Audio drive signal for generating simultaneous stereo sound field

Abstract

A method of generating an audio drive signal based on a two-channel stereo audio signal for driving an upward sounding transducer is disclosed. The method comprises the following steps: receiving a dual-channel stereo audio signal; generating a left audio signal, a right audio signal, and a center audio signal based on the two-channel stereo audio signal, where the left audio signal and the right audio signal are not positively correlated; generating a first audio signal and a second audio signal based on the left audio signal and the right audio signal; providing a left audio drive signal for driving the upward sounding left transducer, the left audio drive signal being obtained by summing the first audio signal and the center audio signal; and providing a right audio drive signal for driving the upward sounding right transducer, where the right audio drive signal is obtained by summing the second audio signal and the center audio signal.

Description

Technical Field

[0001] The present invention relates to a method for generating an audio drive signal based on a two-channel stereo audio signal, the audio drive signal being used to drive an upward-firing transducer. The present invention also relates to an audio system configured to output the audio drive signal. More specifically, the present invention relates to the position, arrangement, and / or drive signals for loudspeakers mounted in front of a confined or small symmetrical listening space and arranged symmetrically about a centerline, to reproduce a simultaneous stereo sound field for two adjacent listening positions within the space. Background Art

[0002] Current and future trends in consumer audio systems include the use of speakers located outside of traditional, conventional locations (e.g., in left-right stereo configurations). The reasons for this particular trend are multifaceted; however, the most compelling reasons and / or application scenarios that serve as the backdrop for this invention include: reducing device size for discreetness in home audio environments (particularly for small living spaces, such as as part of a complete audio-visual system); locating audio system speaker units in optimal interior areas for device packaging in automotive environments, thereby alleviating issues associated with mounting speakers in vehicle door trims while maintaining stereo soundstage width for multiple occupants; and rendering simultaneous, adjacent stereo soundstages for multiple listeners in multiplayer gaming scenarios to provide immersive audio where perceived audio cues align with visual cues. In these envisioned applications, providing stereo audio reproduction from a compact, centralized sound reproduction unit is beneficial. This is the focus of this invention.

[0003] GB 2600 538 A and US 010880648 B2 are examples of previous approaches in this field for this type of centralized stereo equipment. They employ a pair of sound sources that produce first-order directional audio emission characteristics that can be applied to left and right stereo input audio. In short, these devices produce a simultaneous stereo sound field for two adjacent locations by generating directional left and right channel propagation from side-firing sound sources, such that each listener's ipsilateral ear (i.e., the ear on the same side as the sound source) is positioned at the directivity null of the stereo (e.g., left or right) channel intended for reception at the contralateral ear (i.e., the ear on the opposite side of the listener). Simultaneously, a directional lobe corresponding to the channel at each ipsilateral ear position (the most central) is generated for reception. In this way, an effective left-right stereo arc is created, with the possibility of further acoustic support for the contralateral ear through reflections within the environment. Both of the above approaches have the disadvantage that they rely on the orientation of the transducer or waveguide along the horizontal plane, which limits the packaging of the device as it cannot be embedded in a surface and maintain an unobstructed view of the transducer.

[0004] GB 2600 539 A offers a similar solution to the two aforementioned approaches, but with the advantage of deploying an upward-firing sound source. This eliminates the need for the driver to be waveguided or mounted protruding from a surface (such as the dashboard in a car cabin, for example), which can lead to undesirable sound scattering and comb filtering effects from near-field surfaces. Flush mounting the driver also improves the overall system's stealthiness. In one example, the device described in GB 2600 539 A generates simultaneous first-order directivity patterns for left and right channel audio emissions, causing the intended signal for both the ipsilateral and contralateral ears to arrive directly or indirectly via reflections or be significantly attenuated in amplitude via null-steering (consistent with the concepts proposed in GB 2600 538 A and US 010880648 B2).

[0005] However, such prior art systems are known to suffer from a variety of undesirable artifacts, including suppression of audio content common to the left and right audio streams, and comb filtering.

[0006] The object of the present invention is to overcome these problems. Summary of the Invention

[0007] According to a first aspect of the present invention, a method for generating an audio driving signal based on a two-channel stereo audio signal is provided for driving an upward sounding transducer, the method comprising the following steps: receiving a two-channel stereo audio signal; generating a left audio signal based on the two-channel stereo audio signal; , right audio signal and center audio signal , wherein the left audio signal and the right audio signal are not positively correlated; generating a first audio signal based on the left audio signal and the right audio signal and the second audio signal , wherein the first audio signal and the second audio signal are generated according to the following formula: as well as ,in, Is the time delay of the left and right audio signals; provides the left audio drive signal for driving the left transducer for upward sounding , wherein the left audio drive signal is obtained by summing the first audio signal and the center audio signal; and the right audio drive signal is provided for driving the right transducer for upward sound emission , wherein the right audio driving signal is obtained by summing the second audio signal and the center audio signal.

[0008] For example, the method may include providing a left audio drive signal by summing the first audio signal and the center audio signal; and providing a right audio drive signal by summing the second audio signal and the center audio signal.

[0009] Preferably, providing the left audio drive signal comprises applying a gain stage to at least one of the first audio signal and the center audio signal before summing the first audio signal and the center audio signal, and / or providing the right audio drive signal comprises applying a gain stage to at least one of the second audio signal and the center audio signal before summing the second audio signal and the center audio signal.

[0010] According to a second aspect of the present invention, a method for generating an audio driving signal based on a two-channel stereo audio signal is provided for driving an upward sounding transducer, the method comprising the following steps: receiving a two-channel stereo audio signal; generating a left audio signal based on the two-channel stereo audio signal; , right audio signal and center audio signal , wherein the left audio signal and the right audio signal are not positively correlated; generating a first audio signal based on the left audio signal and the right audio signal and the second audio signal , wherein the first audio signal and the second audio signal are generated according to the following formula: as well as ,in, Is the time delay of the left and right audio signals; provides the left audio drive signal for driving the left transducer for upward sounding , wherein the left audio drive signal is obtained from the first audio signal; a right audio drive signal is provided for driving the right transducer for upward sound emission , wherein the right audio drive signal is obtained from the second audio signal; and a center audio drive signal is provided for driving the center transducer for upward sound emission , wherein the center audio driving signal is obtained from the center audio signal.

