Automobile Audio System CROSS-REFERENCE TO RELATED APPLICATIONS This patent application is a continuation-in-part (CIP) of U S Patent Application Serial No 10/290,989, filed on November 8,2002, entitled Automobile Audio System, the entire disclosure of which is incorporated herein by reference BACKGROUND [0001] Audio systems are included m virtually every automobile manufactured today These audio systems are typically designed for use while the doors, tailgates, and other openings to the passenger compartment are closed However, people often choose to listen to their automobile audio system while they are outside of their vehicle, such as while they are camping or "tailgating" at a sporting event Because traditional automobile audio systems are not designed for this type of use, listeners may not enjoy as high a quality sound presentation as they could if the system were designed for outdoor use [0002] The acoustic characteristics of a typical vehicle with all of its doors closed will generally be significantly different than the acoustic characteristics of the same vehicle with its tailgate (or one of its doors) open For example, the linear transfer function from each transducer element to various listening locations will be markedly different m each circumstance Furthermore, opening or closing of the tailgate significantly alters the acoustic characteristics of the cabin space SUMMARY [0003] In one aspect, the invention is embodied in an audio system for a vehicle The audio system includes an audio scarce that generates an audio signal A controller is coupled to the audio source and modifies the audio signal with one of a first and a second signal processing operation in response to a user input The first signal processing operation configures the audio signal for listening inside the vehicle and the second signal processing operation configures the audio signal for listening outside the vehicle A first plurality of transducers is coupled to the controller The first plurality of transducers is driven by the audio signal that is configured forlisterang-inside the vehicle by the first signal processing operation A second plurality of transducers is coupled to the controller The second plurality of transducers is dnven by the audio signal that is configured for listening outside the vehicle by the second signal processing operation [0004] In one embodiment, at least one of the transducers in the second plurahty of transducers is positioned in a location that is external to a passenger compartment of the vehicle For example, the transducer can be mounted to a body panel or mounted to a storage bin that is mechanically coupled to the vehicle [0005] The first and the second plurality of transducers can include at least one common transducer or different transducers The first and the second plurality of transducers can also include different sets of transducers The audio system can also include a visual indicator that indicates that the audio signal is driving the second plurality of transducers The visual indicator can include a brake light, a lamp, a light emitting diode (LED), a strobe, a parking light, a directional light, and a reverse light [0006] In one embodiment, the first signal processing operation includes a different equalization parameter than the second signal processing operation The first and/or the second signal processing operations can substantially attenuate the audio signal before it is coupled to one or more of the transducers m the first and the second plurality of transducers [0007] In one embodiment, the controller includes an amplifier The amplifier can control at least one of a gam and an equalization of the audio signal The controller can also be integrated with the audio source The controller can be coupled to the audio source through a GPIO bus, a RS232 bus, a SCSI bus, a serial bus, a parallel bus, a fibre channel bus, an optical bus, a CAN bus, or a MOST bus [0008] The audio source can include an auxiliary input terminal for coupling an auxiliary device to the audio source The auxiliary device can include a radio, a television, a XM tuner, a SAT tuner, a CD player, a DVD player, a cassette player, a MP3 player, aMD player, a hard drive, a PDA, a video player, a portable audio player, a microphone, and a musical instrument A visual mdicatoi can indicate that the auxiliary device is coupled to the audio source [0009] In one embodiment, the user mput includes operating a switch The switch can be a manual switch, an automatic switch, a sensor, a remote control switch, a switch that ljs integrated with die audio source, and a switch that is integrated with the controller The user input could also include opening at least one of a door, a window, a tailgate, a hatch, a moon roof, a sunroof, a trunk, and a storage bm on the vehicle [0010] The audio source can include an AM tuner, an FM tuner, a TV tuner, a XM tuner, a SAT tuner, a CD player, a DVD player, a cassette player, a MP3 player, a MD player, a video player, and/or a hard dnve In one embodiment, a video display is coupled to the audio source The video display can be configured to direct a video image outside the vehicle The video display can include a liquid crystal display (LCD), a light emitting diode (LED) display, a plasma display, a projection display, and a cathode ray tube (CRT) display [0011] The audio signal can include a stereo audio signal, a matrix surround sound audio signal, a 5 1 surround sound audio signal, a 6 1 surround sound audio signal, a 