control (AGC) circuits comprising noise suppression circuitry, each of said multiplicative AGC circuits coupled to an output of one of said audio bandpass filters, at least one of said AGC circuits comprising;

an absolute value circuit having an input coupled to the output of one of said audio bandpass filters;

a lowpass filter having an input coupled to the output of said absolute value circuit;

a logarithmic element having an input coupled to the output of said lowpass filter;

a second summing junction having a first input coupled to the output of said logarithmic element and a second input equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit;

a noise estimator having an input coupled to the output of said second summing junction;

a third summing junction having a first input coupled to the output of said second summing junction and a second input coupled to the inverted output of said noise estimator;

a second amplifier having an input coupled to the output of said third summing junction, wherein said second amplifier has a gain of G′

;

a fourth summing junction having a first input coupled to the output of said second amplifier;

a fifth summing junction having an input coupled to the output of said noise estimator and having a second input equal to log[atten], wherein atten is a linear attenuation factor;

a third amplifier having an input coupled to the output of said fourth summing junction;

a sixth summing junction having a first input coupled to the output of said third amplifier, a second input coupled to the output of said logarithmic element and a third input equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit;

an exponential element having an input coupled to the output of said sixth summing junction; and

a multiplier having a first input coupled to the output of said exponential element and having a second input coupled to the output of one of said audio bandpass filters;

a first summing junction coupled to an output of said multiplicative AGC circuits;

a first amplifier coupled to the output of said first summing junction; and

an output transducer for converting electrical energy into acoustical energy.

11. An apparatus for processing audio signals apparatus comprising:

an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of audio bandpass filters coupled to an output of said input transducer;

a plurality of multiplicative automatic gain control (AGC) circuits, each of said multiplicative AGC circuits coupled to an output of one of said audio bandpass filters;

a first summing junction coupled to the output of said multiplicative AGC circuits;

a first amplifier coupled to the output of said first summing junction;

an output transducer for converting electrical energy into acoustical energy;

one of said multiplicative AGC circuits comprising;

a logarithmic element having an input coupled to the output of one of said audio bandpass filters, said logarithmic element having a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input coupled to said second output of said logarithmic element, said filter element comprising;

a high pass filter having an input coupled to the output of said logarithmic element;

a lowpass filter having an input also coupled to the output of said logarithmic element;

a second summing junction having a first input coupled to the output of said lowpass filter and having a second input coupled to a value equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit;

a noise estimator having an input coupled to the output of said second summing junction;

a third summing junction having a first input coupled to the output of said second summing junction and a second input coupled to the inverted output of said noise estimator;

a second amplifier having an input coupled to the output of said third summing junction;

a fourth summing junction having an input coupled to the output of said noise estimator and a second input equal to log[atten], wherein atten is a linear attenuation factor;

a fifth summing junction having a first input coupled to the output of said second amplifier and a second input coupled to the output of said fourth summing junction;

a third amplifier having an input coupled to the output of said fifth summing junction; and

a sixth summing junction having a first input coupled to the output of said third amplifier, a second input coupled to the output of said highpass filter, and a third input equal to equal to the logarithm of the threshold value of sound for said multiplicative AGC circuit, wherein said sixth summing junction provides an output equal to the sum of its three inputs;

a delay element having an input coupled to first output of said logarithmic element, wherein said delay element compensates for the delay through said filter element; and

an exponential element having a first input coupled to the output of said delay element and a second input coupled to the output of said filter element.

12. An apparatus for processing audio signals apparatus comprising:

an input transducer for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of audio bandpass filters coupled to an output of said input transducer;

a plurality of multiplicative automatic gain control (AGC) circuits coupled to an output of one of said audio bandpass filters, a plurality of AGC circuits comprising;

an absolute value circuit having an input coupled to the output of one of said audio bandpass filters;

a lowpass filter having an input coupled to the output of said absolute value circuit;

a logarithmic element having an input coupled to the output of said lowpass filter;

a first comparator circuit having an input coupled to the output of said logarithmic element;

a second comparator circuit having an input also coupled to said output of said logarithmic element;

a gain multiplexer having a first select input coupled to the output of said first comparator circuit and also having a second select input coupled to the output of said second comparator circuit;

a normalization multiplexer having a first select input coupled to the output of said first comparator circuit and also having a second select input coupled to the output of said second comparator circuit;

a second amplifier having a first input coupled to the output of said logarithmic element and having a gain equal to the output of said gain multiplexer;

a second summing junction having a first input coupled to the output of said second amplifier and having a second input coupled to the output of said normalization multiplexer;

an exponential element having an input coupled to the output of said second summing junction;

a delay element having an input coupled to the output of one of said audio bandpass filters; and

a multiplier having a first input coupled to the output of said exponential element and having a second in put coup led to the output of said delay element;

a first summing junction coupled to the output of said multiplicative AGC circuits;

a first amplifier coupled to the output of said first summing junction; and

a transducer for converting electrical energy into acoustical energy.