[0011] A common characteristic of all of the aforementioned prior art systems is that they rarely consider the nature of the drive signal received by the output device (i.e., the transducer, also known as the sound source or driver unit). For stereo input drive signals (also known as two-channel stereo audio signals), there is typically a highly correlated signal component common to both the left and right channels. This correlated component is the common portion of the left and right audio streams, often referred to as the "center," and typically includes the centrally positioned elements of the audio mix, namely the human voice and some percussive elements. The superposition of these correlated components, whether in the signal domain prior to acoustic transmission or in air, can lead to a variety of undesirable artifacts, including suppression of the common signal or comb filtering. That is, the common signal is not considered in the first-order polar plots generated by the aforementioned prior art systems.

[0012] The present invention overcomes these problems by utilizing signal processing techniques according to the first and second aspects. In particular, the present invention extracts decorrelated left and right audio signals and a center audio signal channel from conventional stereo content (e.g., via a 2-to-3 channel upmixing technique), and processes at least the left and right audio signals so that, when the resulting left and right audio drive signals (also referred to as left and right audio output signals) are output by upward-facing left and right transducers, they provide directional cancellation zones for the left and right audio content, while simultaneously providing a center audio drive signal (also referred to as a center audio output signal) based on the center audio signal for simultaneous output via two upward-facing transducers or via an upward-facing center transducer.

[0013] In this way, the resulting interference and reflection patterns generated in the sound field near the transducer produce two simultaneous stereo sound fields for two adjacent listeners with a reduced number of artifacts compared to prior art systems, particularly with respect to the correlated components of the original left and right audio channels. Although the method of the present invention is particularly advantageous when the audio drive signal is used to drive a transducer in a confined (and preferably symmetrical) space, those skilled in the art will appreciate that the audio drive signal generated by the present invention will provide acoustic advantages in all listening environments.

[0014] Those skilled in the art will understand that the center audio signal represents the signal (or audio content) common to both channels of a two-channel stereo audio signal. Those skilled in the art will also understand that the left and right signals are not positively correlated, meaning that the normalized correlation coefficient of the two (left and right) signals is less than or equal to 0. The left and right signals can also be described as negatively correlated or substantially decorrelated.

[0015] In one embodiment, generating the first audio signal and the second audio signal includes: generating a copy of each of a left audio signal and a right audio signal; inverting the copy of the right audio signal and the copy of the left audio signal; applying a time delay to each of the inverted copy of the right audio signal and the inverted copy of the left audio signal; summing the delayed and inverted copy of the left audio signal with the right audio signal to generate the first audio signal; and summing the delayed and inverted copy of the right audio signal with the left audio signal to generate the second audio signal.

[0016] In an alternative embodiment, generating the first audio signal and the second audio signal includes: generating an inverted copy of each of the left audio signal and the right audio signal; generating a time-delayed copy of each of the left audio signal and the right audio signal; summing and inverting the inverted copy of the right audio signal with the delayed copy of the left audio signal to generate the first audio signal; and summing and inverting the inverted copy of the left audio signal with the delayed copy of the right audio signal to generate the second audio signal.

[0017] Those skilled in the art will appreciate that in further embodiments, the order of these operations (eg, the inversion and delay operations) may be varied.

[0018] Preferably, wherein the left audio drive signal is derived only from the first audio signal, wherein the right audio drive signal is derived only from the second audio signal, and wherein the center audio drive signal is derived only from the center audio signal.

[0019] Preferably, providing the left audio drive signal comprises applying a gain stage to the first audio signal, and / or wherein providing the right audio drive signal comprises applying a gain stage to the second audio signal, and / or wherein providing the center audio drive signal comprises applying a gain stage to the center audio signal.

[0020] Preferably, wherein the left audio drive signal is a first audio signal, wherein the right audio drive signal is a second audio signal, and wherein the center audio drive signal is a center audio signal.

[0021] Preferably, the method further comprises generating a third audio signal based on the center audio signal , wherein the third audio signal is generated according to the following formula: ,in, is a time delay of the center audio signal, wherein the left audio drive signal is obtained by summing the first audio signal with one of the center audio signal and the third audio signal, wherein the right audio drive signal is obtained by summing the second audio signal with one of the center audio signal and the third audio signal, and wherein the center audio drive signal is obtained by the other of the center audio signal and the third audio signal.

[0022] Preferably, providing the left audio drive signal comprises applying a gain stage to at least one of the first audio signal, the center audio signal, and the third audio signal before one of the center audio signal and the third audio signal is summed with the first audio signal, and / or wherein providing the right audio drive signal comprises applying a gain stage to at least one of the second audio signal, the center audio signal, and the third audio signal before one of the center audio signal and the third audio signal is summed with the second audio signal, and / or wherein providing the center audio drive signal comprises applying a gain stage to the other of the center audio signal and the third audio signal.

[0023] Preferably, one of the center audio signal and the third audio signal is summed with the first audio signal and provided as a left audio drive signal, one of the center audio signal and the third audio signal is summed with the second audio signal and provided as a right audio drive signal, and the other of the center audio signal and the third audio signal is provided as a center audio drive signal.

[0024] In one embodiment, generating the third audio signal includes: generating a copy of the center audio signal; inverting the copy of the center audio signal; and applying a time delay to the inverted copy of the center audio signal. Again, those skilled in the art will appreciate that in alternative embodiments, the order of these operations (e.g., the inversion and delay operations) may vary.

[0025] Preferably, the center audio drive signal is band-limited. In this way, spectrum mismatches can be coped with, for example when a tweeter and two midrange speakers are used and the tweeter cannot reproduce lower frequencies.

[0026] Preferably, the time delay is selected so that, when the left audio drive signal, the right audio drive signal and the center audio drive signal are used to simultaneously drive corresponding upward-facing left transducer, right transducer and center transducer located symmetrically in front of and between the left listener and the right listener, a sound radiation pattern associated with the center audio signal is output having a null point directed towards the ipsilateral ear of the left listener and the ipsilateral ear of the right listener.

[0027] Preferably, the time delay is selected so that, when the left audio drive signal and the right audio drive signal are used to simultaneously drive corresponding upward-facing left and right transducers located symmetrically in front of and between a left listener and a right listener, a sound radiation pattern associated with the left audio signal is output having a null point directed toward the ipsilateral ear of the left listener, and a sound radiation pattern associated with the right audio signal is output having a null point directed toward the ipsilateral ear of the right listener.