7 1 surround sound audio signal, and/or a two-channel down-mixed audio signal One of the first and the second plurality of transducers can include a pair of transducers and the audio source can provide complete surround information to the pair of transducers The surround information is mixed prior to being provided to the pair of transducers [0012] The audio system also includes a remote control receiver The remote control receiver is coupled to a remote control sensor that is located in a tail light assembly on the vehicle The remote control receiver can be an infrared (IR) receiver or a radio-frequency (RF) receiver The remote control receiver can be disabled when the vehicle is m motion To determine the status of the vehicle, a sensor can be coupled to a tachometer in the vehicle The sensor can detect a state of the engine m the vehicle Another sensor can be coupled to a parking brake m the vehicle That sensor can detect a state of the parking brake m the vehicle In one embodiment, audio source determines a status of the vehicle from a vehicle communication bus The system can also include a battery monitor that monitors a state of a battery in the vehicle [0013] A remote control transmitter can communicate with the remote control receiver The remote control transmitter can have a unique transmitter signal The remote control transmitter can control a door lock, a vehicle ignition, a window, a power door, a power seat, a power moon roof, a trunk lid, a communication system, a courtesy light, and/or a vehicle alarm m the vehicle Additionally, the remote control transmitter can control a system configuration, a source selecborira volume, an equalization function, a tuning function, a band selection, a track selection, a seek function, a scan function, a fast forward function, a rewind function, a skip forward function, a skip back function, a fader function, and/or a balance function of the audio source The remote control transmitter can also control a garage door opener, an electee gate, a light, an appliance, and/or a home security system [0014] In another aspect, the invention is embodied m a method for operating an audio system in a vehicle The method includes generating an audio signal The method also includes modifying the audio signal with one of a first and a second signal processing operation in response to a user input The first signal processing operation configures the audio signal for listening inside the vehicle and the second signal processing operation configures the audio signal for listening outside the vehicle The method includes transducing the audio signal that is configured for listening inside the vehicle with a first plurality of transducers The method further includes transducing the audio signal that is configured for listening outside the vehicle with a second plurality of transducers [0015] The first and the second plurality of transducers can include at least one common transducer The method can include visually indicating when the audio signal is transmitted to the second plurality of transducers The method can also include changing an equalization parameter in the audio signal m response to at least one of the first and the second signal processing operations The method can also include remotely controlling the audio system In one embodiment, the method includes coupling an auxiliary device to the audio system and visually indicating when the auxiliary device is coupled to the audio source [0016] The method can also include providing complete surround information to a pair of transducers in either the first or the second plurality of transducers The surround information can be mixed prior to providing the -surround information to the pair of transducers [0017] The user mput can include opening at least one of a door, a window, a tailgate, a hatch, a moon roof, a sunroof, a trunk, and a storage bin on the vehicle The user input can include operating a switch The method can also include remotely controlling a door lock, a vehicle ignition, an air conditioner, a heater, a defroster, trunk lid, a communication system, a window, a power door, a power seat, a power moon roof, a courtesy light, and/or a vehicle alarm in the vehicle The method can also include remotely controlling a system configuration, a source selection, a volume, an equalization function, a tuning function, a band selection, a track selection^seek function, a scan function, a fast forward function, a rewind function, a skip forward function, a skip back function, a fader function, and/or a balance function of the audio system [0018J In one aspect, the invention is embodied in an audio system for a vehicle The audio system includes an audio source that generates an audio signal The audio source is integrated with the vehicle The audio system also includes a remote control receiver that controls a function of the audio source Also includes is a remote control sensor that receives a signal from a remote control transmitter to direct the remote control receiver to control the function of the audio source The remote control sensor is positioned m a location that is external to the passenger compartment of the vehicle [0019] The remote control sensor can be mounted m an exterior light assembly of the vehicle The exterior light assembly can be a left tail light assembly, a right tail light assembly, a left directional light assembly, a right directional light assembly, a parking light assembly, a reverse light assembly, a license plate illuminator