Parallel noise cancellation filters
A noise cancellation filter structure for a noise cancellation enabled audio device, in particular headphone, comprises a noise input for receiving a noise signal and a filter output for providing a filter output signal. A first noise filter produces a first filter signal by filtering the noise signal and a second noise filter produces a second filter signal by filtering the noise signal. The second noise filter has a frequency response with a non-minimum-phase, in particular maximum-phase. A combiner is configured to provide the filter output signal based on a linear combination of the first filter signal and the second filter signal.
1. A noise cancellation filter structure for a noise cancellation enabled audio device, the filter structure comprising
a noise input for receiving a noise signal;
a filter output for providing a filter output signal;
a first noise filter for producing a first filter signal by filtering the noise signal;
a second noise filter for producing a second filter signal by filtering the noise signal, the second noise filter having a frequency response with a non-minimum-phase; and
a combiner configured to provide the filter output signal based on a linear combination of the first filter signal and the second filter signal;
wherein the filter output signal is adapted to be passed via an audio processor to a loudspeaker of the audio device;
wherein the noise input comprises a microphone input for receiving the noise signal;
wherein the filter structure further comprises a compensation output for providing a compensation signal that is generated by processing the filter output signal for being passed via the audio processor to the loudspeaker of the audio device; and
wherein the filter structure is implemented as a feedforward noise cancellation system.
2. The filter structure according to claim 1 , wherein the second noise filter is implemented as an all-pass filter.
3. The filter structure according to claim 2 , wherein the all-pass filter is of at least second order.
4. The filter structure according to claim 1 , wherein the second noise filter is implemented as an infinite-impulse response, IIR, filter.
5. The filter structure according to claim 1 , wherein the first noise filter is implemented as an infinite-impulse response, IIR, filter of at least second order.
6. The filter structure according to claim 1 , wherein the first noise filter and the second noise filter are implemented as digital filters.
7. The filter structure according to claim 6 , wherein the first noise filter and the second noise filter are implemented within a digital signal processor.
8. The noise cancellation filter structure according claim 1 , wherein the noise input is configured for receiving the noise signal from a noise microphone.
9. The noise cancellation filter structure according to claim 1 , wherein the filter output signal is adapted to be used as a basis for a compensation signal in the noise cancellation enabled audio device.
10. The noise cancellation filter structure according to claim 1 , wherein the noise signal represents ambient noise.
11. The filter structure according to claim 1 , further comprising the audio processor being configured to provide an audio output signal based on a combination of the compensation signal with a useful audio signal.
12. A noise cancellation enabled audio device, comprising
a noise cancellation filter structure according to claim 11 ;
a noise microphone coupled to the microphone input; and
the loud speaker for playing the audio output signal.
13. The audio device of claim 12 , wherein the audio device is one of a headphone, an earphone, or a handset.
14. A noise cancellation filter structure for a noise cancellation enabled audio device, the filter structure comprising:
a noise input for receiving a noise signal;
a filter output for providing a filter output signal;
a first noise filter, which has a minimum-phase frequency response, for producing a first filter signal by filtering the noise signal;
a second noise filter for producing a second filter signal by filtering the noise signal, the second noise filter having a frequency response with a non-minimum phase; and
a combiner configured to provide the filter output signal based on a linear combination of the first filter signal and the second filter signal.
15. A signal processing method for a noise cancellation enabled audio device, the method comprising
receiving a noise signal from a microphone;
filtering the noise signal with a first filter characteristic for producing a first filter signal;
filtering the noise signal with a second filter characteristic for producing a second filter signal, the second filter characteristic corresponding to a frequency response with a non-minimum-phase;
producing a filter output signal based on a linear combination of the first filter signal and the second filter signal; and
producing a compensation signal for the audio device by processing the filter output signal for being passed to a loudspeaker of the audio device such that the compensation signal implements a feedforward noise cancellation.
16. The method according to claim 15 , wherein the second filter characteristic implements an all-pass filter.
17. The method according to claim 16 , wherein the all-pass filter is of at least second order.
18. The method according to claim 15 , wherein the noise signal is received from a noise microphone.
19. The method according to claim 15 , wherein the noise signal represents ambient noise.