[0028] Preferably, Calculated according to the following formula:

[0029] in, α is the directionality parameter, where 0 < α ≤ 1, and ,in, (m) is the distance between the centers of the left and right transducers facing upwards, c (m ¹) is the speed of sound in air. Those skilled in the art will understand that αis selected such that, when the left and right audio drive signals are used to simultaneously drive the respective upward-facing left and right transducers located symmetrically in front of and between the left and right listeners, a sound radiation pattern associated with the left audio signal is output having a null point directed toward the ipsilateral ear of the left listener, and a sound radiation pattern associated with the right audio signal is output having a null point directed toward the ipsilateral ear of the right listener. Those skilled in the art will also appreciate that It can be calculated accordingly, mutatis mutandis.

[0030] Preferably, the method further comprises: generating a left audio signal, a right audio signal and a center audio signal by upmixing the two-channel stereo audio signal to generate a left upmix audio signal , right upmix audio signal and center upmix audio signal , wherein the left audio signal, the right audio signal and the center audio signal are obtained by the left upmix audio signal, the right upmix audio signal and the center upmix audio signal respectively.

[0031] Preferably, the left upmix audio signal, the right upmix audio signal and the center upmix audio signal are provided as a left audio signal, a right audio signal and a center audio signal respectively.

[0032] Preferably, the method further comprises: generating a left audio signal, a right audio signal and a center audio signal by upmixing the two-channel stereo audio signal to generate a left upmix audio signal , right upmix audio signal and center upmix audio signal and applying a sound image localization algorithm to the left upmix audio signal, the right upmix audio signal, and the center upmix audio signal to generate a left audio signal, a right audio signal, and a center audio signal, respectively.

[0033] Preferably, the sound image localization algorithm generates the left audio signal, the right audio signal and the center audio signal according to the following formula:

[0034] in, is the compensation gain parameter, and 、 and is the gain factor calculated according to the following formula:

[0035] in, b is a balance parameter ranging from -1 to 1. As can be understood, b = -1 corresponds to left balance, b =1 corresponds to right balance.

[0036] Preferably, the method further comprises applying one or more spectral equalization filters to one or more of the audio drive signals. In this way, spectral coloration introduced by applying zero-steering during the derivation of the audio drive signals from the original stereo audio signal can be compensated.

[0037] Preferably, the method further includes applying a first spectral equalization filter to the left and right audio drive signals, and applying a different second spectral equalization filter to the center audio drive signal. In this manner, different spectral coloration characteristics introduced by different zero-steering configurations of the left and right audio drive signals and the center audio drive signal can be compensated.

[0038] Preferably, the method further comprises applying a gain stage to the one or more audio drive signals.

[0039] Preferably, the method further comprises: applying a time delay to the center audio drive signal or the left and right audio drive signals , so that when the left audio drive signal, the right audio drive signal, and the center audio drive signal are used to simultaneously drive the corresponding upward-facing left transducer, the right transducer, and the center transducer that are symmetrically located in front of and between the left and right listeners, the sound radiation patterns received by the left and right listeners are aligned in time.

[0040] Preferably, the time delay in sampling points is Calculated according to the following formula:

[0041] in, d is the vertical displacement of the upward-facing center transducer relative to a line extending between the upward-facing right and left transducers (m), c is the speed of sound in air (m / s), is the digital sampling rate of the audio driving signal (Hz).

[0042] According to a third aspect of the present invention, there is provided a computer readable medium comprising computer executable code which, when executed on one or more processors of an audio system, causes the system to perform the method of the first or second aspect.

[0043] According to a fourth aspect of the present invention, there is provided a computer program product comprising instructions which, when executed by a computer, cause the computer to perform the method of the first or second aspect.

[0044] According to a fifth aspect of the present invention, there is provided an audio system comprising one or more digital signal processors adapted to perform the method of the first or second aspect.

[0045] Preferably, the audio system further comprises: a left upward-firing transducer configured to be driven by a left audio drive signal; and a right upward-firing transducer configured to be driven by a right audio drive signal.

[0046] Preferably, the audio system further comprises an upward-firing center transducer configured to be driven by a center audio drive signal, wherein the upward-firing center transducer is located between the upward-firing left and right transducers.

[0047] Preferably, wherein the upwardly firing centre transducer is offset from a line extending between the upwardly firing left and right transducers.

[0048] Preferably, the upward-facing center transducer has different acoustic characteristics than the upward-emitting left and right transducers.

[0049] Preferably, the upward-firing left and right transducers have substantially the same acoustic characteristics within an audible bandwidth of 20 Hz to 20,000 Hz.

[0050] According to a sixth aspect of the invention, there is provided a vehicle comprising the audio system according to the fifth aspect, wherein the upwardly facing transducers are mounted in a passenger cabin of the vehicle and are arranged at the front of the passenger cabin in a speaker system arranged symmetrically about a longitudinal axis of the passenger cabin.

[0051] The present invention can also be described as a method for generating drive signals for an arrangement of two or three sound sources, wherein the emission of each sound source contributes to the production of two adjacent, simultaneous stereo sound fields. The method comprises the following steps: receiving a stereo audio signal for stereo reproduction; deriving from the stereo audio signal, by a 2-to-3 channel upmixing technique, first and second highly decorrelated left and right sound field signals and a further center sound field channel representing a signal common to the two received audio signals; and applying axisymmetric zero-point steering about the centerline of the sound source arrangement, steering the generated sound from the decorrelated left and right sound field signals generated by the upward-facing axisymmetric pair of sound sources to the geometric limits of the entire simultaneous sound field.

[0052] In the first aspect of the invention, the center sound field channel is output via a pair of sound sources, while in the second aspect, the center sound field channel is output via an upward-facing center sound source located on the centerline of the sound field, and preferably, an extended center channel sound field area can be generated by applying null-steering to a common third center sound field signal. In this way, the resulting interference and reflection patterns generated in the sound field near the sound source produce two simultaneous stereo sound fields for two adjacent listeners.

[0053] The method of the present invention can also be described as a method for generating audio drive signals configured for use in an audio system comprising left and right upward-facing sound sources, preferably arranged in the center of a confined listening environment, wherein the arrangement is symmetrical about the centerline of the given space. The left and right sound sources are used to produce a simultaneous stereo sound field for the left and right listening positions. This is achieved by first extracting substantially decorrelated left and right channel signal content from the conventional stereo content through a 2-to-3 channel upmixing technique. These decorrelated signals are then fed to two drivers via a selective delay and level compensation stage.

