Patent Publication Number: US-9426562-B2

Title: Passive group delay beam forming

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 12/684,598, entitled “PASSIVE GROUP DELAY BEAM FORMING,” filed on Jan. 8, 2010, which claims priority to U.S. Provisional Application No. 61/143,336, entitled “PASSIVE GROUP DELAY BEAM FORMING,” filed on Jan. 8, 2009, the entire contents of each of which are hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to audio wideband beam steering or forming from multiple sources, and in particular to beam forming by passive group delay. 
     2. Related Art 
     Loudspeaker systems have been implemented as arrays of loudspeakers, or drivers; either stacked and aligned vertically, aligned horizontally, or in two dimensions. The drivers in such configurations may be of the same type, such as tweeters, midrange speakers, or wideband speakers. The drivers may also be connected to cross-over networks, or filters to generate sound in particular frequency ranges. 
     One problem with loudspeaker systems arranged in an array is that the sound generated by multiple drivers does not create a consistent sound field or pattern. This inconsistency in the sound field or pattern distorts the sound and impairs the listening experience of the listener. 
     One solution to the problem is to utilize a digital delay to effectively move the apparent sound from a driver in the array by introducing time delay creating a more consistent coverage. Another solution involves physically placing each driver appropriately in space to create a more consistent sound field. In either solution, the drivers are generally arranged in an arc or spherical shape either through time delay or, physically placed to form an arc or sphere, to provide a desired coverage. 
     A constant beam width transducer (CBT) is a type of sound transducer designed to provide a listening area with a sound beam that projects at a constant angle. The source of sound projects substantially at an angle and forms the listening area within the space defined by the angle sides. One design goal is for CBT&#39;s to project the sound at the same frequency response and volume at any point along any arc of points equidistant to the source. A CBT&#39;s beamwidth is defined as an angle. Studies of CBTs show that a curved line array or spherical array will have a constant beam width of approximately 66% of the total physical arc. The CBT also requires that the elements in the array be ‘shaded.’ That is, the drivers in the center are loudest, and the speakers on either side are attenuated more and more along the arc towards the ends of the array. The time delay or physical curving creates the coverage pattern and the shading smoothes the on- and off-axis response. By using time delay, the arc or sphere can be created from a straight line or flat 2-D array, respectively. This is often preferable for esthetic and space reasons. However providing a separate amp channel and associated digital time delay for each device can be expensive. 
     It would be desirable to provide an arc coverage pattern using a straight or flat speaker array without the need for expensive digital time delay circuitry. 
     SUMMARY 
     In view of the above, a loudspeaker array is provided. The loudspeaker array includes a plurality of loudspeakers. A delay network is included, the delay network having a plurality of stages. Each stage has a stage input and a stage output. The stage output of each stage is coupled to the stage input of a next stage. Each stage output is also connected to at least one of the plurality of loudspeakers. The stage input of the first stage is coupled to an audio signal input. Each stage is configured to add an electrical delay of the audio signal at each subsequent stage. The electrical delay is adjusted such that the plurality of loudspeakers generates sound in a desired radiation pattern. 
     A method is also provided for creating a radiation pattern using a linear loudspeaker array. In an example method, the positions of the loudspeakers in the linear array are set. A delay network is formed by connecting a plurality of delay stages in a ladder configuration. A middle loudspeaker positioned closest to a center of the linear array is connected to the audio signal input. A first loudspeaker pair of loudspeakers positioned on opposite sides of the center of the linear array is connected in series and the pair is connected in parallel with the stage output. Each succeeding loudspeaker pair of loudspeakers positioned on opposite sides of the center of the linear array is connected in series with each other and each succeeding pair is connected in parallel with each succeeding stage output. The component values of components in the delay stages are adjusted to delay propagation of the audio signal through the stage by a predetermined time. 
     Other devices, apparatus, systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The description of examples implementations that follows may be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a block diagram of an example audio system having a loudspeaker array using a delay network. 
