Digital hearing aid system

• US 20050232452A1
• Filed: 06/13/2005
• Published: 10/20/2005
• Est. Priority Date: 04/12/2001
• Status: Active Grant

• Current Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Original Assignee: Sound Design Technologies Limited (ON Semiconductor Corporation)
• Inventors: Ryan, James G., Brown, David R., Armstrong, Stephen W., Sykes, Frederick E.
• Granted Patent: US 7,433,481 B2
• Time in Patent Office: Days
• US Class Current: 381/317
• CPC Class Codes: H04R 2225/43 Signal processing in hearing aids to enhance the speech intelligibility
• CPC Class Codes: H04R 2460/05 Electronic compensation of the occlusion effect
• CPC Class Codes: H04R 25/356 Amplitude, e.g. amplitude shift or compression
• CPC Class Codes: H04R 25/407 Circuits for combining signals of a plurality of transducers
• CPC Class Codes: H04R 25/453 electronically
• CPC Class Codes: H04R 25/505 using digital signal processing

Abstract

A digital hearing aid is provided that includes front and rear microphones, a sound processor, and a speaker. Embodiments of the digital hearing aid include an occlusion subsystem, and a directional processor and headroom expander. The front microphone receives a front microphone acoustical signal and generates a front microphone analog signal. The rear microphone receives a rear microphone acoustical signal and generates a rear microphone analog signal. The front and rear microphone analog signals are converted into the digital domain, and at least the front microphone signal is coupled to the sound processor. The sound processor selectively modifies the signal characteristics and generates a processed signal. The processed signal is coupled to the speaker which converts the signal to an acoustical hearing aid output signal that is directed into the ear canal of the digital hearing aid user. The occlusion sub-system compensates for the amplification of the digital hearing aid user'"'"'s own voice within the ear canal. The directional processor and headroom expander optimizes the gain applied to the acoustical signals received by the digital hearing aid and combine the amplified signals into a directionally-sensitive response.

Claims

1. A method for reducing occlusion in a digital hearing instrument, comprising the steps of:

receiving an intended audio signal from a front microphone circuit;

receiving an occlusion signal from a rear microphone circuit positioned within an ear canal; and

subtracting the occlusion signal from the intended audio signal to generate an audio output signal.

2. The method of claim 1, comprising the further step of amplifying the intended audio signal.

3. The method of claim 1, comprising the further step of:

filtering the occlusion signal to compensate for the magnitude response of the rear microphone circuit.

4. The method of claim 1, wherein the rear microphone circuit includes a rear microphone and an analog-to-digital (A/D) converter.

5. The method of claim 1, wherein the front microphone circuit includes a front microphone and an analog-to-digital (A/D) converter.

6. The method of claim 1, comprising the further steps of:

converting the audio output signal into an acoustical output signal with a speaker circuit; and

directing the acoustical output signal into the ear canal.

7. The method of claim 6, comprising the further step of filtering the audio output signal to compensate for the magnitude response of the speaker circuit.

8. The method of claim 6, wherein the speaker circuit includes a digital-to-analog (D/A) converter and a speaker.

9. The method of claim 2, comprising the further step of:

filtering the audio output signal with a loop filter, wherein the frequency response of the loop filter exhibits greater than unity gain (0 dB) below a pre-selected transition frequency and less than unity gain above the pre-selected transition frequency.

10. The method of claim 9, comprising the further step of:

filtering the amplified intended audio signal with a high frequency equalizer circuit, wherein the frequency response of the high frequency equalizer circuit compensates for the frequency response of the loop filter above the pre-selected transition frequency.

11. A system for reducing occlusion in an ear piece, comprising:

a rear A/D converter that receives an occlusion signal from a rear microphone positioned within an ear canal and generates a digital occlusion signal;

a summation circuit coupled to the rear A/D converter that inverts the digital occlusion signal to generate a digital output signal; and

a digital-to-analog (D/A) converter coupled to the summation circuit that converts the digital output signal into an analog output signal for a speaker positioned within the ear canal.

12. The system of claim 11, further comprising

a front analog-to-digital (A/D) converter that receives an intended signal from a front microphone and generates a digital intended signal;

and

a sound processor coupled between the front A/D converter and the summation circuit that amplifies the digital intended signal and generates an amplified intended signal;

wherein the amplified intended signal is added to the inverted digital occlusion signal by the summation circuit to generate the digital output signal.

13. The system of claim 11, further comprising:

a microphone equalizer circuit coupled between the rear A/D converter and the summation circuit that filters the digital occlusion signal to compensate for the combined magnitude response of the rear microphone and the rear A/D converter.

14. The system of claim 11, further comprising:

a speaker equalization filter coupled between the summation circuit and the D/A converter that filters the digital output signal to compensate for the combined magnitude response of the D/A converter and the speaker.

15. The system of claim 12, further comprising:

a loop filter coupled to the digital output signal of the summation circuit, wherein the frequency response of the loop filter exhibits greater than unity gain (0 dB) below a pre-selected transition frequency and less than unity gain above the pre-selected transition frequency.

16. The system of claim 15, further comprising:

a high frequency equalizer circuit coupled between the sound processor and the summation circuit, wherein the frequency response of the high frequency equalizer circuit applies a transfer function to the amplified intended signal to compensate for the frequency response of the loop filter above the pre-selected transition frequency.

