Patent Publication Number: US-9426561-B2

Title: Microphone arrangement with improved directional characteristic

Description:
INTRODUCTION 
     The invention relates to a microphone arrangement comprising at least two microphones and a signal processing arrangement for deriving a virtual microphone signal from the microphone signals of the at least two microphones. The invention also relates to this signal processing arrangement. A microphone arrangement as defined in the preamble of claim  1 , is known from the published US patent application US2004/0076301. The known microphone arrangement is intended to realise a binaural recording in such a way that a 3D audio playback for a listener is possible. 
     DESCRIPTION OF THE INVENTION 
     The present invention, however, is intended to propose a microphone arrangement, the directional characteristic of which can be modified as desired. One target could be, for example, to keep the directional characteristic constant over an increased frequency range. 
     To this end, the microphone arrangement of the invention is characterised by the features of claim  1 . The signal processing arrangement of the invention is characterised as specified in claim  18 . 
     The invention is motivated by existing arrangements composed of several microphones, the signals of which are combined (microphone arrays). They are normally intended to increase the directivity relative to one microphone. Directivity means that the sound recorded from a desired direction (main direction) is amplified, whilst the sound recorded from other directions is attenuated. There may be several desired directions if necessary. The directivity of such arrangements is based on the running time of the sound, which causes the direction-dependent phase differences between individual microphone signals. The combination of these signals is normally effected by summation (possibly weighted). But because the phase differences are also frequency-dependent, directivity in consequence becomes frequency-dependent which is a disadvantage, because this results in conventional microphone arrays ending up with only a narrow frequency range in which their directional characteristic is optimal. Outside this frequency range, directivity is worse, which is measurable as a reduced directivity index and which is reflected by the fact that outside the main direction the frequency response is not the same as in the main direction, in particular is not flat. 
     The invention introduces a technique by which initially virtual microphone signals are generated from the microphone signals and then the virtual microphone signals are mixed. The virtual microphone signals correspond to such signals as if they were coming from imaginary microphones if these were positioned outside the actual microphone positions. The virtual positions are interpolated or extrapolated from the actual microphone positions. In this way an effect is achieved as if the microphone array were becoming smaller (when interpolated) or becoming larger (when extrapolated). The interpolation or extrapolation of positions corresponds to an interpolation or extrapolation of microphone signals and is thus controllable. When generating virtual microphone signals, the interpolation or extrapolation is controlled, according to the invention, as a function of the frequency in order to make the virtual positions frequency-dependent. As a result the frequency dependency of the directivity of the microphone array can also be modified as desired, and the directional characteristic can be optimised across an increased frequency range, for example in such a way that it remains mostly constant. 
    
    
     
       SHORT DESCRIPTION OF THE FIGURES 
       The invention will now be described with the reference to the drawing by way of some exemplary embodiments, in which 
         FIG. 1  shows a first embodiment of a microphone arrangement according to the invention, 
         FIGS. 2 a , 2 b  and 2 c    show three curves indicating the behaviour of the multiplication factor g[f] as a function of the frequency f, in the microphone arrangement of  FIG. 1 , 
         FIGS. 3 a  and 3 b    show some directional characteristics of a known microphone arrangement of  FIG. 1 , 
         FIG. 4  shows a second embodiment of a microphone arrangement according to the invention, 
         FIGS. 5 a , 5 b  and 5 c    show three curves indicating the behaviour of the multiplication factor g[f] as a function of the frequency f, in the microphone arrangement of  FIG. 4 , 
         FIGS. 6 a  and 6 b    show some directional characteristics of a known microphone arrangement and a microphone arrangement of  FIG. 4 , 
         FIG. 7  shows a third embodiment of a microphone arrangement according to the invention, 
         FIG. 8  shows the position of the microphones of the microphone arrangement according to  FIG. 7 , 
         FIG. 9  shows a fourth embodiment of a microphone arrangement according to the invention, and 
         FIG. 10  shows the position of the microphones of the microphone arrangement according to  FIG. 9 . 
     