         FIG. 2  is a schematic diagram of an example of the loudspeaker array and delay network in  FIG. 1 . 
         FIG. 3  is a schematic diagram of several driver pairs connected to corresponding LC branches from the delay network in  FIG. 2 . 
         FIG. 4  is a graph illustrating the group delay versus frequency at each driver pair in the loudspeaker array in  FIG. 2 . 
         FIG. 5  is a graph illustrating the transfer function shading of drivers in the loudspeaker array in  FIG. 2 . 
         FIG. 6  is the vertical beamwidth of a group delay shaded array versus a straight line array of 16 elements. 
         FIG. 7  is a graph illustrating the beamwidth versus frequency for 2 different arrays of 16 elements of the same size, the arrays having delay networks with different component values. 
         FIG. 8  is a flowchart depicting operation of an example of a method for providing an arc coverage pattern using a linear loudspeaker array. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an example audio system  100  having a loudspeaker array  102  using a delay network  104 . The system  100  includes an audio sound source  106 , such as the audio output of an entertainment system for music and/or multi-media. The loudspeaker array  102  includes a plurality of drivers  102   a - 102   t  aligned vertically. The loudspeaker array  102  may include any number of speakers. Twenty drivers are shown in the loudspeaker array  102  in  FIG. 1 . The drivers  102   a - 102   t  are aligned vertically in  FIG. 1 . However, the loudspeaker array  102  is not limited to any particular linear orientation. In addition, the drivers  102   a - 102   t  are aligned linearly along at least one direction, such as vertical, horizontal or diagonal, when viewed from directly in front of the loudspeaker array  102  as shown in  FIG. 1 . When viewed from the side for a vertically arranged array  102  or from above for a horizontally arranged array  102 , the drivers  102   a - 102   t  in the loudspeaker array  102  may be linearly arranged to form a straight line array. The drivers  102   a - 102   t  may be arranged along a curve to form a curved line array. The drivers  102   a - 102   t  may be partially linearly arranged and partially arranged along a curve. The loudspeaker array  102  may include drivers  102   a - 102   t  configured to generate a sound beam having any shape according to the distribution of the drivers  102   a - 102   t  and direction of projection. The loudspeaker array  102  may also be configured to generate a sound beam having a constant beam width along at least one of its linear dimensions by adjusting the delay and attenuation characteristics as described below with reference to  FIGS. 2-8 . 
     The drivers  102   a - 102   t  may be drivers of any type. For example, the drivers  102   a - 102   t  may be tweeters for generating high frequency audio, woofers for generating low frequency audio, or midrange speakers for generating mid-range frequency audio. Crossover networks may be connected to the delay network  104 , which may be configured to distribute the audio signals to the appropriate drivers (for example, low frequency signals to woofers, high frequency signals to tweeters, and midrange signals to midrange drivers). The drivers  102   a - 102   t  may also be full-range drivers, each able to drive audio through the entire specified range. 
     Example loudspeaker arrays and delay networks are described below in which the loudspeaker arrays include any number of full-range drivers. The size of the drivers used may be selected according to the wavelength of the upper limit of the frequencies of the sound being generated. The drivers are separated by a distance preferably less than one wavelength of the highest frequency. 
     The delay network  104  is connected to the loudspeaker array  102  as described in more detail below with reference to  FIGS. 2 and 3 . The delay network  104  includes a plurality of delay units, or stages,  104   a - 104   r , configured to generate delays in the signals being coupled to the drivers  102   a - 102   t  in the loudspeaker array  102 . The delay units  104   a - 104   r  in  FIG. 1  generate delays that increase for the drivers  102   a - 102   t  from the center of the array to the outside of the array. For example, no delay at all is applied to the signal coupled to the center drivers  102   j - 102   k.  A delay of nT and nT′ is inserted in the signal coupled to each driver on either side of the center drivers  102   j ,  102   k . The largest delay is inserted into the signal coupled to the drivers on the top  102   a  and bottom  102   t  of the array. The components in the delay units  104   a - 104   r  that generate the delay for each driver  102   a - 102   t  are passive components, which include components that do not require a power source for operation, such as for example, inductors, capacitors, and/or resistors. The passive components in the delay network  104  may be selected to generate a flat group delay with frequency such that the loudspeaker array  102  generates sound as though the drivers  102   a - 102   t  were arranged physically or configured with digital delay to provide coverage of a constant beam transducer (“CBT”). In the examples described below, inductors and capacitors are arranged in a cascaded ladder circuit with values selected to provide the desired progressive delay. The delay units  104   a - 104   r  described with reference to  FIGS. 1-4  are implemented using passive components, but may also be implemented using delay units that include active components, such as transistors, integrated circuits, etc. 
