Dual-microphone frequency amplitude response self-calibration

Abstract

A frequency domain method and system for online self-calibrating microphone frequency amplitude response based on noise floor (minima) tracking are disclosed. A cellular telephone or other system with dual microphones may self-calibrate itself on-the-fly. The system selects one of the microphones as a reference and calibrates the frequency response of the two microphones using the first microphone as a reference, so that they have a matched frequency amplitude response. To achieve this on-the-fly calibration, the system uses background noise for calibration purposes. The signal power spectra of the noise minima at the two microphones is used to calibrate the respective microphone frequency response. The system may then adapt the frequency amplitude responses of the two microphones so that the power spectral density from each microphone matches the other, and the system is then calibrated. This calibration could occur any time the device is receiving a noise minima and could be done continuously as the device is being used.

Description

FIELD OF THE INVENTION

The present invention relates to a self-calibration system for use with two or more microphones. In particular, the present invention is directed toward a self-calibration system for use in a cellular telephone or the like, where dual microphones may be used for a noise cancellation circuit or other ambient event detector processes. Other applications may include a microphone array circuit, and noise suppression circuit, or other applications where multiple microphones may be utilized and calibration between microphones may be required.

BACKGROUND OF THE INVENTION

A personal audio device, such as a wireless telephone, may include a noise canceling circuit to reduce background noise in audio signals. One example of such a noise cancellation circuit is an adaptive noise cancellation circuit that adaptively generates an anti-noise signal from a reference microphone signal and injects the anti-noise signal into the speaker or other transducer output to cause cancellation of ambient audio sounds. An error microphone may also be provided proximate the speaker to measure the ambient sounds and transducer output near the transducer, thus providing an indication of the effectiveness of the noise canceling. A processing circuit uses the reference and/or error microphone, optionally along with a microphone provided for capturing near-end speech, to determine whether the noise cancellation circuit is incorrectly adapting or may incorrectly adapt to the instant acoustic environment and/or whether the anti-noise signal may be incorrect and/or disruptive and then take action in the processing circuit to prevent or remedy such conditions.

Examples of such adaptive noise cancellation systems are disclosed in published U.S. Patent Application 2012/0140943, published on Jun. 7, 2012, and Published U.S. Patent Application 2012/0207317, published on Aug. 16, 2012, both of which are incorporated herein by reference. Both of these references are assigned to the same assignee as the present application, and one names at least one inventor in common and thus are not “Prior Art” to the present application. However, they are provided to facilitate the understating of noise cancellation circuits as applied in the field of use. These references are provided by way of background only to illustrate one problem solved by the present invention. They should not be taken as limiting the present invention to any one type of multi-microphone application or noise cancellation circuit.

Referring now to FIG. 1, a wireless telephone 10 is shown in proximity to a human ear 5. Wireless telephone 10 includes a transducer, such as speaker SPKR that reproduces distant speech received by wireless telephone 10, along with other local audio events such as ringtones, stored audio program material, injection of near-end speech (i.e., the speech of the user of wireless telephone 10) to provide a balanced conversational perception, and other audio that requires reproduction by wireless telephone 10, such as sources from web-pages or other network communications received by wireless telephone 10 and audio indications such as battery low and other system event notifications. A near-speech microphone NS is provided to capture near-end speech, which is transmitted from wireless telephone 10 to the other conversation participant(s).

Wireless telephone 10 includes active noise canceling circuits and features that inject an anti-noise signal into speaker SPKR to improve intelligibility of the distant speech and other audio reproduced by speaker SPKR. A reference microphone R is provided for measuring the ambient acoustic environment and is positioned away from the typical position of a user's mouth, so that the near-end speech is minimized in the signal produced by reference microphone R. Prior art noise cancellation circuits rely on the use of two microphones (E and R). The embodiment of FIG. 1 also provides a third microphone, near-speech microphone NS, in order to further improve the noise cancellation operation by monitoring the ambient disturbance to the noise cancellation system when wireless telephone 10 is in close proximity to ear 5. Exemplary circuit 14 within wireless telephone 10 includes an audio CODEC integrated circuit 20 that receives the signals from reference microphone R, near speech microphone NS and error microphone E and interfaces with other integrated circuits such as an RF integrated circuit 12 containing the wireless telephone transceiver.