    
    
     DESCRIPTION OF THE FIGURES 
       FIG. 1  shows a first embodiment of the microphone arrangement according to the invention. The microphone arrangement is provided with two microphones  100 ,  102  and a signal processing arrangement  105  for deriving a virtual microphone signal from the microphone signals of the two microphones  100  and  102 . The signal processing arrangement  105  is provided with a first and a second input  108  and  109  for receiving the microphone signals of the two microphones  100  and  102 , respectively. A first and a second multiplication circuit  110 ,  111  is provided with signal inputs coupled with the first and second inputs  108 ,  109  of the signal processing arrangement, respectively, with control inputs for receiving respective first and second control signals, respectively, and with signal outputs. The signal processing arrangement  105  further includes a control signal generator  112  for generating the first and second control signals. An arrangement  114  for power-corrected summation is provided, with a first and a second input coupled with the output of the first and second multiplication circuits  110 ,  111 , respectively, and with an output. The arrangement  114  is configured for power-corrected summation of the signals offered at its first and second inputs and for providing a power-corrected summed overall signal to the output. 
     Power-corrected summation arrangements, as understood here, are known from the literature. In this respect reference should be made to the WO2011/057922A1 and the previously filed but not yet published PCT/EP2012/069799 of the same applicant, in particular to the description of FIGS. 2, 6 and 7, which are therefore regarded as being hereby incorporated by reference. 
     A signal combining arrangement  116  is provided, with a first input  117  coupled with the output of the power-corrected summation arrangement  114 , a second input  118  coupled with one of the at least two microphones, in this case microphone  102 , and with an output  119  coupled with the output  120  of the signal combining arrangement  116 . 
     The first multiplication circuit  110  is configured for multiplying the signal at its input with a multiplication factor A·(1−g) 1/2  under the influence of the first control signal of the control signal generator  112 . The second multiplication circuit  111  is configured for multiplying the signal at its input with a multiplication factor B·g 1/2  under the influence of the second control signal of the control signal generator  112 . According to the invention, g is frequency-dependent and thus indicated as g[f], and A and B are constant values, the absolute values of which are preferably equal 1. Further, A=B or A=−B applies. 
       FIG. 2 a    shows, what the frequency-dependent behaviour of the multiplication factor g[f] might look like. In this embodiment, A=−B applies. 
     In  FIG. 2 a   , the multiplication factor g[f] between a first frequency value f 0  and a second frequency value f 0  shows an increasingly diminishing value f 2  as the frequency increases. Below the frequency value f 2 , g[f] is a constant value V, preferably equal 1. Above the first frequency value f 0 , g[f] is constant in turn, preferably equal zero. In the frequency range between f 2  and f 0 , g[f] decreases continuously as the frequency increases. 
     The mode of operation of the microphone arrangement as shown in  FIG. 1  with the behaviour for g[f] as shown in  FIG. 2 a    will now be explained in detail with reference to  FIG. 3 a   .  FIG. 3 a    shows the directional characteristics of a microphone arrangement with two microphones as shown in  FIG. 1 , which are arranged at a distance D from each other and the output signals of which are directly added together. For low frequencies the directional characteristic is as shown by  311 , i.e., spherical. For increasing frequencies the directional characteristic changes as indicated by the directional characteristics  312 ,  313  and  314 . Here the directional characteristic  313  is assumed to be the desired directional characteristic because the directivity of the microphone arrangement is at its highest. Directivity is defined as the ratio of sensitivity in a main direction versus mean sensitivity of the microphone arrangement in all directions. The spherical characteristic  311  is too sensitive for sound from directions outside the main directions, and the same applies to the directional characteristic  314 . The frequency f 0 , at which the optimal directional characteristic occurs, depends on the distance D, as follows:
 
 f   o   =C/ (2 ·D )
 
wherein C is the speed of sound.
 