assembly, a high-level brake light assembly, a courtesy light assembly, a running light assembly, a head light assembly, and/or a fog light assembly The extenor light assembly can include a curved reflector that focuses the signal from the remote control transmitter The remote control sensor can be an infrared (BR) sensor or a radio-frequency (RF) sensor The remote control sensor can be mounted to a body panel of the vehicle [0020] The function can include a system configuration selection, a source selection, a volume, an equalization function, a tunmg function, a band selection, a track selection, a seek function, a scan function, a fast forward function, a rewind function, a skip forward function, a skip back function, a fader function, and a balance function of the audio source The remote control transmitter can control a door lock, a vehicle ignition, a heater, an air conditioner, a defroster, a window, a power door, a power seat, a power moon roof, a convertible top, a courtesy light, vehicle telematics, a communication system, a trunk lid, a suspension load leveling system, and/or a vehicle alarm m the vehicle The remote control receiver can be disabled when the vehicle is in motion [0021] In another aspect, the invention is embodied m an apparatus for controlling an electncal component m a vehicle The apparatus includes a remote control receiver that is coupled to the electncal component m the vehicle and a remote control sensor that is coupled to the remote control receiver The remote control sensor is mounted m a light assembly and at least a portion of the light assembly is positioned on the exterior of the vehicle A remote control transmitter communicates a control signal to the remote control sensor The remote control receiver controls the electncal component in response to receiving the control signal from the remote control sensor [0022] The remote control sensor can be an infrared (IR) sensor or a radio-frequency (RF) sensor The light assembly can include a left tail Jight assembly, a right tail light assembly, a left directional light assembly, a right directional light assembly, a parking light assembly, a reverse light assembly, a license plate illuminator assembly, a high-level brake light assembly, a courtesy light assembly, a running light assembly, a head light assembly, and/or a fog light assembly [0023] The electncal component can include an audio system, a door lock, a vehicle ignition, a heater, an air conditioner, a defroster, a window, a power door, a power seat, a power moon roof, a convertible top, a courtesy light, vehicle telematics, a communication system, a trunk lid, a suspension load leveling system and/or a vehicle alarm [0024] The remote control transmitter can control a function of an audio system in the vehicle The function can include a system configuration selection, a source selection, a volume, an equalization function, a tuning function, a band selection, a track selection, a seek function, a scan function, a fast forward function, a rewind function, a skip forward function, a skip back function, a fader function, and/or a balance function of the audio system BRIEF DESCRIPTION OF THE DRAWINGS [0025] The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, m which like numerals indicate like structural elements and features m various figures The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention [0026] FIG 1A is a top view of an automobile having a dual-mode audio system [0027] FIG IB is a top view of an automobile having another dual mode audio system [0028] FIG 2A is a flow-chart illustrating the operation of a mode detection process m a dual-mode audio system [0029] FIG 2B is a flow-chart illustrating the operation of a mode detection process in a tn-mode audio system [0030] FIG 3 is a flow-chart illustrating the operation of a battery monitoring process m a dual-mode audio system [0031] FIG 4 is a diagram of a sport utility vehicle having a rear set of controls and rear transducers mounted on its tailgate [0032] FIG 5A-D are side views of an automobile having rear transducers mounted in four different locations [0033] FIG 6 illustrates a block diagram of an audio system according to the invention [0034] FIG 7 illustrates a perspective view of a vehicle including an embodiment of the audio system of FIG 6 [0035] FIG 8 illustrates a side view of a vehicle including an embodiment of the audio system of FIG 6 DETAILED DESCRIPTION [0036] A vehicle audio system, according to one aspect of the invention, is able to operate m two modes one mode where one or more parameters of the audio system are configured to better optimize sound quality while the doors (e g , including tailgates mj sport utility vehicles, vans and pick-up trucks and hatches m hatch-backed vehicles) are closed and a second mode where one or more parameters of the audio system are configured to better optimize sound quality while one or more of the doors, such as a tailgate, are open [0037] Additionally, users can operate the automobile audio system while the engine is not running and thus, dram power from the vehicle's battery It is therefore desirable for a vehicle audio system to monitor one or more operating conditions of the battery and adjust system performance in order to extend the maximum period of time the audio system can operate from battery power It is also desirable to monitor one or more operating conditions of the battery to ensme operation of the audio system does not result m a condition where sufficient