Abstract:
The inventive mechanism uses several sub-systems to generate outputs from the stereo input signal. A first sub-system synthesizes the phantom center output, which places the monaural center image between the left and right speakers in front of the listener. A second sub-system synthesizes the virtual surround (or rear) output signals, which places the sound images to the sides of the listener. A third sub-system synthesizes the left and right stereo outputs, and expands the locations of the left and right sound images.

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of and commonly assigned U.S. application Ser. No. 09/058,047 now U.S. Pat. No. 6,198,826, entitled QSOUND SURROUND SYNTHESIS FROM STEREO filed Apr. 9, 1998, which is incorporated herein by reference, which is a continuation-in-part of and commonly assigned U.S. application Ser. No. 08/858,586 now U.S. Pat. No. 6,126,730, entitled FULL SOUND ENHANCEMENT USING MULTI-INPUT SOUND SIGNALS filed May 19, 1997, which is incorporated herein by reference. The present application is related to co-pending and commonly assigned U.S. application Ser. No. 08/858,594, entitled METHOD AND SYSTEM FOR SOUND EXPANSION, filed May 19, 1997, which is incorporated herein by reference. The present application is related to commonly assigned U.S. Pat. No. 5,105,462, issued to Lowe, et al, entitled SOUND IMAGING METHOD AND APPARATUS, filed May 2, 1991, and issued Apr. 14, 1992, the disclosure of which is incorporated herein by reference. The present application is also related to commonly assigned U.S. Pat. No. 5,208,860, issued to Lowe, et al, entitled SOUND IMAGING METHOD AND APPARATUS, filed Oct. 31, 1991, and issued May 4, 1993, the disclosure of which is incorporated herein by reference. The present application is also related to commonly assigned U.S. Pat. No. 5,440,638, issued to Lowe, et al, entitled STEREO ENHANCEMENT SYSTEM, filed Sep. 3, 1993, and issued Aug. 8, 1995, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This application relates in general to audio signal processing, and in particular to a decoding mechanism which synthesizes virtual surround channels from input signals having surround information encoded along with left, right and center audio signals. 
     BACKGROUND OF THE INVENTION 
     A recent trend in the audio industry is the use of matrix surround encoded stereo, for example Dolby Surround encoding, in audio signals. These audio signals may accompany analog or digital video signals, which together form a television signal, VCR signal, or CD or DVD signal. In Dolby systems, the encoder combines four different signals, i.e. left, right, center, and surround and produces two output signals, i.e. left total and right total. A further discussion of Dolby encoding, Surround decoders, and Pro Logic decoders may be found in “Dolby Pro Logic Surround Decoder Principles or Operation” by Roger Dressler, which is hereby incorporated by reference. The main difference between matrix surround decoders, for example a Dolby Pro Logic decoder and other decoders, is the way steering and relative balance between output channels is achieved. 
     The Surround decoder receives the two output signals, left total and right total, from the encoders and produces three output signals, i.e. left, right, and surround. The common or mono information contained in the left and right channels of a matrix surround encoded recording carries the center channel information, and thus the center signal is reproduced as a phantom image between the left and right speakers. The Surround decoder is typically a passive decoder as it uses (L−R) difference amplifier. The Pro Logic decoder receives the two output signals, left total and right total from the encoder, and produces four output signals, i.e. left, right, center, and surround. However, the Pro Logic decoder uses an adaptive matrix to continuously monitor the encoded audio signals, evaluate the inherent sound field dominance, and apply processing in the same direction and proportion to that dominance. 
     A problem with existing systems is that a listener has to use more than two speakers to get the desired surround effect from a sound source. This not only costs more in terms of the number of speakers to be purchased to get the desired sound effect, but also requires a room of adequate size so that the different speakers producing the left, right, center and surround sound may be placed at sufficient distances from the listener and also from each other to produce the desired sound effects. Thus, listeners with limited resources are not able to afford the multiple speaker systems or are unable to find means for placing the speakers to achieve the desired sound effects. 
