Surround Sound Recording for Mobile Devices

Abstract

A microphone arrangement and a method using the microphone arrangement for recording surround sound in a mobile device, where the microphone arrangement comprises a first and a second microphone and arranged at a first distance to each other and configured to obtain a stereo signal, and comprises a third microphone configured to obtain a steering signal together with at least one of the first and second microphone or with a fourth microphone. The microphone arrangement also comprises a processor configured to separate the stereo signal into a front stereo signal and a back stereo signal based on the steering signal.

Claims

1. A microphone arrangement for recording surround sound in a mobile device, comprising: a first microphone arranged to obtain a first audio signal of a stereo signal; a second microphone arranged to obtain a second audio signal of the stereo signal; a third microphone configured to obtain a third audio signal; and a processor coupled to the first microphone, the second microphone, and the third microphone and configured to: obtain a steering signal based on the third audio signal and another audio signal obtained by another microphone of the microphone arrangement; and separate the stereo signal into a front stereo signal and a back stereo signal based on the steering signal.

2. The microphone arrangement according to claim 1, wherein the microphone arrangement comprises a fourth microphone arranged to obtain a fourth audio signal, and wherein the processor is further configured to obtain the steering signal based on the third audio signal and at least one of the first audio signal, the second audio signal, and the fourth audio signal.

3. The microphone arrangement according to claim 1, wherein the steering signal comprises direction-of-arrival (DOA) information, and wherein the processor is further configured to combine the DOA information with at least a part of the stereo signal to obtain the front and back stereo signals.

4. The microphone arrangement according to claim 3, wherein the processor is further configured to: determine a direct-sound component and a diffuse-sound component of the stereo signal, and combine the DOA information only with the direct-sound component of the stereo signal to obtain the front stereo signal and the back stereo signal.

5. The microphone arrangement according to claim 3, wherein the processor is further configured to determine the DOA information based on a first inter-channel-level-difference (ICLD) between the third audio signal and the another audio signal, wherein the first ICLD bases on a difference between time or frequency representations, in particular power spectra of the third audio signal and the another audio signal.

6. The microphone arrangement according to claim 5, wherein the third microphone and the another microphone are omnidirectional sound pressure microphones, and wherein the processor is further configured to: process the third audio signal and the another audio signal such that two virtual sound pressure gradient microphones directed to opposite directions are formed; and obtain the first ICLD on the basis of the output signals of the two virtual sound pressure gradient microphones.

7. The microphone arrangement according to claim 3, wherein the processor is further configured to determine the DOA information additionally based on a second ICLD between the third audio signal and the another audio signal, wherein the second ICLD bases on a difference between time or frequency representations, in particular power spectra, between the third audio signal and the another audio signal, and wherein the difference being caused by a shadowing effect of a housing of the microphone arrangement disposed at least partly between the third microphone and the another microphone.

8. The microphone arrangement according to claim 7, wherein the processor is further configured to: set the first ICLD to determine the DOA information for frequencies of the stereo signal at or below a determined frequency threshold value; and set the second ICLD to determine the DOA information for frequencies of the stereo signal above the determined frequency threshold value.

9. The microphone arrangement according to the claim 8, wherein the determined threshold value depends on a second distance between the third microphone and one of the first, second, and the fourth microphone.

10. The microphone arrangement according to claim 5, wherein the processor is further configured to bias the first or the second ICLD towards the third microphone or the another microphone.

11. The microphone arrangement according to claim 3, wherein the processor is further configured to bias the DOA information towards one of the third microphone or the another microphone.

12. The microphone arrangement according to claim 1, wherein the third microphone and the another microphone are directional microphones and are directed to opposite directions, or wherein the first microphone and the second microphone are directional microphones and are directed towards the opposite direction.

13. The microphone arrangement according to claim 1, wherein the processor is further configured to determine a center signal from the stereo signal.

14. The microphone arrangement according to claim 1, wherein a fourth microphone of the microphone arrangement is configured to obtain a center signal.