     It is noted that the description below describes examples of delay networks in which the delay units (such as delay units  104   a - 104   r ) are applied symmetrically about the center drivers (such as center drivers  102   j  and  102   k ). That is, the delays generated by each delay unit are equal and the delay network is configured to increment the sum of delays at each driver positioned away from the center drivers. In other examples, the delay network  104  need not be symmetrical. Each delay unit in the delay network may have a unique delay value and different attenuation characteristics that a designer may configure to generate a desired constant beam width pattern. 
       FIG. 2  is a schematic diagram of an example of the loudspeaker array and delay network in  FIG. 1 . The example  200  in  FIG. 2  includes a 20-element loudspeaker array  202  and a cascaded LC ladder network (“ladder network”)  204 , which is one example of the delay network  104  shown in  FIG. 1 . The loudspeaker array  202  includes  20  drivers  202   a - 202   t  arranged linearly. The configuration in  FIG. 2  is horizontal, however, a vertical configuration may be used as well. 
     Assuming a horizontal configuration, the driver  202   a  is located on one end of the array. The remaining drivers  202   b - 202   t  are then aligned in order such that the driver  202   t  is on the opposite end of the driver  202   a . The driver pair of driver  202   j  and  202   k  (center drivers  202   j,    202   k ) is positioned at the center of the loudspeaker array  202 . 
     Assuming a vertical configuration, the driver  202   a  is positioned at the top of the loudspeaker array  202  and the driver  202   t  is positioned at the bottom of the loudspeaker array  202 . The center drivers  202   j ,  202   k  are positioned in the middle of the vertical loudspeaker array  202 . In the description that follows, a vertical configuration is assumed. However, examples of the described implementations are not limited to vertical configurations. 
     The ladder network  204  is connected to an input signal V i . The ladder network  204  includes delay units, or stages, formed with inductors L 1 -L 9  and capacitors C 1 -C 9  connected to form a cascaded ladder of LC branches with taps used to connect to the drivers  202   a - 202   t  in the loudspeaker array  202 . Each stage includes a stage input and a stage output. The stages are configured such that the inductors L 1 -L 9  are connected in series with the input signal V i  and the capacitors C 1 -C 9  are connected in parallel with pairs of drivers between the inductors. The stage output for each stage in the ladder network  204  in  FIG. 2  is the stage input for the next stage in the ladder network  204 . The stage output for the first stage is the stage input for the second stage. The stage output for the second stage is the stage input for the third stage. As shown in  FIG. 2 , each capacitor in the LC branches forming the stages connects to the node between each inductor. The taps to the ladder network  204  are at each stage output, which is the node connecting the capacitor between the inductors. The values of the inductors L 1 -L 9  and capacitors C 1 -C 9  are selected to insert the appropriate delay to the signal being coupled to the corresponding drivers. The ladder network  204  includes a load resistance R L , representing the load resistance of two drivers connected in series. 
     The configuration of the stages in  FIG. 2  is recognizable to those of ordinary skill in the art to be a low pass filter. While the topology is the same as a low pass filter, the values of the components are radically different. The component values are mistuned. That is, the component values are sized to create flat group delay with frequency, which is not done with low pass filters. The component values are also sized to create relatively flat attenuation over a broad frequency range. As shown in  FIG. 5 , the first 4 or 5 transfer functions (from the center out) are flat. The group delay along the ladder is cumulative as is seen in  FIG. 4 . 
