REALISATION OF CONTROLLER TRANSFER FUNCTION FOR ACTIVE NOISE CANCELLATION

Abstract

An apparatus for realizing an active noise cancellation control law transfer function between a sensing microphone and a speaker. The apparatus includes a multiplicity of filters. Each filter is operable over a different frequency range. At least one filter has an adjustable parameter whereby the filter can be adjusted such that the filters cumulatively realize a required control law transfer function. The adjustable parameter may in one embodiment be the amplitude. In other embodiments, it may be other parameters.

Description

FIELD OF THE INVENTION

This invention relates to active noise cancellation systems, and has application to both feedback and feedforward control architectures, or combinations of these.

BACKGROUND

All active noise cancellation products, whether they are based on a feedback or feedforward control architecture (or a combination of these two architectures) require a tailored transfer function between the noise sensing device (typically one or more sensing microphones) and the device that creates the acoustic response required to cancel the sensed noise (typically a speaker). In this document the transfer function between the sensing microphone (s) and the speaker is referred to as the control law transfer function. This transfer function facilitates the realisation of noise cancellation over a suitable bandwidth whilst minimising noise amplification and and/or instability outside this bandwidth.

Classically the control law transfer function has been realised through use of an analog filter which consists of a fixed combination of active and passive components. Such a realisation has the following disadvantages:

• • 1. Its fixed nature does not allow adjustment of production variation within the electro-acoustics to which it interfaces.
  • 2. Its fixed nature does not allow easy accommodation of a range of electro-acoustic designs.
  • 3. Its fixed nature does not permit dynamic adjustment of the control law to provide optimised noise cancellation based on the prevailing noise field.
  • 4. The component count of the implementation is high and typically does not lend itself to integration within an integrated circuit.

OBJECT

It is an object of the present invention to provide a method or apparatus for realising an active noise cancellation control law transfer function which will ameliorate at least one of the foregoing disadvantages, or which, alternatively, will at least provide a useful alternative to existing solutions.

SUMMARY

In one aspect the disclosed subject matter provides apparatus for realising an active noise cancellation control law transfer function between a sensing microphone and a speaker, the apparatus including a plurality of filters, each filter being operable over a different frequency range, and at least one filter having at least one adjustable parameter whereby the filter can be adjusted such that the filters cumulatively realise a required control law transfer function.

Each filter may include at least one adjustable parameter. In some embodiments the adjustable parameter is amplitude. In other embodiments the adjustable parameter is bandwidth.

The adjustable filter may comprise a parametric filter.

In one embodiment the adjustable parameter is dynamically adjustable. An adjustment controller may be provided to adjust the adjustable parameter.

In another aspect the disclosed subject matter broadly provides a method of active noise cancellation for apparatus including a speaker and sensing microphone and a plurality of filters, each filter being operable over a different frequency range, and at least one filter having at least one adjustable parameter whereby the filter can be adjusted, the method including:

• • determining one or more acoustic or electro-acoustic characteristics of the apparatus, and;
  • adjusting the adjustable parameter dependent on the characteristic such that the filters cumulatively realise a control law transfer function to implement active noise cancellation.

In another aspect the disclosed subject matter broadly provides a method of active noise cancellation for apparatus including a speaker and sensing microphone and a plurality of filters, each filter being operable over a different frequency range, and at least one filter having at least one adjustable parameter whereby the filter can be adjusted, the method including:

• • sensing the noise cancellation performance of the apparatus, and;
  • adjusting the adjustable parameter dependent on the sensed performance such that the filters cumulatively realise a control law transfer function to implement active noise cancellation.

The method may further comprise dynamically adjusting the adjustable parameter.

Further aspects will become apparent from the following description.

BRIEF DRAWING DESCRIPTION

One or more embodiments of the invention will be described below with reference to the drawings in which:

FIG. 1: Is a diagrammatic representation of a known feedback active noise cancellation system.

FIG. 2: Is a diagram showing gain plotted against frequency for the desired response for a known control law a transfer function for an active noise cancellation system such as that shown diagrammatically in FIG. 1

FIG. 3: Is a circuit diagram showing one example of a possible circuit realisation of a control law transfer function such as that represented in FIG. 2.

FIG. 4: Is a diagrammatic representation of a feedback active noise cancellation system according to one embodiment of the invention.

