GB2455826A

GB2455826A - Adaptive noise cancellation

Info

Publication number: GB2455826A
Application number: GB0810999A
Authority: GB
Inventors: Anthony James Magrath; Richard Clemow
Original assignee: Wolfson Microelectronics PLC
Current assignee: Cirrus Logic International UK Ltd
Priority date: 2007-12-21
Filing date: 2008-06-16
Publication date: 2009-06-24
Anticipated expiration: 2028-06-16
Also published as: WO2009081189A1; GB0725114D0; GB0810999D0; GB2455826B

Abstract

A method of calibrating and operating a noise cancellation system, the system being for use in a device comprising a microphone 20,22 for detecting ambient noise and generating a noise signal; a signal processor 24 for generating a noise cancellation signal from the noise signal; and a speaker 28 for receiving the noise cancellation signal such that the speaker generates a sound signal in which the detected ambient noise has been at least partially cancelled. The method comprising: applying a test signal to the speaker to generate a test sound; detecting the test sound with the microphone; and adapting the signal processor on the basis of the detected test sound. The method may be applied to digital signals, and may involve separating the test sound from the noise. The test signal may be a band-limited noise signal, a pseudo-random data pattern or a sinusoidal function.

Description

GAIN CALIBRATION BASED ON DEVICE PROPERTIES

This invention relates to a noise cancellation system, and in particular to a noise cancellation system having a filter that can easily be adapted based on the properties of the device in which the system is being used, in order to improve the noise cancellation performance.

BACKGROUND

Noise cancellation systems are known, in which an electronic noise signal representing ambient noise is applied to a signal processing circuit, and the resulting processed noise signal is then applied to a speaker, in order to generate a sound signal. In order to achieve noise cancellation, the generated sound should approximate as closely as possible the inverse of the ambient noise, in terms of its amplitude and its phase.

In particular, feedforward noise cancellation systems are known, for use with headphones or earphones, in which one or more microphones mounted on the headphones or earphones detect an ambient noise signal in the region of the wearer's ear. In order to achieve noise cancellation, the generated sound then needs to approximate as closely as possible the inverse of the ambient noise, after that ambient noise has itself been modified by the headphones or earphones. One example of modification by the headphones or earphones is caused by the different acoustic path the noise must take to reach the wearer's ear, travelling around the edge of the headphones or earphones.

The microphone used to detect the ambient noise signal and the loudspeaker used to generate the sound signal from the processed noise signal will in practice also modify the signals, for example being more sensitive at some frequencies than at others. One example of this is when the speaker is closely coupled to the ear of a user, causing the frequency response of the loudspeaker to change due to cavity effects.

In practice, whilst the manufacture of noise cancellation devices is largely standardized across a particular class of device (e.g. a particular model of mobile phone, or headset, etc), the manufacturing process is not sufficiently accurate that all devices can be treated identically. That is, after manufacture, even devices that are supposed to be identical will have slight variations in the speaker and microphone sensitivities, as well as in the shape of the device.

Thus, the signal processing circuit should ideally be able to compensate for all of these effects. In order to be able to achieve this compensation, a relatively complex filter, for example a digital filter such as an infinite response (lIR) filter may be useful.

SUMMARY OF INVENTION

According to a first aspect of the present invention, there is provided a method of calibrating a noise cancellation system, the noise cancellation system being for use in a device comprising a microphone for detecting ambient noise and generating a noise signal; a signal processor for generating a noise cancellation signal from the noise signal; and a speaker for receiving the noise cancellation signal such that the speaker generates a sound signal therefrom, in which the detected ambient noise has been at least partially cancelled, the method comprising: applying a test signal to the speaker to generate a test sound; detecting the test sound with the microphone; and adapting the signal processor on the basis of the detected test sound.

This has the advantage that the noise cancellation system can be calibrated without having to separately detect the sensitivity of the speaker and the microphone.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: Figure 1 illustrates a noise cancellation system in accordance with an aspect of the invention; Figure 2 illustrates a signal processing circuit in accordance with an aspect of the invention in the noise cancellation system of Figure 1; Figure 3 is a flow chart, illustrating a method of calibrating a noise cancellation system in accordance with an aspect of the invention; and Figure 4 illustrates a signal processing circuit in accordance with the present invention when embodied in a feedback noise cancellation system.

DETAILED DESCRIPTION

Figure 1 illustrates in general terms the form and use of a noise cancellation system in accordance with the present invention.

