Field of the Invention.

The invention relates to a microphone system and particularly, but not exclusively, to a micro-electrical-mechanical system (MEMS) microphone formed from two or more omnidirectional MEMS microphone assemblies.

Background of the Invention.

Typically, a MEMS microphone assembly comprises an electrically charged floating membrane or diaphragm (hereinafter“membrane”) susceptible to movement by an incident sound or pressure wave (hereinafter “pressure wave”) positioned adjacent to a fixed conductive plane normally comprising a conductive backplate fixed in position relative to the membrane at rest. Normally, the fixed backplate is perforated. Movement of the membrane relative to the fixed backplate in response to the incident pressure wave causes variations over time in capacitance between the membrane and the fixed backplate. The variations in capacitance can be translated into an electrical signal representative of pressure variations experienced by the membrane, i.e. an electrical signal representation of the pressure wave incident on the membrane.

A consequence of current manufacturing processes for MEMs microphone assemblies is that the microphone assemblies are omnidirectional in nature. That is, their responses to incident pressure waves are independent of the angle of incidence of the pressure wave on the membrane. As such, it is not immediately apparent how such microphone assemblies could be used to construct an Ambisonic microphone which must be unidirectional.

Objects of the Invention.

An object of the invention is to mitigate or obviate to some degree one or more problems associated with known unidirectional microphones.

The above object is met by the combination of features of the main claims; the sub claims disclose further advantageous embodiments of the invention.

Another object of the invention is to provide a unidirectional microphone system formed from two or more omnidirectional microphone assemblies to allow positioning of the microphone system in three-dimensional space without exhibiting any bias to any particular incident pressure wave source direction.

One skilled in the art will derive from the following description other objects of the invention. Therefore, the foregoing statements of object are not exhaustive and serve merely to illustrate some of the many objects of the present invention.

Summary of the Invention.

The present invention concerns a microphone system comprising a first microphone assembly for providing a first electrical signal representation of a pressure wave incident thereon and a second microphone assembly for providing a second electrical signal representation of the pressure wave where said second microphone assembly is spaced a selected distance d from the first microphone assembly. A signal processor combines said first and second electrical signals to provide a unidirectional output signal preferably by inverting the second electrical signal, delaying the second electrical signal by a selected time period dependent on the distance d and the speed of the incident pressure wave, and adding said inverted and delayed second electrical signal to the first electrical signal. The first and second MEMS microphone assemblies are preferably omnidirectional MEMS microphone assemblies. They combine to provide a cardioid polar response pattern.

The microphone system preferably comprises an Ambisonic microphone unit comprising a multiple of four first microphone assemblies arranged equidistant from a centre point of an imaginary or real sphere. The multiplicity of microphone assemblies may be arranged in a tetrahedral configuration. In one embodiment, each of said first microphone assemblies has associated with it a second microphone assembly spaced by the selected distance d from its respective first microphone assembly, where d is less than a radius R of the imaginary or real sphere. In another embodiment, each of said first microphone assemblies is associated with a single, shared second microphone assembly positioned at the centre of the imaginary or real sphere such that the selected distance d from each first microphone assembly to the single, shared second microphone assembly is equal to a radius R of the imaginary or real sphere.

Other aspects of the invention are in accordance with the appended claims. The summary of the invention does not necessarily disclose all the features essential for defining the invention; the invention may reside in a sub-combination of the disclosed features.

Brief Description of the Drawings.

The foregoing and further features of the present invention will be apparent from the following description of preferred embodiments which are provided by way of example only in connection with the accompanying figures, of which:

Figure 1 is a schematic block diagram of a typical MEMS microphone assembly;

Figure 2 is a schematic diagram of a membrane shown in isolation for the microphone assembly of Fig. 1;

Figure 3 is a schematic block diagram of a microphone system in accordance with an embodiment of the invention;

Figure 4A is a cardioid polar pattern at 5KHz for the microphone system of Fig. 3;

Figure 4B is a cardioid polar pattern at l5KHz for the microphone system of Fig. 3;

Figure 5 is a schematic diagram of an Ambisonic microphone unit in accordance with an embodiment of the invention; and

Figure 6 is a schematic diagram of an Ambisonic microphone unit in accordance with another embodiment of the invention.

Description of Preferred Embodiments.

The following description is of preferred embodiments by way of example only and without limitation to the combination of features necessary for carrying the invention into effect.