DESCRIPTION OF ONE OF MORE EMBODIMENTS

Referring to FIG. 1, a known active noise cancellation system is shown in which a speaker or driver 1 delivers sound in a selected region, usually the enclosed space between a headset or earphone and the user's inner ear, for example. The sound from loudspeaker 1 is sensed by sensing microphone(s) 2, which also senses any noise N in the selected region. A controller 3 receives the output from the sensing microphone(s) 2 and applies an appropriate control law to actuate the speaker 1 such that the noise N is effectively cancelled.

In order to provide an appropriate signal to speaker 1, the controller 3 must realise a suitable control law transfer function between the sensing microphone (s) and speaker.

Turning now to FIG. 2, a plot of gain against frequency for a control law transfer function known for use in the active noise cancellation system as broadly outlined in FIG. 1 is shown. As can be seen from FIG. 2, the general form of the function includes an amplitude peak at around 300 Hz, a notch at approximately 8 kHz, and a rising characteristic above 10 kHz.

In order to realise the function shown, a solution is proposed which uses a plurality of filters having one or more adjustable parameters. The filters are each operable over a different selected frequency range in a similar manner to a multi-channel audio equaliser. Thus the filters may cumulatively realise a required control law transfer function. In one embodiment, the proposed solution uses parametric filters, although those skilled in the art will appreciate that other forms of filter may be used. Those skilled in the art will also realise that the resultant circuit construction may take a variety of physical forms, and may in some embodiments be provided in the form of an integrated circuit with few, if any, additional components.

A parametric filter allows adjustment of centre frequency, quality factor (Q) and amplitude. Therefore, a parametric filter provides significant flexibility for an application such as the realisation of a selected frequency band of a control law transfer function.

Control law transfer functions are invariably based on a minimum phase system, and therefore the amplitude and phase characteristics are uniquely related. Accordingly, one only needs to realise the desired amplitude response and the phase response will automatically follow, or vice versa. Hence in some embodiments the adjustable parameter for one or more of the filters may simply be amplitude.

In one embodiment a plurality of parametric filters may be used together, in a manner similar to use of a parametric equaliser, to realise a control law transfer function. In particular, if there is a sufficient number of filters (for example being analogous to a parametric equaliser with a number of channels) and range of adjustment, any amplitude shape i.e. any gain profile with respect to frequency can be realised over a selected bandwidth and so it is possible to realise any desired or required control law transfer function. In some embodiments, multiple filter parameters of multiple filters are adjustable. In other embodiments, only a single parameter of a single filter of the plurality of filters may be adjustable.

In practice, it is usually necessary to rationalise the number of channels, i.e. the number of filters and the range of adjustment in order to minimise circuit complexity. However, if appropriate informed design choices are made, then because of the inherent flexibility of each parametric filter, a wide range of control law transfer functions can still be approximated to a sufficient level of accuracy.

Therefore, turning to the control law transfer function characteristic illustrated in FIG. 2, it will be seen that this may be sufficiently well matched with a filter arrangement that is analogous to a “2.5” channel parametric equaliser, i.e. two parametric filters and a shelving filter. The first and second channels correspond to two parametric filters that are fully featured and cover the 300 Hz peak at 8 KHz notch. The third channel corresponds to a shelving filter with a frequency and amplitude adjustment only (i.e. no Q).

The amplitude of a parametric equaliser is typically centred around 0 dB, so a separate adjustable gain stage is used to realise the final control or transfer function. In this example around 12 dB of gain is provided by this stage.

Turning now to FIG. 3, a typical circuit to realise the control or transfer function of FIG. 2 using the parametric equaliser approach is shown. In FIG. 3, the “ESRC” block provides the overall system gain, the 40 k potentiometers set the channel amplitude, the 1 k adjustable resistors set the channel Q (which determines the bandwidth over which the filter operates) for the peak and the notch, and the frequency of the shelf, the 100 k adjustable resistors set the channel frequency for the peak and the notch.

For integration into an integrated circuit it is typically necessary to have C2 and C3 as external components owing to the low corner frequencies that they realise. For the other capacitors it is possible to integrate them by rescaling (i.e. reducing the capacitor value and increasing the resistor values) whilst still meeting noise floor specifications. The integrated resistors can be realised by a number of means such as “switch cap” or “transconductance”.

The external component count is therefore only two capacitors (per left or right channel). The adjustable resistor settings can be programmed into the integrated circuit as a one time programmable (OTP) setting.

