Parallel noise cancellation filters

Abstract

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.

Claims

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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The improved signal processing concept will be described in more detail in the following with the aid of drawings.

(2) Elements having the same or similar function bear the same reference numerals throughout the drawings. Hence their description is not necessarily repeated in following drawings.

(3) In the drawings:

(4) FIG. 1 shows an example headphone with several noise paths;

(5) FIG. 2 shows an example implementation of a filter structure according to the improved signal processing concept;

(6) FIGS. 3A and 3B show example frequency responses; and

(7) FIG. 4 shows an example implementation of a noise cancellation system employing the improved signal processing concept.

DETAILED DESCRIPTION

(8) FIG. 1 shows a schematic example of a headphone HP, which is particularly shown as an earphone having an earphone rubber tip TIP placed in the ear canal EC of a user. The headphone HP has a speaker SP seated inside a housing HS and featuring for example a noise cancellation microphone FF_MIC. In this example, the microphone FF_MIC is placed inside the housing HS to act as a feed-forward microphone sensing primarily ambient noise. For example, the ambient noise can enter the housing HS through a rear vent so as not to restrict the movement of the membrane of the speaker SP. The housing HS further features a front vent opening, commonly implemented to avoid damage to the speaker SP on placing the earphones in the ear.

(9) As can be seen in FIG. 1, sound from the speaker SP and the environment can enter the ear canal EC to reach the user's eardrum ED. For the ambient noise, there are several noise paths NP1, NP2, NP3, NP4 shown, each having different physical properties. For example, the first noise pass NP1 goes between the earphone rubber tip TIP and the ear canal EC. The second noise pass NP2 goes through the earphone rubber tip TIP. The third noise pass NP3 goes via the earphone rear vent and through the speaker and the noise path NP4 goes through the earphone front vent. Inter alia, each of the noise paths NP1 to NP4 has a different acoustic path length. In particular, each noise signal resulting from the respective noise paths NP1 to NP4 has a different amplitude and phase based on the pathway that they have taken. Moreover, the noise signals recombine at the eardrum ED, respectively the drum reference point, DRP, thereby effecting notches at some frequencies, where the noise signals have a phase difference of 180°.

(10) Due to the nature of how these notches are formed, they could be highly damped in amplitude and sometimes are almost not noticeable. However, even when they are substantially damped, the phase response can appear to be very undamped.

(11) Accordingly, there is a non-linear relationship between the damping of the amplitude and phase response. This behavior preferably is matched with an ANC filter.

(12) For example, FIG. 2 shows an implementation of a filter structure with a first noise filter CF filtering a noise signal NO. A second noise filter AF is connected in parallel and filters the same noise signal NO. The outputs of the first and the second noise filter CF, AF are provided to a combiner CMB that includes a gain stage after the second noise filter AF, a summer for summing up a first filter signal provided by the first noise filter CF and an amplified version of a second filter signal produced by the second noise filter AF. The output of the summer is provided to a further gain stage in the combiner CMB for producing a filter output signal OUT. Despite the specific implementation of the combiner CMB shown in FIG. 2, generally spoken, the combiner provides a linear combination of the first filter signal and the second filter signal to produce the filter output signal OUT.

(13) The first noise filter CF may have a conventional filter structure as used in noise cancellation applications, for example an infinite impulse response, IIR, filter of at least second order. The second noise filter AF is implemented with a frequency response with a non-minimum phase, in particular maximum phase. For example, the second noise filter AF is implemented as an all-pass filter providing a specific delay to the processed signal resulting from its phase response, but having a flat amplitude response. For example, the all-pass filter may be of a second order or higher and is implemented as an IIR filter. In other implementations, the all-pass filter could also easily be implemented as a FIR filter or even as an analog allpass filter or as an analog delay line.

(14) Preferably, the first and the second noise filter CF, AF are implemented as digital filters, for example in a DSP. The filter structure shown in FIG. 2 permits some independence between amplitude and phase of its frequency response, which when implemented as part of a noise cancellation system can increase the noise cancellation bandwidth.

(15) For example, during operation, as the phase of the all-pass filter reaches −180°, the two paths sum to create a notch.

(16) Referring now to FIGS. 3A and 3B, example frequency responses of the parallel filter structure of FIG. 2 where the first noise filter CF is implemented to match the acoustic response of the audio device respectively headphone HP. In this example, the acoustic response is assumed to be flat for simplicity. The second noise filter AF is implemented as an all-pass filter with a phase of −180 degrees at the notch frequency.

