Telecoil hum filter

1. A system comprising:

a telecoil operable to receive a magnetic signal and convert the magnetic signal to an input audio signal; and

a telecoil hum filter comprising:

a primary signal path;

a secondary signal path;

a high-pass filter on the primary signal path configured to filter the input audio signal to provide a high-pass filtered input audio signal;

a comb notch filter comprising:

a delay module configured to generate a delayed signal by applying a delay to the input audio signal, wherein the delay is based on a fundamental hum frequency, and

a comb notch filter summing module configured to generate a comb notch filter output signal by adding the input audio signal and the delay signal;

at least one low-pass filter on the secondary signal path configured to filter one or more of the input audio signal and the comb notch filter output signal, wherein a combination of the at least one low-pass filter and the comb notch filter generate a secondary signal path input audio signal; and

a telecoil hum filter summing module configured to add the high-pass filtered input audio signal and the secondary signal path input audio signal to generate a telecoil hum filtered signal.

2. The system of claim 1 , wherein the comb notch filter comprises a low-pass filter configured to filter the input audio signal.

3. The system of claim 2 , wherein the low-pass filter comprises a cutoff frequency of 1 kHz.

4. The system of claim 2 , wherein the delay module and the low-pass filter form a digital finite impulse response filter.

5. The system of claim 4 , wherein the digital finite impulse response filter comprises:

a digital delay line having a plurality of delay elements configured to delay a sample of the input audio signal;

a plurality of filter taps, each of the plurality of filter taps configured to apply a filter coefficient to one of the input audio signal or an output of one of the plurality of delay elements, and

a finite impulse response filter summing module configured to add the outputs of the filter taps to generate the delayed signal.

6. The system of claim 1 , wherein the delay is equal to:

1

2

*

freq

where freq is the fundamental hum frequency.

7. The system of claim 1 , wherein the telecoil hum filter comprises an inverter configured to invert one of the secondary signal path input audio signal or the high-pass filtered input audio signal.

8. The system of claim 1 , wherein the at least one low-pass filter comprises:

a first low-pass filter on the secondary signal path configured to filter the input audio signal to generate a low-pass filtered input audio signal, wherein the low-pass filtered input audio signal is provided as the input audio signal to the comb notch filter; and

a second low-pass filter on the secondary signal path configured to filter the comb notch filter output signal to provide the secondary signal path input audio signal.

9. A method comprising:

receiving a magnetic signal at a telecoil;

converting the magnetic signal to an input audio signal at the telecoil;

filtering, by a high-pass filter on a primary signal path, the input audio signal to provide a high-pass filtered input audio signal;

generating a delayed signal at a delay module of a comb notch filter, wherein the delayed signal is generated by applying a delay to the input audio signal, wherein the delay is based on a fundamental hum frequency;

generating a comb notch filter output signal at a comb notch filter summing module by adding the input audio signal and the delay signal;

filtering, by at least one low-pass filter on a secondary signal path, one or more of the input audio signal and the comb notch filter output signal, wherein a combination of the at least one low-pass filter and the comb notch filter generate a secondary signal path input audio signal; and

adding, by a telecoil hum filter summing module, the high-pass filtered input audio signal and the secondary signal path input audio signal to generate a telecoil hum filtered signal.

10. The method of claim 9 , comprising filtering the input audio signal at a low-pass filter of the comb notch filter.

11. The method of claim 10 , wherein the applying the delay to the input audio signal and the filtering the input audio signal comprises:

delaying a sample of the input audio signal by each of a plurality of delay elements of a digital delay line;

applying a filter coefficient by each of a plurality of filter taps, wherein the filter coefficient is applied to one of the input audio signal or an output of one of the plurality of delay elements; and

adding, by a finite impulse response filter summing module, the outputs of the filter taps to generate the delayed signal.

12. The method of claim 9 , wherein the delay is equal to:

1

2

*

freq

where freq is the fundamental hum frequency.

13. The method of claim 9 , comprising inverting, by an inverter, one of the secondary signal path input audio signal or the high-pass filtered input audio signal.

14. The method of claim 9 , wherein the filtering by the at least one low-pass filter comprises:

filtering, by a first low-pass filter on the secondary signal path, the input audio signal to generate a low-pass filtered input audio signal, wherein the low-pass filtered input audio signal is provided as the input audio signal to the comb notch filter; and

filtering, by a second low-pass filter on the secondary signal path, the comb notch filter output signal to provide the secondary signal path input audio signal.

15. The method of claim 9 , comprising selectively providing a bass boost to the comb notch filter output signal.

16. The method of claim 9 , comprising:

comparing the input audio signal and the comb notch filter output signal; and

selecting one of the input audio signal or the comb notch filter output signal based on the comparison.

17. A non-transitory computer readable medium having stored thereon, a computer program having at least one code section, the at least one code section being executable by a machine for causing the machine to perform steps comprising:

receiving an input audio signal from a telecoil;

filtering, by a high-pass filter on a primary signal path, the input audio signal to provide a high-pass filtered input audio signal;

generating a delayed signal at a delay module of a comb notch filter, wherein the delayed signal is generated by applying a delay to the input audio signal, wherein the delay is based on a fundamental hum frequency;

generating a comb notch filter output signal at a comb notch filter summing module by adding the input audio signal and the delay signal;

filtering, by at least one low-pass filter on a secondary signal path, one or more of the input audio signal and the comb notch filter output signal, wherein a combination of the at least one low-pass filter and the comb notch filter generate a secondary signal path input audio signal; and

adding, by a telecoil hum filter summing module, the high-pass filtered input audio signal and the secondary signal path input audio signal to generate a telecoil hum filtered signal.

18. The non-transitory computer readable medium of claim 17 , comprising filtering the input audio signal at a low-pass filter of the comb notch filter, wherein the applying the delay to the input audio signal and the filtering the input audio signal comprises:

delaying a sample of the input audio signal by each of a plurality of delay elements of a digital delay line;

applying a filter coefficient by each of a plurality of filter taps, wherein the filter coefficient is applied to one of the input audio signal or an output of one of the plurality of delay elements; and

adding, by a finite impulse response filter summing module, the outputs of the filter taps to generate the delayed signal.

19. The non-transitory computer readable medium of claim 17 , wherein the delay is equal to:

1

2

*

freq

where freq is the fundamental hum frequency.