     Another problem occurs when Dolby encoded audio materials are decoded on QSound systems. QSound systems use “Q-filters” in the processing of audio signals. The Q-filters could be part of a “QXpander circuit,” wherein QXpander is a registered trademark of QSound. Also, the term “QXpander circuit” is a descriptive term used for the purpose of this application to refer to the filters more specifically described in U.S. Pat. No. 5,440,638 to Lowe et al., which is hereby incorporated by reference. The Q-filters could be Q1 filters. The term “Q1 filter” is also a descriptive term used for the purpose of this application to refer to the filters more specifically described in U.S. Pat. Nos. 5,105,462 and 5,208,860 both to Lowe et al., wherein each of these patents are hereby incorporated by reference. A Q-filter adjusts the amplitude and phase of an input signal on a frequency dependent basis. Note that the Q-filters use phase inverted signals during processing to achieve the QSound virtual audio image effects. Consequently, if an input signal to a Q-filter is already inverted from Dolby encoding, then the Q-filter system will re-invert the input signal and then proceed with processing of an improper, re-inverted signal. A re-inverted or non-inverted signal will adversely affect the expansion mechanisms of the Q-filter. Thus, the output signal will result in the incorrect placement of sound images. In other words, the surround images would appear to be located at the left and right speakers, and not placed to the sides or rear of the listener. Note that these effects would occur on other expansion mechanisms that use phase inversion. 
     Therefore, there is a need in the art for a mechanism which will correctly apply expansion mechanisms to both non-inverted signals and inverted signals. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved by a system and method which correctly operate on both non-inverted signals and inverted signals. 
     The inventive mechanism uses several sub-systems to generate outputs from the stereo input signal. In the preferred embodiment, a first sub-system synthesizes the phantom center output, which places the monaural center image between the left and right speakers in front of the listener. A second sub-system synthesizes the virtual surround (or rear) output signals via Q-filters, a delay device, an attenuator, and summers, and places the sound image to the sides of the listener. A third sub-system synthesizes the left and right stereo outputs, and via a QXpander circuit expands the locations of the left and right sound images. The QXpander circuit includes a Q-filter and summers. Thus, the inventive mechanism operates on center, surround, and stereo information contained in a matrix encoded stereo input signal, and processes each type of information differently to achieve the proper placement of the different sound images resulting from the different information. 
     A technical advantage of the present invention is to separately operate on center, surround, and stereo information contained within a matrix encoded stereo input signal. 
     Another technical advantage of the present invention is that center information contained within the stereo input signal is retained during processing of the input signal. 
     A further technical advantage of the present invention is that matrix encoded surround information contained within the stereo input signal is properly decoded and processed using Q-filters to create left and right surround sound images. 
     A further technical advantage of the present invention is that stereo information contained within the matrix encoded stereo input signal is operated on by a QXpander circuit which expands the sound images of the left and right stereo information. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of the preferred embodiment of the inventive mechanism; 
         FIG. 2  is an plan view of the sound images produced by the inventive mechanism; and 
         FIG. 3  is a schematic representation of an alternative embodiment of the inventive mechanism. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  depicts a preferred embodiment of the inventive mechanism. In the inventive arrangement  100  of  FIG. 1 , a two-channel stereo input from an A/V source, which includes encoded surround information, is converted into output signals. Left input  26  and right input  27  include four different signal portions-left, right, center, and surround. The input signals are processed into left output  28  and right output  29 . Although only two outputs are shown, other outputs can be provided for rear or surround channels, a center channel, and/or a sub-woofer channel. The inventive arrangement  100  of  FIG. 1  is subdivided into three sub-systems. 