[0054] Considering the left edge of the reproduced sound field, the upmixed left stereo content is produced by left and right upward-facing sources through acoustic cancellation in the ipsilateral ear region of the left listener. This cancellation method is derived from the theory of null-steering applied to the first-order directivity patterns (e.g., dipole, cardioid, or hypercardioid) of two ideal hemispherical sources representing two upward-firing drivers. The effect of null-steering is threefold. First, the cancellation region directed toward the ipsilateral ear of the left listener (for left edge reproduction) imposes a perceptible interaural level difference (ILD) between the direct sound components reaching the ipsilateral and contralateral ears at the left listening position. Second, when null-steering is applied, the maximum amplitude of the reproduced source directivity pattern is directed toward the left side of the space, parallel to the centerline. This has the effect of producing strong first-order reflections on surfaces occupying the area to the front left of the left listener. These two effects combine to cause the left edge to be perceived as originating toward the front left corner of the environment. Meanwhile, the third effect is the creation of a rightward amplitude maximum, which delivers the upmixed left stereo content to the ipsilateral ear of the right listener, as intended. For the right border of the reproduced sound field, the same process is carried out symmetrically about the center line of the space, thereby completing a simultaneous stereo sound field for two adjacent listeners.

[0055] In view of the aforementioned upmixing of 2- to 3-channel audio content, in addition to presenting the extracted left / right channel content, another focus is on reproducing the remaining channel, often referred to as the "center channel." The basic approach (corresponding to the first aspect of the present invention) generates a center audio signal at equal levels from left and right upward-facing sources, summing it with the extracted and processed left and right channel upmix signals. Another approach (embodied in the second aspect of the present invention) employs an additional source located at the centerline of the space between the two existing sources. This source generates the center audio signal, while the left and right sources generate the extracted and processed left and right channel upmix signals.

[0056] This approach reproduces stereo center content, which is perceived as originating from the speaker system (in the centerline region of the space). To simultaneously shift the center content toward the forward line of sight of two adjacent listeners, a preferred embodiment of the present invention (also encompassed by the second aspect of the invention) utilizes null steering. By processing the extracted stereo center signal and combining the processed content for output to all three aforementioned sound sources, the cancellation region of center content is directed toward the ipsilateral ears of both listeners. This, in turn, has the effect of directing acoustic energy toward surfaces in front of the listeners (if present), while simultaneously reducing the ILD of direct sound components reaching both occupants. Consequently, strong reflections from surfaces in front of and to the sides (if present) of the two occupants complement the aforementioned lower ILD, causing the stereo center audio content to be perceived as originating from a more central region of the effective arc.

[0057] As will be understood by those skilled in the art, the present invention can be implemented in various ways depending on the application. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of an exemplary listening environment including a speaker system configured to generate simultaneous left and right effective arcs for reception by left and right listeners; Figure 2 is a schematic plan view of an exemplary listening environment showing first-order polar radiation patterns corresponding to left audio content produced by upward-facing left and right transducers; Figure 3 shows that by manipulating the directivity parameter α A selection of common polar patterns generated; Figure 4 is a flow chart showing steps of a method for generating left and right audio drive signals based on a two-channel stereo audio signal; Figure 5 is a schematic diagram showing an audio system including left and right transducers facing upward; Figure 6 It shows that according to Figure 4A flowchart of the method steps for generating left and right drive output signals according to a specific embodiment of the present invention; Figure 7 is a schematic diagram showing an audio system including an upward-facing left transducer, a right transducer, and a center transducer; Figure 8 is a flow chart illustrating steps of a method for generating a left audio drive signal, a right audio drive signal, and a center audio drive signal based on a two-channel stereo audio signal; Figure 9 It shows that according to Figure 8 A flowchart of a method for generating a left audio drive signal, a right audio drive signal, and a center audio drive signal according to a specific embodiment of the present invention; Figure 10 It shows that according to Figure 8 A flowchart of a method for generating a left audio drive signal, a right audio drive signal, and a center audio drive signal according to another embodiment of the present invention; Figure 11A and Figure 11B is a schematic plan view of an exemplary listening environment showing first-order polar radiation patterns corresponding to center audio content produced by upward-facing left, right, and center transducers; Figure 12A and Figure 12B is a schematic plan view of an exemplary listening environment showing different arrangements of an upward-facing left transducer, a right transducer, and a center transducer; Figure 13 is a schematic diagram showing an audio system including upward-facing left, right and center transducers, wherein the center audio drive signal is time delayed. DETAILED DESCRIPTION

[0059] The present invention can be implemented in several different ways depending on a given listening environment and sound reproduction purpose / scenario. Example implementations of some expected application scenarios will be described below with reference to the accompanying drawings.

[0060] The present invention is directed to providing audio drive signals (also referred to as audio output signals) configured to generate two simultaneous stereo sound fields for adjacent listening positions receiving the same stereo program material when the audio drive signals are used to drive a centrally arranged configuration of two or more sound sources (referred to herein as a speaker system). The present invention is necessary, for example, in the design and deployment of stereo audio reproduction systems where aesthetic considerations, physical size, spacing, location, or packaging constraints are paramount.

[0061] An example application scenario of the present invention is described in Figure 1, which illustrates an exemplary listening environment 10, including a speaker system 12 configured to generate simultaneous left and right active arcs for reception by left and right listeners 20 and 22. This example relates to the field of sound reproduction systems in confined, enclosed spaces, such as vehicle cabins, medium-sized audiovisual environments, or small offices, though it is noted that the present invention is not limited to such environments and can also be implemented in unconfined listening environments. As shown, the present invention seeks to generate two sound field active arcs (where "L" represents the leftmost boundary of the stereo active arc; "R" represents the rightmost boundary of the stereo active arc) designed to have a width significantly greater than the physical dimensions of the speaker system 12 itself. This general example deployment will be used to explain the components of the present invention and its mode of operation in detail.

[0062] The speaker system 12 includes a pair of transducers 14, 16 (also referred to as drive units or sound sources) positioned adjacent to each other. In particular, the speaker system 12 includes a left transducer 14 and a right transducer 16. In other embodiments, as will be discussed further below, the speaker system 12 may also include a center transducer 18.