In general, the noise cancellation techniques measure ambient acoustic events (as opposed to the output of speaker SPKR and/or the near-end speech) impinging on reference microphone R, and by also measuring the same ambient acoustic events impinging on error microphone E, the noise cancellation processing circuits of illustrated wireless telephone 10 adapt an anti-noise signal generated from the output of reference microphone R to have a characteristic that minimizes the amplitude of the ambient acoustic events at error microphone E. Since acoustic path P(z) (also referred to as the passive forward path) extends from reference microphone R to error microphone E, the noise cancellation circuits are essentially estimating acoustic path P(z) combined with removing effects of an electro-acoustic path S(z) (also referred to as secondary path) that represents the response of the audio output circuits of CODEC IC 20 and the acoustic/electric transfer function of speaker SPKR including the coupling between speaker SPKR and error microphone E in the particular acoustic environment, which is affected by the proximity and structure of ear 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10, when wireless telephone is not firmly pressed to ear 5.

The dual microphone (R and NS) system of FIG. 1 is widely used in mobile telephony for uplink noise suppression. In order to protect the noise cancellation system, oversight software requires audio signals from R and NS microphones in order to detect certain situations, such as close talk, wind noise, howling, and the like. Close talk, as the term is known, occurs when the near-end user is talking while holding the phone to his/her ear. Wind noise occurs when wind buffets the microphone, producing loud buffeting noises. Howling occurs when an anti-noise signal is picked up by microphone R, and it is played out speaker SPKR. The speaker output gets coupled back to the reference microphone R and sets up a positive feedback loop. Howling can occur, for example, if a user cups their hand from the speaker back to the reference microphone R, or if there is some internal leakage path. Scratching is a term used to describe physical contact with a microphone, which produces a loud scratching noise.

Gain mismatch between the two microphones can reduce robustness and increase failures in detecting situations, such as close talk, scratch, howling and the like. If the gain from the two microphones differs, then the signal levels from the microphones will be different from one another, even when transmitting the same sound levels. In actual practice, some gain mismatch between the microphones is inevitable, due to manufacturing tolerances, microphone mounting and placement and the like. The absolute difference of amplitude frequency response could vary in a range of 0 to 10 dB or more.

Factory calibration of the microphones is one solution but provides only a partial solution to the problem. Microphone gain calibration provides only an overall gain calibration instead of a frequency response calibration. Moreover, even if calibrated at the factory, microphone response may drift over time.

Thus, it remains a requirement in the art to provide a way for calibrating a dual-microphone system when in use in the field, which provides a frequency response calibration in real-time.

SUMMARY OF THE INVENTION

A cellular telephone or other system with dual microphones self-calibrates itself on-the-fly. The system selects one of the microphones as a reference and calibrates the frequency response of the two microphones using the first microphone as a reference so that they have a matched frequency amplitude response.

To achieve this on-the-fly calibration, the system uses background noise for calibration purposes. While ambient (background) noise changes all the time, it usually falls back to the noise floor or “minima” at some time. The system tracks the slowly-changing ambient noise “minima” and uses this “minima” as a calibration signal. The signal power spectra of the noise minima at the two microphones are used to calibrate the respective microphone frequency response.

This technique is based on two assumptions. First, it assumes that the ambient noise is a diffused noise field, that is, not from a single point source or the like. Alternatively, the noise is from far field (a distance away from the microphone) so as to behave like a diffused noise field. With one or both assumptions, the noise power spectral density (PSD) from each microphone rshould be very close to one another if frequency amplitude responses of the two microphones are matched. The system may then adapt the frequency amplitude responses of the two microphones so that the PSD from each microphone matches the other, and the system is then calibrated. This calibration could occur any time the device is receiving noise and could be done continuously as the device is being used.

Noise minima is usually stationary or pseudo-stationary, or much more stationary than speech. The noise minima is proportionate to the noise power, as set forth, for example, in I. Cohen and B. Berdugo, Noise Estimation by Minima Controlled Recursive Averaging for Robust Speech Enhancement, IEEE Signal Processing Letters, Vol. 9, No. 1, January 2002, pp 12-15, incorporated herein by reference. Thus, the difference of the noise minima of the microphone signals yields the difference of the microphone gain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating how dual microphones may be used in a noise cancellation circuit in a cellular telephone.