13. An apparatus for processing audio signals comprising:

a first means for converting acoustical energy into electrical energy corresponding to said acoustical energy;

plurality means for audio bandpass filtering coupled to the output of said first means for converting acoustical energy;

a plurality of means for multiplicative automatic gain control (AGC), each of said means for multiplicative AGC coupled to said means for audio bandpass filtering, one of said means for multiplicative AGC comprising;

a means for generating a logarithmic value coupled to the output of one of said means for audio bandpass filtering, said means for generating a logarithmic value having a first output carrying a signal indicating the sign of a signal at said input of said means for generating a logarithmic value and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said means for generating a logarithmic value;

a means for filtering coupled to said second output of said means for generating a logarithmic value, said means for filtering comprising;

a means for lowpass filtering having an input coupled to said second output of said means for generating a logarithmic value;

a means for band pass filtering having an input also coupled to said second output of said means for generating a logarithmic value;

a means for high pass filtering having an input also coupled to said second output of said means for generating a logarithmic value;

a second means for summing having a first input coupled to the output of said means for lowpass filtering and having a second input equal to −

log[e_max], where e_max is the maximum allowable value of the audio envelope for which AGC gain is applied;

a second means for amplifying having an input coupled to the output of said second means for summing;

a third means for amplifying having an input coupled to the output of said means for band pass filtering; and

a third means for summing having a first input coupled to the output of said means for high pass filtering, a second input coupled to the output of said second means for amplifying, a third input coupled to the output of said third means for amplifying and a fourth input equal to log[atten], wherein said third means for summing provides an output equal to the sum of its four inputs, and wherein atten is a linear attenuation factor;

a means for delaying having an input coupled to the first output of said means for generating a logarithmic value, wherein said means for delaying element compensates for the delay through said means for filtering element;

a means for providing an exponential value having a first input coupled to the output of said means for delay and a second input coupled to the output of said means for filtering element; and

a fourth means for amplifying coupled to the output of said means for providing an exponential value, wherein said fourth means for amplifying has a gain equal to (e_max), where e_max is the maximum allowable value of the audio envelope for which AGC gain is applied;

a first means for summing coupled to an output of said means for multiplicative AGC;

a first means for amplifying coupled to an output of said first means for summing; and

a means for converting electrical energy into acoustical energy.

14. An apparatus for processing audio signals comprising:

a means for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of means for audio bandpass filtering coupled to the output of said means for converting acoustical energy;

a plurality of means for multiplicative automatic gain control (AGC) comprising noise suppression circuitry, each of said means for multiplicative AGCcoupled to an output of one of said means for audio bandpass filtering, at least one of said AGC circuits comprising;

an absolute value circuit having an input coupled to the output of one of said means for audio bandpass filtering;

a means for lowpass filtering having an input coupled to the output of said absolute value circuit;

a means for providing a logarithmic value having an input coupled to the output of said means for lowpass filtering;

a second means for summing having a first input coupled to the output of said means for providing a logarithmic value and a second input equal to the logarithm of the threshold value of sound for said means for multiplicative AGC;

a means for estimating noise having an input coupled to the output of said second means for summing;

a third means for summing having a first input coupled to the output of said second means for summing and a second input coupled to the inverted output of said means for estimating noise;

a second means for amplifying having an input coupled to the output of said third means for summing, wherein said second means for amplifying has a gain of G′

;

a fourth means for summing having a first input coupled to the output of said second means for amplifying;

a fifth means for summing having an input coupled to the output of said means for estimating noise and having a second input equal to log[atten], wherein atten is a linear attenuation factor;

a third means for amplifying having an input coupled to the output of said fourth means for summing;

a sixth means for summing having a first input coupled to the output of said third means for amplifying, a second input coupled to the output of said means for providing a logarithmic value and a third input equal to the logarithm of the threshold value of sound for said means for multiplicative AGC;

an means for providing an exponential value having an input coupled to the output of said sixth means for summing; and

a means for delaying having a first input coupled to the output of said means for providing an exponential value and having a second input coupled to the output of one of said means for audio bandpass filtering;

a first means for summing coupled to an output of said means for multiplicative AGCs;

a first means for amplifying coupled to the output of said first means for summing; and

an means for converting electrical energy for converting electrical energy into acoustical energy.

