METHOD FOR DETERMINING A RESPONSE FUNCTION OF A NOISE CANCELLATION ENABLED AUDIO DEVICE

Abstract

In a method for determining a response function of a noise cancellation enabled audio device, the audio device is placed onto a measurement fixture, wherein a loudspeaker of the audio device faces an ear canal representation of the measurement fixture. A first and a second response function between an ambient sound source and a test microphone located within the ear canal representation are measured while parameters of a noise processor of the audio device are set to a proportional transfer function with respective first and second gain factors being different from each other. A model response function is determined based on the first and the second response function and on the first and the second gain factor.

Claims

1. A method for determining a response function of a noise cancellation enabled audio device, in particular headphone, the method comprising: placing the audio device onto a measurement fixture, wherein a loudspeaker of the audio device faces an ear canal representation of the measurement fixture; measuring a first response function between an ambient sound source and a test microphone located within the ear canal representation while parameters of a noise processor of the audio device are set to a proportional transfer function with a first gain factor; measuring a second response function between the ambient sound source and the test microphone while parameters of the noise processor are set to a proportional transfer function with a second gain factor being different from the first gain factor; determining a model response function for the noise processor based on the first response function, the second response function and the first and the second gain factor.

2. The method according to claim 1, further comprising: determining an ambient-to-ear response function based on the first and/or the second response function; and determining an overall processor response function based on the first response function, the second response function and the first and the second gain factor; wherein the model response function is determined from the ambient-to-ear response function and the overall processor response function.

3. The method according to claim 1, wherein the model response function F is determined according to the formula $F = a .Math. .Math. 1 - \frac{X (a .Math. .Math. 2 - a .Math. .Math. 1)}{Y - X},$ with a1 being the first gain factor, a2 being the second gain factor, X being the first response function and Y being the second response function.

4. The method according to claim 1, further comprising: measuring a third response function between the ambient sound source and the test microphone while parameters of the noise processor are set to a proportional transfer function with a third gain factor being different from the first gain factor and the second gain factor; wherein the model response function is determined based on the first, the second and the third response function, and the first, the second and the third gain factor.

5. The method according to claim 4, further comprising: determining an ambient-to-ear response function based on the first response function or on the first, the second and the third response function; and determining an overall processor response function based on the first, the second and the third response function and on the first, the second and the third gain factor; wherein the model response function is determined from the ambient-to-ear response function and the overall processor response function.

6. The method according to claim 4, wherein the model response function F is determined according to the formula $F = a .Math. .Math. 1 - \frac{X (a .Math. .Math. 3 - a .Math. .Math. 2)}{Z - Y},$ with a1 being the first gain factor, a2 being the second gain factor, a3 being the third gain factor, X being the first response function, Y being the second response function and Z being the third response function.

7. The method according to claim 5, further comprising determining a leakage response function based on the first, the second and the third response function and on the first, the second and the third gain factor; wherein the overall processor response function is determined further based on the leakage response function.

8. The method according to claim 1, wherein the first gain factor equals zero.

9. The method according to claim 8, wherein the noise processor disabled and/or muted during measurement of first response function.

10. A method for determining a response function of a noise cancellation enabled audio device, in particular headphone, the method comprising: placing the audio device onto a measurement fixture, wherein a loudspeaker of the audio device faces an ear canal representation of the measurement fixture; measuring a first response function between an ambient sound source and a test microphone located within the ear canal representation while parameters of a noise processor of the audio device are set to a predefined transfer function in combination with a first gain factor; measuring a second response function between the ambient sound source and the test microphone while parameters of the noise processor are set to the predefined transfer function in combination with a second gain factor being different from the first gain factor; determining a model response function for the noise processor based on the predefined transfer function, the first response function, the second response function and the first and the second gain factor.

11. The method according to claim 10, further comprising: measuring a third response function between the ambient sound source and the test microphone while parameters of the noise processor are set to the predefined transfer function in combination with a third gain factor being different from the first gain factor and the second gain factor; wherein the model response function is determined based on the predefined transfer function, the first, the second and the third response function, and the first, the second and the third gain factor.