     The taps to the ladder network  204  are connected to the drivers  202   a - 202   t  such that the shortest delays are provided to the signals coupled to the drivers in the center of the array and the delays increasing to the signals coupled to the drivers extending up and down from the center drivers  202   j ,  202   k . The drivers  202   a - 202   t  are driven in driver pairs physically positioned symmetrically about the center of the loudspeaker array  202 . In the example shown in  FIG. 2 , the center drivers  202   j ,  202   k  are positioned vertically at the center of the array. The next driver pair  202   i ,  202   l  are arranged with driver  202   i  positioned above center driver  202   j  and driver  202   l  positioned below center driver  202   k . The subsequent driver pairs are arranged similarly from the center to the top and bottom. The driver pairs are connected to the ladder network  204  such that the signal is coupled to one terminal (for example, the ‘+’ terminal) of one driver in the pair. The other terminal (for example, the ‘−’ terminal) is connected to a terminal (for example, the ‘+’ terminal) of the other driver in the driver pair. The opposite terminal (for example, the ‘−’ terminal) of the other driver in the driver pair is connected to a common connection that connects one terminal of half of the drivers in the array  202 . That is, the common connection connects one terminal of the other driver in each driver pair. An opposite terminal of the driver pair is connected to the ladder network  204  to receive the delayed signal. 
     As shown in  FIG. 2 , the center drivers  202   j ,  202   k  are connected to the audio signal input Vi such that the audio signal coupled to the center driver pair  202   j ,  202   k  is not delayed. The LC branch formed with inductor L 1  and capacitor C 1  provides the first delay, which is inserted to the signal coupled to the first driver pair  202   i ,  202   l . The LC branch formed with inductor L 2  and capacitor C 2  provides the second delay, which is added to the first delay and inserted to the signal coupled to the second driver pair  202   h ,  202   m . Each succeeding branch formed by inductors L 3 -L 9  and capacitor C 1 -C 9  provides a progressively greater delay to each succeeding driver pair such that the delay is increasing for the drivers closest to the top and bottom. Effectively, each driver pair (top and bottom) of transducers is tapped off the ladder at further increments in group delay so the outside transducers receive delay from all sections of the ladder thereby receiving the greatest delay. The group delay yields an apparent curving of the array in the vertical dimension. 
       FIG. 3  is a schematic diagram of several driver pairs connected to corresponding stages formed by the LC branches in the delay network in  FIG. 2 .  FIG. 3  shows the center driver pair  202   j ,  202   k ; the next driver pair  202   i ,  202   l  after the center driver pair  202   j ,  202   k ; and the next driver pair  202   h ,  202   m  after the previous driver pair  202   i ,  202   l . The ladder network includes the first stage formed with the LC branch of inductor L 1  and capacitor C 1 ; and the second stage formed with the LC branch of inductor L 2  and capacitor C 2 . Each stage of the ladder network includes a load resistance (e.g., R 1  and R 2 ) representing the load resistance of the driver pairs connected to that stage. The succeeding LC branches are not shown for purposes of providing clarity of the description but could continue ad infinitum. 
     The ladder network includes an audio input signal generator  302  coupled to the input of the ladder network. As shown in  FIG. 3 , the first tap in the ladder network connects directly to the first driver pair  202   j ,  202   k . The first driver pair  202   j ,  202   k  is the center driver pair, which receives the audio signal without delay. The second tap in the ladder network between inductor L 1  and inductor L 2  is connected to the second driver pair  202   i ,  202   l . The first driver  202   i  in the second driver pair receives the delay and signal attenuation provided by the first LC branch formed by inductor L 1  and L 1 . Thus, the first delay is inserted to the signal coupled to the first driver on top of the center driver  202   j , which is driver  202   i ; and to the first driver below the center driver  202   k , which is driver  2021 . The third tap in the ladder network between inductor L 2  and inductor L 3  is connected to the third driver pair  202   h ,  202   m . The first driver  202   h  in the third driver pair receives the delay and signal attenuation provided by both the first LC branch formed by inductor L 1  and L 1  and the second LC branch formed by inductor L 2  and C 2 . Thus, the second delay is inserted to the signal coupled to the second driver on top of the center driver  202   j , which is driver  202   h;  and to the second driver below the center driver  202   k , which is driver  202   i.    