(17) FIGS. 3A and 3B show three different constellations of the filter structure with different gain settings in the combiner CMB. The curve GST refers to a constellation where the gain of the all-pass pass is smaller than the gain of the conventional path with the first noise filter. The curve GEQ corresponds to a basically equal gain in the all-pass path and the conventional path. The third curve GGT corresponds to a constellation where the gain of the all-pass path is greater, in particular significantly greater than the gain of the conventional path.

(18) As can be seen, depending on the gain relationship, an effective filter function can be achieved that has a high damping for the amplitude at the notch frequency while leaving the phase undamped at the frequency. Such behavior may better match the real-life acoustic properties of the earphone or headphone, thereby resulting in better noise cancellation performance.

(19) Referring now to FIG. 4, an implementation of the filter structure described in conjunction with FIG. 2 in a noise cancellation system is shown. In particular, the noise signal NO is provided by the microphone FF_MIC, which may be a feed-forward microphone as shown in FIG. 1. The filter output signal OUT, carrying a compensation signal for the ambient noise, is combined in an audio processor AUD with a useful audio signal S0 to provide a speaker output signal provided to the speaker SP. The processor AUD may be as simple as adding up the audio signal S0 with the filter output signal OUT such that the speaker signal contains both the useful signal and an anti-noise signal produced from the ambient noise via the filter structure. More complex functions of the processor AUD are not excluded.

(20) Generally speaking, an arrangement like that shown in FIG. 4 allows to perform active noise cancellation with improved performance by receiving a noise signal NO, filtering the noise signal NO with a first filter characteristic for producing a first filter signal and filtering the noise signal NO with a second filter characteristic for producing a second filter signal. In particular, the first filter characteristic may be the filter characteristic of the first noise filter CF and the second filter characteristic may be the non-minimum phase characteristic or all-pass characteristic of the second noise filter AF. A filter output signal OUT is produced based on a linear combination of the first filter signal and the second filter signal. A compensation signal can be produced for the audio device based on the filter output signal OUT.

(21) The filter structure for active noise cancellation according to the improved signal processing concept allows various applications, some of which are described as further examples in the following.

(22) In one example implementation, a feed-forward noise cancellation headphone, earphone or handset comprises a feed-forward microphone, a speaker and a filter where the earphone has multiple pathways, through which noise can enter the ear such that the combination of two or more of these noise sources creates a notch shape in the ambient to ear transfer function and the filter has a response which matches said notch in amplitude and phase up to the notch resonant frequency.

(23) In one example implementation, a feed-forward noise cancellation headphone, earphone or handset comprises a feed-forward microphone, a speaker and a filter where the earphone has multiple pathways through which noise can enter the ear such that the combination of two or more of these noise sources creates a notch shape in the ambient to ear transfer function and the filter has a response which matches said notch in amplitude by no less than 3 dB and in phase by no less than 20 degrees across a bandwidth of more than 100 Hz at any point in the octave directly below the notch resonant frequency.

(24) In one example implementation, a feed-forward noise cancellation headphone, earphone or handset comprises a feed-forward microphone, a speaker and a filter where the earphone has multiple pathways through which noise can enter the ear such that the combination of two or more of these noise sources creates a notch shape in the ambient to ear transfer function and the filter is a non-minimum phase filter.

(25) In one example implementation, a filter topology for a noise cancellation headphone, earphone or handset is represented by an all-pass filter in parallel to a conventional noise cancellation filter, or by any mathematically equivalent arrangement that will produce the same response.

(26) In some implementations, a filter shape for a noise cancellation headphone, earphone or handset is arranged with at least one parallel path which contains one of an all-pass filter, a non-minimum phase filter or a simple time delay.

(27) It should be noted that in all of the implementations described above, neither of the microphones nor the speaker SP are essential parts of a noise cancellation system according to the improved signal processing concept. Even the audio processor AUD could be provided externally. For example, such a noise cancellation system could be implemented both in hardware and software, for example in a signal processor. The noise cancellation system can be located in any kind of audio player, like a mobile phone, an MP3 player, a tablet computer or the like. However, the noise cancellation system could also be located within the audio device, e.g. a mobile handset or a headphone, earphone or the like.