battery charge (to allow the vehicle to be restarted) is no longer available [0038] It should be noted that the terms automobile and vehicle are used synonymously in this description and refer to any self-propelled passenger vehicle used for land transport, including cars, trucks, pick-up trucks, sport utility vehicles, and the like Additionally, the term tailgate refers to a ranged door of a vehicle which provides access to a rear opening of the vehicle or provides access to a cargo area A tailgate may be hmgedly connected to the top, bottom, or side of the frame of the vehicle [0039] As shown in FIG 1A, a vehicle (e g, an SUV) 10 includes an audio system 20 that is configured to operate m two modes a conventional mode and an open mode [0040] The audio system includes a first system configuration and a second system configuration The first system configuration corresponds to the conventional mode The conventional mode is a mode of operation m which audio system 20 is configured for play when all of the doors of the vehicle are closed and the pnmary listening position of interest is the seating area within the passenger compartment In this mode, sound is radiated within the passenger compartment as m a conventional automobile audio system The conventional mode includes a first plurality of transducers that are positioned to radiate sound for listening mside the vehicle [0041] The second configuration corresponds to the open mode or the tailgate mode The open mode is a mode of operation in which audio system 20 is configured for play when the tailgate 11 is open In this mode, sound is radiated through the open tailgate and/or through the vehicle's body panels or other openings to the outside environment In one embodiment, a second plurality of transducers is located m the rear portion of the vehicle, such as on the tailgate, hatch, oi rear door The second plurality of transducers can also be located m the bed or the liner of a pick-up truck, for example The open mode includes the second plurality of transducers that are positioned to radiate sound for listening outside the vehicle In one embodiment, the first and second plurality of transducers can include one or more common transducers [0042] The primary hstenmgjgosition of interest in the open mode is outside of the vehicle, typically in line with the open tailgate 11 In other embodiments, the open mode of operation can be configured for play when one or more doors (including a tailgate) are open In some embodiment'?, die open mode of operation can be configured to operate when a window, a tailgate, a hatch, a moon roof, a sunroof, a trunk, and a storage bin on the vehicle are in an open position [0043] Because the acoustic characteristics of the vehicle are significantly different when all of the doors are closed versus when one or more doors are opened, and because the listening positions of primary concern are different m the different modes, the audio system 20 adjusts system configuration in order to better optimize performance m each mode As will be explained in greater detail below, adjustment of system configuration may include altering the system topology for each mode of operation, as well as performing different signal processing operations such as equalization, signal mixing, amplification, dynamic range control, spatial enhancement processes and other signal processing techniques on the channels of audio data in each mode of operation The system topology defines how different signals provided by an audio source are routed to the different transducers of the audio system 10044] Additionally, audio system 20 features a battery monitoring process which monitors one or more operating conditions of the automobile battery 30 and takes one or more actions based on whether the system 20 is primarily running off of the battery and the state of one or more of the monitored operating conditions As will be explained in greater detail below, the battery monitoring process takes various actions including triggering'an alarm and altering the system configuration in order to conserve battery power [0045] Referring again to FIG 1A, audio system 20 includes one or more audio sources (not shown) such as a stereo radio, CD player, DVD player, and/or an MP3 player, which may be mounted m or near a front control console 21, a rear control console 22, or at another location withm the automobile 10 The front and rear control consoles 21,22 each may contain a set of controls (e g, volume, source selection, track selection, play, pause, etc ) for the audio sources included within the system 20 [0046] The audio system 20 also includes two pairs of front transducers 24a-24b, 24c-24d, a pair of side transducers 26a-26b, a pair of rear transducers 28a-28b, and a woofer 29 For example, a first plurality of transducers corresponding to the first system configuration can include the two pairs of front transducers 24a-24b, 24c-24d, the pair of side transducers 26a-26b, the pair of lear transducers 28a-28b, and the woofer 29 A second plurality of transducers corresponding to the second system configuration can include the pair of rear transducers 28a-28b, and the woofer 29 However, any combination of transducers can be included in the first and/or the second system configurations [0047] It should be noted that the arrangement of elements including the transducers, as well as the number of elements shown m FIG 1A, were chosen to illustrate the novel features of the invention It should be understood that