15. An apparatus for processing audio signals apparatus comprising:

a means for converting acoustical energy means for delaying for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of means for audio bandpass filtering coupled to an output of said means for converting acoustical energy;

a plurality of means for multiplicative automatic gain control (AGC), each of said means for multiplicative AGCcoupled to an output of one of said means for audio bandpass filtering;

a first means for summing coupled to the output of said means for multiplicative AGCs;

a first means for amplifying coupled to the output of said first means for summing;

a means for converting electrical energy into acoustical energy;

one of said means for multiplicative AGC comprising;

a means for providing a logarithmic value having an input coupled to the output of one of said means for audio bandpass filtering, said means for providing a logarithmic value having a first output carrying a signal indicating the sign of a signal at said input of said means for providing a logarithmic value and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said means for providing a logarithmic value;

a means for filtering having an input coupled to said second output of said means for providing a logarithmic value, said means for filtering comprising;

a means for high pass filtering having an input coupled to the output of said means for providing a logarithmic value;

a means for lowpass filtering having an input also coupled to the output of said means for providing a logarithmic value;

a second means for summing having a first input coupled to the output of said means for lowpass filtering and having a second input coupled to a value equal to the logarithm of the threshold value of sound for said means for multiplicative AGC;

a means for estimating noise having an input coupled to the output of said second means for summing;

a third means for summing having a first input coupled to the output of said second means for summing and a second input coupled to the inverted output of said means for estimating noise;

a second means for amplifying having an input coupled to the output of said third means for summing;

a fourth means for summing having an input coupled to the output of said means for estimating noise and a second input equal to log[atten], wherein atten is a linear attenuation factor;

a fifth means for summing having a first input coupled to the output of said second means for amplifying and a second input coupled to the output of said fourth means for summing;

a third means for amplifying having an input coupled to the output of said fifth means for summing; and

a sixth means for summing having a first input coupled to the output of said third means for amplifying, a second input coupled to the output of said means for high pass filtering, and a third input equal to equal to the logarithm of the threshold value of sound for said means for multiplicative AGC, wherein said sixth means for summing provides an output equal to the sum of its three inputs;

a means for delaying having an input coupled to first output of said means for providing a logarithmic value, wherein said means for delaying compensates for the delay through said means for filtering; and

a means for providing an exponential value having a first input coupled to the output of said means for delaying and a second input coupled to the output of said means for filtering.

16. An apparatus for processing audio signals apparatus comprising:

a means for converting acoustical energy means for delaying for converting acoustical energy into electrical energy corresponding to said acoustical energy;

a plurality of means for audio bandpass filtering coupled to an output of said means for converting acoustical energy;

a plurality of means for multiplicative automatic gain control (AGC) coupled to an output of one of said means for audio bandpass filtering, a plurality of AGC circuits comprising;

an absolute value circuit having an input coupled to the output of one of said means for audio bandpass filtering;

a means for lowpass filtering having an input coupled to the output of said absolute value circuit;

a means for providing a logarithmic value having an input coupled to the output of said means for lowpass filtering;

a first comparator circuit having an input coupled to the output of said means for providing a logarithmic value;

a second comparator circuit having an input also coupled to said output of said means for providing a logarithmic value;

a gain multiplexer having a first select input coupled to the output of said first comparator circuit and also having a second select input coupled to the output of said second comparator circuit;

a normalization multiplexer having a first select input coupled to the output of said first comparator circuit and also having a second select input coupled to the output of said second comparator circuit;

a second means for amplifying having a first input coupled to the output of said means for providing a logarithmic value and having a gain equal to the output of said gain multiplexer;

a second means for summing having a first input coupled to the output of said second means for amplifying and having a second input coupled to the output of said normalization multiplexer;

an means for providing an exponential value having an input coupled to the output of said second means for summing;

a means for delaying having an input coupled to the output of one of said means for audio bandpass filtering; and

a means for delaying having a first input coupled to the output of said means for providing an exponential value and having a second input coupled to the output of said means for delaying;

a first means for summing coupled to the output of said means for multiplicative AGCs;

a first means for amplifying coupled to the output of said first means for summing; and

a means for converting electrical energy into acoustical energy.