Reference in this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments, but not other embodiments. Referring to Fig. 1 and provided by way of example only is a typical MEMS microphone assembly 10 comprising a housing 12 accommodating an electrically charged (V _b ) floating membrane 14. The membrane is spaced from and generally arranged parallel to a fixed conductive plane comprising a conductive backplate 16 which is fixed in position relative to the housing 12. The membrane 14 is supported by springs or other biasing means 18 to enable it to move, e.g. flex, as represented by arrow 20 when a pressure wave indicated by arrow 22 passing through a front window 24 of the housing 12 is incident on it, although in this example of a MEMS microphone assembly 10, an incident pressure wave may be admitted to the housing by a rear window 26 of the housing 12. Consequently, in this example of a MEMS microphone assembly 10, the fixed backplate 16 has a perforated structure, although this is not always the case with MEMS microphone assemblies.

Movement of the membrane 14 in response to the incident pressure wave 22 causes variations over time in capacitance between the membrane 14 and the fixed backplate 16. The variations in capacitance can be translated by an operational amplifier (Op-Amp) 28 or the like into an electrical signal representative of the pressure variations experienced by the membrane 14, i.e. an into an electrical signal representation of the pressure wave 22 incident on the membrane 14.

The interior volume 30 of the housing 12 comprising the acoustic cavity in which the membrane 14 is suspended by the springs 18 is, in effect, open to the atmosphere all around the membrane 14 which causes the MEMS microphone assembly 10 to be inherently omnidirectional.

It will be understood that Fig. 1 is merely one example of typical a MEMS microphone assembly and that the invention as will be described hereinafter is applicable to any typical MEMS microphone assembly structures.

Consider now Fig. 2 which shows a membrane 100 for a MEMS microphone assembly (now shown). The membrane 100 is shown in isolation for reasons of clarity, but it will be understood that the membrane 100 forms part of a MEMS microphone assembly of a similar type shown by way of example in Fig. 1. The membrane 100 is suspended from a fixed frame 102 by a biasing mechanism 104 comprising, in this example, a plurality of spring members 106 arranged around the periphery of the membrane 100. The spring members 106 are integrally formed with the membrane 100 and the fixed frame 102. Arrow 108 depicts the desired direction for a unidirectional response of the membrane 100, but, as will be understood from the foregoing, it is not possible to achieve such a response with a current omnidirectional MEMS microphone assembly as illustrated by Figs. 1 and 2.

Where a pressure wave such as a sound wave approaches a front surface 100 A of the membrane as viewed in Fig. 2 at say about 343 metres/second (m/s) for a normal room temperature environment, the pressure wave causes the membrane 100 to move or flex, from which can be obtained an electrical signal representation of the pressure wave in the manner described with respect to Fig. 1, but absent of any information on the pressure wave’s direction or angle of incidence to the front surface 100A of the membrane 100. Similarly, if an identical pressure wave approaches a rear surface of the membrane 100, it will cause the membrane 100 to move or flex in a same manner enabling a same electrical signal representation of the pressure wave to be obtained, but again absent of any information on its direction or angle of incidence to the membrane 100, although, in this case, the electrical signal representation of the pressure wave will be inverted with respect to the electrical signal representation obtained for the pressure wave incident on the front surface 100A of the membrane 100.

As indicated, an inherent feature of current MEMS microphone assemblies is that they are omnidirectional in nature, namely that their responses to incident pressure waves are independent of the angle of incidence of the pressure wave on the membrane. Consequently, although it is not possible to obtain a unidirectional response from a current omnidirectional MEMS microphone assembly, current MEMS technology allows MEMS microphone assemblies to be manufactured at very low cost and at very small sizes so there is a desire to make use of the cost and size advantages afforded by current MEMS technology.

The invention therefore seeks to create a unidirectional microphone system from two or more omni-directional microphone assemblies to allow said microphone system to be positioned in three-dimensional (3D) space with no bias as to any particular incident pressure wave source direction.

Fig. 3 shows a schematic block diagram of a microphone system 200 in accordance with an embodiment of the invention. The microphone system 200 comprises a first microphone assembly 202 for providing a first electrical signal representation of a pressure wave incident thereon and a second microphone assembly 204 for providing a second electrical signal representation of the pressure wave. The pressure wave and its preferred direction of propagation are represented by arrow 206. The second microphone assembly 204 is spaced a selected or calculated distance d from the first microphone assembly 202. More specifically, a membrane 208 of the first microphone assembly 202 is spaced the selected or calculated distance d from a membrane 210 of the second microphone assembly 204.