     The first sub-system is used to provide a phantom center output. As shown in  FIG. 2 , the sound image of the phantom center location  33  appears to listener  30  to be placed between left  31  and right  32  speakers. Note that the left and right speakers are connected to left output  28  and right output  29  of  FIG. 1 , respectively. This sub-system may include summers  10 ,  24 ,  25 , as well as multiplier  14 . Both inputs include center information as common information. For example, left input  26  would include center information C as well as left information L, and right input  27  would include center information C as well as right information R. Summer  10  combines left input  26  and right input  27  in phase. Thus, the output of summer  10  is L+R+2C. For the sake of simplicity assuming that stereo information is not present, i.e. only center information is present, then the output of summer  10  would be 2C, which is a mono signal. This output from summer  10  may be modified by multiplier  14 , and recombined with the left and right input signals in summers  24  and  25 , respectively. Multiplier  14  may have a control input (not shown) which is used to set the amount of attenuation or amplification that the multiplier applies to its input signal. The control input may be preset during manufacturing, or may be variably set by a processor or listener. Note that attenuator  13  and delay device  12  have similar control inputs which may be used to set the attenuation in attenuator  13  and the delay in delay device  12 . Although in the preferred embodiment, an attenuator  13  is used, a multiplier may be used in place of attenuator  13  without departing from the scope of the present invention. Multiplier  14  could be used to reduce or increase the output from summer  10 , to turn down or increase the effect of the phantom center channel. It has been empirically shown that the phantom center sub-system is desirable so as to improve the imaging of the phantom center. Thus, in the preferred embodiment, the center information is passed to left output  28  and right output  29  after being added in summers  24  and  25  to the outputs of the second and third sub-systems from summers  22 ,  23 , and summers  20 ,  21  respectively. The high degree of expansion of the stereo information and the surround information due to processing through Q-filters  16 ,  15 ,  17  may tend to dominate the center information. Thus, the center subsystem compensates for Q-filters  16 ,  15 ,  17 . 
     The second sub-system is used to provide a virtual surround channel. As shown in  FIG. 2 , the sound images of the surround channel location  34  appear to the listener  30  to be placed at the left and right sides of the listener  30 . The second sub-system includes summers  11 ,  18 ,  19 ,  22 ,  23 , delay device  12 , attenuator  13 , as well as Q-filters  15  and  17 . Q-filters  15 ,  16  and  17  could be Q1 filters, which is described in U.S. Pat. Nos. 5,105,462 and 5,208,860 both to Lowe et al., wherein each of these patents are hereby incorporated by reference. Surround information would be included as common information to both inputs  26 ,  27 . However, the Dolby encoder encodes such information in the two inputs with opposite phases. For example, left input  26  could include surround information S as well as left information L, and right input  27  could include phase inverted surround information −S as well as right information R. Summer  11  combines the left input and right inputs in opposite phase. For example, one of the input signals, say the right input (R−S)  27  may be phase inverted by summer  11  and combined with the other input signal, say the left input (L+S)  26  to produce L+S−(R−S) or (L−R+2S). Thus, the output of summer  11  is (L−R+2S). Although summer  11  includes an inverter, the invertor could be located separately from the summer. Similarly, summers  18 ,  19  and  21  shown to have invertors, could have the invertors located separately. The inverter multiplies a signal by −1, and therefore the polarity of the amplitude is changed. Any positive amplitude becomes negative and any negative amplitude becomes positive. For the sake of simplicity assume that stereo information is not present, i.e. only mono-surround information is present, then the output from summer  11  would be 2S, which is a mono signal. 
     The output from summer  11  is then input to delay device  12  and attenuator  13 . In the preferred embodiment, the delay effectuated by delay device  12  is typically 0.5 ms while the attenuation effectuated by attenuator  13  is typically 3 dB. Although the delay and attenuation have been described as being 0.5 ms and 3 dB respectively, a range of values for both delay and attenuation may be used without departing from the scope of the present invention. Thus, in alternative embodiments, the attenuation may be in the range 2 dB to 6 dB and the delay may be in the range 167 μs to 667 μs. Attenuation is desirable to prevent the apparent location of the image of the sound from being shifted to one side because of the delay in the other signal. Thus, two separate signals are created from the same signal because of delay device  12  and attenuator  13  with one of the signals being delayed and the other being attenuated. The delay and attenuation create a stereo-like effect from a monaural input. 
     The delayed signal is then passed through Q-filter  15  which contributes to the expansion of the sound image. The signal from delay device  12  is also fed to summer  18 . The output from Q-filter  15  is phase inverted by summer  19  and added to the signal passing through attenuator  13 . Similarly, the attenuated signal is passed through Q-filter  17 , which contributes to the expansion of the sound image. The signal from attenuator  13  is also fed to summer  19 . The output from Q-filter  17  is phase inverted and added to the signal passing through delay device  12  by summer  18 . Thus, virtualization of the surround sound may be achieved in this manner to provide a highly expanded stereo surround signal from a mono or stereo signal. In the preferred embodiment, the expanded surround information is then added in summers  22  and  23  to the output of summers  20 ,  21 , respectively. The outputs of summers  22  and  23  form one of the inputs to summers  24  and  25 , respectively. The output from summers  24  and  25  go to left output  28  and right output  29 . 