[0063] Each transducer 14, 16 is oriented upward so that the transducers 14, 16 (i.e., the diaphragms of the drive units) are oriented coplanar with the azimuth plane in a standard spherical coordinate system relative to an observer at the origin. In addition, the downward-facing portions of the transducers 14, 16 are acoustically decoupled and operate in independent (sealed) volumes to avoid acoustic interaction effects within the overall enclosure. Therefore, with ideal sound source characteristics, the sound radiated from each transducer 14, 16 can be modeled using monopole characteristics in the positive elevation direction of the above-mentioned coordinate system (i.e., each sound source is assumed to have hemispherical directivity within the available bandwidth). It is also assumed that the characteristics of all transducers 14, 16 are substantially similar within the audible bandwidth limits and within the use / operation bandwidth. Although this is not a limitation on the loudspeaker system 12, it simplifies the description of the present invention. When used, the loudspeaker system 12 is provided with respect to the centerline of the listening environment 10 (i.e., as shown in FIG. 1 ). Figure 1 The center line (shown as a central longitudinal axis in dashed lines) is symmetrically arranged between and in front of two adjacent listening positions (corresponding to the positions of left listener 20 and right listener 22, respectively). The area between the center line and the left and right boundaries of listening environment 10 defines the maximum extent of the effective arc of two simultaneous stereo reproductions in front of the listening positions.

[0064] Considering the reproduction of material at the left and right boundaries of simultaneous active arcs, it is first necessary to derive a 2-to-3-channel upmix from the input stereo audio format such that the resulting upmixed left and right channel signals are highly decorrelated. The reason for this requirement will become apparent after first describing the reproduction of two simultaneous, adjacent active arcs. The left and right transducers 14 and 16 are responsible for providing the left and right sound field boundaries for the two adjacent active arcs. For simplicity, an idealized 2D planar model will be used to describe the operating mode of each transducer 14 and 16.

[0065] Regarding the reproduction of the simultaneous left boundary (i.e. the audio content or sound radiation pattern corresponding to the left channel in a two-channel stereo audio signal), Figure 2 An exemplary scenario is shown in which the left and right transducers 14, 16 are driven by audio drive signals (also referred to as audio output signals) such that the overall summed radiation pattern describes a first-order polar response function in the azimuth plane (again, considering an ideal hemispherical source directivity in the azimuth plane). The method of generating such an audio drive signal to produce a first-order directivity pattern will be referenced later. Figure 4 Description. For reproduction of the left boundary, the radiation pattern is induced to produce a sound pressure level null 28 in the direction of the region of the ipsilateral ear of the left listener 20, and to produce a region of relatively high sound level output directed towards the region of the ipsilateral ear of the right listener 22 (wherein the term "ipsilateral ear" refers to the ear located on the same side of the listener as the loudspeaker system 12). This can be achieved by driving each transducer 14, 16 based on the upmixed extracted left channel signal, but the left audio drive signal driving the left transducer 14 includes an inverted and time-delayed version of the upmixed extracted left channel signal relative to the right drive signal driving the right transducer 16. Time Delay According to the radiation function parameters Calculations are based on Southern, A., and Murphy, D., “Low complexity directional sound sources for finitedifference time domain room acoustic models,” presented at the 126th Audio Engineering Society (AES) Convention, Munich, Germany, May 2009:

[0066] in, τ Defined as the displacement between the centers of the left and right transducers 14, 16 d (m) and the speed of sound in air c (m ¹), so (s), andα is the directivity parameter, where 0 < α ≤ 1, which describes the directionality of the 2D monopole ( α = 0) to a 2D dipole ( α = 1) (see, for example, Southern, A. and Murphy, D., “Low complexity directional sound sources for finite difference time domain room acoustic models,” in Proceedings of the 126th Audio Engineering Society (AES) Convention, Munich, Germany, May 2009). For example, α = 0.5, the resulting radiation function parameter describes the well-known cardioid polar diagram. α An example of a first-order directional function of a value is Figure 3 As shown. α The desired sound pressure level zero point can be set at all angles is directed upward (relative to the centerline of the listening environment 10) because the directivity function is derived from the dipole ( α =1) transition to heart shape ( α = 0.5). Given the desired radiation function parameter values ​​for the null angle, the time delay of the upmixed extracted left channel signal driving the leftmost driver (relative to the left channel signal driving the rightmost driver) is applied. Defined as:

[0067] Null-steering the left boundary audio in this manner has three important effects. First, null point 28 is defined as being directed toward the ipsilateral ear of left listener 20, resulting in an increase in the interaural level difference (ILD) between the sound pressure levels experienced by the ipsilateral and contralateral ear regions in the left listening position, favoring the contralateral ear. The contralateral ear is the ear located on the opposite side of the listener relative to loudspeaker system 12.

[0068] Secondly, simultaneously, the directivity pattern generated by the null-steering in this manner provides a local amplitude radiation maximum 30 in the direction of the left edge of the listening environment 10 (perpendicular to the centerline), resulting in strong early reflections perceived in the contralateral ear region of the left listener 20. This further increases the ILD favoring the contralateral ear, so that the left edge of the sound field perceived by the left listener 20 is perceived as originating in the left edge region of the listening environment 10. In this way, a virtual secondary sound source is created in the left edge region of the listening environment 10, providing the left edge of the reproduced stereo effective arc for the left listener 20.

[0069] Thirdly, and simultaneously, the radiation pattern formed to provide the aforementioned left-of-centerline effect also induces an amplitude radiation maximum 32 with a wider surrounding lobe directed toward the right edge of the enclosure (again perpendicular to the centerline). This then provides sufficient sound pressure level output in the direction of the ipsilateral ear of the right listener 22 to reproduce the left edge signal in the desired manner, so that it is perceived as originating from the area of ​​the loudspeaker system 12 as the left edge of the simultaneous effective arc for the right listener 22.

[0070] Another benefit of the null-steering operation stems from the symmetry of the resulting sound radiation pattern about a line perpendicular to the centerline. This directs the null away from listeners 20, 22, and in some embodiments, toward a vertical planar wall or screen surface of listening environment 10. As a result, early first-order specular reflections between loudspeaker system 12 and the ipsilateral ear of left listener 20, which would otherwise exist, are suppressed. This further reinforces the direction of arrival perceived by left listener 20 as being towards the left edge of the listening environment.

[0071] The directional sound cancellation and enhancement zones, combined with the early reflection patterns in the front region of the listening environment 10, together create the left boundaries of the two simultaneously active arcs. The right boundaries of the simultaneously active arcs are created in an axisymmetric manner about the centerline of the loudspeaker system 12 by correspondingly creating the left boundaries (i.e., each transducer 14, 16 is driven based on the upmixed extracted right channel signal, but the right audio drive signal driving the right transducer 16 contains an inverted and delayed version of the upmixed extracted right channel signal relative to the left audio drive signal driving the left transducer 14). That is, by Figure 2 The radiation pattern shown is mirrored about the centerline of the listening environment 20, creating an identical arrangement for the right boundary of the effective arc. The right boundary reproduction is performed simultaneously with the left boundary reproduction by signal summation, which will now be referred to as Figure 4 Provide a description.