The first and second microphone assemblies 202, 204 may be accommodated in separate respective housings 212, 214 or they may be accommodated in a same housing 216 as depicted by dashed lines 218. It is necessary, however, that whatever the spaced arrangement of the first and second microphone assemblies 202, 204, it is necessary to know the distance d between the respective membranes 208, 210. It is also necessary to know the speed of the pressure wave, but an estimate of said speed may be employed. For example, a sound wave speed or velocity of 343m/s may be used for envisaged applications of the invention where the microphone system 200 of the invention will typically be utilized in a room temperature environment for detecting sound signals. That being said, the microphone system 200 may be equipped with a module for measuring the speed or velocity of an incident pressure wave.

Each of the first and second microphone assemblies 202, 204 has a respective fixed backplate 220, 222 placed adjacent to its respective membrane 208, 210.

The first and second microphone assemblies 202, 204 may have generally the same structure as shown for the typical MEMS microphone assembly 10 of Fig. 1, although it will be understood that the microphone system 200 in accordance with the invention may comprise any combination of two or more known microphone assemblies, particularly two or more known MEMS microphone assemblies.

Whatever the spaced arrangement of the first and second microphone assemblies 202, 204, the outputs 224, 226 of the first and second microphone assemblies 202, 204 are each connected to a signal processor 228 which is configured to combine the first and second electrical signal representations obtained from the first and second microphone assemblies 202, 204 to provide a unidirectional output signal 230. The signal processor 228 includes a non-transitory memory 232 which may be configured to store machine readable instructions which, when executed by the signal processor 228, implements the methods herein described. Alternatively, the first and second electrical signal representations obtained from the first and second microphone assemblies 202, 204 may be transmitted to a remote signal processor. The signal processor 228 may also provide the module for measuring the speed or velocity of an incident pressure wave. Furthermore, the memory 232 may be arranged to store the output signals from the first and second microphone assemblies 202, 204 including the outputted first and second electrical signal representations of the incident pressure wave 206.

It can be seen from Fig. 3 that the second microphone assembly 204 is placed behind the first microphone assembly 202 with respect to a desired direction of the incident pressure wave as depicted by arrow 206. Placing the second microphone assembly 204 at a selected or calculated distance behind the first microphone assembly 202 makes it possible to gain information on the direction of the incident pressure wave 206.

A pressure wave reaching the first microphone assembly 202 from the desired direction depicted by arrow 206 can be considered as reaching the membrane 208 of said first microphone assembly 202 at a time t and reaching the membrane 210 of the second microphone assembly 204 at a slightly later time of d/c seconds where c is the speed or velocity of the pressure wave and d is the separation distance between the first and second membranes 208, 210 in metres. For convenience, t can be equated to time zero at the first membrane 208.

Conversely, a pressure wave reaching the membrane 208 of the first microphone assembly 202 at time t = 0 from an opposite direction to arrow 206 will already have encountered the membrane 210 of the second microphone assembly 204 at a time ( t - d/c) seconds earlier.

It is therefore possible to take the outputs 224, 226 of the first and second microphone assemblies 202, 204 and manipulate these using the signal processor 228 to beamform the output 230 of the microphone system 200.

Taking the outputs 224, 226 of the first and second microphone assemblies 202, 204 for the situation where the incident pressure wave is incident on the membranes 208, 210 in a direction opposite to the desired direction of arrow 206, a method in accordance with the invention comprises the steps at the signal processor 228 of: (a) capturing an electrical signal representation of the pressure wave from the second membrane 210; (b) inverting said captured electrical signal representation; (c) adding a time delay of d/c seconds to said captured electrical signal representation; (d) storing said inverted and time delayed electrical signal representation in the memory 232; (e) capturing an electrical signal representation of the pressure wave from the first membrane 208; and (f) adding the stored inverted and time delayed electrical signal representation from the memory 232 to the captured electrical signal representation of the pressure wave from the first membrane 208. As the pressure wave should result in the generation of identical electrical signal representations by the first and second membranes 208, 210 save for the fact that one is inverted with respect to the other and one occurs at a time d/c seconds before the other, it will be appreciated that the method steps (a) to (f) result in the stored inverted and time delayed electrical signal representation from the memory 232 cancelling out the captured electrical signal representation of the pressure wave from the first membrane 208, i.e. resulting in a zero signal output 230.