the invention is not limited to use of the particular configuration of transducers and placements shown For example, as shown in FIG IB, another dual-mode audio system 20' can include the same transducers shown m FIG 1A as well as a front center transducer 25' located proximate to the front console 21 or in the dashboard of the vehicle 10' and a rear center transducer 23* located proximate to the rear of the vehicle 10' The audio systems 20,20' of FIG 1A and FIG IB can be configured as surround sound audio systems The surround sound audio system can be a matrix surround sound system, a 5 1 surround sound system, a 6 1 surround sound system, a 7 1 surround sound system, and a two-channel down-mixed surround sound system [0048] FIG 2A and FIG 3 illustrate an operation of the audio system 20 shown in FIG 1A [0049] As shown in FIG 2A, the audio system 20 includes a mode detection process 100 which detects when the system 20 is activated 102 and when a switching event 108,114 occurs that switches the system 20 between the first system configuration and the second system configuration A switching event 108,114 occurs when a user changes the state of a switch provided withm the system 20 One or more manual switches may be located on the front control panel 21 (shown in FIG 1A), rear control panel 22, on a remote control device, on a key fob, or in another location Since it is possible that significant changes would be made jn system operation when the mode is changed, having a manual switch keeps from dramatically changing system operation while passengers remain inside the vehicle [0050] In other embodiments, a switching event may occur automatical ly For example, a switching event may occur automatically when the system detects that a tailgate or door is open and the vehicle engine is noirunnmg (which can be sensed by the state of the ignition interlock, the RPM of the engine, or some other means) It may also be desirable to sense if passengers remain in the vehicle cabin, which could be done using seat switches, ultrasonic motion detectors, or some other method Thus, in other embodiments, the system may automatically switch modes from the conventional mode to the open mode when the tailgate or door is open, the engine is off, and no passengers are occupying the vehicle cabin [0051] Referring again to FIG 2A, when the mode detection process 100 detects that the system 20 has been turned on 102, the process 100 will place the system 20 m a conventional mode of operation 104 By automatically reverting to a conventional mode when the system 20 is turned on, the system 20 avoids a situation where the driver gets into the vehicle and turns on the system after it had been switched to open mode However, m other embodiments, a mode detection process m a dual-mode audio system could be configured to start up m different modes depending on how the system was turned on or the state of a switch For example, if the system was turned on using controls mounted m the front of the vehicle, the system may assume that someone would be mside the vehicle and the system should start in conventional mode If the system were turned on usmg controls that were easily accessible from outside the vehicle (such as controls located in rear console 22, or controls located on a remote control), the system may be configured to start up in open mode [0052] When the system 20 is placed m the conventional mode of operation 104, the system 20 is m a first system configuration 106 which, as will be explained m more detail below, has a first system topology and performs a first set of signal processing operations which are configured for play with the doors of the vehicle closed and the primary listening position is withm the passenger's compartment The system 20 remains m its first system configuration 106 until a switching event is detected 108 [0053] If a switching event is detected 108, mode detection process 100 places the system in the open mode of operation 110 and the system 20 is changed over to a second system configuration 112 which has a second system topology and performs a second set of signal processing operations that is configured for play with the tailgate of the vehicle opened and the primarily listening position being outside of the vehicle In one embodiment, the position is proximate to a rear opening of the vehicle In another embodiment, the position is proximate to a side opening of the vehicle [0054] If another switching event 114 is detected, then the mode detection process 100 reverts the system 20 back to the conventional mode of operation 104 and the system is placed in the first system configuration 106 [0055] In each system configuration, 106,112, the system 20 has a certain system topology which defines how different signals provided by an audio source are routed to the various transducers m the system In the first system configuration 106, the system topology is arranged m order to better optimize the system for play with the doors of the automobile closed and the primary listening area is inside the passenger compartment of the vehicle Similarly, m the second system configuration 112, the system topology is arranged in order to better optimize the system for play with a window, door, moon roof, or rear tailgate opened and the primary listening position outside the vehicle near the opening For example, m an audio system 20 having a transducer configuration similar to the arrangement m FIG 1A and an audio source which produces 5 1 channels of audio