12. The method according to claim 11, wherein each of the response functions measured between the ambient sound source and the test microphone is measured without accessing any test point within the audio device.

13. The method according to claim 11, each of the response functions measured between the ambient sound source and the test microphone is measured without the audio device being disassembled during the respective measurements.

14. The method according to claim 11, wherein the audio device and the noise processor are enabled for feedforward noise cancellation.

15. The method according to claim 11 further comprising determining parameters of a filter function of the noise processor based on the model response function.

16. The method according to claim 1, wherein each of the response functions measured between the ambient sound source and the test microphone is measured without accessing any test point within the audio device.

17. The method according to claim 1, wherein each of the response functions measured between the ambient sound source and the test microphone is measured without the audio device being disassembled during the respective measurements.

18. The method according to claim 1, wherein the audio device and the noise processor are enabled for feedforward noise cancellation.

19. The method according to claim 1, further comprising determining parameters of a filter function of the noise processor based on the model response function.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The improved measurement concept will be described in more detail in the following with the aid of drawings. 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.

[0040] In the drawings:

[0041] FIG. 1 shows an example headphone worn by a user with several sound paths from an ambient sound source;

[0042] FIG. 2 shows an example implementation of a measurement configuration according to the improved measurement concept;

[0043] FIG. 3 shows an example implementation of a method according to the improved measurement concept; and

[0044] FIG. 4 shows an example frequency response of a model response function.

DETAILED DESCRIPTION

[0045] FIG. 1 shows an example configuration of a headphone HP worn by a user with several sound paths from an ambient sound source. The headphone HP shown in FIG. 1 stands as an example for any noise cancellation enabled audio device and can particularly include in-ear headphones or earphones, on-ear headphones or over-ear headphones. Instead of a headphone, the noise cancellation enabled audio device could also be a mobile phone or a similar device.

[0046] The headphone HP in this example features a microphone FF_MIC, which is particularly designed as a feedforward noise cancellation microphone, and a loudspeaker LS. Internal processing details of the headphone HP are not shown here for reasons of a better overview.

[0047] In the configuration shown in FIG. 1, several sound paths exist, of which each can be represented by a respective response function or transfer function. For example, an ambient-to-ear sound path AE represents the sound path from an ambient sound source to a user's eardrum through the user's ear canal. A sound path from the ambient sound source to the microphone FF_MIC can be represented by the response function AM, also called ambient-to-mic response function AM. A response function or transfer function of the headphone HP, in particular between the microphone FF_MIC and the loudspeaker LS, can be represented by a processor function P which may be parameterized as a noise cancellation filter during regular operation. The specification DE represents the acoustic path between the headphone's loudspeaker LS and the eardrum, and may be called a driver-to-ear response function. A further path, G, can be taken into account from the headphone HP to the feedforward microphone FF_MIC which occurs through internal and/or external leakages in the headphone HP. This path G may represent a Driver to Feedforward Microphone FF MIC response and may also be called a leakage response or leakage path.

[0048] Accordingly, during operation, one direct sound path, namely the sound path AE and one combined sound path from the ambient sound source to the eardrum exist. The combined sound path results from the combination of sound path AM, processor path P, which incorporates the frequency responses of all the electrical elements of the noise cancellation electronics, and the driver-to-ear sound path DE. The combined sound paths may be written as AM.P.DE.

[0049] For optimum noise cancellation performance, the processor noise path P may be parameterized to represent more or less the model response function F as defined in equation (1), such that

[00004] $\begin{matrix} P F = - \frac{A .Math. E}{AM . DE} . & (4) \end{matrix}$

[0050] The determination of the model response function F will be explained in more detail in conjunction with an example implementation of a measurement configuration as shown in FIG. 2, and an example flow diagram of a corresponding method as shown in FIG. 3.