     In addition to the group delay being inserted at the signal coupled to each driver pair, the signal is progressively attenuated. The signal received by the drivers at the ends is attenuated relative to the signal at the center drivers  202   j ,  202   k.    
     The graphs in  FIGS. 4 and 5  illustrate the group delay and magnitude attenuation provided by an example ladder network  204 . These two effects of the ladder network  204  operate similar to the CBT concept with time delay and amplitude shading creating a constant width coverage beam at frequencies in which the wave length is smaller than the size of the array. 
       FIG. 4  is a graph illustrating the group delay versus frequency at each driver pair in the loudspeaker array in  FIG. 2 . Each curve in the graph represents the delay inserted at the signal at each tap in the ladder network  204  through the frequency range of operation. As shown in  FIG. 4 , the delay is increasingly greater at each successive tap starting from the tap at the audio signal input, which is connected to the center drivers  202   j ,  202   k . The delay is longest at the tap after the LC branch formed by inductor L 9  and capacitor C 9 , which connect to the drivers at the top (at  202   a ) and bottom (at  202   t ) of the loudspeaker array. 
       FIG. 5  is a graph illustrating shading of drivers in the loudspeaker array in  FIG. 2 . Each curve in the graph in  FIG. 5  represents the amplitude at each tap in the ladder network  204  through the frequency range of operation. As shown in  FIG. 5 , the signal is increasingly attenuated at each successive tap starting from the tap at the audio signal input, which is connected to the center drivers  202   j ,  202   k.    
       FIG. 6  is a graph illustrating the beamwidth versus frequency for a group delay derived array versus straight line array. The graphs are beamwidth plots for a 16-element array of one meter high. The graph for the group delay derived array shows beamwidth for a group delay derived with a broad vertical beam of 40 degrees (above 800 Hz). 
       FIG. 7  is a graph illustrating the beamwidth versus frequency for 2 different arrays of 16 elements of the same size, the arrays having delay networks with different component values. The graph in  FIG. 7  is a beamwidth plot for an 16-element array of one meter high with two different sets of component values to derive a narrow pattern and a wide pattern. The graph illustrates the comparison between a coverage of 15 degrees (above 5 kHz) versus 40 degrees (above 800 Hz).  FIG. 7  shows how the beamwidth may be varied by adjusting the component values of the passive components in the ladder delay network. 
     It is noted that the beamwidth plots of the 16-element array in  FIG. 7  are identical below 1 kHz. This is because below 1 kHz, the coverage is defined by the height of the array, which in this case is one meter. 
     It is also noted that  FIGS. 6-7  illustrate performance of vertically-oriented arrays. The loudspeaker arrays may also be oriented horizontally. The term ‘beamwidth’ refers to a width in the direction of the array configuration. 
       FIG. 8  is a flowchart depicting operation of an example of a method for providing an arc coverage pattern using a linear loudspeaker array. The method illustrated in  FIG. 8  may be implemented using a computer program having a user interface that permits user interaction for setting component values, loudspeaker positions, configuring views for data analysis, and setting any other parameter. The computer program may be developed as an application using a suitable programming language, or may be implemented as a macro or a sequence of instructions in an application such as a spreadsheet, a database, or suitable alternatives. The example method illustrated in  FIG. 8  allows a user to determine component values for use in a selected network to create an arc coverage pattern with a linearly arranged loudspeaker array. The method also allows the user to optimize performance of the network by ensuring that a constant beam width is achieved at a desired level over the desired frequency range. 