15. A method of surround sound recording in a mobile device, comprising: obtaining a first audio signal of a stereo signal with a first microphone; obtaining a second audio signal of the stereo signal with a second microphone; obtaining a third audio signal with a third microphone; obtaining a steering signal based on either the third audio signal and the first audio signal or the second audio signal or based on a fourth audio signal obtained by a fourth microphone; and separating the stereo signal into a front stereo signal and a back stereo signal based on the steering signal.

16. A mobile device for recoding surround sound, comprising: a non-transitory memory comprising instructions; and one or more processors in communication with the memory, wherein the one or more processors execute the instructions to perform, a method comprising the following operations: obtaining a first audio signal of a stereo signal with a first microphone; obtaining a second audio signal of the stereo signal with a second microphone; obtaining a third audio signal with a third microphone; obtaining a fourth audio signal with a fourth microphone; obtaining a steering signal based on the third audio signal and one of the first audio signal, the second audio signal, or the fourth audio signal; and separating the stereo signal into a front stereo signal and a back stereo signal based on the steering signal.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0062] The above-described aspects and implementation forms of the present disclosure will be explained in the following description of specific embodiments in relation to the enclosed drawings.

[0063] FIG. 1 shows an example of a microphone arrangement according to an embodiment of the present disclosure with four microphones mounted on a mobile device;

[0064] FIG. 2 shows a top view of the mobile device of FIG. 1, wherein two microphones for obtaining the steering signal are placed to benefit from a shadowing of the housing of the mobile device, and two microphones for recording the stereo signal are placed close to the sides of the mobile device;

[0065] FIG. 3 shows an illustration of a delay-and-subtract operation applied to two omnidirectional microphone signals, in order to yield a first-order directive signal;

[0066] FIG. 4 shows a tangent function for post-processing of the first ICLD based on the two omnidirectional microphone input signals;

[0067] FIG. 5 shows a post-processing function for DOA estimation from the first and second ICLD;

[0068] FIG. 6 shows a top view of the mobile device of FIG. 1, wherein the microphones for obtaining the stereo signal are remotely placed to capture an enlarged stereo image;

[0069] FIG. 7 shows a frequency dependence of a normalized cross-correlation;

[0070] FIG. 8 shows a block diagram of a multichannel signal generation unit based on a front-back separation obtained from the steering signal, and based on direct-sound and diffuse-sound components extracted from the stereo signal; and

[0071] FIG. 9 shows a flowchart diagram of method steps of a method according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0072] Generally, the microphone arrangement of the present disclosure requires at least two pairs of microphone, namely one pair (the LR pair) to record left/right stereo information (the stereo signal), and one pair (the FB pair) to record a signal for obtaining a front/back separation parameter (the steering signal). The two pairs of microphones may be composed of at least three microphones. In the case of three microphones, a first and a second microphone form the LR pair, and a third microphone forms together with the first and/or the second microphone the FB pair. Preferably, at least four microphones are used, wherein a first microphone and a second microphone form the LR pair, and a third microphone and a fourth microphone form the FB pair.

[0073] The two microphones used as the FB pair are preferably placed such that one points towards the front and one points towards the back of a mobile device, in order to benefit from a shadowing effect caused by the housing of the mobile device for a better front/back discrimination. The FB pair microphones can be of low grade, since they are only relevant for information extraction for the steering signal, and not directly generate audio signals for the sound recording. The two microphones used as the LR pair are preferably placed on the sides (left and right) of the mobile device, and preferably point towards the same direction (to avoid shadowing effects), e.g. to the back of the mobile device, however they could also point to the front. For mobile devices having large enough form factors, the LR pair microphones are thus already ideally suited to capture a relevant stereo image. The LR pair microphones are preferably of higher grade, since they are relevant for generating high-quality audio signals for the sound recording.