[0072] Figure 4 FIG4 is a flow chart illustrating a method 40 for generating a left audio drive signal and a right audio drive signal based on a two-channel stereo audio signal according to aspects of the present invention. In step 42, a two-channel stereo audio signal is received. The two-channel stereo audio signal is composed of a left channel audio signal and a right channel audio signal.

[0073] In step 44, a left audio signal is generated based on the two-channel stereo audio signal. , right audio signal and center audio signal In particular, the left audio signal, the right audio signal and the center audio signal can be extracted from the two-channel stereo audio signal by a suitable 2-to-3-channel upmixer. The audio signals are generated so that the left audio signal and the right audio signal are not positively correlated, for reasons that will be explained below.

[0074] At step 46, a first audio signal is generated based on the left audio signal and the right audio signal. and the second audio signal , according to the following formula:

[0075] in, is a time delay, chosen to steer the radiation null 28 (by generating a specific radiation pattern) as previously described.

[0076] At step 48, the left audio drive signal for driving the left transducer 14 is The first audio signal and center audio signal The sum is obtained, for example according to the following formula:

[0077] At step 49, the right audio drive signal for driving the right transducer 16 is By the second audio signal and center audio signal The sum is obtained, for example, as follows:

[0078] These equations lead to the aforementioned necessity of deriving highly decorrelated left and right audio signals from the received two-channel stereo audio signal via a suitable 2-to-3-channel upmixer. As is evident from the signal equations above, any common correlation content between the original left and right audio signals will result in signal cancellation via comb filtering, due to the summation of such common content with delayed versions in both the left and right audio drive signals. When generating the left and right audio drive signals, this situation should be avoided as much as possible within the operational constraints of the chosen upmixer to maintain optimal independence between the reproduced left and right boundary signals, thereby preserving the width of the resulting effective arc.

[0079] The center audio signal extracted from the received two-channel stereo audio signal is defined as the (highly) correlated signal content common to the original left-channel audio signal and the right-channel audio signal extracted using the 2-to-3-channel upmixing algorithm. Figure 4 In the method, the center audio drive signal is configured to be output simultaneously through the left and right transducers 14, 16 to reproduce the center channel content. Optionally, a gain stage can be applied to any of the center, left, or right audio drive signals to weight the prominence of the signal content in the output and to adapt to the acoustic characteristics of the transducers 14, 16 and their location in space. This also applies to the method 80 described below.

[0080] Figure 5 is a schematic diagram illustrating an audio system 50 that includes a speaker system 12 comprising left and right transducers 14, 16 facing upward. The audio system 50 also includes one or more speakers configured to perform Figure 4 method for a digital signal processor. In particular, Figure 5 An illustration of the signal processing required to provide a first order polar radiation pattern from the left and right transducers 14, 16 is provided.

[0081] Figure 6 It shows that according to Figure 4 A flowchart of the steps of a method for generating left and right audio drive signals in accordance with a specific embodiment of the present invention. In this embodiment, a copy of a left audio signal and a copy of a right audio signal are generated. Next, the copy of the right audio signal and the copy of the left audio signal are inverted, and then a time delay is applied to the inverted copies of the right and left audio signals. The right audio signal is summed with a delayed and inverted copy of the left audio signal to generate a first audio signal, while the left audio signal is summed with a delayed and inverted copy of the right audio signal to generate a second audio signal. Finally, the center audio signal is summed with each of the first and second audio signals to create left and right audio drive signals.

[0082] Those skilled in the art will appreciate that in alternative embodiments, the inversion and delay operations may be applied in a different order. For example, in one embodiment, the time delay may be applied before the inversion. In another embodiment, generating the first audio signal and the second audio signal may include generating inverted copies of the left and right audio signals, generating time-delayed copies of the left and right audio signals, summing and inverting the inverted copy of the right audio signal with a delayed copy of the left audio signal to generate the first audio signal, and summing and inverting the inverted copy of the left audio signal with a delayed copy of the right audio signal to generate the second audio signal.

[0083] Next, an alternative aspect of the invention will be described in which left, right and center audio drive signals are generated based on a two-channel stereo audio signal for driving the left, right and center transducers 14, 16, 18, respectively.

[0084] Figure 7 1 is a schematic diagram illustrating an audio system 70 in which the speaker system 12 further includes a center transducer 18. The center transducer 18 is located symmetrically between the left transducer 14 and the right transducer 16. The center transducer 18 is offset from a line extending between the centers of each of the left and right transducers 14, 16. The center transducer 18 is configured to output a center audio drive signal that can be adjusted based on the Figure 8 method to generate.

[0085] Figure 84 is a flowchart illustrating a method 80 for generating a left audio drive signal, a right audio drive signal, and a center audio drive signal based on a two-channel stereo audio signal.

[0086] Next, at step 87, a left audio drive signal is provided based on the first audio signal. At step 88, a right audio drive signal is provided based on the second audio signal. At step 89, a center audio drive signal is provided based on the center audio signal. The left, right, and center audio drive signals are configured to be output simultaneously through the left, right, and center transducers 14, 16, 18, respectively.

[0087] In one embodiment, for example, Figure 9 As shown in the specific method of , the left audio drive signal can be based only on the first audio signal, the right audio drive signal can be based only on the second audio signal, and the center audio drive signal can be based only on the center audio signal. This can be defined by the following formula:

[0088] However, if the center audio content is output only through the center transducer 18, this will result in the center audio content being perceived as originating from the center transducer 18. It is desirable to provide a perceived center channel sound field that is extended and exists outside the physical dimensions of the speaker system 12 so that the center channel contribution is heard as originating from a more central location within the effective arc. Thus, in another embodiment, for example, Figure 10 As shown in the specific method (note that the dashed lines in the flowchart represent an alternative sequence of steps), the third audio signal can be generated based on the center audio signal , wherein the third audio signal is generated according to the following formula:

[0089] in, is a time delay applied to the center audio signal. Then, the left audio drive signal can be generated based on the sum of the first audio signal and one of the center audio signal and the third audio signal, the right audio drive signal can be generated based on the sum of the second audio signal and one of the center audio signal and the third audio signal, and the center audio drive signal can be generated based on the other of the center audio signal and the third audio signal. For example, the left audio drive signal , right audio drive signal and center audio drive signal It can be defined according to the following formula:

[0090] or:

[0091] Processing the center audio signal in this manner (i.e., applying the inversion and time delay operations) results in two simultaneous radiation patterns when the left, right, and center audio drive signals are simultaneously output by the left, right, and center transducers 14, 16, 18, respectively. Figure 11A and Figure 11B As shown, the time delay According to the above The same method is used to calculate the radiation pattern so that the resulting radiation pattern has sound pressure level nulls 100 and 106 pointing to the ipsilateral ears of the left listener 20 and the right listener 22. Note that this center channel reproduction method is symmetrical about the center line of the listening environment 10. In addition, the time delay may be chosen so that the global radiation maxima 102, 108 are directed away from the listeners 20, 22. Thus, strong reflections originating from the front and sides (if present) of the listening environment 10 complement the reduced ILD provided by the directivity null, so that the stereo center audio content is perceived as originating from the center region of the simultaneous effective arc of playback.