     Note that the surround sub-system removes the center channel information. Since the center channel information is encoded in-phase in the left and right inputs, summer  11  with the inverted input will cancel the center information during the L−R combination. Similarly, the phantom center sub-system removes the surround information. Since the surround information is encoded in opposite phase in the left and right inputs, summer  10  will cancel the surround information during the L+R combination. 
     The third sub-system is used to provide an expanded stereo sound image. As shown in  FIG. 2 , sound images  35  and  36  of the expanded stereo channels appear to listener  30  to be located outside of the left and right speakers  31  and  32 . In the preferred embodiment, this sub-system includes multiplier  54 , Q-filter  16 , and summers  20  and  21 . Multiplier  54 , Q-filter  16  and summers  20 ,  21  comprise a single filter QXpander circuit, although a double filter QXpander circuit may also be used, wherein QXpander is a registered trademark of QSound. A single filter and a two filter QXpander circuit is described in co-pending and commonly assigned U.S. application Ser. No. 08/858,586, which is hereby incorporated herein by reference. A two filter QXpander circuit is described in U.S. Pat. No. 5,440,638 to Lowe et al., which is hereby incorporated herein by reference. The Q-filter  16  may be a Q1 filter. 
     Since the input to the expansion sub-system is the output from summer  11 , the signal delivered to Q-filter  16  would not have any center information but could contain surround information, i.e. the signal delivered to Q-filter  16  is L−R+2S. Assume, that the signal from summer  11  does not contain any surround information, then the output of summer  11  is L−R which is inputted into Q-filter  16 , which adjusts the amplitude and phase of the signal on a frequency dependent basis. Note that multiplier  54  can modify the signal prior to input to Q-filter  16  by either boosting or attenuating the signal. The Q-filtered signal is recombined with the left and right input signals by summers  20  and  21 , respectively. Note that the filtered signal from Q-filter  16  is inverted by summer  21  and combined with the right input signal. The inversion at summer  21  ensures that the signal from the left input has been inverted as part of the Q-processing. The sound image created by this Q-filtering is shown in  FIG. 2 , where the images  35 ,  36  have been hard panned to the left and right of the left and right speakers, respectively. In an alternative embodiment, the signal from left input  26  may be inverted at summer  11  instead of the signal from right input  27  being inverted at summer  11 . In such an embodiment, the signal from Q-filter  16  into summer  20  is inverted instead of the signal fed into summer  21 . 
     Note that the mono center information is not processed by Q-filter  16 , and that only the difference information (L−R) from summer  11  is processed by Q-filter  16 . By virtue of its operation the information processed by Q-filter  16  and then summed back with the left and right signals at summers  20  and  21  respectively, create an expanded stereo effect. The surround information is not virtualized by this portion of the circuit. Note that the output from Q-filter  16  is inverted by summer  21  and combined with the right input signal, while the output from Q-filter  16  is combined in-phase with the left signal. These signals are then combined with the outputs from summers  18  and  19 , the surround channel virtualization portion of the circuit, in summers  22  and  23 , and the output from multiplier  14 , the center channel portion of the circuit, in summers  24  and  25 . 
     Furthermore, the monophonic, center channel information is not present in any of the Q-filters  15 ,  16 , and  17 . Matrix surround encoding allows the sound signals to be steered to four separate channels—left, right, center, and surround. However, it is often desirable to simultaneously steer to either the left and right channels or the center and surround channels. Thus, a matrix encoded stereo input signal may be in the left and right channel mode or the center and surround channel mode at any given time. If the input signal is in the center/surround mode, then Q-filters  15  and  17  may be used to synthesize two channels which are then processed so as to create two virtual surround channels upon playback. This process is known as Omni-to-3D and discussed in greater detail in copending U.S. patent application Ser. No. 08/858,594, which is hereby incorporated herein by reference. The delay device  12  and attenuator  13  create a synthetic stereo difference from the monophonic surround channel information received from the summer  11 . In the preferred embodiment, acoustic images  34  of two widely placed surround speakers is produced by dividing the difference signal from summer  11  into two channels, passing them through two filters  15 ,  17 , inverting the filtered signals and combining them with the other synthesized stereo channel. If the input signal is in the left/right mode, then QXpander circuit comprising of summer  11 , Q-filter  16  and summers  20 ,  21  produces an expanded stereo effect. 