[0074] FIG. 1 shows a microphone arrangement 100 in a device according to an embodiment of the present disclosure, or a device, here a tablet or smartphone, comprising the microphone arrangement. The embodiment is a specific embodiment of the above described general microphone arrangement. The microphone arrangement 100 includes four microphones 101-104 (designated as m1-m4 in FIG. 2) and a processor 105, e.g. a processor 105. The microphones 101-104, m1-m4 can be mounted onto a mobile device 200 as illustrated in FIG. 1. The mobile device 200 can be a tablet, smart phone, mobile phone, laptop, camera, computer, or any other portable device with the capability to record sound. A first microphone 102 m2 and a second microphone 103 m3 are configured to obtain a stereo signal. In FIG. 1 these microphones 102 m2 and 103 m3, which form the LR pair, are placed, as is preferred, at the sides of the mobile device 200, and are separated by a first distance d.sub.1 for capturing a relevant stereo image. A third microphone 101 m1 and a fourth microphone 104 m4 are configured to obtain a steering signal. In FIG. 1 these two microphones 101 m1 and 104 m4, which form the FB pair, are placed, as is preferred, in the center of the mobile device 200. Thereby, one microphone points towards the front of the mobile device 200, and the other microphone points towards the back of the mobile device 200, in order to enable a front/back discrimination based on the steering signal (DOA, 1-DOA).

[0075] As noted above, the fourth microphone 104 may be omitted, and instead the third microphone 101 may be configured to obtain the steering signal (DOA, 1-DOA) together with at least one of the first microphone 102 and the second microphone 103. In other words, the two necessary pairs of microphones (LB pair and FB pair) may be formed from just the three microphones 101-103, whereby at least one microphone of the LB pair microphones 102 and 103 is also used as microphone for the FB pair.

[0076] The microphone arrangement 100 further includes a processor 105, which is configured to separate the stereo signal obtained by the LR pair microphones 102 and 103 into a front stereo signal (FL, FR) and a back stereo signal based on the steering signal (DOA, 1-DOA) obtained by the FB pair microphones 101 and 104. In FIG. 1 the processor 105 is provided as a separate unit. In this case, the processor 105 is preferably integrated into the housing of the mobile device 200. The processor 105 could even be a processor of the mobile device 200. However, the processor 105 can also be part of one or more of the microphones 101-104. That is, for instance, the processor 105 may be configured to separate the stereo signal of the first and second microphones 102 and 103 into the front and back stereo signals, based on the audio signal obtained by the third microphone 101. Alternatively, the first and second microphones 102 and 103 may be provided, from at least the third microphone 101, with the steering signal (DOA, 1-DOA), and may use the steering signal (DOA, 1-DOA) together with the captured stereo signal, in order to output the front stereo signal (FL, FR) and back stereo signal (BL, BR), respectively.

[0077] At least the microphones configured to obtain the steering signal (DOA, 1-DOA), i.e. in FIG. 1 the third and fourth microphones 101 and 104, may be, in particular omnidirectional, sound pressure microphones, which are configured to measure a sound field's sound pressure at one point. In this case, when the wave length of the sound is large compared to a body size of the microphones, e.g. double the body size or larger, the measured sound pressure does not depend on a DOA information of the sound. That means a sound pressure microphone has an omnidirectional characteristic.

[0078] Advantageously, the microphones 101 and 104 are even two virtual sound pressure gradient microphones, which are directed to opposite directions. Such pressure gradient microphones aim at measuring the sound pressure gradient relative to a certain direction. In practice, the sound pressure gradient may be approximated by measuring the difference in sound pressure between two points (using two closely spaced omnidirectional microphones, like the microphones 101 and 104). Additionally, a delay may be applied to one obtained microphone signal, which is subtracted from the other obtained microphone signal, which relates to the directional response of an obtained difference signal. That is, the processor 105 is preferably configured to apply a delay-and-subtract processing resulting in two virtual sound pressure gradient microphones 101 and 104, which are directed to opposite directions.

[0079] The measurement of a sound pressure difference with a delay between two points (represented by the third and the fourth microphone 101 and 104) spaced apart by a second distance d.sub.2 is illustrated in FIG. 2. Given the arrangement of the omnidirectional microphones 101 and 104, as illustrated in FIG. 2, two virtual cardioid signals, x.sub.f(t) and x.sub.b(t) in time domain, X.sub.f(k,i) and X.sub.b(k,i), in a suitable time-frequency domain such as the short-time Fourier transform (STFT) domain, wherein t is the time index, k is the spectrum time index and i is the frequency index, can be derived based on gradient processing (as described, for instance, by C. Faller, “Conversion of two closely spaced omnidirectional microphone signals to an xy stereo signal”, Preprint 129th Cony. Aud. Eng. Soc., November 2010).