In another embodiment one or more parameters of one or more of the filters is dynamically adjustable. Therefore, the adjustable resistor settings for the embodiment shown in FIG. 3 may be reprogrammable settings permitting dynamic adjustment or change of control law transfer functions. FIG. 4 shows a system according to an embodiment of the invention in which the control law transfer function is realised by a controller 3 incorporating a plurality of filters (such as the arrangement shown in FIG. 3), at least one of the filters having an adjustable parameter. The FIG. 4 embodiment includes a dynamic performance adjustment controller 4 which sensing the noise cancellation performance of the apparatus comprising the system by monitoring the noise signal detected by the sensing microphone 2 and sends the necessary signals or instructions to the controller 3 to dynamically adjust one or more adjustable parameters of one or more of the filters that realise the control law transfer function. In one embodiment the adjustment controller 4 includes a digital signal processor which periodically monitors the signal from noise sensing microphone 2 and determines which parameters of the controller 3 require adjustment. Appropriate output signals (either analog or digital) are generated and provided to the controller 3 to make the required adjustment. The sensing microphone signal can then be used again to determine whether the adjustment is successful, and to what extent further adjustment is required.

As mentioned above, the invention may be used in association with an active noise cancellation device such as an active noise cancelling headset or earphone. In some embodiments the filters may be part of the device. In others, the filters may be remotely associated with the device, for example being provided in a remote control module. It will be seen that the invention allows continual or periodic monitoring of one or more acoustic or electro-acoustic characteristics of the active noise cancelling device so that the filter(s) can be adjusted to realise a control law transfer function dependent on the determined characteristic(s).

Similarly, the invention allows a generic filter circuit to be used which can be adjusted for different models or forms of active noise cancelling device. Furthermore, the invention may allow each specific device produced from a production line to be tested for one or more acoustic or electro-acoustic characteristics, so that the control law transfer function for each specific device may be adjusted to optimise performance of that device to account for manufacturing tolerances.

From the foregoing it will be seen that a solution is proposed which addresses the major limitations of the classical analog realisation of a control law transfer function. In particular, the invention provides an adjustable transfer function that is amenable to integration with a low external part count.

Those skilled in the art to which the invention relates will appreciate that the invention offers an elegant and viable alternative to a digital control law transfer function realisation. A digital realisation has the disadvantages of requiring high speed, low latency analog to digital and digital to analog conversion as well as the digital signal processing elements. This all comes at a cost in terms of power consumption, noise floor and price amongst others.

Those skilled in the art will also appreciate that the invention may equally be employed on a digital platform.

Claims

1. Apparatus for realising an active noise cancellation control law transfer function between a sensing microphone and a speaker, the apparatus including a plurality of filters, each filter being operable over a different frequency range, and at least one filter having at least one adjustable parameter whereby the filter can be adjusted such that the filters cumulatively realise a required control law transfer function.

2. Apparatus as claimed in claim 1 wherein each filter includes at least one adjustable parameter.

3. Apparatus as claimed in claim 2 wherein the adjustable parameter is amplitude.

4. Apparatus as claimed in claim 2 wherein the adjustable parameter is bandwidth.

5. Apparatus as claimed in claim 1 wherein the adjustable filter comprises a parametric filter.

6. Apparatus as claimed in claim 1 wherein the adjustable parameter is dynamically adjustable.

7. Apparatus as claimed in claim 6 further comprising an adjustment controller to adjust the adjustable parameter.

8. A method of active noise cancellation for apparatus including a speaker and sensing microphone and a plurality of filters, each filter being operable over a different frequency range, and at least one filter having at least one adjustable parameter whereby the filter can be adjusted, the method including: determining one or more acoustic or electro-acoustic characteristics of the apparatus, and;adjusting the adjustable parameter dependent on the characteristic such that the filters cumulatively realise a control law transfer function to implement active noise cancellation.

9. A method of active noise cancellation for apparatus including a speaker and sensing microphone and a plurality of filters, each filter being operable over a different frequency range, and at least one filter having at least one adjustable parameter whereby the filter can be adjusted, the method including: sensing the noise cancellation performance of the apparatus, and;adjusting the adjustable parameter dependent on the sensed performance such that the filters cumulatively realise a control law transfer function to implement active noise cancellation.

10. A method as claimed in claim 9 further comprising dynamically adjusting the adjustable parameter.