     However, there is no active steering between the left/right mode and the center/surround mode, i.e. the signal continually passes through all the Q-filters  15 ,  16 ,  17 . Thus, both the stereo expansion and Omni-to-3D are processed continually. The inventive mechanism  100  allows the dominance of one enhancement over the other by achieving a relative balance between the two subsystems. This relative balance may be achieved empirically. The relative balance between stereo expansion and omni-to-3D produces enhancements in the sound coming from the speakers which could not be achieved by the individual subsystems. 
     Although summers  20 ,  22 , and  24  have been shown as separate summers for clarity, they could be combined and a single summer used instead to provide the desired effect. Similarly, although summer  21  includes a phase inverter, in an alternative embodiment a separate phase inverter could be used to invert the signal from Q-Filter  16  into summer  21 , and summers  21 ,  23 ,  25  could be combined to provide the desired output signal. 
       FIG. 3  depicts an alternative embodiment  300  of the inventive mechanism of the present invention. In this embodiment the first sub-system includes summers  41 ,  43  and  44 , as well as multiplier  49 . Center information would be included as common information to both inputs. For example, left input  26  would include center information C as well as left information L, and right input  27  would include center information C as well as right information R. Summer  41  combines the left input  26  and right input  27  in phase. Thus, the output of summer  41  is L+R+2C. For the sake of simplicity assume that stereo information is not present, i.e. only center information is present, then the output would be 2C, which is a mono signal. This output from summer  41  is then modified by multiplier  49 , and recombined with the left and right input signals by summers  43  and  44 , respectively. Multiplier  49  has a control input (not shown) which is used to set the amount of amplification or attenuation that the multiplier applies to its input signal. In the embodiment shown in  FIG. 3 , the value of multiplier  49  is set to 0.55. However, a range of values may be used depending on the particular implementation. Thus, in alternative embodiments, the value of multiplier  49  may be varied within a 15% range of the above value. The control input may be preset during manufacturing, or may be variably set by a processor or listener. Note that each of the multipliers  50 ,  51  and  52  would have similar control inputs. In the embodiment shown in  FIG. 3 , the value of multiplier  50  is set to 0.75. However, a range of values may be used depending upon the particular implementation. Thus, in alternative embodiments, the value of multiplier  50  may be varied within a 15% range of the above values. Similarly, in the embodiment shown in  FIG. 3 , the value of multiplier  52  is set to 0.75. However, a range of values may be used depending upon the particular implementation. Thus, in alternative embodiments, the value of multiplier  52  may be varied within a 15% range of the above values. In the embodiment shown in  FIG. 3 , the value of multiplier  51  is set to 0.80. However, a range of values may be used depending upon the particular implementation. Thus, in alternative embodiments, the value of multiplier  51  may be varied from 0.24 to 1.20 without departing from the scope of the present invention. In theory, the value of multiplier  51  may be allowed to approach infinity. 
     Multiplier  49  would be used to reduce the output of summer  41 , to turn down the effect of the phantom center channel. Note that center information would be passed to left output  28  through summers  43 ,  45  and  47  and right output  29  through summers  44 ,  46 , and  48 . However, it has been empirically shown that the phantom center sub-system is desirable so as to improve the imaging of the phantom center. The high degree of expansion of the stereo information from the application of Q-filter  53  tends to dominate the center information. Thus, the center subsystem compensates for the Q-filter  53 . 
     The second sub-system of the alternative embodiment includes summers  42 ,  45  and  46 , as well as multipliers  50  and  52 . Surround information would be included as common information to both inputs  26 ,  27 . However, the Dolby encoder encodes such information in the two inputs with opposite phases. For example, left input  26  would include surround information S as well as left information L, and right input  27  would include surround information −S as well as right information R. Summer  42  combines the left input  26  and right input  27  in opposite phase. Thus, the output of summer  42  is (L+S)−(R−S) or (L−R+2S). Although summer  42  includes an inverter, the inverter could be located separately from the summer. Similarly, summers  45  and  48  also shown to have invertors, could have the invertors located separately. For the sake of simplicity assume that stereo information is not present, i.e. only surround information is present, then the output would be 2S, which is a mono signal. This output is then modified by multipliers  50  and  52 , and recombined with the left and right input signals by summers  45  and  46 , respectively. Surround image placement  34  shown in  FIG. 2  is enhanced from the opposite phase of signals from summers  45  and  46 , i.e. note that the output of summer  42  is recombined with each input signal such that the surround signal from summer  42  is phase flipped with respect to the phase of the surround signal present in the input signals. In other words, summer  45  combines the left input signal (L+S) with the phase flipped output of summer  42  (−2S) to yield −S, if multiplier  50  is set to 1. If multiplier  50  is set to a value greater than 1, i.e. for boosting the signal, then the output of summer  45  will be less than −S, e.g. −2S. 