It will be understood that steps (b), (c) and (d) may be performed in any order. It will also be appreciated that the method may include storing the captured electrical signal representation of the pressure wave from the first membrane 208 prior to step (f).

As the method defined above is a continuous process, only pressure waves from the exact opposite direction of the desired direction 206 are completely cancelled out. Pressure waves coming from behind, but at less than opposite angles are cancelled to lesser degrees.

Taking the outputs 224, 226 of the first and second microphone assemblies 202, 204 for the situation where the incident pressure wave is incident on the membranes 208, 210 in a same direction as the desired direction of arrow 206, a further method in accordance with the invention comprises the steps at the signal processor 228 of: (i) capturing an electrical signal representation of the pressure wave from the first membrane 208; (ii) storing said captured electrical signal representation in the memory 232; (iii) capturing an electrical signal representation of the pressure wave from the second membrane 210; (iv) inverting said captured electrical signal representation from the second membrane 210; (v) adding a time delay of d/c seconds to said inverted electrical signal representation from the second membrane 210; and (vi) adding the stored electrical signal representation from the memory 232 to the inverted and time delayed electrical signal representation of the pressure wave from the second membrane 210. The net effect is that the stored electrical signal representation from the memory 232 is added to the inverted and time delayed electrical signal representation of the pressure wave from the second membrane 210 at time t = 2*d/c seconds. This time delay comprises a first time delay of d/c seconds being the later time at which the pressure wave is incident on the second membrane 210 and a second time delay of d/c seconds being the time delay added by the signal processor 228.

The pressure wave arriving from the desired direction of arrow 206 is sampled first by the first membrane 208 (t = 0 seconds) and eventually sampled by the second membrane 210 at t = d/c seconds. The addition of the further delay of d/c seconds and the inversion of the signal from the second membrane 210 results in this signal manifesting itself as some distortion in the electrical signal representation of the pressure wave from the first membrane 208, but no complete cancellation occurs.

It will be understood that steps (ii), (iii) and (iv) may be performed in any order. It will also be appreciated that the method may include storing the captured electrical signal representation of the pressure wave from the second membrane 208 prior to step (v).

As the method defined above is a continuous process, pressure waves from the desired direction 206 are largely retained. Consequently, the continuous method results in a cardioid polar pattern for the microphone system 200 as illustrated by Figs. 4A and 4B.

Preferably, the inverted and time delayed signal from the second membrane 210 is attenuated prior to being added in order to mitigate to some degree the distortion it causes to the signal from the first membrane 208. The degree of attenuation may be determined at a final test stage of the assembled microphone system of Fig. 3.

Additionally, or alternatively to attenuating the added signal, it is possible to utilize one or more additional microphone assemblies in the cascaded manner illustrated in Fig. 3 with the outputs of all of the microphone assemblies being fed to the signal processor 228. The amount of distortion would be reduced for each additional microphone assembly added to the string of assemblies.

Where, for example, a third microphone assembly is added to the microphone system 200 of Fig. 3, the third microphone assembly is preferably spaced by a selected or calculated distance d’ from said first microphone assembly 202, where d’ is greater than d.

It is possible to create an Ambisonic microphone unit based on the foregoing embodiments of the invention by using four or more microphone systems of the type shown in Fig. 3 or of similar type, each having a cardioid polar pattern and each being formed from two or more microphone assemblies. It is desirable to make the microphone assemblies 202, 204 as small as possible. This requires that distance d is minimized. However, a constraint on minimizing distance d is the sampling time required for obtaining the outputs 224, 226 of the first and second microphone assemblies 202, 204 and the processing time required by the signal processor 228 to process said outputs 224, 226. The distance d must be of a size which provides sufficient time to acquire and process the outputs 224, 226, i.e. the time taken for the pressure wave to travel distance d must be such as to allow the two time-separated outputs 224, 226 to be acquired, to be fed to the signal processor 228 and processed thereby.