     It is the object of the invention to maintain this optimal directional characteristic  313  constant for an increased frequency range. This is achieved in the following way: Signal processing in the circuit parts  110 ,  111 , and  114  leads to a virtual microphone signal of a virtual microphone Mv at the output of the device  114 , which microphone is situated either between the two microphones  100  and  102  (whereby an interpolation of the microphone signals is performed by the circuit parts  110 ,  111  and  114 ) or outside the two microphones  100  and  102  (whereby an extrapolation of the microphone signals is performed by the circuit parts  110 ,  111  and  114 ). In consequence the virtual microphone signal of the virtual microphone (which is present at the output of the arrangement  114 ) and the microphone signal of the microphone  102  are combined in the signal combining arrangement  116  for deriving the output signal at the output  120 . The distance between the virtual microphone and the microphone  102  is smaller for an interpolation than the distance between the microphones  100  and  102  and larger for an extrapolation. 
     An extrapolation in the signal processing arrangement  105  is achieved in case A=−B. For example A could be equal to 1. If we assume this, then this means for the signal processing arrangement  105  that the multiplication factor in the multiplication circuit  111  is equal to −g 1/2  and the multiplication factor in the multiplication circuit  110  is equal to (1−g) 1/2 . Extrapolation means that the distance D EXT  between the virtual microphone Mv and the microphone  102  is larger than D, and thus the frequency at which the optimal directional characteristic occurs is below f 0 , e.g., occurs at f 1 , as indicated by the directional characteristic  316  in  FIG. 3 a   . Because of the frequency dependency of g[f], as indicated in  FIG. 2 a   , this means that this optimal directional characteristic is largely maintained in a frequency range between f 0  and f 2  as indicated by the frequency characteristics  313  and  316  in  FIG. 3 a   . Since g[f] is constant above f 0 , preferably equal to zero, the directional characteristic of the microphone arrangement for frequencies above f 0  remains unchanged. 
     For f&lt;f 2 , g cannot increase beyond the value 1 because g=1 is the maximum possible value, for which (1−g) 1/2  can be calculated. 
     It should be mentioned that in the above description the correlation between D EXT , depending on the frequency, and g[f] is as follows:
 
 D   EXT ( f )/ D≈ 1+ g[f ] for  f   2   &lt;f&lt;f   0  
 
Further,
 
 f   0   /f≈D   EXT ( f )/ D  
 
applies.
 
     An interpolation in the signal processing arrangement  105  is achieved in case A=B, wherein the multiplication factor g[f] behaves as a function of the frequency, as indicated in  FIG. 2 b   . For frequencies below f 0 , g[f] is equal to a constant, preferably equal to zero. For frequencies above f 0 , the multiplication factor g[f] increases in value as the frequency increases. Preferably, the multiplication factor g[f] continuously increases in value above f 0  as the frequency increases. 
     The interpolation will now be described with reference to  FIG. 3 b   . For simplicity&#39;s sake let it be assumed that A=B=1. This means that in the signal processing arrangement  105  in  FIG. 1  the multiplication factor in the multiplication circuit  111  is g 1/2  and the multiplication factor in the multiplication circuit  110  is (1−g) 1/2 . For an interpolation, the distance between the virtual microphone M v  and microphone  102  is smaller than D, and thus the frequency, at which the optimal directional characteristic occurs, is above f 0 , e.g., at f 3 , as indicated in  FIG. 3 b    by the directional characteristic  317 . Due to the frequency dependency of g[f], as indicated in  FIG. 2 b   , this means that this optimal directional characteristic is now largely maintained in a frequency range above f 0 , as indicated by the frequency characteristics  313  and  317  in  FIG. 3   b.    
     It should be mentioned that in the above description the correlation between D INT , depending on the frequency, and g[f] is as follows:
 
 D   INT ( f )/ D≈ 1− g[f ] for  f≧f   0  
 
Further,
 
 f   0   /f≈D   INT ( f )/ D  
 
applies.
 