[0080] One way of converting the sound pressure signals of the two preferably omnidirectional microphones 101 and 104 into pressure gradient signals is to apply a delay-and-subtract processing in order to obtain a directional signal towards the front and back of the microphone arrangement 100, i.e. a positive and negative x-direction, respectively, as shown in FIG. 3.

[0081] Front and back pointing pressure gradient signals, x.sub.f(t) and x.sub.b (t), are computed as:

x.sub.f(t)=h(t)*(m.sub.1(t)−m.sub.4(t−τ))

x.sub.b(t)=h(t)*(m.sub.4(t)−m.sub.1(t−τ))

where, m.sub.1(t) and m.sub.4(t) denote the time-domain signals of the microphones 101 and 104, respectively, * denotes an optional linear convolution with h(t) being an impulse response of a free-field response correction filter. The delay r relates to the directional response of the virtual cardioid microphones and depends on the distance between the two microphones and the desired directivity:

[00001]$\tau =\frac{\mathrm{ud}}{c\left(1-u\right)},$

where, d represents the distance between the microphones, and c the celerity of sound. In a preferred embodiment, this distance is very small and compatible with mobile device applications. It is then in the range 2 to 10 millimeters (mm).

[0082] The parameter u controls the directivity and can be defined as:

[00002]$u=\frac{\cos \left(\frac{\pi }{2}+\phi \right)}{\cos \left(\frac{\pi }{2}+\phi \right)-1},$

wherein φ can be a value between 0 and π/2.

[0083] Further, x.sub.f(t) and x.sub.b(t) are converted to a time/frequency representation X.sub.f(k,i) and X.sub.b(k,i), e.g., using STFT.

[0084] The front and back power spectra are respectively estimated as:

P.sub.f(k,i)=E{X.sub.f(k,i)X.sub.f(k,i)*}

P.sub.b(k,i)=E{X.sub.b(k,i)X.sub.b(k,i)*}.  (1)

[0085] In the above formula (1), E{ . . . } denotes short-time averaging (temporal smoothing), and * the conjugate complex.

[0086] In order to estimate the DOA information of the sound, the level difference between the front and back signals captured by the microphones 101 and 104, i.e. the two parts of the obtained steering signal (DOA, 1-DOA), can be used. This level difference is also denoted as a first inter-channel level difference (ICLD). In particular, the processor 105 is configured to determine the DOA information based on the first ICLD of the microphones 101 and 104, which are configured to obtain the steering signal (DOA, 1-DOA).

[00003]$\begin{array}{cc}{\mathrm{ICLD}}_1\left(k,i\right)=20.Math..Math.\log .Math..Math.10.Math.\left(\frac{P_f\left(k,i\right)}{P_b\left(k,i\right)}\right).& \left(2\right)\end{array}$

[0087] This first ICLD measure in formula (2) is in particular limited and translated to the interval [−1, 1] for post-processing and for DOA information estimation:

[00004]$\begin{array}{cc}{\mathrm{icld}}_1\left(k,i\right)=\frac{\max .Math.\left\{g_{{\mathrm{ICLD}}_1},\min .Math.\left\{{\mathrm{ICLD}}_1\left(k,i\right),g_{{\mathrm{ICLD}}_1}\right\}\right\}}{g_{{\mathrm{ICLD}}_1}},& \left(3\right)\end{array}$

[0088] In the formula (3), g.sub.ICLD (in decibel (dB)) is a limiting gain.

[0089] The first ICLD bases generally on a difference between time/frequency representations, in particular power spectra, of the input signals obtained by the microphones 101 and 104. The processor 105 is preferably configured to determine the DOA information of the sound based on this first ICLD of the microphones 101 and 104, which are configured to obtain the steering signal (DOA, 1-DOA).

