Abstract:
A dynamically variable spectral matrix surround system decodes two-channel stereo into multi-channel surround. In one embodiment, the true stereo signal is present in left and right front and left and right surround channel outputs. When a dominant center channel signal appears, the system subtracts center channel audio from the critical voice band only. The higher frequency portion of the spectrum will remain true stereo at all times. In another embodiment, the front center signal bandwidth is determined. A dynamically variable portion of the audio spectrum is inverted and added to the opposite channel, thereby dynamically subtracting the bandwidth of the front center signal from the left front, left surround, right front and right surround channels but leaving the portion of the audio spectrum that does not contain front center information unaltered. The input is divided into two frequency bands. The low frequency portion remains true stereo at all times because only high frequencies are processed by cancellation steering. By dynamically varying the cancellation bandwidth in the left and right output channels, the typical audible dominance of the difference signals is greatly reduced. When the input contains a dominant left or right signal, the center front and surround channels are steered down in level so as to produce the output only in the front channels. When a dominant surround signal is present in the input, the front channels are steered down in level. Therefore, allows the system produces an output only in the channel where the originally encoded signal was intended.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to audio sound systems and more particularly concerns audio sound systems which can decode two-channel stereo into multi-channel sound, commonly referred to as “surround” sound. Typical prior art systems have utilized a variable output matrix for decoding a given signal into multi-channel outputs. Surround matrix systems capable of providing more than two output channels are well known. The Dolby Prologic® system is to date perhaps the best known example of a variable output matrix system that can decode a stereo encoded signal into four channels. For several years there has been a desire to increase the number of output channels in a matrix system to five or more. There has also been a desire to provide stereo performance in the rear surround channels. This is especially desirable when using a matrix system to decode non-encoded stereo music. U.S. Pat. Nos. 5,319,713 and 5,333,201 disclose a surround system which provide a stereo surround signal by steering a mono L−R signal in multiple bands. While this system will provide a stereo perception by steering dominant left or right signals in multiple bands, it lacks the finer detail or resolution of a true stereo signal. The surround system disclosed in U.S. Pat. No. 5,796,844 will provide a true stereo left and right surround signal when no dominant front center signal is present. When a dominant front center signal is present the &#39;844 patent system reverts to mono in the surround channels or must compromise the front to rear separation of the center information. As a result, the &#39;844 system frequently produces a mono signal in the surround channel outputs when there is a dominant front center signal. The left to right separation of the surround channels is one of the most important aspects of the surround system performance as perceived by the listener. The better the left/right stereo separation of the system, including the surround channels, the better the perceived performance of the system. In most of the matrix systems available today, the low frequency portion of the spectrum is dynamically changing when there is any active steering in the matrix. This will tend to produce subtle but noticeable instability at the bass frequencies. Furthermore, all of the matrix surround systems exhibit a noticeable increase in reverberation when decoding non-encoded stereo music compared to a stereo playback. 
     It is, therefore, a primary object of the current invention to provide a dynamic spectral matrix surround system which maintains maximum true stereo performance in the left and right front and left and right surround channels. It is also an object of the invention to provide a dynamic spectral matrix surround system which maintains true stereo operation in the high frequency portion of the spectrum when there is no high frequency center channel information present. A further object of the invention is to provide a dynamic spectral matrix surround system which affords maximum perceived removal of the front center signal in the left and right front and left and right surround channels while simultaneously providing maximum stereo separation. Another object of the invention is to provide a dynamic spectral matrix surround system which improves the stability of the bass frequencies during the dynamic steering of the matrix. Yet another object of the present invention is to provide a dynamic spectral matrix surround system that is compatible with all matrix encoded material, as well as all non-encoded stereo material. And it is an object of the present invention to provide a dynamic spectral matrix surround system which reproduces non-encoded stereo material with a more correctly balanced level of difference information, thereby reducing the typical increase of originally recorded reverberation. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a dynamically variable spectral matrix surround system is provided which can decode two-channel stereo material into multi-channel surround. The left input is fed to both the left front and left surround channels. The right input is fed to both the right front and right surround channels. The center channel output receives a summed left and right signal. In one embodiment, the true stereo signal is present in the left and right front and the left and right surround channel outputs. When a dominant center channel signal appears, the system will provide cancellation of the center channel audio in the critical voice band only. The higher frequency portion of the spectrum will remain true stereo at all times. In another embodiment of the invention, the front center signal bandwidth is determined. A dynamically variable portion of the audio spectrum is inverted and added to the opposite channel, thereby dynamically subtracting the bandwidth of the front center signal from the left front, left surround, right front and right surround channels. The portion of the audio spectrum that does not contain front center information is unaltered and thus remains true stero in the left front, left surround, right front and right surround output channels. This greatly improves the true stereo soundfield for the listener while simultaneously reducing the typical increase of audible difference signals. The net result is a decoded output with a closer level of difference information to that of the original stereo input source material. The input is divided into two frequency bands with a 24 db per octave crossover at approximately 200 Hz. The low frequency portion of the spectrum remains true stereo at all times, due to the fact that only frequencies above 200 Hz are processed by cancellation steering. By dynamically varying the cancellation bandwidth in the left and right output channels, the typical audible dominance of the difference signals is greatly reduced. This provides a surround system with a much closer sonic balance of difference information to that of the original stero recording. When the input contains a dominant left or right signal, the center front and surround channels are steered down in level so as to produce the output only in the front channels. When a dominant surround signal is present in the input, the front channels are steered down in level. This allows the system to produce an output only in the channel where the originally encoded signal was intended. The dynamic spectral matrix surround system provides a higher level of left to right separation in all channels than was previously available with a matrix decoding system. It maintains this higher level of left to right separation regardless of the encoded direction of the input signal. The low frequency portion of the spectrum maintains true stereo performance at all times. The center channel attenuation in the left and right channels is greater than that typically obtained with a matrix system, thereby improving the channel separation. The difference information present in the input signal decodes with a much closer balance with that of the original stereo signal. 
     In its simplest four speaker form, the process for dynamically decoding two channel stereo into multi-channel sound includes the steps of feeding left and right input signals to left and right front and surround channel outputs, respectively, summing the left and right input signals to provide a summed signal, determining when the summed signal is dominant, and subtracting the right and left input signals from the left and right surround channel outputs, respectively, when the summed signal is dominant. 
     If center front and/or surround speakers are also desired, the process further includes the steps of feeding the summed signal to a center front channel output and/or differencing the right and left input signals to provide a center surround signal at a center surround channel output. 
     The process can be enhanced in the four speaker systems by filtering the left and right input signals over a preselected bandwidth to provide left and right filtered signals for subtraction from the right and left surround channel ouputs, respectively, when the summed signal is dominant. Similarly, the five or six speaker systems can be enhanced by filtering the summed signal over the preselected bandwidth to provide a center front signal at a center front channel output and/or differencing the right and left input signals to provide a differenced signal and filtering the differenced signal over the preselected bandwidth to provide a center surround signal at a center surround channel output. Any of these filtered systems can be further enhanced by dynamically filtering rather than fixed filtering the left, right, summed and differenced signals. 
     In the basic, fixed filtered and dynamically filtered four, five or six speaker systems, further enhancement can be achieved by splitting the left and right input signals into left and right bass and high frequency band signals, respectively, and using the high frequency band signals in place of the broad band input signals in the system, recombining the bass band signals with the left and right high frequency band outputs of the system for the left and right front channel outputs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a partial block diagram/partial schematic diagram of the dynamic spectral matrix surround system; 
         FIG. 2  is a block diagram of the steering voltage generator of  FIG. 1 ; 
         FIG. 3  is a block diagram of the left front steering circuit of  FIG. 1 ; 
         FIG. 4  is a block diagram of the right front steering circuit of  FIG. 1 ; 
         FIG. 5  is a block diagram of the center front steering circuit of  FIG. 1 ; 
         FIG. 6  is a block diagram of the left surround steering circuit of  FIG. 1 ; 
         FIG. 7  is a block diagram of the right surround steering circuit of  FIG. 1 ; 
         FIG. 8  is a block diagram of the center surround steering circuit of  FIG. 1 ; 
         FIG. 9  is a block diagram of the left surround steering circuit, which includes an additional dynamic filtering enhancement; 
         FIG. 10  is a block diagram of the right surround steering circuit which includes an additional dynamic filtering enhancement; 
         FIG. 11  is a block diagram of simplified implementation of the steering voltage generator circuit of  FIG. 1 ; 
         FIG. 12  is a block diagram of a simplified left front surround steering circuit; and 
         FIG. 13  is a block diagram of a simplified right front surround steering circuit. 