     Therefore, due to the microphone arrangement according to  FIG. 1 , an enlargement of the frequency range for which the optimal directional characteristic is maintained, is possible only towards low frequencies, or only towards higher frequencies, depending upon the values for A and B. In the first case A=−B, and preferably: A=1 and B=−1. In the second case A=B, and preferably A=B=1. 
       FIG. 2 c    shows a behaviour of the multiplication factor g[f] as a function of f, which for frequencies below f 0  is equal to the behaviour of the multiplication factor in FIG.  2   a , and for frequencies above f 0  is equal to the behaviour of the multiplication factor in  FIG. 2 b   . In this way the extrapolations and interpolations are combined which means that the microphone arrangement in  FIG. 1  has a directional characteristic which in a frequency range between f 1  and f 3  has a largely optimal directional characteristic, as indicated by  313 ,  316  and  317  in  FIGS. 3 a    and  3   b.    
       FIG. 4  shows a second exemplary embodiment of the microphone arrangement according to the invention. 
     The microphone arrangement according to  FIG. 4  shows great similarities with the microphone arrangement of  FIG. 1 . The circuit parts in the signal processing arrangement  405 , which in  FIG. 4  are designated  410 ,  411 ,  412 ,  414 , and  416 , are similar to the circuit parts  110 ,  111 ,  112 ,  114 ,  116  of the signal processing arrangement  105  in  FIG. 1 . The signal processing arrangement  405  in  FIG. 4  is further provided with a third and a fourth multiplication circuit  421 ,  422 . The third and fourth multiplication circuits  421  and  422  are provided with signal inputs coupled with the first or the second input  408  or  409  of the signal processing arrangement  405 , with control inputs for receiving respective first or second control signals, and with signal outputs. 
     An arrangement  423  for power-corrected summation is provided with a first and a second input coupled with the output of the third or fourth multiplication circuit  421 ,  422 , and an output. The arrangement  423  is configured for power-corrected summation of the signals offered at its first and second inputs and for providing a power-corrected summed overall signal at the output which is coupled with the second input  418  of the signal combining arrangement  416 . 
     The third multiplication circuit  421  is configured for multiplying the signal at its input with a multiplication factor B·g 1/2 , under the influence of the second control signal. The fourth multiplication circuit  422  is configured for multiplying the signal at its input with a multiplication factor A·(1−g) 1/2  under the influence of the first control signal. Both control signals are generated by the control signal generator  412 . Exactly as already mentioned with reference to  FIG. 1 , g is frequency-dependent according to the invention and A and B are constant values, the absolute values of which are preferably equal 1. Further, A=B or A=−B applies. 
     The arrangement  423  is preferably identical with the arrangement  414 . 
       FIG. 5 a    shows what the frequency-dependent behaviour of the multiplication factor g[f] could look like. In this case A=−B. 
     The multiplication factor g[f] in  FIG. 5 a    shows a frequency value which decreases for an increasing frequency between a first frequency value f 0  and a second frequency value f 12 . Below the frequency value f 12 , g[f] is a constant value V, preferably equal 1. Above the first frequency value f 0 , g[f] is again constant, preferably equal zero. In the frequency range between f 12  and f 0 , g[f] continuously decreases as the frequency increases. 
     The mode of operation of the microphone arrangement of  FIG. 4  with a behaviour for g[f] as shown in  FIG. 5 a    will now be explained in detail with reference to  FIG. 6 a   .  FIG. 6 a    shows the directional characteristics of a microphone arrangement with two microphones, as shown in  FIG. 4 , which are arranged at a distance D from each other and the output signals of which are directly added together. 
     For low frequencies, the directional characteristic as indicated with  611 , is again spherical. For increasing frequencies, the directional characteristic changes as has already been described with reference to  FIG. 3 a    and as indicated by the directional characteristics  612 ,  613  and  614 . The directional characteristic  613  is again assumed as being the desired directional characteristic, for the same reasons as already explained in conjunction with  FIG. 3 a   . The frequency f 0 , at which the optimal directional characteristic occurs, is given by
 
 f   0   =C/ (2· D )
 
wherein C is the speed of sound.
 
     It is the object of the invention to keep the optimal directional characteristic  613  largely constant for an increased frequency range. This is achieved as follows. Signal processing in the circuit parts  410 ,  411  and  414  leads, as already explained with reference to  FIGS. 3 a  and 3 b   , to a virtual microphone signal of a virtual microphone at the output of the arrangement  414 , which microphone is situated either between the two microphones  408  and  409  (whereby an interpolation of the microphone signals is performed by the circuit parts  410 ,  411  and  414 ) or which is situated outside the two microphones  408  and  409  (whereby an extrapolation of the microphone signals is performed by the circuits parts  410 ,  411  and  414 ). 
     Exactly the same applies, of course, to the signal processing in the circuit parts  421 ,  422  and  423 . This means that a microphone signal of a virtual microphone is also generated at the output of the arrangement  423 . 
     An extrapolation in the microphone arrangement of  FIG. 4  is achieved for the case A=−B. A, for example, could be equal to 1. At the output of the arrangement  414  a microphone signal of a virtual microphone M v1  is then present, and at the output of the arrangement  423  the microphone signal of a virtual microphone M v2  is then present. The positions of both virtual microphones are shown in  FIG. 6 a   . Extrapolation in this case means that the distance D EXT2  between the two virtual microphones M V1  and M V2  is not only larger than D but also larger than D EXT  in  FIG. 3   a.    
     Thus, the frequency range at which the desired directional characteristic is largely maintained, may be enlarged towards even lower frequencies, i.e., in a frequency range between f 0  and f 12 , in  FIG. 6 a   . Since g[f] is constant above f 0 , preferably equal to zero, the directional characteristic of the microphone arrangement for frequencies above f 0  remains unchanged. 
     For f&lt;f 12 , g cannot increase beyond the value 1 for decreasing frequencies because g=1 is the maximum possible value for which (1−g) 1/2  can be calculated. 
     It should be mentioned that in the above description the correlation between D EXT , dependent on the frequency, and g[f] is as follows:
 