     At step  802  in  FIG. 8 , the desired beamwidth and the desired bandwidth are determined. The beamwidth and bandwidth specifications may be entered into memory, or may be requested from the user via a user interface query. The user interface query may be a menu-driven interface, an electronic form, or any suitable alternative form of data entry. 
     At step  804 , the driver spacing is determined. The spacing is the distance between the drivers. The driver spacing may be provided in memory or requested from the user via a user interface. In general, the driver spacing should be less than one wavelength (A) of the highest frequency being controlled. 
     At step  806 , the number of drivers to be used in the linear array is determined, driver spacing is determined. The number of drivers may be provided in memory or requested from the user via a user interface. In general, the number of drivers should be selected so that the height of the linear array is longer than one wavelength (Λ) of the lowest frequency being controlled. 
     At step  808 , a ladder network is generated. The ladder network may be defined by the topology of the stages, the components and component values. The configuration of each stage may be pre-defined in memory and offered to the user as alternatives from which to choose. 
     At step  810 , a model transfer function is generated for the group delay or the attenuation at each transducer. The group delay or attenuation is generated as a function of frequency. The transfer function may be generated as a graph, but may be any user readable output. An example of a generated transfer function is shown at  FIG. 4 . 
     At step  812 , an acoustical model illustrating how the transducers will sum in space is generated. The model includes the group delay or attenuation, and may be displayed as beamwidth vs. the frequency.  FIGS. 6 and 7  depict examples of an acoustical model that may be generated to illustrate the beamwidth. 
     At step  814 , the component values of the components in the stages of the ladder network may be adjusted to obtain a constant beamwidth over the desired frequency range. The component values may be selected from a broad range of values for each component. The values are selected to provide a near constant beamwidth at the desired frequency range. An initial set of values are selected for optimization by further fine tuning of the values. At step  816 , the component values are fine-tuned for the most constant beamwidth. Step  816  performs a local search. A computational optimizer may be used in step  816  to fine tune the values until values are found that result in the most constant beamwidth at the target value over the required range. Optimizers have an initial condition (or a seed), and will find the local minima, maxima, or fixed values. The computational optimizer may use the component values found in step  814  as a seed. 
     At decision block  818 , the acoustical model is checked to determine if it controls up to the highest frequency. If it does not (“No” branch), a smaller driver and driver spacing are selected at step  820  and the method goes back to step  806 . If control up to the highest frequency is attained (“Yes” branch), the acoustical model is checked to determine if it controls down to the lowest frequency at decision block  822 . If it is not (“No” branch), additional drivers are added to the ladder network at step  824 . The method then continues to step  808  to generate a new ladder network. If control to the lowest frequency is attained at decision block  822  (“Yes” branch), the beamwidth is checked over the entire range at the target value. If the beamwidth is not constant (“No” branch), new seed component values are selected at step  814 . If the beamwidth is constant (“Yes” branch), the design is complete. 
     While examples of implementations have been described above, various modifications may be implemented in other configurations. For example, a variable pattern control can be achieved using ganged switches that change the value of the components at the same time. The sound pattern may also be made to steer up or down if each half (for example, the top half and the bottom half) is driven with different ladder networks. A wider pattern coverage may also be achieved by adding physical curving of the array, so the array is not perfectly straight. The additional curving could be applied to only one half or to both asymmetrically. In the described implementations, the center drivers received the signal without a delay. In another implementation, a ground plane version may be created by providing the ladder delay from one end to the other of the array and positioning the non-delayed end perpendicular to a boundary. 
     The foregoing description of an implementation has been presented for purposes of illustration and description. It is not exhaustive and does not limit the claimed inventions to the precise form disclosed. Modifications and variations are possible in light of the above description or may be acquired from practicing the invention. For example, the described implementation includes software that optimizes the component values but the invention may be implemented as a combination of hardware and software or in hardware alone. Note also that the implementation may vary between systems. The claims and their equivalents define the scope of the invention.