[0051] FIG. 2 shows an example implementation of a measurement configuration according to the improved measurement concept including an ambient sound source AS comprising an ambient amplifier ADR and an ambient speaker ASP for playing a test signal TST. The noise cancellation enabled audio device HP comprises the microphone FF_MIC, whose signal is processed by a noise processor PROC and output via the loudspeaker LS. The noise processor PROC features a control interface CI, over which processing parameters of the noise processor PROC can be set, like filter parameters or gain factors a1, a2, a3 for respective proportional transfer functions. The audio device HP is placed onto a measurement fixture MF, which may be an artificial head with an ear canal representation EC, at the end of which a test microphone ECM is located for recording a measurement signal MES via a microphone amplifier MICAMP. It should be noted that at least the measurement fixture MF and the ambient sound source AS are represented with their basic functions, namely playing a test signal TST and recording a measurement signal MES without excluding more sophisticated implementations.

[0052] Referring now to FIG. 3, an example block diagram showing a method flow of a method for determining a response function of a noise cancellation enabled audio device, in particular headphone, is shown. The method may be operated with the example measurement setup shown in FIG. 2.

[0053] As shown in block 310, as a prerequisite the audio device is placed onto the measurement fixture MF, such that a loudspeaker LS of the audio device HP faces the ear canal representation EC of the measurement fixture MF.

[0054] Block 320 includes the measuring of two or more response functions X, Y and, optionally, Z. Each of the response functions is measured between the ambient sound source AS and the test microphone ECM located within the ear canal representation EC that preferably emulates the position of a user's eardrum.

[0055] According to the improved measurement concept, for each of the response functions to be measured parameters of the noise processor PROC are set to a proportional transfer function with a specific gain factor. For example, the first response function X is measured with the first gain factor chosen to a factor a1, the second response function Y is measured with the second gain factor set to a factor a2, and the third, optional, response function Z is measured with the third gain factor set to a factor a3. All gain factors a1, a2 and a3 are chosen differently.

[0056] Measurement of the response functions X, Y and Z for example is performed by playing an appropriate test signal TST from the ambient sound source AS and recording an associated response signal MES with the test microphone ECM. The response functions X, Y and Z can then be determined from the test signal TST and the corresponding response signal MES. For example, the measured response functions X, Y and Z represent a frequency response having phase and amplitude over a given frequency range. Such frequency responses may also be represented with a complex notation with real part and imaginary part, which is well-known in the field of signal processing.

[0057] Referring now to block 330 of FIG. 3, a model response function F is determined based on at least the first and the second response functions X, Y and the associated gain factors a1, a2. In some implementations, also the optional third response function Z and the corresponding third gain factor a3 may be used.

[0058] The model response function F represents the ideal response of the noise processor PROC for an optimum noise cancellation performance based on the measurements performed before.

[0059] Hence, in optional block 340, a filter function for the processor PROC can be determined based on the model response function F. In particular, parameters of a filter function of the processor PROC can be determined, for example with various design tools for adapting the filter parameters to the model response function F as close as possible or technically feasible.

[0060] Finally, the filter parameters determined this way can be used for normal operation of the audio device, e.g. if the audio device or headphone is used by a user.

[0061] Referring to FIG. 4, an example frequency response of a model response function F is shown with its amplitude in the upper diagram and its phase in the lower diagram.

[0062] The filter function e.g. is designed such that the frequency response of the model response function F is matched as close as possible.

[0063] Referring back to FIG. 3, in the following various implementations of the method for determining the model response function will be explained in more detail.

[0064] For example, if the influence of the leakage path G is neglected, a response function M at the test microphone's ECM position basically results in the ambient-to-ear response function AE and a combination of the response function AM, the processor transfer function P and the driver to ear response function DE. This can hence be represented by

M=AE+AM.P.DE,(5)

with AM.P.DE representing the aforementioned combination.

[0065] In some implementations, two different measurements for a first response function X and a second response function Y are performed, wherein parameters of the noise processor PROC are set to a proportional transfer function with the first gain factor a1 for the first response function X and with the second gain factor a2 for the second response function Y. With equation (5), the first response function X can be written as

X=AE+AM.a1.DE(6)

and the second response function Y can be written as

Y=AE+AM.a2.DE,(7)

with a1, respectively a2, representing the processor transfer function P of equation (5).