FIG. 2 is a block diagram illustrating dual-microphone frequency amplitude response self-calibration.

FIG. 3 is a graph illustrating a sample of ambient noise signals, along with corresponding noise minima calculation.

DETAILED DESCRIPTION OF THE INVENTION

Dual-microphone frequency amplitude response self-calibration is disclosed in the context of a two-microphone system, for example, using a near speech (NS) microphone for receiving a voice signal and a reference microphone (R) for measuring ambient noise for the noise cancellation circuit. However, dual-microphone frequency amplitude response self-calibration may be applied to other systems as well, including the three-microphone system disclosed in FIG. 1. In such a system, two microphones maybe calibrated relative to the third microphone, and two corrective gain adjustments made relative to that microphone. However for the purposes of the following discussion, only two microphones NS an R are assumed.

Referring to FIG. 2, in block 110 a noise minima tracker tracks noise minima for the calibration of the two microphones. Microphones R and NS, by way of example, may output audio signals in response to ambient noise and the like. The diagram of FIG. 2 has been simplified for the purposes of illustration. The audio signals from microphones R and NS may be suitably digitized in an A/D converter (not shown) to process the signals in the digital domain if desired. An input filter 100 may be provided for one or both microphones R and NS. For the purposes of illustration of the dual-microphone frequency amplitude response self-calibration, only one input filter is illustrated, although in practice, two such filters may be provided. The input filter may adjust the gain of a microphone (e.g., microphone NS in this example) by altering the frequency profile of the microphone signal.

Noise minima may be tracked in the frequency domain as illustrated in block 110. In the routine shown in FIG. 2, minima is tracked for a channel x, where x represents one of the two microphones on a cell phone (in this case, reference microphone R). Minima values for both microphones R and NS are then calculated. This routine may be enabled as a software portion of the microprocessor or may be performed in hardware. For the purposes of testing and illustrating dual-microphone frequency amplitude response self-calibration, it is shown as a software routine. The same routine is then performed for the channel y for near speech microphone NS.

Once the noise minima for both microphones have been tracked in block 110, in block 120, a calibrator calibrates the amplitude of each frequency bin. First, the gain difference between the two microphones R and NS is calculated per frequency bin from the minima of two microphones in step 110. The gain difference g[k] represents a ratio between the minima of the two microphones receiving the same ambient noise signal. The value g[k] is the microphone gain difference per frequency bin and may be calculated as follows:g[k]=alpha*g[k]+(1-alpha)*xMinEnv[k]/yMinEnv[k]  (1)where xMinEnv[k] represents the minima level for a particular frequency bin k, for the signal x (e.g., Reference Microphone R) and yMinEnv[k] represents the minima level for a particular frequency bin k, for the signal y (e.g., Near Speech Microphone NS) and alpha represents a smoothing factor that smoothly updates the gain difference.

The order in which the noise minima (x versus y) are calculated is not necessarily important. Similarly, either microphone may be used as the reference microphone relative to the other, by suitably altering the numerator and denominator of equation (1) above.

As illustrated in block 150, from this gain difference, the amplitude and profile of a compensation filter 100 to one or both microphones may be adjusted so that the amplitude and frequency response of the filtered microphone outputs are normalized with regard to one another. The outputs from microphones R and NS are now suitably calibrated relative to one another as the signal levels from both microphones will be equivalent to one another for a given input. These calibrated microphone signals may then be passed to other ambient event detection processes 170 in the cell phone, such as noise cancellation or the like, for use as inputs for those processes. As the microphones are now calibrated relative to one another, the noise cancellation circuit, for example, will operate more effectively, as the relative signal strengths as well as frequency response for each of microphones R and NS will be equivalent for an equivalent audio input.

Block 120 outputs the gain difference per frequency bin g[k], where k represents an individual frequency bin. Frequency gain difference g[k] may be calculated according to equation (1) above, representing a ratio between the minima of the two microphones receiving the same ambient noise signal. As a cellular phone ages, it is possible a microphone may be aging, malfunctioning, broken, or clogged. Thus, in step 130, a determination is made whether the microphone is broken or clogged. If gain g[k] is out of a reasonable range, i.e., greater than 20 dB, then a determination is made that one of the two microphones R, NS is broken or clogged or damaged as determined in microphone condition detector block 140. In block 160, the user may be notified via a message on the device that one of the microphones is broken, clogged, or damaged, and the user may be directed to take the device for servicing. The device may also try to compensate for this error by shutting off or attenuating the noise cancellation circuit or taking other reparative action.