Specifically, Figure 1 shows an earphone 10, being worn on the outer ear 12 of a user 14. Thus, Figure 1 shows a supra-aural earphone that is worn on the ear, although it will be appreciated that exactly the same principle applies to circumaural headphones worn around the ear and to earphones worn in the ear such as so-called ear-bud phones. The invention is equally applicable to other devices intended to be worn or held close to the user's ear, such as mobile phones and other communication devices.

Ambient noise is detected by microphones 20, 22, of which two are shown in Figure 1, although any number more or less than two may be provided. Ambient noise signals generated by the microphones 20, 22 are combined, and applied to signal processing circuitry 24, which will be described in more detail below. In one embodiment, where the microphones 20, 22 are analogue microphones, the ambient noise signals may be combined by adding them together. Where the microphones 20, 22 are digital microphones, i.e. where they generate a digital signal representative of the ambient noise, the ambient noise signals may be combined alternatively, as will be familiar to those skilled in the art. Further, the microphones could have different gains applied to them before they are combined, for example in order to compensate for sensitivity differences due to manufacturing tolerances.

This illustrated embodiment of the invention also contains a source 26 of a wanted signal. For example, where the noise cancellation system is in use in an earphone, such as the earphone 10, that is intended to be able to reproduce music, the source 26 may be an inlet connection for a wanted signal from an external source such as a sound reproducing device. In other applications, for example where the noise cancellation system is in use in a mobile phone or other communication device, the source 26 may include wireless receiver circuitry for receiving and decoding radio frequency signals. In other embodiments, there may be no source, and the noise cancellation system may simply be intended to cancel the ambient noise for the user's comfort.

The wanted signal, if any, from the source 26 is applied through the signal processing circuitry 24 to a loudspeaker 28, which generates a sound signal in the vicinity of the user's ear 12. In addition, the signal processing circuitry 24 generates a noise cancellation signal that is also applied to the loudspeaker 28.

One aim of the signal processing circuitry 24 is to generate a noise cancellation signal, which, when applied to the loudspeaker 28, causes it to generate a sound signal in the ear 12 of the user that is the inverse of the ambient noise signal reaching the ear 12.

In order to achieve this, the signal processing circuitry 24 needs to generate from the ambient noise signals generated by the microphones 20, 22 a noise cancellation signal that takes into account the properties of the microphones 20, 22 and of the loudspeaker 28, and also takes into account the modification of the ambient noise that occurs due to the presence of the earphone 10.

Figure 2 shows in more detail the form of the signal processing circuitry 24. An input is connected to receive an input signal, for example directly from the microphones 20, 22. This input signal is amplified in an amplifier 41 and the amplified signal is applied to an analog-digital converter 42, where it is converted to a digital signal. The digital signal is applied to an adaptive digital filter 44, and the filtered signal is applied to an adaptable gain device 46. Those skilled in the art will appreciate that in the case where the microphones 20, 22 are digital microphones, wherein an analog-digital converter is incorporated into the microphone capsule and the input 40 receives a digital input signal, the analog-digital converter 42 is not required.

The resulting signal is applied to a first input of an adder 48, the output of which is applied to a digital-analog converter 50. The output of the digital-analog converter 50 is applied to a first input of a second adder 56, the second input of which receives a wanted signal from the source 26. The output of the second adder 56 is passed to the loudspeaker 28. Those skilled in the art will further appreciate that the wanted signal may be input to the system in digital form. In this instance, the adder 56 may be located prior to the digital-analog converter 50, and thus the combined signal output from the adder 56 is converted to analog before being output through the speaker 28.

Thus, the filtering and level adjustment applied by the filter 44 and the gain device 46 are intended to generate a noise cancellation signal that allows the detected ambient noise to be cancelled.

As mentioned above, the noise cancellation signal is produced from the input signal by the adaptive digital filter 44 and the adaptive gain device 46. These are controlled by a control signal, which is generated by applying the digital signal output from the analog-digital converter 42 to a decimator 52 which reduces the digital sample rate, and then to a microprocessor 54.

In this illustrated embodiment of the invention, the adaptive filter 44 is made up a first filter stage 80, in the form of a fixed lIR filter 80, and a second filter stage, in the form of an adaptive high- pass filter 82.

The microprocessor 54 generates a control signal, which is applied to the adaptive high-pass filter 82 in order to adjust a corner frequency thereof. The microprocessor 54 generates the control signal on an adaptive basis in use of the noise cancellation system, so that the properties of the filter 44 can be adjusted based on the properties of the detected noise signal.