 D   EXT ( f )/ D≈ ½+ g[f ] for  f   12   &lt;f&lt;f   0  
 
Further,
 
 f   0   /f≈D   EXT ( f )/ D  
 
applies.
 
     An interpolation in the microphone arrangement of  FIG. 4  is achieved for the case A=B, wherein the multiplication factor g[f] behaves as a function of the frequency as indicated in  FIG. 5 b   . For frequencies below f 0 , g[f] is equal to a constant, preferably equal zero. For frequencies above f 0  the multiplication factor g[f] increases in value as the frequency increases. Preferably the multiplication factor g[f] above f 0  continuously increases in value as the frequency increases. 
     The interpolation will now be described with reference to  FIG. 6 b   . For simplicity&#39;s sake it is assumed that A=B=1. 
     The microphone signal of a virtual microphone M v1  is then present at the output of the arrangement  414 , and the microphone signal of a virtual microphone M v2  is then present at the output of the arrangement  423 . The positions of both virtual microphones are shown in  FIG. 6 b   . The interpolation means in this case that the distance D INT2  between the two virtual microphones M v1  and M v2  is not only smaller than D, but also smaller than D INT  in  FIG. 3   b.    
     Thus the frequency range, at which the desired directional characteristic is largely maintained, can be enlarged towards higher frequencies, i.e., in the frequency range above f 0  in  FIG. 6 b   . Since g[f] remains constant, preferably equaling zero for frequencies below f 0 , the directional characteristic of the microphone arrangement for frequencies below f 0  remains unchanged. 
     It should be mentioned that in the above description the correlation between D INT , dependent on the frequency, and g[f] is as follows:
 
 D   INT ( f )/ D≈ ½− g[f ] for  f≧f   0  
 
Further,
 
 f   0   /f≈D   INT ( f )/ D  
 
applies.
 
