Separate inner and outer hair cell loss compensation

1. A method for compressively amplifying an audio signal, comprising:

inputting the audio signal to a fast signal level estimator (fast SLE) having a fast low-pass filter characterized by a rise time constant and a decay time constant;

inputting the audio signal to a slow signal level estimator (slow SLE) having a slow low-pass filter characterized by a rise time constant and a decay time constant;

subtracting the output of the slow SLE from the output of the fast SLE and inputting the result to a fast level-to-gain transformer;

inputting the output of the slow SLE to a slow level-to-gain transformer, wherein the slow level-to-gain transformer is characterized by expansion when the output of the slow SLE is below a specified threshold; and,

amplifying the audio signal with a gain adjusted by a summation of the outputs of the fast level-to-gain transformer and the slow level-to-gain transformer, wherein the output of the fast level-to-gain transformer is multiplied by a weighting factor computed as a function of the output of the slow SLE before being summed with the output of the slow level-to-gain transformer.

2. The method of claim 1 wherein the fast level-to-gain transformer is characterized by having higher and lower compression thresholds.

3. The method of claim 2 wherein the fast level-to-gain transformer is characterized by outputting a constant gain value at input levels below the lower compression threshold.

4. The method of claim 2 wherein the fast level-to-gain transformer is characterized by compression limiting at input levels above the higher compression threshold.

5. The method of claim 1 wherein the rise time constant and the decay time constant of the fast low-pass filter are both less than the rise time constant and the decay time constant of the slow low-pass filter.

6. The method of claim 1 wherein the rise time constant and the decay time constant of the fast low-pass filter are both between 1 millisecond and 10 milliseconds and the rise time constant and the decay time constant of the slow low-pass filter are both between 100 milliseconds and 1000 milliseconds.

7. The method of claim 1 wherein the rise time constant is less than the decay time constant for both the fast low-pass filter and the slow low-pass filter.

8. The method of claim 1 wherein the rise time constant and the decay time constant of the fast low-pass filter are approximately 1 millisecond and 10 milliseconds, respectively, and the rise time constant and the decay time constant of the slow low-pass filter are approximately 100 milliseconds and 1000 milliseconds, respectively.

9. The method of claim 1 wherein the slow level-to-gain transformer is characterized by expansion below a specified threshold of the output of the slow SLE that corresponds to speech.

10. The method of claim 1 wherein the slow level-to-gain transformer is characterized by a region of linear gain at high input levels to minimize distortion.

11. The method of claim 1 further comprising adjusting the gains of a plurality of amplifiers for a plurality of frequency-specific channels corresponding to frequency components of the audio signal with a fast SLE and a slow SLE for each frequency-specific channel.

12. The method of claim 11 wherein, for each frequency-specific channel, the input to the slow SLE is a narrowband frequency component of the audio signal and the input to the fast SLE is a wideband frequency component of the audio signal.

13. A method for compressively amplifying an audio signal, comprising

inputting the audio signal to a slow signal level estimator (slow SLE) having a slow low-pass filter characterized by a rise time constant and a decay time constant;

subtracting the output of the slow SLE from the audio signal and inputting the result to a fast signal level estimator (fast SLE) having a fast low-pass filter characterized by a rise time constant and a decay time constant;

inputting the output of the slow SLE to a slow level-to-gain transformer;

inputting the output of the fast SLE to a fast level-to-gain transformer; and

amplifying the audio signal with a gain adjusted by a summation of the outputs of the fast level-to-gain transformer and the slow level-to-gain transformer.

14. The method of claim 13 further comprising multiplying the output of the fast level-to-gain transformer by a weighting factor before being summed with the output of the slow level-to-gain transformer.

15. The method of claim 14 further comprising computing the weighting factor as a function of the output of the slow SLE.

16. A method for compressively amplifying an audio signal, comprising:

filtering the audio signal into a plurality of frequency components;

inputting the frequency components of the audio signal to a plurality of corresponding frequency-specific channels each of which includes first and second fast signal level estimators (fast SLEs) and a slow signal level estimators (slow SLEs) wherein each slow SLE has a slow low-pass filter characterized by a rise time constant and a decay time constant and each fast SLE has a fast low-pass filter characterized by a rise time constant and a decay time constant;

for each frequency-specific channel, inputting a narrowband component of the audio signal to the first slow SLE and inputting a wideband component of the audio signal to the second slow SLE and to the fast SLE;

for each frequency-specific channel, subtracting the output of the second slow SLE from the output of the fast SLE and inputting the result to a fast level-to-gain transformer;

for each frequency-specific channel, inputting the output of the first slow SLE to a slow level-to-gain transformer;

for each frequency-specific channel, amplifying the audio signal with a gain adjusted by a summation of the outputs of the fast level-to-gain transformer and the slow level-to-gain transformer; and,

multiplying the output of the fast level-to-gain transformer by a weighting factor computed as a function of the output of the first slow SLE before summing with the output of the slow level-to-gain transformer.

17. The method of claim 16 wherein the fast level-to-gain transformer is characterized by having higher and lower compression thresholds.

18. The method of claim 17 wherein the fast level-to-gain transformer is characterized by outputting a constant gain value at input levels below the lower compression threshold.

19. The method of claim 17 wherein the fast level-to-gain transformer is characterized by compression limiting at input levels above the higher compression threshold.

20. The method of claim 16 wherein the rise time constant and the decay time constant of the fast low-pass filter are both less than the rise time constant and the decay time constant of the slow low-pass filter.