[0066] Taking equations (6) and (7), the following equation can be derived

YX=AM.(a2a1).DE,(8)

resulting in the following expression for the combined response of ambient-to-microphone AM and driver-to-ear DE:

[00005] $\begin{matrix} AM . D .Math. E = \frac{Y - X}{a .Math. 2 - a .Math. 1} . & (9) \end{matrix}$

[0067] Starting, for example, from equation (6), the ambient-to-ear response function AE can be derived as

[00006] $\begin{matrix} A .Math. E = X - AM . a .Math. .Math. 1. .Math. DE = X - a .Math. .Math. 1 .Math. \frac{Y - X}{a .Math. 2 - a .Math. 1} . & (10) \end{matrix}$

[0068] Inserting the expressions of equations (9) and (10) into equation (1), the model response function F can be written as

[00007] $\begin{matrix} F = a .Math. 1 - \frac{X (a .Math. 2 - a .Math. 1)}{Y - X} . & (11) \end{matrix}$

[0069] In summary, the model response function F is determined when the headphone or other audio device is fully assembled and no access to internal test points or the like is necessary.

[0070] Equation (11) can be simplified, for example by choosing the first gain factor a1 to be zero, such that no signals are transferred from the audio device's microphone FF_MIC to its loudspeaker LS. Besides actually setting filter parameters of the processor transfer function P to achieve the zero gain factor, this can also be achieved by disabling and/or muting the noise processor PROC during measurement of the first response function X. In such a configuration, the model response function F simplifies to

[00008] $\begin{matrix} F = - a .Math. 2 .Math. \frac{X}{Y - X} . & (12) \end{matrix}$

[0071] In some implementations also a third measurement can be performed, i.e. a third response function Z can be measured with a third gain factor a3 for the proportional transfer function of the noise processor PROC. Taking into account equation (5) again, this results in

Z=AE+AM.a3.DE.(13)

[0072] Similar to equation (9) above, the combined response AM.DE can now be determined from equations (7) and (13), resulting in

[00009] $\begin{matrix} AM . DE = \frac{Z - Y}{a .Math. 3 - a .Math. 2} . & (14) \end{matrix}$

[0073] In analogy to equation (10), the ambient-to-ear response function AE can be determined as

[00010] $\begin{matrix} A .Math. E = X - AM . a .Math. .Math. 1. .Math. DE = X - a .Math. .Math. 1 .Math. \frac{Z - Y}{a .Math. 3 - a .Math. 2} . & (15) \end{matrix}$

[0074] Using equation (1), the model response function F for example results in

[00011] $\begin{matrix} F = a .Math. 1 - \frac{X (a .Math. 3 - a .Math. 2)}{Z - Y}, & (16) \end{matrix}$

wherein it would be apparent to the skilled reader that other combinations of the three measured response functions X, Y, Z were possible.

[0075] If the first gain factor a1 is chosen to be zero, as described above, equation (16) simplifies to

[00012] $\begin{matrix} F = - \frac{X (a .Math. 3 - a .Math. 2)}{Z - Y} . & (17) \end{matrix}$

[0076] Moreover, if for example the second and the third gain factor a2, a3 are chosen to a2=+1 and a3=1, equation (17) further simplifies to

[00013] $\begin{matrix} F = \frac{2 .Math. X}{Z - Y} . & (18) \end{matrix}$

[0077] While in the previous example implementations the leakage response G has been neglected, it can be considered in implementations as described in the following. For example, performing the measurement of the three response functions X, Y, Z as described above, these can be represented as

X=(AE+AM.a1.DE)/(1G.a1.DE),(19)

Y=(AE+AM.a2.DE)/(1G.a2.DE)(20)

and

Z=(AE+AM.a3.DE)/(1G.a3.DE).(21)

[0078] With the three measurements, it is possible to determine the three unknowns AE, AM.DE and G.DE for finally finding a representation of the model response function F according to equation (1).