The calibration system, while disclosed in the context of noise cancellation, may be used for a number of applications, for example, in a cellular telephone, where multiple microphones are used to detect what are known as ambient events. These ambient events may include wind noise, scratch, howling, and close talk, as discussed above, or any scenario where signals from dual microphones need to be closely compared.

Equation (1) may be implemented in software as illustrated in Table I below. First, a value xMinEnv[k] (which will be g[k], eventually) is set to the minima of a previous value xTempEnv[k] or a power spectral density value for the frequency bin k. If the detector status is not equal to “OTHERS” (meaning there are no other ambient noise events detected) the value xTempEnv[k] is then calculated using Equation (1) above. If there are any ambient event detection results (from a plurality of such detectors in the system, not shown) other than “OTHERS”, which means there are no special events, alpha_min is used to update the Temp Envelope; otherwise, alpha_min_disturb is used to update it. This is different from the aforementioned paper by Cohen and Berdugo, in which they use a single smoothing factor because there are no other detectors involved.

The program then updates xMinEnv[k] to be the minima of itself or the PSD, and xTempEnv[k] likewise. The process is repeated for each frequency bin k within a desired range (e.g., frequency response range of the cellular telephone device, or a selected sub-range thereof).

TABLE I
Minima update algorithm:
For each frequency bin:
For every N frames
Update xMinEnv[k] = min(xTempEnv[k], xBlockPow[k]);
Update xTempEnv[k] = alpha*xTempEnv[k] + (1-alpha)*
xBlockPow[k]
If other detectors (if available in the system) says
there's no disturbance, using smoothing factor alpha = alpha_min,
if there is disturbance, using alpha = alpha_min_disturb
If there's no other detectors in the system, using
alpha = alpha_min
For the frames within the N frames
Update xMinEnv[k] = min(xMinEnv[k], xBlockPow[k]);
Update xTempEnv[k] = min(xTempEnv[k], xBlockPow[k]);
Where:
k denotes the k-th frequency bin.
xBlockPow [k] denotes the block power for the k-th bin at
channel x
xMinEnv[k] denotes the minima for the k-th bin at channel x
xTempEnv[k] denotes the temporary minima for the k-th
bin at channel x
alpha_min_disturb is larger than alpha_min, which
means when disturbance occurs, update the temporary
minima slower.

FIG. 3 is a graph illustrating a sample of ambient noise signals from microphones R and NS, along with a noise minima calculation. The Y-axis of the graph represents Sound Pressure Level (SPL) for one frequency bin in decibels (dB) and the X-axis of the graph represents time in seconds. The solid thin line represents a raw ambient signal for the NS microphone, and the dark solid line below it represents the minima calculated for the NS microphone. The dashed thin line represents a raw ambient signal for the R microphone, and the dark dashed line below it represents the minima calculated for the R microphone. The difference in minima between the two microphones is illustrated in FIG. 3.

In the dual-microphone frequency amplitude response self-calibration system and method, noise minima is calculated for each frequency bin at each microphone. From these noise minima calculations, a frequency gain difference g[k] may be calculated according to equation (1) above, representing a ratio between the minima of the two microphones receiving the same ambient noise signal. This ratio may then be used to correct the frequency response of one microphone relative to the other, so that for a given equivalent input, both microphones output the same or similar signal.

While disclosed in terms of calibrating by frequency bin, the dual-microphone frequency amplitude response self-calibration system and method may also be used to self-calibrate microphones by altering the wideband gain of one or more microphones. The frequency response of each microphone may be calculated in a similar manner as illustrated above in connection with FIG. 2, but the calibration factor for input filter 100 may be made by altering the wideband gain of the microphone rather than on a frequency bin basis.