In the case where distance d is say lOmm and taking the velocity of sound at room temperature as 343m/s, there will be a delay of 29.4psecs between the first and second membranes 208, 210 of the first and second microphone assemblies 202, 204 detecting the same sound pressure wave. To enable a pressure wave travelling from the opposite of the desired direction to be cancelled, the microphone system 200 must be configured such that the steps of sampling the pressure wave at the second microphone assembly 204, inverting said sampled pressure wave and then adding said inverted and sampled pressure wave to the pressure wave sampled at the first microphone assembly 202 can be conducted within 29.4psecs. This is achievable using a 48kHz pressure wave sampling rate at each of the first and second microphone assemblies 202, 204 which enables one sample every 20.8psecs thereby providing 9psecs for the signal processing steps.

It will be understood from the foregoing that the distance d is directly related to the selected sampling rate and signal processor speed. Consequently, when seeking to minimize the size of distance d, one first determines the preferred or required sampling rate, selects a preferred or required signal processor having a known processing speed and then determines the minimum size of distance d therefrom. The spacing between a membrane and its capacitor plate in the microphone assemblies 202, 204 is typically lmm.

Ambisonic audio sampling relies on sampling sound pressure variations on the surface of an imaginary or real sphere, of radius R, where R is the distance from a centre point to the acoustic sampling plane of the microphone. The acoustic sampling plane could be a diaphragm cone connected to a coil surrounding a permanent magnet of a dynamic microphone, or a piezo-electric crystal for a piezo-electric microphone, or the moving conductive plate of a condenser microphone, or the moving membrane of a MEMS microphone assembly. Ambisonic sampling relies on measuring or sampling the sound waves at exact points on the surface of the real or imaginary sphere, where the mathematical solution of the wave equation using spherical harmonics can be used to de-convolute the solution for the sum of all incident plane waves on the spherical surface into the independent C,U,Z cartesian (or polar coordinate) components using Bessel functions. To achieve this, each microphone sensing element should be uni-directional or have a cardioid polar pattern.

An Ambisonic microphone unit comprises an array of 4, 8, 32, ..etc. unidirectional or cardioid microphone systems placed equidistantly on a surface of an imaginary or real sphere of radius R. Sampling the sound waves at points on the surface of the sphere coincident with the microphone systems is essentially the same as sampling the solution to the wave equation for any number of planar waves hitting the spherical surface from any directions of incidence. Using known complex mathematics involving spherical Bessel functions to a degree dependent on the number of microphones, information on what sound came from what direction can be obtained.

To create a unidirectional or cardioid microphone system using MEMS technology, it is necessary to use two or more MEMS microphone assemblies to create each unidirectional or cardioid microphone system as hereinbefore described.

In an embodiment of a four microphone Ambisonic microphone unit 300 as schematically depicted in Fig. 5, it is necessary to use eight MEMS microphone assemblies 304 to create the four MEMS microphone systems 302. Each microphone system 302 may be of a type as depicted, for example, in Fig. 3. The microphone systems 302 are arranged in an Ambisonic format such as a tetrahedral configuration as shown in Fig. 5.

It can be seen therefore that, for an N microphone Ambisonic microphone unit, at least 2*N MEMS microphone assemblies are required.

The resulting Ambisonic microphone unit 300 comprises a multiple of four first microphone systems 302 arranged equidistant from a centre point 301 of an imaginary or real sphere 303 and preferably arranged in a tetrahedral configuration. Each of the first microphone systems 300 comprises a first microphone assembly 304A and a second microphone assembly 304B, where said second microphone assembly 304B is spaced by the selected distance d from its respective first microphone assembly 304A, where d is less than the radius R of the imaginary or real sphere. In one embodiment, each of the first microphone assemblies 304A may have associated with it a third microphone assembly (not shown) spaced by a selected distance d’ from its respective first microphone assembly 304A, where d’ is greater than d but less than the radius R of the imaginary or real sphere.

It is, however, possible to form an Ambisonic microphone unit using a reduced number of microphone assemblies, namely to reduce the number of microphone assemblies from 2*N microphone assemblies to as few as N + 1 microphone assemblies as depicted in Fig. 6. This involves creating an Ambisonic microphone unit 400 by placing, for example, four primary microphone assemblies 404A (Fig. 1) on a surface of the imaginary or real sphere having a radius R and placing a single secondary microphone assembly 404B at the centre point of the imaginary or real sphere. The secondary microphone assembly 404B is used as the common, second microphone assembly for each of the four microphone systems formed by connecting an output of the secondary microphone assembly 404B to the respective signal processors of the four primary microphone assemblies 404A. As each of the primary and secondary microphone assemblies 404A,B is omnidirectional, its orientation with respect to other ones of the primary and secondary microphone assemblies 404A,B is not critical. The four primary microphone assemblies 404A are preferably arranged in a tetrahedral configuration.