[0092] Another important factor to consider is the positioning of the center transducer 18 along the centerline of the listening environment 10, that is, relative to the left and right transducers 14, 16 and the left and right listeners 20, 22. In a preferred embodiment, the center transducer 18 is arranged so that a line connecting the center point of the center transducer 18 and the center points of the left and right transducers 14, 16 can be extrapolated to areas in the adjacent listening area that are not in the ipsilateral ear area of ​​either listener 20, 22. This positioning factor is depicted in Figure 12A In, and given that Figure 12B The scene shown, its importance is shown, in Figure 12B , the position of the center transducer 18 relative to the left and right transducers 14, 16 significantly changes the extrapolated line of sight of the intersection lines so that they point toward the ipsilateral ears of the two listeners 20, 22. This arrangement requires careful selection of the signal delay factors so that the null point of the center channel signal is simultaneously directed toward the ipsilateral ears of the two listeners 20, 22.

[0093] In summary, method 40 provides a method for generating audio drive signals for driving only the left and right transducers 14, 16. In this case, the center audio signal extracted by upmixing can be used to drive the left and right transducers 14, 16 simultaneously without further processing of the signal. Therefore, the perceived sound source area of ​​the center audio signal content will be approximately the size of the speaker system 12 of the left and right transducers 14, 16 in width. Alternatively, method 80 provides a method for generating audio drive signals for driving the left and right transducers 14, 16 and a center transducer 18 that is additionally located on the longitudinal centerline of the listening environment 10. In this case, when the center transducer 18 is longitudinally offset relative to the left and right transducers 14, 16, it is advantageous to delay or advance the center audio drive signal to accommodate the wavefront time alignment at the listening positions of the left and right listeners 20, 22.

[0094] For example, see Figure 13 , which shows an audio system 130, including left transducer, right transducer and center transducer 14, 16, 18, wherein the center transducer 18 is offset relative to the left and right transducers 14, 16 along the longitudinal axis in the direction of the listening position by a distance d (m), applied to the center audio drive signal The sample points are time delayed to provide proper time alignment, where: (Sampling point) For a given wave speed c (m / s) and digital audio sampling rate Those skilled in the art will also appreciate that if the center transducer 18 is instead offset behind the left and right transducers 14, 16 relative to the left and right listeners 20, 22, then a delay may instead be applied to the left and right audio drive signals.

[0095] The present invention can also be implemented by including a sound image localization algorithm to move the perceived acoustic position of the center audio signal extracted by the upmix to the left or right in space to the boundary of the valid arc. The following formula shows the calculation of the gain coefficient:

[0096] in, b is a balance parameter ranging from -1 (i.e. left-balanced) to 1 (i.e. right-balanced), 、 and are the resulting gain coefficients for the left, right, and center channels, respectively.

[0097] The gain is applied as follows:

[0098] in, is a constant make-up gain value that can be applied to maintain a consistent volume as the signal is panned. 、 and The left upmix audio signal, the right upmix audio signal, and the center upmix audio signal are generated by applying a 2-to-3-channel upmix to a two-channel stereo audio signal. It will be appreciated that in an alternative embodiment where the sound image localization algorithm is not applied, the relationship between the left upmix audio signal, the right upmix audio signal, and the center upmix audio signal and the left audio signal, the right audio signal, and the center audio signal is typically defined according to the following formula:

[0099] In other implementations, the relationships between the left upmix audio signal, the right upmix audio signal, and the center upmix audio signal and the left audio signal, the right audio signal, and the center audio signal may be defined according to the following formula instead:

[0100] That is, the left audio signal, the right audio signal, and the center audio signal may correspond to inverted versions of the left upmix audio signal, the right upmix audio signal, and the center upmix audio signal. In this case, a subsequent inversion operation is applied when generating each of the left audio drive signal, the right audio drive signal, and the center audio drive signal. It will be appreciated that when applying the above-described sound image localization algorithm, the left upmix audio signal, the right upmix audio signal, and the center upmix audio signal may also be inverted.

[0101] The tonal alignment and spectral balance of the overall sound field produced by loudspeaker system 12 can be altered (in any embodiment) through sophisticated signal processing routines. Of particular note, the two sound sources, when working together to produce a first-order directivity pattern (as previously described), exhibit a first-order high-pass filter response in the resulting overall sound field. The resulting spectral imbalance of energy in the low- to high-frequency range caused by these response characteristics is compensated for by employing appropriate filtering strategies tuned to the spectral characteristics produced by the selected first-order directivity function / functions (to achieve the desired null-steering configuration).

Claims

1. A method for generating an audio drive signal based on a two-channel stereo audio signal, for driving an upward-firing transducer, the method comprising the following steps: Receive two-channel stereo audio signals; Generate a left audio signal based on the two-channel stereo audio signal , right audio signal and center audio signal , wherein the left audio signal and the right audio signal are not positively correlated; Generate a first audio signal based on the left audio signal and the right audio signal and the second audio signal , wherein the first audio signal and the second audio signal are generated according to the following formula: as well as ,in, is the time delay between the left audio signal and the right audio signal; Provides a left audio drive signal for driving the upward-firing left transducer , wherein the left audio drive signal is obtained by summing the first audio signal and the center audio signal; and Provides the right audio drive signal for driving the upward-firing right transducer , wherein the right audio driving signal is obtained by summing the second audio signal and the center audio signal.

2. The method according to claim 1, wherein Providing the left audio drive signal comprises applying a gain stage to at least one of the first audio signal and the center audio signal before summing the first audio signal and the center audio signal, and / or Wherein providing the right audio drive signal comprises applying a gain stage to at least one of the second audio signal and the center audio signal before summing the second audio signal and the center audio signal.