       FIG. 6 c    shows a behaviour of the multiplication factor g[f] as a function of f, which for frequencies below f 10  is equal to the behaviour of the multiplication factor in  FIG. 6 a    and for frequencies above f 10  is equal to the behaviour of the multiplication factor in  FIG. 6 b   . In this way, the extrapolation and the interpolation are combined, which means that the microphone arrangement in  FIG. 4  has a directional characteristic which in a frequency range between f 4  (see  FIG. 6 a   ) and f 5  (see  FIG. 6 b   ) has a largely optimal directional characteristic, as indicated by  613 ,  616  and  617  in  FIGS. 6 a    and  6   b.    
     Additionally, it should be mentioned that the rising and falling parts of the progression of the multiplication factor g[f] as a function of the frequency as shown in  FIGS. 2 a , 2 b , 2 c , 5 a , 5 b  and 5 c   , behave like parts of a hyperbolic curve. This follows from the inverse proportionality to the frequency in the above-mentioned formulae. 
       FIG. 7  shows a third exemplary embodiment of the microphone arrangement according to the invention. In this case the microphone arrangement comprises three microphones  700 ,  702  and  703 . The signal processing arrangement  705  is now constructed as follows: The circuit parts in the signal processing arrangement  705  indicated in  FIG. 7  by  710 ,  711 ,  712 ,  714 , and  716 , are similar to the circuit parts  110  and  111  and  112  and  114  and  116  of the signal processing arrangement  105  in  FIG. 1 , respectively. The third microphone  403  is coupled with a third input  707  of the signal processing arrangement  705 . The signal processing arrangement  705  is further provided with a third and a fourth multiplication circuit  721  and  722 . The signal inputs of the multiplication circuits  721  and  722  are coupled with the second input  709  and the third input  707  of the signal processing arrangement  705 , respectively. Control inputs of the multiplication circuits  721  and  722  are coupled with the control signal generator  712  for receiving respective first and second control signals, respectively. Signal outputs of the two multiplication circuits  721  and  722  are coupled with associated inputs of an arrangement  723  for power-corrected summation. One output of the arrangement  723  is coupled with a third input  715  of the signal combining arrangement  716 . The arrangement  723  is configured for power-corrected summation of the signals offered at its first and second inputs and for providing a power-corrected summed overall signal at the output. The third multiplication circuit  721  is configured for multiplying the signal at its input with a multiplication factor B×g 1/2  under the influence of the second control signal. The fourth multiplication circuit  722  is configured for multiplying the signal at its input with a multiplication factor A×(1−g) 1/2  under the influence of the first control signal. 
     Both control signals are generated by the control signal generator  712 . Just as already indicated with reference to  FIG. 1  according the invention the multiplication factor g is frequency-dependent, and A and B are constant values the absolute values of which are preferably equal 1. Further: A=B or A=−B. The frequency-dependent behaviour of the multiplication factor g[f] in the embodiment of  FIG. 7  is again as already described with reference to  FIGS. 2 a    to  2   c.    
     The arrangement  723  is preferably identical with the arrangement  714 . 
     The three microphones  700 ,  702  and  703  need not necessarily lie on a straight line.  FIG. 8  shows the position of the three microphones  700 ,  702  and  703 , which in this case are positioned on intersecting lines. In the embodiment of  FIG. 7  two virtual microphone signals are again generated. The first virtual microphone signal is present at the input  717  of the signal combining arrangement  716  and is derived from the microphone signals of the microphones  700  and  702 . The second virtual microphone signal is present at the input  715  of the signal combining arrangement  716  and is derived from the microphone signals of microphones  702  and  703 . 
     Let it be assumed that in the microphone arrangement of  FIG. 7  an extrapolation is performed for obtaining the two virtual microphone signals. This has the effect as if two virtual microphones had been realised. Specifically speaking, as if the microphone  700  were no longer at the position indicated in  FIG. 8 , but further away from the microphone  702  on the connection line  800  through the two microphones  700  and  702 , e.g., at the position  804 . Similarly it seems as if the microphone  703  is not at the indicated position, but further away from the microphone  702  on the connection line  802  through the two microphones  702  and  703 , e.g., at the position  806 . The position of the microphone  702  does not change. Due to this other position for the two virtual microphone signals another directional characteristic of the microphone arrangement, of course, is created which can now be modified as desired. 
     Yet another embodiment of a microphone arrangement with three microphones is shown in  FIG. 9 . The microphone signals of two microphones  900  and  902  are processed in the circuit part  905  which can be constructed as shown in  FIG. 1 or 4 , in order to obtain an output signal S 1  at the output  920 . The output signal S 1  and the microphone signal of the microphone  903  are then brought together in a circuit part  910  in order to obtain the output signal S 2  of the microphone arrangement. The circuit part  910  may again look like the circuit part  105  shown in  FIG. 1  (and as can indeed be seen in  FIG. 9 ) or like the circuit part  405  shown in  FIG. 4 . 
     The positions of the virtual microphones arise as shown in  FIG. 10 . In this case, a first extrapolation is now performed on the microphone signals of the microphones  900  and  902 , whereby a virtual microphone signal S 1  of a first virtual microphone at the position  1004  is derived at the output  920  in  FIG. 9 . Thereafter a second extrapolation is performed on the microphone signals of the first virtual microphone at the position  1004  and the microphone  903 , which leads to a second virtual microphone signal of a virtual microphone at the position  1007 , whereby the second virtual microphone signal is present on the line  930  in  FIG. 9 . The output signal S 2  at the output of the microphone arrangement is therefore the combination of the two first and second virtual microphone signals. 
     In conclusion, it should be mentioned that the invention is not limited to the exemplary embodiments shown in the description of the figures. As such various modifications are possible which however, all fall within the scope of the invention. As such the microphone arrangement may be comprised of more than three microphones. The microphones need not necessarily lie on a straight line.