The resulting Ambisonic microphone unit 400 comprises a multiple of four first microphone assemblies 404A arranged equidistantly from a centre point of the imaginary or real sphere having the radius R. In this configuration of an Ambisonic microphone unit 400, the single, shared secondary microphone assembly 400B is positioned at the centre of the imaginary or real sphere such that the selected distance d from each primary microphone assembly 400 A to the single, shared secondary microphone assembly 400B is equal to the radius R of the sphere.

In one embodiment, each of the primary microphone assemblies 400 A may have associated with it a third microphone assembly (not shown) spaced by a selected distance d’ from its respective primary microphone assembly, where d’ is less than the radius R of the imaginary or real sphere.

In the embodiments of Figs. 5 and 6, it is preferred that the first microphone system or microphone assembly is a MEMS microphone system or assembly and that the second microphone system or assembly is a MEMS microphone system or assembly. In the embodiments of Figs. 5 and 6, it is preferred that the microphone systems or assemblies are omnidirectional.

In the embodiments of Figs. 5 and 6, it is also preferred that an Ambisonic sound signal output of the microphone units is an“A” format signal, a“B” format signal, or a“C” format signal.

It is, however, possible to form an Ambisonic microphone unit generally in accordance with Figs. 5 and 6 where the microphone systems or assemblies each comprise an analogue condenser microphone assembly or system of respectively similar configurations as shown in Fig. 1 and Fig. 3.

Where the Ambisonic microphone unit’s array of microphone systems comprises omnidirectional microphone assemblies which accept sound from all directions, electrical signals of the microphone assemblies contain information about sounds coming from all directions. Processing of these sounds allows the selection of a sound signal coming from a given direction. Thus, a microphone array can comprise many known arrangements which enable selection of sound coming from a given direction by using known algorithms to process one or many channel signals of a captured surround sound field.

In embodiments where the microphone systems (Fig. 3) are arranged in a tetrahedral array and preferably a B format tetrahedral array, preferably pairs of MEMS microphone assemblies are provided to provide eight channels. The use of small MEMs microphone assemblies enables the size of the array to be miniaturized. The pairs of MEMS microphone assemblies may be spatially offset within a pair and/or between the pairs. More preferably, each pair of MEMs microphone assemblies is arranged with one spaced a small distance behind the other. Each pair of MEMS microphone assemblies may be sufficiently displaced to provide a single cardioid pattern beam formed from the two omnidirectional MEMs assemblies. As such, the signal from the first MEMS assembly may be delayed in time and then combined with the signal from the second MEMS assembly to cancel out signals from behind the pair of MEMs assemblies to provide a controlled cardioid polar pattern. The eight channel arrangement so formed provides for better manipulation of the cardioid pickup pattern by the Ambisonic microphone unit array enabling much more accurate and tight beam forming. The invention also provides a method of generating a unidirectional output signal from a microphone system comprising the steps of arranging a first microphone assembly to provide a first electrical signal representation of a pressure wave incident thereon, arranging a second microphone assembly at a selected distance d from the first microphone assembly and arranging said second microphone assembly to provide a second electrical signal representation of the pressure wave incident thereon, and providing a signal processor to combine said first and second electrical signals to provide said unidirectional output signal.

It should be understood that the elements shown in the figures, may be implemented in various forms of hardware, software or combinations thereof. Preferably, these elements are implemented in a combination of hardware and software on one or more appropriately programmed general-purpose devices, which may include a processor, memory and input/output interfaces.

The present description illustrates the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope.

Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

In the claims hereof, any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements that performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. It is thus regarded that any means that can provide those functionalities are equivalent to those shown herein.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only exemplary embodiments have been shown and described and do not limit the scope of the invention in any manner. It can be appreciated that any of the features described herein may be used with any embodiment. The illustrative embodiments are not exclusive of each other or of other embodiments not recited herein. Accordingly, the invention also provides embodiments that comprise combinations of one or more of the illustrative embodiments described above. Modifications and variations of the invention as herein set forth can be made without departing from the spirit and scope thereof, and, therefore, only such limitations should be imposed as are indicated by the appended claims.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word“comprise” or variations such as“comprises” or“comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art.