3. A method for generating an audio drive signal based on a two-channel stereo audio signal, for driving an upward-firing transducer, the method comprising the following steps: Receive two-channel stereo audio signals; Generate a left audio signal based on the two-channel stereo audio signal , right audio signal and center audio signal , wherein the left audio signal and the right audio signal are not positively correlated; Generate a first audio signal based on the left audio signal and the right audio signal and the second audio signal , wherein the first audio signal and the second audio signal are generated according to the following formula: as well as ,in, is the time delay between the left audio signal and the right audio signal; Provides a left audio drive signal for driving the upward-firing left transducer , wherein the left audio driving signal is obtained from the first audio signal; Provides the right audio drive signal for driving the upward-firing right transducer , wherein the right audio drive signal is obtained from the second audio signal; and Provides the center audio drive signal for driving the upward-firing center transducer , wherein the center audio driving signal is obtained from the center audio signal.

4. The method according to claim 3, wherein: The left audio drive signal is derived only from the first audio signal, wherein the right audio drive signal is derived only from the second audio signal, and wherein the center audio drive signal is derived only from the center audio signal.

5. The method according to claim 4, wherein Providing the left audio drive signal includes applying a gain stage to the first audio signal, and / or Wherein, providing the right audio drive signal comprises: applying a gain stage to the second audio signal, and / or Wherein providing the center audio drive signal comprises applying a gain stage to the center audio signal.

6. The method according to claim 3, further comprising: Generate a third audio signal based on the center audio signal , wherein the third audio signal is generated according to the following formula: ,in, is the time delay of the center audio signal, The left audio driving signal is obtained by summing the first audio signal with one of the center audio signal and the third audio signal. The right audio driving signal is obtained by summing the second audio signal with one of the center audio signal and the third audio signal, and The center audio driving signal is obtained from the other of the center audio signal and the third audio signal.

7. The method according to claim 6, wherein: providing the left audio drive signal comprises applying a gain stage to at least one of the first audio signal, the center audio signal, and the third audio signal before summing one of the center audio signal and the third audio signal with the first audio signal, and / or wherein providing the right audio drive signal comprises applying a gain stage to at least one of the second audio signal, the center audio signal, and the third audio signal before summing one of the center audio signal and the third audio signal with the second audio signal, and / or Wherein providing the center audio drive signal comprises applying a gain stage to the other of the center audio signal and the third audio signal.

8. The method according to claim 6 or 7, wherein: The time delay is selected so that, when the left audio drive signal, the right audio drive signal and the center audio drive signal are used to simultaneously drive corresponding upward left transducer, upward right transducer and upward center transducer symmetrically located in front of and between a left listener and a right listener, a sound radiation pattern associated with the center audio signal is output, the sound radiation pattern having a null point directed toward the ipsilateral ear of the left listener and the ipsilateral ear of the right listener.

9. The method according to any one of claims 3 to 8, further comprising: Applying a time delay to the center audio drive signal or to the left and right audio drive signals , so that when the left audio drive signal, the right audio drive signal, and the center audio drive signal are used to simultaneously drive the corresponding upward left transducer, upward right transducer, and upward center transducer symmetrically located in front of and between the left listener and the right listener, the sound radiation patterns received by the left listener and the right listener are aligned in time.

10. The method according to claim 9, wherein: The time delay Calculated according to the following formula: in, d is the vertical displacement of the upward center transducer relative to a line extending between the upward right transducer and the upward left transducer, c is the speed of sound in air, is the digital sampling rate of the audio driving signal.

11. The method according to any one of claims 3 to 10, wherein The center audio drive signal is band limited.

12. A method according to any one of the preceding claims, wherein The time delay is selected so that, when the left audio drive signal and the right audio drive signal are used to simultaneously drive corresponding upward-facing left transducers and upward-facing right transducers symmetrically located in front of and between a left listener and a right listener, a sound radiation pattern associated with the left audio signal having a null point directed toward the ipsilateral ear of the left listener is output, and a sound radiation pattern associated with the right audio signal having a null point directed toward the ipsilateral ear of the right listener is output.

13. A method according to any one of the preceding claims, wherein Calculated according to the following formula: in, α is the directionality parameter, where 0 < α ≤ 1, and ,in, (m) is the distance between the centers of the upward-facing left transducer and the upward-facing right transducer, and c (m ¹) is the speed of sound in air.

14. The method according to any one of the preceding claims, further comprising: The left audio signal, the right audio signal, and the center audio signal are generated by: Upmix the two-channel stereo audio signal to generate a left upmix audio signal , right upmix audio signal and center upmix audio signal , wherein the left audio signal, the right audio signal and the center audio signal are respectively obtained from the left upmix audio signal, the right upmix audio signal and the center upmix audio signal.

15. The method according to claim 14, further comprising: A sound image localization algorithm is applied to the left upmix audio signal, the right upmix audio signal, and the center upmix audio signal to generate the left audio signal, the right audio signal, and the center audio signal, respectively.

16. The method according to claim 15, wherein The sound image localization algorithm generates the left audio signal, the right audio signal, and the center audio signal according to the following formula: in, is the compensation gain parameter, and 、 and is the gain factor calculated according to the following formula: in, b is a balance parameter ranging from -1 to 1.

17. The method according to any one of the preceding claims, further comprising: One or more spectral equalization filters are applied to one or more of the audio drive signals.

18. The method according to any one of the preceding claims, further comprising: A gain stage is applied to one or more of the audio drive signals.

19. A computer program product comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of the preceding claims.

20. An audio system comprising one or more digital signal processors adapted to perform the method according to any one of claims 1 to 18.

21. The audio system of claim 20, further comprising: an upward-firing left transducer configured to be driven by the left audio drive signal; as well as An upward-firing right transducer is configured to be driven by the right audio drive signal.

22. The audio system of claim 21 , further comprising: An upward-firing center transducer is configured to be driven by the center audio drive signal, wherein the upward-firing center transducer is located between the upward-firing left transducer and the upward-firing right transducer.

23. The audio system of claim 22, wherein The upward-facing center transducer has different acoustic characteristics than the upward-firing left transducer and the upward-firing right transducer.

24. An audio system according to any one of claims 21 to 23, wherein The upward-firing left transducer and the upward-firing right transducer have substantially the same acoustic characteristics within an audible bandwidth of 20 Hz to 20,000 Hz.

25. A vehicle comprising an audio system according to any one of claims 21 to 24, wherein The upward-facing transducers are mounted within a passenger cabin of the vehicle and are arranged at the front of the cabin in a speaker system that is symmetrically arranged about a longitudinal axis of the cabin.

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