Hearing aid system comprising a matched filter and a measurement method

1. A hearing aid system, comprising:

an input transducer inside a hearing aid for converting an input sound signal into an electrical input signal, the input sound signal including an information signal part of a known waveform and a background noise part;

an A/D converter for converting the electrical input signal to a digital input signal;

a signal path including a signal processing unit for processing the digital input signal to at least adapt the digital input signal to a hearing profile of a user of the hearing aid, the signal processing unit providing a processed output signal;

an output transducer configured to convert a digital or analogue electrical output signal to an output sound signal;

a signal generator inside the hearing aid configured to generate a predefined source signal, the predefined source signal being added to an output of the signal processing unit or being directly provided as input of the output transducer;

a matched filter matched to the known waveform receiving said electrical input signal in analogue or digital form, the matched filter configured to improve the identification of the information signal part of the known waveform from the noisy input signal; and

an adder configured to add the source signal to the output of the signal processing unit, wherein

the information signal part of the known waveform is generated by said signal generator.

2. A hearing aid system according to claim 1 , further comprising:

a D/A converter for converting a processed output signal to an analogue electrical output signal.

3. A hearing aid system according to claim 1 , further comprising:

a selective connection configured to provide that the source signal is connected directly to the D/A converter or output transducer.

4. A hearing aid system according to claim 1 , wherein

the signal generator is configured to generate a signal with a predefined waveform s(t).

5. A hearing aid system according to claim 4 , wherein

the matched filter has an impulse response of a predefined shape s(−t+Δt) for a certain range of t, where Δt is a certain time delay.

6. A hearing aid system according to claim 1 comprising an alternative path comprising the matched filter.

7. A hearing aid system according to claim 2 , wherein

the digital input signal is fed to the matched filter.

8. A hearing aid system according to claim 6 wherein the alternative path further comprises a detection unit for evaluating the signal from the matched filter.

9. A hearing aid system according to claim 8 wherein the output of the matched filter is fed to the detection unit.

10. A hearing aid system according to claim 8 wherein the output of the detection unit is connectable to the signal processing unit.

11. A hearing aid system according to claim 1 wherein the signal processing unit is connectable to the signal generator to allow the signal generator to be controlled from the signal processing unit.

12. A hearing aid system according to claim 1 , further comprising:

a programming interface to an external programming unit.

13. A hearing aid system according to claim 12 wherein the output of the detection unit is connectable to the external programming unit via the programming interface.

14. A hearing aid system according to claim 12 wherein the signal generator is connectable to the external programming unit via the programming interface.

15. A hearing aid system according to claim 8 , wherein

the detection unit comprises an evaluation part for evaluating the detected signal from the matched filter to define the current acoustic environment of the hearing aid system based on a comparison with values of the detected signal from the matched filter for pre-defined acoustic environments stored in a memory.

16. A hearing aid system according to claim 8 , further comprising:

a post-processing unit configured to modify the adaptation of the input signal to a user's hearing profile performed by the signal processing unit based on the output of the detection unit.

17. A hearing aid system according to claim 16 , wherein

the hearing aid system is switched into a low power mode based on pre-defined criteria.

18. A method of reducing feedback in a hearing aid system based on a critical gain measurement, on a hearing aid including an input transducer for converting an input sound signal to an electrical input signal, a signal generator inside the hearing aid configured to generate a predefined source signal, the predefined source signal being added to an output of the signal processing unit or being directly provided as input of the output transducer, and an output transducer for converting a processed electrical output signal to a processed sound output, the method comprising:

generating a source signal having a predefined waveform s(t) with the signal generator inside the hearing aid;

converting the generated source signal into an acoustic signal having a predefined output level P o with the output transducer of the hearing aid and a predefined frequency or bandwidth;

measuring the input level P i of the generated sound at the input transducer as determined by the level of the electrical input signal from the input transducer of the hearing aid;

the critical gain at the frequency or in the frequency band of the generated sound as the difference P o -P i between the output and input levels on a dB scale, where the critical gain is defined as the maximum difference between the output and input sound levels on a dB scale above which the hearing aid starts to howl due to acoustic feedback;

varying the frequency or frequency band of the generated sound to obtain a relationship between frequency and critical gain, wherein

the measuring the input level P i of the generated sound at the input transducer includes

using a matched filter which is adapted to receive the generated sound by having an impulse response s(−t+Δt) for a certain range of t, where Δt is a certain time delay.

19. A method according to claim 18 wherein the predefined waveform s(t) is periodic in that s(t)=s(t+m·T 0 ), where m is an integer and T 0 is a time period.

20. A method according to claim 18 wherein the hearing aid comprises a signal path comprising a signal processing unit for adapting the input signal to a user's hearing profile and an alternative path comprising the matched filter.

21. A method according to claim 18 , further comprising:

communicating with a programming unit whereon fitting software runs and from which the gain measurement can be controlled.

22. A method according to claim 18 , wherein

Δt is in the order of the group delay of the matched filter.

23. A method according to claim 18 , further comprising:

evaluating a signal from the matched filter in a detector unit.

24. A method according to claim 20 , further comprising:

evaluating a detector signal output from a detector unit by the signal processing unit with a view to the current acoustic environment; and

modifying the signal processing in accordance with said evaluating the detector signal.

25. A method according to claim 24 , further comprising:

storing characteristics of relevant acoustic environments for comparison with current values of the detector signal;

comparing the stored characteristics with the current values of the detector signal; and

modifying one or more signal processing parameters based on said comparing and predefined criteria.

26. A method according to claim 23 , further comprising:

switching the hearing aid system into a low power mode based on pre-defined criteria and a comparison of current output signals from the detector unit with stored ones for active acoustic environments.

27. A hearing aid system according to claim 17 , wherein

said predefined criteria include a comparison of current output signals from the detector with stored signals representative of active acoustic environments.

28. A hearing aid system according to claim 17 , wherein

a low power mode is a mode where power consumption is reduced to less than 20% of normal power consumption.

29. A hearing aid system according to claim 17 , wherein

the hearing aid is completely switched off.