However, the invention is equally applicable to systems in which the filter 44 is fixed. In this context, the word "fixed" means that the characteristic of the filter is not adjusted on the basis of the detected noise signal.

However, the characteristic of the filter 44 can be adjusted in a calibration phase, which may for example take place when the system 24 is manufactured, or when it is first integrated with the microphones 20, 22 and speaker 28 in a complete device, or whenever the system is powered on, or at other irregular intervals.

More specifically, the characteristic of the fixed hR filter 80 can be adjusted in this calibration phase by downloading to the filter 80 a replacement set of filter coefficients, from multiple sets of coefficients stored in a memory 90.

Further, the gain applied by the adjustable gain element 46 can similarly be adjusted in the calibration phase. Alternatively, a change in the gain can be achieved during the calibration phase by suitable adjustment of the characteristic of the fixed hR filter 80.

In this way, the signal processing circuitry 24 can be optimized for the specific device with which it is to be used.

Figure 3 is a flow chart, illustrating a method in accordance with an aspect of the invention. As mentioned above, the signal processing circuitry needs to generate a noise cancellation signal that, when applied to the speaker 28, produces a sound that cancels as far as possible the ambient noise heard by the user. The amplitude of the noise cancellation signal that produces this effect will depend on the sensitivity of the microphones 20, 22 and of the speaker 28, and on the degree of coupling from the speaker 28 to the microphones 20, 22 (for example, how close is the speaker 28 to the microphones 20, 22?), although this can be assumed to be equal for all devices (such as mobile phones) of the same model. The method proceeds from the recognition that, although these two parameters cannot easily be measured, what is actually important is their product. The method in accordance with the invention therefore consists of applying a test signal, of known amplitude, to the speaker 28 and detecting the resulting sound with the microphones 20, 22. The amplitude of the detected signal is a measure of the product of the sensitivity of the microphones 20, 22 and that of the speaker 28.

In step 110, a test signal is generated in the microprocessor 54. In one embodiment of the invention, the test signal is a digital representation of a sinusoidal signal at a known frequency. As discussed above, the aim of this calibration process is to compensate for the differences between devices, even though these devices are nominally the same. For example, in a mobile phone or similar device, the gain of the microphone may be 3dB more or less than its nominal value. Similarly, the gain of the speaker may be 3dB more or less than its nominal value, with the result that the product of these two may be 6dB more or less than its nominal value. In addition, the speaker will typically have a resonant frequency, somewhere within the audio frequency range. It will be appreciated that making measurements of the relative gains of two speakers will give misleading results, if one measurement is made at the resonant frequency of the speaker and the other measurement is made away from the resonant frequency of that speaker, and that, if the two speakers have different resonant frequencies, this situation may arise even if the gain measurements are made at the same frequency.

Therefore, the test signal preferably comprises a digital representation of a sinusoidal signal at a known frequency, where that known frequency is well away from any expected resonant frequency of the speaker, and hence such that all devices of the same class are expected to have generally similar properties, except for the general sensitivities of their microphones and speakers.

In alternative embodiments, the test signal may be a band-limited noise signal, or a pseudo-random data-pattern such as a maximum-length sequence.

In step 112, the test signal is applied from the microprocessor 54 to the second input of the adder 48, and thus applied to the speaker 28.

In step 114, the resulting sound signal is detected by the microphones 20, 22, and a portion of the detected signal is passed to the microprocessor 54.

In step 116, the microprocessor 54 measures the amplitude of the detected signal.

This can be done in different ways. For example, the total amplitude of the detected signal may be measured, but this will result in the detection not only of the test sound, but also of any ambient noise. Alternatively, the detected sound signal can be filtered, and the amplitude of the filtered sound signal detected. For example the detected sound signal can be passed through a digital Fourier transform, allowing the component of the sound signal at the frequency of the test signal to be separated out, and its amplitude measured. As a further alternative, the test signal can contain a data pattern, and the microprocessor 54 can be used to detect the correlation between the detected sound signal and the test signal, so that the detected amplitude can be determined to be the amplitude that results from the test signal, rather than from ambient noise.

In step 118, the signal processor is adapted based on the detected amplitude. For example, the gain of the adaptive gain element 46 can be adjusted.

It will be apparent to those skilled in the art that the present invention is equally applicable to so-called feedback noise cancellation systems.