date (l e , front left, front right, front center, left surround, and nght surround of full bandwidth (20-20kHz) audio data plus a sixth channel of low frequency audio data), the first and second system configuration 106, 112 can have a system topology as shown m Table I Table I (Table Removed) [0056] In other separate embodiments, many other transducer arrangements and routing of audio data are possible For example, an audio system may have a transducer arrangement as shown in FIG IB and may have an audio souice which produces 6 1 channels of audio data including a front left, front nght, front center, left surround, nght surround, and center surround of full bandwidth (20-20kHz) of audio data plus a sixth channel of low frequency audio data In this example, the system configuration 106,112 may have the system topology as shown m Table II Table II (Table Removed) [0057] In other embodiments, a system configuration may include a topology m which one or more transducers are shut off m different modes of operaoon For example, refernng again to FIG 1 A, the front left and nght transducers 24a-24d may be shut off in the open mode of play leaving the side transducers to output left and nght surround sound Alternatively, the side transducers may be shut off m the open mode, leaving the front transducers to output the left and right surround sound Similarly, the rear transducers 28a-28b may be shut off in the conventional mode leaving the side transducers to output the surround sound [0058] Other embodiments may shut off front transducers 24a-24d (shown in FIG 1 A) and side transducers 26a-26b m open mode In this arrangement, left, right and center surround signals would be mixed with the left, right and center front signals that are fed to transducers 28a-28b [0059] In some embodiments, the audio source can provide complete surround information to the rear right 28a and the rear left transducers 28b In one embodiment, the audio source is a DVD source and a digital video disk (DVD) includes a two-channel down-mixed audio track that is fed to the rear right 28a and the rear left transducers 28b Alternatively, the audio source can down-mix the surround information into two channels that are fed to the rear right 28a and the rear left transducers 28b [0060] In addition to changing the system topology m each mode of operation, the first and second system configuration 106,112 can also perform different signal processing operations m each mode Signal processing operations may include operations such as equalization, amplification, signal mixing, spatial enhancement, dynamic range control and other signal processing techniques on one or more channels of audio data in order to alter the frequency response (both magnitude and phase as a function of frequency), polanty, and the magnitude of the voltage level of the signals delivered to each of the transducer channels in each the mode of operation [0061] The first and second system configurations 106,112, may perform different equalization signal processing operations in order to provide for a different frequency response of the system 20 in each mode of operation Equalization signal processing may be performed using any of the various techniques known m the art For example, equalization signal processing may be performed on each channel of audio data by passing the data through one or more digital filters whose filter coefficients are stored m memory and provided to the digital signal processor Sets of Filter coefficients corresponding to each mode of operation would be stored in memory, and the system could switch sets of coefficients according to commands issued by the mode detection process 100 (FIG 2A) [0062] Equalization processing operations may also be implemented in the analog domam by providing physically separate circuits with separate sets of filters for each mode of operation Different circuits would be switched mto or out of the signal path in accordance with commands issued by the mode control process 100 Alternatively, there may be one physical circuit m which the performance can be dynamically adjusted through use of I variable gam circuits, voltage controlled filters, switchable electrical component values, switched capacitor filters, or any other form of adjustable or programmable analog filters or signal processors [0063] Equalization signal processing operations, whether implemented m the digital or analog domain, should be designed to provide a smoother frequency response of the audio system in each mode of play as compared to the frequency response of the system with no equalization Furthermore, the frequency response of the system measured at the desired listening positron for the open mode of operation (outside of the vehicle m line with the open door or tailgate) should be smoother using the signal processing designed for operation m the open mode than the response would be if the conventional mode signal processing were used For each mode of operation, the overall measured frequency response of the system measured at each desired listening position will generally be similar in character However, the frequency response of the signal processing used for each mode will generally be significantly different For purposes of discussion, it should be noted that we will sometimes refer to signal processing used for equalization as a 'channel of equalization' in this specification The channel of equalization may be accomplished using either analog or digital techniques We will also sometimes