Various noise cancellation systems rely on the accuracy of the microphone signals in order to create an effective noise cancellation signal, which is subtracted from the speech signal. By providing this on-the-fly calibration, the dual-microphone frequency amplitude response self-calibration system and method provide improved noise cancellation, as the error signal is measured more accurately. In addition, the dual-microphone frequency amplitude response self-calibration system and method can also detect the presence of a damaged, broken, or clogged microphone, and can alert the user of this problem and/or disable or modify operation of the noise cancellation system to compensate for this problem.

While disclosed in the context of a cellular telephone with an adaptive noise cancellation system, the present invention may be applied to other types of noise cancellation systems as well as other systems using multiple microphones. For example, the dual-microphone frequency amplitude response self-calibration system and method may be applied to noise cancellation headsets for use in aviation and other applications such as dual microphone noise suppression, microphone array, beamforming and the like. The dual-microphone frequency amplitude response self-calibration system and method may also be used for stereo microphones and other multiple microphone setups, where microphones may require calibration with respect to one another.

While the preferred embodiment and various alternative embodiments of the invention have been disclosed and described in detail herein, it may be apparent to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.

Claims

1. In a multiple microphone system having at least two microphones, a method of self-calibration, comprising: receiving ambient noise signals from the at least two microphones;tracking noise minima in a time domain for each of the ambient noise signals from the at least two microphones by tracking the noise minima of each of the ambient noise signals from the at least two microphones for a predetermined number of frequency bins;calculating an amplitude calibration value based on a ratio of the noise minima of each of the ambient noise signals from two of the at least two microphones by calculating the amplitude calibration value for each of the predetermined number of frequency bins based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones; andaltering gain of at least one of the at least two microphones to calibrate one of the at least two microphones relative to another of the at least two microphones based on the amplitude calibration value.

2. The method of claim 1, further comprising: comparing the amplitude calibration value based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones with a predetermined value,if the amplitude calibration value based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones is greater than the predetermined value, determining that one or more of the at least two microphones is broken, malfunctioning, or clogged, andnotifying a user that one or more of the at least two microphones is broken, malfunctioning, or clogged.

3. The method of claim 1, wherein calculating the amplitude calibration value for each frequency bin further comprises smoothing amplitude calibration value changes over time by multiplying the amplitude calibration value by a predetermined smoothing factor.

4. The method of claim 3, wherein calculating the amplitude calibration value for each frequency bin based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones further comprises calculating a value g[k] as follows: g[k]=alpha*g[k]+(1-alpha)*xMinEnv[k]/yMinEnv[k]where xMinEnv[k] represents a minima level for a particular frequency bin k for a signal x from one of the at least two microphones and yMinEnv[k] represents a minima level for a particular frequency bin k for the signal y from another of the at least two microphones and alpha represents the predetermined smoothing factor.

5. A self-calibrating multiple microphone system, comprising at least two microphones, comprising: the at least two microphones each receiving at least ambient noise signals;a noise minima tracker receiving the ambient noise signals from each of the at least two microphones and tracking noise minima in a time domain for each of the ambient noise signals from the at least two microphones by tracking the noise minima of each of the ambient noise signals from the at least two microphones for a predetermined number of frequency bins;a calibrator calculating an amplitude calibration value based on a ratio of the noise minima of each of the ambient noise signals from two of the at least two microphones by calculating the amplitude calibration value for each of the predetermined number of frequency bins based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones; andan input filter coupled to at least one of the at least two microphones having a gain profile altered by the calculated amplitude calibration value to calibrate one of the at least two microphones relative to another of the at least two microphones.

6. The system of claim 5, further comprising: a microphone condition detector comparing the amplitude calibration value based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones with a predetermined value, and if the amplitude calibration value based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones is greater than the predetermined value, determining that one or more of the at least two microphones is malfunctioning, broken, or clogged, and notifying a user that one or more of the at least two microphones is malfunctioning, broken, or clogged.

7. The system of claim 5, wherein calculating the amplitude calibration value for each frequency bin further includes smoothing amplitude calibration value changes over time by multiplying the amplitude calibration value by a predetrmined smoothing factor.

8. The system of claim 7, wherein the calibrator calculates the amplitude calibration value for each frequency bin based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones as g[k] as follows: g[k]=alpha*g[k]+(1-alpha)*xMinEnv[k]/yMinEnv[k]where xMinEnv[k] represents a minima level for a particular frequency bin k for a signal x from one of the at least two microphones and yMinEnv[k] represents a minima level for a particular frequency bin k for the signal y from another of the at least two microphones and alpha represents the predetermined smoothing factor.