The feedback method is based upon the use, inside the cavity that is formed between the ear and the inside of an earphone shell, or between the ear and a mobile phone, of a microphone placed directly in front of the loudspeaker. Signals derived from the microphone are coupled back to the loudspeaker via a negative feedback loop (an inverting amplifier), such that it forms a servo system in which the loudspeaker is constantly attempting to create a null sound pressure level at the microphone.

Figure 4 shows an example of signat processing circuitry according to the present invention when implemented in a feedback system.

The feedback system comprises a microphone 120 positioned substantially in front of a loudspeaker 128. The microphone 120 detects the output of the loudspeaker 128, with the detected signal being fed back via an amplifier 141 and an analog-to-digital converter 142. A wanted audio signal is fed to the processing circuitry via an input 140.

The fed back signal is subtracted from the wanted audio signal in a subtracting element 188, in order that the output of the subtracting element 188 substantially represents the ambient noise, i.e. the wanted audio signal has been substantially cancelled.

Thereafter, the processing circuitry is substantially similar to that in the feed forward system described with respect to Figure 2. The output of the subtracting element 188 is fed to an adaptive digital filter 144, and the filtered signal is applied to an adaptable gain device 146.

The resulting signal is applied to an adder 148, where it is summed with the wanted audio signal received from the input 140.

Thus, the filtering and level adjustment applied by the filter 144 and the gain device 146 are intended to generate a noise cancellation signal that allows the detected ambient noise to be cancelled.

As mentioned above, the noise cancellation signal is produced by the adaptive digital filter 144 and the adaptive gain device 146. These are controlled by a control signal, which is generated by applying the signal output from the subtracting element 188 to a decimator 152 which reduces the digital sample rate, and then to a microprocessor 154.

In this illustrated embodiment of the invention, the adaptive filter 144 is made up a first filter stage 180, in the form of a fixed lIR filter 180, and a second filter stage, in the form of an adaptive high-pass filter 182.

The microprocessor 154 generates a control signal, which is applied to the adaptive high-pass filter 182 in order to adjust a corner frequency thereof. The microprocessor 54 generates the control signal on an adaptive basis in use of the noise cancellation system, so that the properties of the filter 144 can be adjusted based on the properties of the detected noise signal.

However, the invention is equally applicable to systems in which the filter 144 is fixed.

In this context, the word "fixed" means that the characteristic of the filter is not adjusted on the basis of the detected noise signal.

However, the characteristic of the filter 144 can be adjusted in a calibration phase, which may for example take place when the system is manufactured, or when it is first integrated with the microphones 120 and speaker 128 in a complete device, or whenever the system is powered on, or at other irregular intervals.

More specifically, the characteristic of the fixed IIR filter 180 can be adjusted in this calibration phase by downloading to the filter 180 a replacement set of filter coefficients, from multiple sets of coefficients stored in a memory 190.

Further, the gain applied by the adjustable gain element 146 can similarly be adjusted in the calibration phase. Alternatively, a change in the gain can be achieved during the calibration phase by suitable adjustment of the characteristic of the fixed llR filter 180.

In this way, the signal processing circuitry can be optimized for the specific device with which it is to be used.

The microprocessor 154 further generates a test signal, as described previously, and outputs the test signal to an adding element 150, where it is added to the signal output from the adding element 148. The combined signal is then output to a digital-analog converter 152, and output through a speaker 128.

It will be clear to those skilled in the art that the implementation may take one of several hardware or software forms, and the intention of the invention is to cover all these different forms.

Noise cancellation systems according to the present invention may be employed in many devices, as would be appreciated by those skilled in the art. For example, they may be employed in mobile phones, headphones, earphones, headsets, etc. Furthermore, it will be appreciated that the present invention is applicable to any device comprising both a speaker and a microphone. For example, in such devices the present invention may be useful to give a first estimate of the sensitivity of one of, or both of, the speaker and the microphone Examples of such devices include audio/video record/playback devices, such as dictation devices, video cameras, etc. The skilled person will recognise that the above-described apparatus and methods may be embodied as processor control code, for example on a carrier medium such as a disk, CD-or DVD-ROM, programmed memory such as read only memory (firmware), or on a data carrier such as an optical or electrical signal carrier. For many applications, embodiments of the invention will be implemented on a DSP (digital signal processor), ASIC (application specific integrated circuit) or FPGA (field programmable gate array). Thus the code may comprise conventional program code or microcode or, for example code for setting up or controlling an ASIC or FPGA. The code may also comprise code for dynamically configuring re-configurable apparatus such as re-programmable logic gate arrays. Similarly the code may comprise code for a hardware description language such as Verilog TM or VHDL (very high speed integrated circuit hardware description language). As the skilled person will appreciate, the code may be distributed between a plurality of coupled components in communication with one another. Where appropriate, the embodiments may also be implemented using code running on a fleld-(re-)programmable analogue array or similar device in order to configure analogue/digital hardware.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims

1. A method of calibrating a noise cancellation system, the noise cancellation system being for use in a device comprising a microphone for detecting ambient noise and generating a noise signal, a signal processor for generating a noise cancellation signal from the noise signal; and a speaker for receiving the noise cancellation signal such that the speaker generates a sound signal therefrom, in which the detected ambient noise has been at least partially cancelled, the method comprising: applying a test signal to the speaker to generate a test sound; detecting the test sound with the microphone; and adapting the signal processor on the basis of the detected test sound.

2. A method as claimed in claim 1, further comprising: selecting the test signal such that it has a frequency within a frequency range over which all devices of a class are expected to have similar properties.

3. A method as claimed in claim 1 or 2, further comprising: selecting the test signal such that it takes the form of a band-limited noise signal.

4. A method as claimed in claim 1 or 2, further comprising: selecting the test signal such that it takes the form of a pseudo-random data pattern.

5. A method as claimed in claim 1 or 2, further comprising: selecting the test signal such that it takes the form of a digital representation of a sinusoid at a predetermined frequency.

6. A method as claimed in claim 5, wherein the predetermined frequency is separated from a resonant frequency of the speaker.

7. A method as claimed in any preceding claim, comprising: measuring an amplitude of the detected test sound; and adapting a gain of the signal processor on the basis of the measured amplitude.

8. A method as claimed in claim 7, further comprising: separating the detected test sound from detected ambient noise.

9. A method as claimed in claim 8, wherein the step of separating the detected test sound from detected ambient noise comprises detecting the noise signal at a predetermined frequency of the test sound.

10. A method as claimed in claim 8, wherein the step of separating the detected test sound from detected ambient noise comprises cross-correlating the detected sound with the test signal.

11. A noise cancellation system, comprising a signal processor for generating a noise cancellation signal from a noise signal, the system being suitable for use in a device comprising a microphone for detecting ambient noise and generating the noise signal, a signal processor; and a speaker for receiving the noise cancellation signal such that the speaker generates a sound signal therefrom, in which the detected ambient noise has been at least partially cancelled, the signal processor being adapted to: apply a test signal to the speaker to generate a test sound; detect the test sound; and adapt the signal processor on the basis of the detected test sound.

12. A noise cancellation system as claimed in claim 11, wherein said test signal has a frequency within a frequency range over which all devices of a class are expected to have similar properties.

13. A noise cancellation system as claimed in claim 11 or 12, wherein said test signal takes the form of a band-limited noise signal.

14. A noise cancellation system as claimed in claim 11 or 12, wherein said test signal takes the form of a pseudo-random data pattern.

15. A noise cancellation system as claimed in claim 11 or 12, wherein said test signal takes the form of a digital representation of a sinusoid at a predetermined frequency.

16. A noise cancellation system as claimed in claim 15, wherein the predetermined frequency is separated from a resonant frequency of the speaker.

17. A noise cancellation system as claimed in any preceding claim, wherein a gain of the signal processor is adapted on the basis of a measured amplitude of the detected test sound.

18. A noise cancellation system as claimed in claim 17, the detected test sound is separated from detected ambient noise.

19. A noise cancellation system as claimed in claim 18, wherein the detected test sound is separated from detected ambient noise by detecting the noise signal at a predetermined frequency of the test sound.

20. A noise cancellation system as claimed in claim 81, wherein the detected test sound is separated from detected ambient noise by cross-correlating the detected sound with the test signal.

21. An integrated circuit, comprising: a noise cancellation system as claimed in any one of claims 11-20,

22. A mobile phone, comprising: an integrated circuit as claimed in claim 21.

23. A pair of headphones, comprising: an integrated circuit as claimed in claim 21.

24. A pair of earphones, comprising: an integrated circuit as claimed in claim 21.

A headset, comprising: an integrated circuit as claimed in claim 21.

26. An audio playback/record device, comprising: an integrated circuit as claimed in claim 21.

27. A dictation device, comprising: an integrated circuit as claimed in claim 21.