[0090] Because of the spacing distance d.sub.2 between the two microphones 101 and 104, frequency aliasing will occur in the estimated pressure gradient signals for frequencies above the threshold value:

[00005]$\begin{array}{cc}{f}_1=\frac{c}{4.Math.d},& \left(4\right)\end{array}$

[0091] In formula (4), c stands for celerity of sound and d (=d.sub.2) is the distance between the microphones 101 and 104. This distance d.sub.2 is typically related to the thickness of the mobile device 200, as shown in FIG. 2, which can be, for example 1 cm or even only 0.5 centimetres (cm). In this frequency region (usually corresponding to high frequencies above 10 kHz) the determination of the front/back separation, i.e. the DOA information, in the steering signal (DOA, 1-DOA) can take advantage of a shadowing effect caused by the housing of the mobile device 200, the housing being arranged between the two microphones 101 and 104. The shadowing effect leads to a gain difference between the omnidirectional input signals of the two microphones 101 and 104, M.sub.1(k,i) and M.sub.4(k,i), and a second ICLD may be derived:

[00006]$\begin{array}{cc}{\mathrm{ICLD}}_2\left(k,i\right)=20.Math..Math.\log .Math..Math.10.Math.\left(\frac{M_1\left(k,i\right)}{M_4\left(k,i\right)}\right).& \left(5\right)\end{array}$

[0092] Again the ICLD measure (5) is translated to the interval [−1, 1] for post-processing and DOA information estimation:

[00007]$\begin{array}{cc}{\mathrm{icld}}_2\left(k,i\right)=\frac{\max .Math.\left\{g_{{\mathrm{ICLD}}_2},\min .Math.\left\{{\mathrm{ICLD}}_2\left(k,i\right),g_{{\mathrm{ICLD}}_2}\right\}\right\}}{g_{{\mathrm{ICLD}}_2}},& \left(6\right)\end{array}$

[0093] In the above formula (6), gICLD (in dB) is again a limiting gain. Additionally since the two omnidirectional power spectra M.sub.1 and M.sub.4 are potentially not matched and/or not calibrated to catch front/back gain difference in the steering signal (DOA, 1-DOA), the ICLD measurement of formula (5) may be biased towards one direction (front or back of the microphone arrangement 100). Thus, slight gain differences are not relevant, and in order to minimize the influence of small gain differences icld.sub.2 may be post-processed using the following

[00008]$\begin{array}{cc}{\mathrm{icld}}_2\left(k,i\right)=\frac{\tan \left(t_{{\mathrm{icld}}_2}.Math.{\mathrm{icld}}_2\left(k,i\right)\right)}{\tan \left(t_{{\mathrm{icld}}_2}\right)},& \left(7\right)\end{array}$

[0094] Therein, ticld is a parameter controlling the influence of small gain differences as shown in FIG. 4. A parameter ticld=π/2 will lead to a configuration, in which only large measured gain difference values between the microphones 101 and 104 will yield a non-zero icld.sub.2(k, i), whereas a smaller parameter ticld<π/2 will tend to a more linear function.

[0095] The second ICLD bases generally on a gain difference between respective input signals of said microphones 101 and 104, the gain difference being caused by the shadowing effect of the housing of the microphone arrangement 100 (or the mobile device 200) disposed at least partly between said microphones 101 and 104. The processor 105 is preferably configured to determine the DOA information of the sound based on this second ICLD of the microphones 101 and 104 configured to obtain the steering signal (DOA, 1-DOA).

[0096] A total ICLD over the full frequency range can then be derived as:

[00009]$\begin{array}{cc}\mathrm{icld}\left(k,i\right)=\{\begin{array}{cc}{\mathrm{icld}}_1\left(k,i\right)& i\le i_1\\ {\mathrm{icld}}_2\left(k,i\right)& \mathrm{otherwise}\end{array},& \left(8\right)\end{array}$

[0097] In the formula (8), i.sub.1 is the frequency index corresponding to the aliasing frequency fl as defined in the formula (4). The front-back separation represented by the DOA information may be derived by transforming the total ICLD in formula (8) into a value in the interval [0, 1] as:

[00010]$\begin{array}{cc}\mathrm{doa}\left(k,i\right)=\frac{1}{2}+\frac{1}{2}.Math.\frac{\arctan \left(t_{\mathrm{doa}}.Math.\mathrm{icld}\left(k,i\right)\right)}{\arctan \left(t_{\mathrm{doa}}\right)}& \left(9\right)\end{array}$

[0098] In the specific time-frequency tile (k,i), a DOA information doa(k,i)=1 corresponds to sound coming from the front direction of the microphone arrangement 100, and a DOA information doa(k,i)=0 corresponds to sound coming from the back direction of the microphone arrangement 100. Intermediate values lead to DOA information representing sound coming from certain angles to the microphone arrangement 100, which can be derived as (1−doa(k,i))π. Thereby, tdoa denotes a parameter controlling the front-back separation strength shown in FIG. 5. The larger the parameter tdoa is, the more the front-back separation will be emphasized in the steering signal (DOA, 1-DOA).

[0099] Generally, the processor 105 is preferably configured to use the first ICLD to determine the DOA information for frequencies of the steering signal (DOA, 1-DOA) at or below a determined threshold value, and to use the second ICLD to determine the DOA information for frequencies of the steering signal (DOA, 1-DOA) above the determined threshold value.

[0100] While the microphones 101 and 104 are dedicated to obtain the steering signal (DOA, 1-DOA) (i.e. are the FB pair for determining front-back separation), the two other microphones 102 and 103, as illustrated in FIG. 6, directly yield a stereo image as the stereo signal. As the distance d.sub.1 between these two microphones 102 and 103 is typically large when placed at opposite sides of a mobile device 200 (usually above 100 mm), the omnidirectional to stereo processing (as proposed in C. Faller, “Conversion of two closely spaced omnidirectional microphone signals to an xy stereo signal”, Preprint 129th Cony. Aud. Eng. Soc., November 2010) does not apply without too strong limitations, mainly aliasing starting already at a very low frequency. However, the rather large distance d.sub.1 and the opposite placement of the microphones are suited to directly yield an enlarged stereo image as the stereo signal.

[0101] Based on this naturally captured stereo signal, the surround multichannel generation is helped by direct-sound and diffuse-sound component extraction in both the left and right channels, i.e. the channels captured by the microphones 102 and 103, respectively. Analogously to the diffuse-sound extraction used for the virtual cardioids (described by C. Tournery et al., “Converting stereo microphone signals directly to mpeg-surround”, Preprint 128th Cony. Aud. Eng. Soc., 5 2010), here the diffuse-sound component is estimated based on the two omnidirectional power spectra M2(k,i) and M3(k,i). Rather than considering a constant normalized cross-correlation θdiff over all frequencies, a Gaussian model is preferably derived approximating the curves (as proposed in R. K. Cook et al., “Measurement of correlation coefficients in reverberant sound fields”, Journal of the Acoustical Society of America, 27(6):1072-1077, 1955) as shown in FIG. 7:

[00011]$\begin{array}{cc}{\theta }_{\mathrm{diff}}\left(i\right)=\exp \left(-\frac{i^2}{2.Math.i_c^2}\right),& \left(10\right)\end{array}$

[0102] In formula (10) i.sub.c is the index of the Gaussian frequency model. The resulting diffuse power spectrum is P.sub.diff, and two Wiener gain filters to retrieve the direct left and right sounds are, respectively:

[00012]$\begin{array}{cc}{W}_2\left(k,i\right)=\sqrt{\frac{M_2\left(k,i\right)-P_{\mathrm{diff}.Math.}\left(k,i\right)}{M_2\left(k,i\right)}}.Math..Math.W_3\left(k,i\right)=\sqrt{\frac{M_3\left(k,i\right)-P_{\mathrm{diff}.Math.}\left(k,i\right)}{M_3\left(k,i\right)}},& \left(11\right)\end{array}$

[0103] Analogously, the diffuse-sound components in both left and right channels are retrieved from the filters as:

[00013]$\begin{array}{cc}{V}_2\left(k,i\right)=\sqrt{\frac{P_{\mathrm{diff}.Math.}\left(k,i\right)}{M_2\left(k,i\right)}}.Math..Math.V_3\left(k,i\right)=\sqrt{\frac{P_{\mathrm{diff}.Math.}\left(k,i\right)}{M_3\left(k,i\right)}}& \left(12\right)\end{array}$