     
    
    
     While the invention will be described in connection with several preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , left and right stereo source signals L and R are applied to left and right inputs  9 L and  9 R. The stereo input signals L and R are buffered by buffer amplifiers  10 L and  10 R, providing buffered signals sufficient to drive the input crossover sections. The output of the left buffer amplifier  10 L drives the inputs of both a high-pass filter  11 L and a bass or low-pass filter  12 L. The output of the right buffer amplifier  10 R drives the inputs of both a high-pass filter  11 R and a bass or low-pass filter  12 R. The left filters  11 L and  12 L provide a 24 db per octave crossover for the left input signal L. 24 db per octave crossover filters are commonly known and used by the skilled artisan. One advantage of this type of filter is that the bands can be recombined to avoid any peaks or dips in the final frequency response as a result of phase coherency at the crossover point. The 24 db per octave filters allow the bass frequency signals to be crossed over at a higher crossover point and still provide excellent removal of all voice band audio from the unaltered bass portion of the audio spectrum. The output L B  of the left low-pass filter  12 L is fed directly to a summing amplifier  60  where the bass portion of the audio spectrum can be re-combined with the left channel, upper frequency portion of the audio spectrum after the upper band signal L H  has been processed. The left filters  11 L and  12 L provide a 24 db per octave crossover for the left input signal L at approximately 200 hz. This allows the bass frequency signal L B  to be crossed over at a higher crossover point and still provide excellent removal of all voice band audio from the unaltered bass portion of the audio spectrum. This further provides a high band steered signal, with better removal of all bass frequency components, to avoid any dynamic modulation at the bass frequencies that can otherwise cause audible side effects in other systems. This allows the low-band portion of the audio spectrum to retain true stereo performance while simultaneously improving system performance to avoid the above mentioned audible low frequency modulation. 
     The output R B  of the right bass or low-pass filter  12 R is fed directly to another summing amplifier  70  where the bass portion of the audio spectrum can be re-combined with the right channel, upper frequency portion of the audio spectrum after the upper band signal R H  has been processed. This provides the above mentioned left channel improvements for the right channel also. It should be noted that the left and right channel bass frequency signals L B  and R B  at the outputs of the low-pass filters  12 L and  12 R can also be summed with the left and right surround final outputs S L  and S R  to provide bass information in the outputs of these other channels. The outputs L B  and R B  of the low pass filters  12 L and  12 R can also be summed and added to the center channel outputs F C  and S C  if so desired. The above addition of the bass frequency signals L B  and R B  in the surround channels S L  and S R  is particularly desirable in automotive applications. 
     The left and right high-pass outputs L H  and R H  of the high pass filters  11 L and  11 R are applied to the inputs of sum and difference amplifiers  20  and  30  respectively. The output of the summing amplifier  20  provides a summed output L H +R H  of the high band portions L H  and R H  of the left and right input signals L and R. The difference amplifier  30  produces a left minus right difference output L H −R H  of the high band portions L H  and R H  of the left and right input signals L and R. The output L H +R H  of the summing amplifier  20  feeds the input of the center front steering circuit  51  and also provides one of the inputs to a steering voltage generator  40 . The output L H −R H  of the difference amplifier  30  is fed to the input of the center surround steering circuit  53  and also to one of the inputs of the steering generator  40 . The center surround output channel S C  may be omitted in lower cost implementations of the invention or in systems that do not require this additional channel S C , such as automotive and/or PC sound systems. 
     The high passed left and right outputs L H  and R H  of the high pass filters  11 L and  11 R are fed to the inputs of the left and right front and left and right surround steering circuits  50 ,  52 ,  54  and  55 , respectively. The processed output signal  56  of the left front steering circuit  50  is fed to the other inputs of the first summing amplifier  60 . The low-passed bass frequencies at the output L B  of the left low pass filter  12 L combined with the left front steering output  56  form a composite left front output signal F L  at the left front channel output  80 . The processed output signal  57  of the right front steering circuit  52  is fed to the other input of the other summing amplifier  70 . The low-passed bass frequencies at the output R B  of the right low pass filter  12 R combined with the left front steering output  57  form a composite right front output signal F R  at the right front channel output  82 . The processed outputs of the center front steering circuit  51 , the center surround steering circuit  53 , the left surround steering circuit  54  and the right surround steering circuit  55  drive the other system channel outputs  81 ,  83 ,  84  and  85  to provide front center, surround center and surround left and right output signals F C , S C , S L  and S R , respectively. 