refer to the frequency response of the signal processing as the 'frequency response of the channel of equalization' [0064] First and second systejaa configurations 106,112, may include different amplification processing operations of the audio signals applied to a transducer for each mode of operation In other words, the first system configuration 106 may include signal processing operations which adjust the amplification of the audio signals m one manner, and the second system configuration may include signal processing operations which adjust the amplification of the audio signals m a different manner Adjusting the amplification of the audio signals may be performed using any of the techniques known m the art Adjusting gam can be done m multiple places withm the signal path of an audio signal, and the system is not limited m the locations where gain adjustment occurs For example, the amplification of an audio signal applied to a transducer may be adjusted by changing the gam of an amplifier in the signal path of a particular transducer In a digital system, the gam for each channel for each mode of operation may be determined by a multiplication coefficient or set of filter coefficients stored m memory and supplied to a digital signal processor m order to control the level of the signal supplied to one or more transducers in each mode of operation The gam may also be adjusted in the analog domain by controlling a variable gam analog amplifier (or other known methods of controlling gam m an analog system) located m the signal path of each channel of audio data [0065] The first and second system configurations 106,112 may perform different signal mixing signal processing functions in each mode of operation Signal mixing operations involve summing various signals together m various proportions Mixing may occur m one or both modes of operation Summing can be accomplished using op amp summer circuitry in the analog domain, or data values can be directly summed by a microprocessor or digital signal processor [00661 The first and second system confi guration 106,112 may include different spatial enhancement signal processing m each mode of operation Spatial enhancement signal processing generally improves the spatial character of the sound field created by the system and may be implemented using any of the techniques known m the art One spatial enhancement technique for two-channel audio mvolves determining first sum and difference signals by alternately adding and subtracting first and second signals from each other (hereinafter referred to as a matrix operation) Next, some form of signal processing is applied at least to the difference signal (or possibly to both the sum and difference signals, where the processing applied to the sum and difference signals is different), then a second matrix operation (take sum and difference of processed sum and difference signals) is performed to generate third and fourth signals These third and fourth signals are now spatially enhanced versions of the first and second signals Other enhancement techniques might only operate on a difference signal In these methods, a difference of two signals is taken The difference signal is modified m some manner, then added back to one of the original channels and subtracted from the other of the original channels [0067] In the open mode of Operation spatial enhancement may be performed on the signals provided to the rear transducers (e g, transducers 2Sa-28b m FIG 1 A) since these transducers will most likely be the primary transducers providing sound to listeners outside the vehicle Spatial enhancement may be performed on the complete signals applied to the rear transducers, or may be apphed to only a portion of the signals applied to the rear J transducers For example, for reproduction of a surround sound sipal source, center channel information may be applied to each rear transducer equally, without any spatial enhancement processmg Simultaneously, left and nght channel signals may be applied to left and right transducers respectively These could be apphed with or without spatial enhancement processmg Similarly, left and nght surround signals could be apphed with or without spatial enhancement processing to left and nght transducers respectively, where the spatial enhancement processmg used could either be the same as or different from that applied to the left and nght channel signals In ope particular embodiment, the second configuration feeds center channel signals equally to rear transducers 28a-28b {shown in FIG 1 A) without spatial enhancement, front left and nght signals to left and nght rear transducers 28a-28b respectively without spatial enhancement, and spatially enhanced left and nght surround signals to left and nght rear transducers 28a-28b, lespectively [0068] Other combinations of performing spatial enhancement processmg of vanous signals are also possible For example, a system such as that descnbed m co-pending application titled "Audio Signal Processing" having U S Senal Number 09/886,868 filed on June 21,2001 and assigned to Bose® Corporation, which is herein incorporated by reference, could be used in the rear of the vehicle, where spatial enhancement processmg is used with the configuration of transducers disclosed [0069] It should be understood that spatial enhancement is not limited to the second system configuration dunng the open mode of operation, but may also be performed on the audio signals provided to one or more sets of transducers