17. A digital hearing instrument, comprising:

a microphone that receives an acoustical signal and generates an analog signal;

a variable pre-amplifier coupled to the microphone that receives the analog signal and also receives a gain control signal, wherein the gain control signal determines the value of a first gain that is applied by the variable pre-amplifier to the analog signal to generate an amplified analog signal;

an analog-to-digital (A/D) converter coupled to the variable pre-amplifier that converts the amplified analog signal to generate a digital signal;

a headroom expander coupled to the A/D converter that applies a second gain to the digital signal to generate a headroom expander output signal and that monitors an energy level of the digital signal to generate the gain control signal, wherein the headroom expander is configured to adjust the gain control signal to decrease the value of the first gain and to increase the value of the second gain when the energy level of the digital signal rises above a pre-selected level;

a sound processor coupled to the headroom expander that amplifies the headroom expander output to generate a processed output signal; and

and a speaker that converts the processed output signal into an acoustical output signal.

18. The digital hearing instrument of claim 17, further comprising:

a digital-to-analog (D/A) converter coupled between the sound processor and the speaker that converts the processed output signal from the digital domain into the analog domain.

19. The digital hearing instrument of claim 17, further comprising:

a digital-to-analog (D/A) converted coupled between the headroom expander and the pre-amplifier that converts the gain control signal from the digital domain into the analog domain.

20. The digital hearing instrument of claim 17, wherein the headroom expander comprises:

a multiplier that receives the digital signal and a second gain control signal and applies the second gain to the digital signal to generate the headroom expander output, wherein the second gain control signal determines the value of the second gain;

a level detector that receives the digital signal and generates an energy level signal that is approximates the energy level of the digital signal; and

a threshold and gain control circuit coupled to the level detector that monitors the energy level signal and generates the gain control signal and the second gain control signal when the energy level of the digital signal rises above the pre-determined level.

21. The digital hearing instrument of claim 20 wherein the headroom expander further comprises:

a delay circuit coupled between the threshold and gain control circuit and the multiplier that applies a time delay to the second gain control circuit.

22. The digital hearing instrument of claim 20, wherein the value of the first gain is the inverse of the value of the second gain.

23. The digital hearing instrument of claim 20, wherein the headroom expander decreases the value of the first gain and proportionally increases the value of the second gain when the energy level of the digital signal rises above the pre-selected level.

24. The digital hearing instrument of claim 20, wherein the sound processor comprises:

a band-split filter coupled to the headroom expander that filters the headroom expander output signal into a plurality of channel audio signals;

a plurality of channel processors coupled to the band-split filter that each apply a channel-specific gain to one of the channel audio signals to generate a conditioned channel signal; and

a summation circuit coupled to the plurality of channel processors that sums the conditioned channel signals from the channel processors and generates the processed output signal.

25. The digital hearing instrument of claim 24, wherein the sound processor further comprises:

a pre-filter coupled between the headroom expander and the band-split filter that applies a transfer function to the headroom expander output signal.

26. The digital hearing instrument of claim 24, wherein the sound processor further comprises:

a post-filter coupled between the summation circuit and the speaker that applies a transfer function to the processed output signal.

27. The hearing instrument of claim 24, wherein the sound processor further comprises:

a pre-filter coupled between the headroom expander and the band-split filter that converts the headroom expander output signal from the acoustic domain into the cochlear domain; and

a post-filter coupled between the summation circuit and the speaker that converts the processed output signal from the cochlear domain into the acoustic domain.

28. The digital hearing instrument of claim 24, wherein the sound processor further comprises:

a notch filter coupled between the summation circuit and the speaker that attenuates a narrow band of frequencies in the processed output signal.

29. The digital hearing instrument of claim 28, wherein the narrow band of frequencies is adjustable.

30. The digital hearing instrument of claim 24, wherein the sound processor further comprises:

a volume control circuit coupled between the summation circuit and the speaker that receives a volume control input and amplifies that processed output signal by a gain, wherein the volume control circuit determines the gain as a function of the volume control input.

31. The digital hearing instrument of claim 24, wherein the sound processor further comprises:

an automatic gain control (AGC) output circuit coupled between the summation circuit and the speaker that reduces distortion by filtering pathological signals from the processed output signal.

32. The hearing instrument of claim 24, wherein the sound processor further comprises:

a squelch circuit coupled between the summation circuit and the speaker that expands low energy level signals in the processed output signal to attenuate noise, wherein low energy level signals are signals having an energy level below a pre-selected threshold value.

33. A method of optimizing the operating point of an analog-to-digital (A/D) converter in an input stage of a digital hearing instrument, comprising the steps of:

receiving an analog audio signal from a microphone;

applying a first gain to the analog audio signal to generate an amplified analog audio signal;

converting the amplified analog audio signal into a digital audio signal;

applying a second gain to the digital audio signal to generate a conditioned audio input signal;

monitoring an energy level of the digital audio signal; and

if the energy level of the digital audio signal rises above a pre-selected level, then decreasing the first gain and increasing the second gain.

34. The method of claim 33, wherein the second gain is increased in proportion to the decrease in the first gain.