Multi-sensory speech enhancement using a clean speech prior

1. A method of determining an estimate for a noise-reduced value representing a portion of a noise-reduced speech signal, the method comprising:

generating an alternative sensor signal using an alternative sensor other than an air conduction microphone;

generating an air conduction microphone signal;

using the alternative sensor signal and the air conduction microphone signal to estimate a value for a channel response of the alternative sensor signal; and

using the channel response and a prior probability distribution of the noise-reduced value to estimate the noise-reduced value.

2. The method of claim 1 wherein the prior probability distribution of the noise reduced value is defined by a variance.

3. The method of claim 2 further comprising determining the variance of the distribution based on the air conduction microphone signal.

4. The method of claim 3 wherein determining the variance based on the air conduction microphone signal comprises forming a sum of energy values for frames of the air conduction microphone signal.

5. The method of claim 4 wherein the frames of the air conduction microphone signal all contain speech.

6. The method of claim 3 wherein determining the variance of the distribution further comprises determining the variance based on a variance of ambient noise.

7. The method of claim 6 wherein determining the variance of the distribution further comprises determining a variance associated with a current frame of the noise-reduced speech signal based on a current frame of the air conduction microphone signal and a variance of the distribution associated with a preceding frame of the noise-reduced speech signal.

8. The method of claim 7 wherein determining the variance of the distribution further comprises limiting the values of the variance so that the variance always exceeds some minimum value.

9. The method of claim 8 wherein the minimum value is a percentage of the variance of the ambient noise.

10. A computer-readable storage medium having computer-executable instructions for performing steps comprising:

determining a channel response for an alternative sensor using an alternative sensor signal and an air conduction microphone signal;

determining a variance for a prior probability distribution for a clean speech value based on the air conduction microphone signal; and

using the channel response and the variance for the prior probability distribution for a clean speech value to estimate a clean speech value.

11. The computer-readable storage medium of claim 10 wherein determining the variance for the prior probability distribution further comprises determining the variance for the prior probability distribution based on a distribution of ambient noise.

12. The computer-readable storage medium of claim 11 wherein determining the variance for the prior probability distribution based on the air conduction microphone signal comprises forming a sum of energy values for frames of the air conduction microphone signal.

13. The computer-readable storage medium of claim 11 wherein determining the variance for the prior probability distribution further comprises determining a variance for the prior probability distribution associated with a current clean speech value based on a variance for a prior probability distribution associated with an earlier clean speech value.

14. The computer-readable storage medium of claim 13 wherein determining the variance of the prior probability distribution further comprises taking a weighted sum of the variance for a prior probability distribution associated with an earlier clean speech value and the difference between the energy of a frame of the air conduction microphone signal and the variance of the distribution of ambient noise.

15. The computer-readable storage medium of claim 10 wherein determining the variance of the prior probability distribution further comprises setting a minimum value for the variance of the prior probability distribution.

16. The computer-readable storage medium of claim 15 wherein the minimum value for the variance is a function of a variance for a distribution of ambient noise.

17. A method of identifying a clean speech value for a clean speech signal, the method comprising:

determining a channel response of an alternative sensor to ambient noise;

determining a parameter of a prior probability distribution for clean speech values from a value of an air conduction microphone signal; and

using the channel response and the prior probability distribution for clean speech values to determine a clean speech value.