[0079] Taking an example implementation for such a configuration with the three gain factors a1, a2 and a3 chosen to be a1=0, a2=+1 and a3=1, equations (19) , (20) and (21) simplify to

X=AE,(22)

Y=(AE+AM.DE)/(1G.DE)(23)

and

Z=(AEAM.DE)/(1+G.DE).(24)

[0080] With these simplifications, the combined leakage response G.DE, abbreviated as L, can be expressed as

[00014] $\begin{matrix} L = G . DE = \frac{2 * X - Y - Z}{Z - Y} . & (25) \end{matrix}$

[0081] The combined response function AM.DE can then be expressed as

AM.DE=.Math.(Y.Math.(1L)Z.Math.(1+L)).(26)

[0082] Finally, using equations (22), (26) and (25), equation (1) can be rewritten as

F=2.Math.X/(Y.Math.(1L)Z.Math.(1+L)).(27)

[0083] In alternative implementations, it is also possible to use an approach where the noise processor PROC implements different but known and predefined filter transfer functions P for each measurement instead of only using the proportional transfer functions with respective gain factors a1, a2 and, optionally a3. After making measurements for the first, second and, optionally, third response functions X, Y and Z, one can compensate for the known response functions implemented by the noise processor PROC.

[0084] For example, different but known filters for the two or three measurements can be implemented, which can improve the signal-to-noise ratio of the measurements. One would have to correct for these known filter shapes after calculating the individual first, second and, optionally, third response functions X, Y and Z. Preferably, the predefined filter transfer functions only differ by an overall gain factor applied.

[0085] Accordingly, in such implementations, the filter transfer function P of the noise processor PROC may be set to a predefined transfer function R in combination with the respective gain factors a1, a2 and, optionally a3, such that two or three known filter functions result. This is similarly accomplished using the control interface CI. Based on equation (5), this results in equations similar to equations (6), (7) and (13), namely:

X=AE+AM.R.a1.DE,(28)

Y=AE+AM.R.a2.DE,(29)

and, optionally

Z=AE+AM.R.a3.DE.(30)

[0086] The model response function F for the noise processor PROC is determined based on the predefined transfer function R, the response functions X, Y, and optionally Z, and on the gain factors a1, a2 and, optionally, a3.

[0087] For example, the result of all the calculations yield an answer F/R instead of the desired answer F, which can be compensated for due to knowledge of the predefined transfer function R. Detailed implementation of the necessary equations can be readily derived by the skilled person from the description above for the implementation using gain factors a1, a2 and, optionally a3 only.

[0088] As mentioned before, the model response function F as determined with each of the example implementations described above, can be used as a model to design appropriate filter parameters for the transfer function P of the noise processor PROC. For example, respective filter parameters can be determined offline, having knowledge of the model response function F, and afterwards be transferred to the audio device or headphone HP via the control interface CI.

[0089] For example, a main beneficiary of the improved measurement concept is the acoustical engineer who designs the ANC headphone. The improved measurement concept allows the engineer to make more accurate measurements of a reference headphone design and in a more convenient way. It has a secondary application area on a headphone production line where it would allow measurements to be made that could be used to select the optimum ANC filter for each unit as it is produced.

REFERENCE NUMERALS

[0090] HP audio device

[0091] FF_MIC microphone

[0092] LS loudspeaker

[0093] AM ambient-to-microphone response function

[0094] AE ambient-to-ear response function

[0095] DE driver-to-ear response function

[0096] G leakage response function

[0097] P processor transfer function

[0098] F model response function

[0099] PROC noise processor

[0100] CI control interface

[0101] ASS ambient sound source

[0102] ADR ambient driver

[0103] ASP ambient speaker

[0104] EC ear canal representation

[0105] ECM test microphone

[0106] MICAMP microphone amplifier

[0107] TST test signal

[0108] MES measurement signal

[0109] MF measurement fixture

METHOD FOR DETERMINING A RESPONSE FUNCTION OF A NOISE CANCELLATION ENABLED AUDIO DEVICE

Inventors

Cpc classification

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H04R2460/01

ELECTRICITY

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H04R5/04

ELECTRICITY

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G10K11/17813

PHYSICS

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G10K2210/3055

PHYSICS

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H04R5/033

ELECTRICITY

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G10K2210/3057

PHYSICS

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H04R3/04

ELECTRICITY

Classification Explorer

G10K11/17873

PHYSICS

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G10K2210/30232

PHYSICS

International classification

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H04R3/04

ELECTRICITY

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H04R5/033

ELECTRICITY

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H04R5/04

ELECTRICITY

Abstract

Claims

Description