9. A self-calibrating cellular telephone including at least two microphones, comprising: the at least two microphones on the self-calibrating cellular telephone each receiving at least ambient noise signals;a noise minima tracker receiving the ambient noise signals from each of the at least two microphones and tracking noise minima in a time domain for each of the ambient noise signals from the at least two microphones by tracking the noise minima of each of the ambient noise signals from the at least two microphones for a predetermined number of frequency bins;a calibrator calculating an amplitude calibration value based on a ratio of the noise minima of each of the ambient noise signals from two of the at least two microphones by calculating the amplitude calibration value for each of the predetermined number of frequency bins based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones; andan input filter coupled to at least one of the at least two microphones having a gain profile altered by the calculated amplitude calibration value to calibrate one of the at least two microphones relative to another of the at least two microphones.

10. The self-calibrating cellular telephone of claim 9, further comprising: a microphone condition detector comparing the amplitude calibration value based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones with a predetermined value, and if the amplitude calibration value based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones is greater than the predetermined value, determining that one or more of the at least two microphones is malfunctioning, broken, or clogged, and notifying a user that one or more of the at least two microphones is malfunctioning, broken, or clogged.

11. The self-calibrating cellular telephone of claim 9, wherein calculating the amplitude calibration value for each frequency bin further includes smoothing amplitude calibration value changes over time by multiplying the amplitude calibration value by a predetermined smoothing factor.

12. A self-calibrating cellular telephone including at least two microphones, comprising: the at least two microphones on the self-calibrating cellular telephone each receiving audio signals including ambient noise signals;a noise minima tracker receiving the ambient noise signals from the at least two microphones and tracking noise minima for each of the ambient noise signals from the at least two microphones;a calibrator calculating an amplitude calibration value based on a ratio of the noise minima of each of the ambient noise signals from two of the at least two microphones; andan input filter coupled to at least one of the at least two microphones having a gain profile altered by the calculated amplitude calibration value to calibrate one of the at least two microphones relative to another of the at least two microphones,wherein the noise minima tracker tracks the noise minima of each of the ambient noise signals from the at least two microphones for a predetermined number of frequency bins,wherein the calibrator calculates the amplitude calibration value for each frequency bin based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones,wherein calculating the amplitude calibration value for each frequency bin further includes smoothing amplitude calibration value changes over time by multiplying the amplitude calibration value by a predetermined smoothing factor, andwherein the calibrator calculates the amplitude calibration value for each frequency bin based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones as g[k] as follows: g[k]=alpha*g[k]+(1-alpha)*xMinEnv[k]/yMinEnv[k]where xMinEnv[k] represents a minima level for a particular frequency bin k for a signal x from one of the at least two microphones and yMinEnv[k] represents a minima level for a particular frequency bin k for the signal y from another of the at least two microphones and alpha represents the predetermined smoothing factor.

13. A self-calibrating cellular telephone including at least two microphones, comprising: the at least two microphones on the self-calibrating cellular telephone each receiving audio signals including ambient noise signals;a noise minima tracker receiving the ambient noise signals from the at least two microphones and tracking noise minima for each of the ambient noise signals from the at least two microphones;a calibrator calculating an amplitude calibration value based on a ratio of the noise minima of each of the ambient noise signals from two of the at least two microphones; andan input filter coupled to at least one of the at least two microphones having a gain profile altered by the calculated amplitude calibration value to calibrate one of the at least two microphones relative to another of the at least two microphones,wherein the noise minima tracker tracks the noise minima of each of the ambient noise signals from the at least two microphones for a predetermined number of frequency bins,wherein the calibrator calculates the amplitude calibration value for each frequency bin based on the ratio of the noise minima of each of the ambient noise signals from the two of the at least two microphones as g[k] as follows: g[k]=alpha*g[k]+(1-alpha)*xMinEnv[k]/yMinEnv[k]where xMinEnv[k] represents a minima level for a particular frequency bin k for a signal x from one of the at least two microphones and yMinEnv[k] represents a minima level for a particular frequency bin k for the signal y from another of the at least two microphones and alpha represents a predetermined smoothing factor.