     Note that the surround sub-system removes the center channel information. Since the center channel information is encoded in phase in the left and right inputs, the summer  42  with the inverted input will cancel the center information during the L−R combination. Similarly, the phantom center sub-system removes the surround information. Since the surround information is encoded in opposite phase in the left and right inputs, the summer  41  will cancel the surround information during the L+R combination. 
     The third sub-system of the alternative embodiment includes Q-filter  53 , summers  47  and  48 , and multiplier  51 . Since the input to the expansion sub-system is the output from summer  42 , then the signal delivered to Q-filter  53  would not have any center information, but could contain surround information, i.e. L−R+2S. The Q-filter  53  could be a Q1 filter. Assume, that the signal from summer  42  does not contain any surround information, thus the output of summer  42  is (L−R) which is inputted into Q-filter  53 , which adjusts the amplitude and phase of the signal on a frequency dependent basis. Note that multiplier  51  can modify the signal prior to input to Q-filter  53  by either boosting or attenuating the signal. The Q-filtered signal is recombined with the left and right input signals by summers  47  and  48 , respectively. Note that the filtered signal from Q-filter  53  is inverted by summer  48  and combined with the output of summer  46 . The inversion at summer  46  ensures that the signal from the left input has been inverted as part of the Q-processing. The sound image created by Q-filtering is shown in  FIG. 2 , where the images  35 ,  36  have been hard panned to the left and right of the left and right speakers, respectively. In an alternative embodiment, the signal from left input  26  may be inverted at summer  42  instead of the signal from right input  27  being inverted at summer  42 . In such an embodiment, the signal from Q-filter  53  into summer  47  is inverted instead of the signal fed into summer  48 . 
     Note that the mono center information is not processed by Q-filter  53 , and that only the difference information (L−R) from summer  42  is processed by Q-filter  53 . By virtue of its operation the information processed by Q-filter  53  and then summed back with the left and right signals at summers  47  and  48  respectively, create an expanded stereo effect. The surround information is not virtualized by this portion of the circuit. Note that the output from Q-filter  53  is inverted by summer  48  and combined with the output from summer  46 , while the output from Q-filter  53  is combined in-phase with the output from summer  45 . Signals from summers  43  and  44 , the center channel portion of the circuit, are combined with the outputs from multipliers  50  and  52 , the surround channel virtualization portion of the circuit, in summers  45  and  46 . These signals are then combined with the output from Q-filter  53 , the expanded stereo portion of the circuit in summers  47  and  48 . 
     Although summer  45  includes a phase inverter, in an alternative embodiment, a separate phase inverter could be used to invert the signal from multiplier  50 , and summers  43 ,  45 ,  47  could be combined to provide the desired output signal. Similarly, a separate phase inverter could be used to invert the signal from Q-filter  53  into summer  48 , and summers  44 ,  46 ,  48  could be combined to provide the desired output. 
     The Q-filters of  FIGS. 1 and 3 , are IIR or Infinite Impulse Response type filters. These types of filters have a feedback loop, which causes the filter response to last longer. The filters could alternatively be of the FIR or Finite Impulse Response type. The Q-filters can also be implemented as IIR or FIR filters in digital or analog domain. The Q-filters in  FIGS. 1 and 3  are preferably multi-stage filters, for example 2-stage filter and 3-stage filter. However, all of the filters could comprise only one stage. 
     Therefore, the inventive mechanism of  FIGS. 1 and 3  operate on center, surround, and stereo information, and process each type of information differently to achieve the proper placement of the different sound images resulting from the different information. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.