[0104] The gains in the formulas (11) and (12) are preferably limited using a maximum allowed attenuation gdiff. Eventually, four output signals are derived serving as basis for the generation of the surround multichannel signals. First of all the direct-sound component from the left:

X.sub.l,dir(k,i)=W.sub.2(k,i)M.sub.2(k,i).  (13)

[0105] Then the direct-sound component from the right:

X.sub.r,dir(k,i)=W.sub.3(k,i)M.sub.3(k,i).  (14)

[0106] And the diffuse-sound components from the left and right, respectively:

X.sub.l,diff(k,i)=V.sub.2(k,i)M.sub.2(k,i)  (15)

X.sub.r,diff(k,i)==V.sub.3(k,i)M.sub.3(k,i),  (16)

[0107] These four generated signals (13-16) are combined with the help of the DOA information of the formula (9) into multichannel output signals. As a first step the target generated output format is a 5.1 standard surround signal including successively front left (FL), front right (FR), center (C), low frequency effects (LFE), rear left (RL), and rear right (RR).

[0108] Thereby, FL is composed of the direct sound of the left channel coming from the front direction and the left diffuse sound, FR is composed of the direct sound of the right channel coming from the front direction and the right diffuse sound, RL is composed of the direct sound of the left channel coming from the back direction and the left diffuse sound low-pass filtered, and RR is composed of the direct sound of the right channel coming from the back direction and the right diffuse sound low-pass filtered.

[0109] Optionally, the diffuse signals can be low-pass-filtered before adding them to the surround channels BL and BR. Low-pass-filtering these signals has the beneficial effect of simulating a room response, thus creating the perception of reflections from a virtual listening room.

[0110] The generation of these four output channels by the processor 105 is summarized in the block diagram in FIG. 8. Given an optional low-pass filter with a frequency response GLP(k,i), and a possible time delay d.sub.R, the four pre-defined output channels are obtained by:

X.sub.FL(k,i)=doa(k,i)X.sub.l,dir(k,i)+X.sub.l,diff(k,i)  (17)

X.sub.FR(k,i)=doa(k,i)X.sub.r,dir(k,i)+X.sub.r,diff(k,i)  (18)

X.sub.BL(k,i)=(1−doa(k,i))X.sub.r,dir(k,i)+G.sub.LP(k,i)X.sub.r,diff(k−d.sub.R,i)  (19)

X.sub.BR(k,i)=(1−doa(k,i))X.sub.r,dir(k,i)+G.sub.LP(k,i)X.sub.r,diff(k−d.sub.R,i)  (20)

[0111] Optionally, a center channel is obtained either from left/right channel mixing of the stereo signal obtained by the microphones 102 and 103, or by directly using the fourth microphone 104 (in this case this microphone should be high-grade as the microphones 102 and 103).

[0112] In FIG. 9 a method 900 of surround sound recording in a mobile device is shown. In a first step 901 of the method 900, a stereo signal is obtained with the first microphone and the second microphone. The microphones are distanced from each other by the first distance dr. In a second step 902 a steering signal is obtained with the third microphone, either together with the fourth microphone, or together with one or both of the first and second microphones. In a third step 903 of the method 900, the stereo signal is separated into a front stereo signal and a back stereo signal based on the steering signal. The separation is preferably performed by the processor, but can also be performed by one of the microphones or by the mobile device.

[0113] In summary, the present disclosure provides a microphone arrangement and method to record surround sound using mobile devices by employing cheap omnidirectional microphones. The present disclosure is fully stereo (left/right) backward compatible. The left/right separation in the stereo signal obtained by the LR pair microphones is wide enough, even when using omnidirectional microphones thanks to the typical sizes of mobile devices. The back (optionally front) microphones of the FB pair are only used for extraction of the DOA information of the sound, and thus can be chosen to be of lower-grade, and do not need to be calibrated. The present disclosure avoids front-back confusion (i.e. a lack of front/back information), which exists in the conventional recording of stereo signals.

[0114] The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed disclosure, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.