     The operation of the steering circuits  50 – 55  will be described in greater detail later. The outputs L H , and R H  of the high-pass filters  11 L and  11 R also fed to the input of the steering voltage generator  40 . In operation, the four inputs L H , R H , L H +R H  and L H −R H  to the steering voltage generator  40  are used to produce the steering voltages L/R, C, S and F that control the audio path steering circuits  50 – 55  described above. The four output steering voltages L/R, C, S and F are provided at outputs,  118 ,  119 ,  120  and  121 , respectively. These four steering voltages L/R, C, S and F are fed to the steering voltage inputs of the audio path steering circuits  50 – 55  as designated by the L/R, C, S, and F references. 
     The operation of the steering voltage generator  40  of  FIG. 1  is described in greater detail with reference to  FIG. 2 . The steering voltage generator  40  receives the four input signals L H , R H , L H +R H  and L H −R H  previously described with reference to  FIG. 1 . As previously mentioned, these signals L H , R H , L H +R H  and L H −R H  have had all low band audio information removed by the left and right high pass filters  11 L and  11 R. This enhances the performance of the steering voltage generator  40  by only detecting the high-band portion of the audio spectrum where virtually all of the directional audio information is contained. As shown in  FIG. 2 , the high-passed left input signal L H  is applied to an input  114  and feeds a left logging circuit  41 L. The output of the left logging circuit  41 L feeds the input of a full-wave rectifier circuit  42 L. As will be apparent to the skilled artisan, the order of these two circuits can be changed with no real change in the net result. The logging and full-wave rectifier circuits  41 L and  42 L shown are the equivalent of those described in my U.S. Pat. No. 5,771,295. Other forms of level detection can be used, such as peak averaging, but with degraded system performance. The output of the full-wave rectifier  42 L is equal to the absolute value of the logarithm of the high passed left input signal L H  applied to the logging circuit  41 L. The high-passed right input signal R H  is applied at another input  115  and fed to another logging circuit  41 R. The output of the logging circuit  41 R feeds the input of another full-wave rectifier circuit  42 R. The output of this full-wave rectifier  42 R is equal to the absolute value of the logarithm of the high passed right input signal R H  applied to the second logging circuit  41 R. The design and operation of logging circuits and absolute value circuits are well known to the skilled artisan. Therefore, a more detailed description of these circuits is not required for the skilled artisan to build the invention. It will also be understood by anyone skilled in the art that the output of the full-wave rectifier circuits will be linear in volts per decibel. The output of the first full wave rectifier  42 L feeds the positive input of a difference amplifier  43 . The output of the second full wave rectifier  42 R feeds the negative input of another difference amplifier  43 . The resulting output of the first difference amplifier  43  is positive when there is a dominance in the left input signal L and negative when there is a dominance in the right input signal R. 
     The high-passed left plus right input signal L H +R H  is applied to a third input  116  and feeds a third logging circuit  45 C. The output of the third logging circuit  45 C feeds the input of a third full-wave rectifier circuit  46 C. The high-passed left minus right input L H −R H  signal is applied to a fourth input  117  and feeds a fourth logging circuit  45 S. The output of the fourth logging circuit  45 S feeds the input of a fourth full-wave rectifier circuit  46 S. 
     The output of the third full-wave rectifier  46 C feeds the positive input of another difference amplifier  47 . The output of the fourth full wave rectifier  46 S feeds the negative input of the difference amplifier  47 . The resulting output of the difference amplifier  47  is positive when there is a dominance in the left plus right input signal L+R and negative when there is a dominance in the left minus right input signal L−R. The output of the first difference amplifier  43  feeds both a variable low-pass filter  48  and a filter control circuit  44 . The output of the second difference amplifier  47  feeds both a second varible low-pass filter  49  and the filter control circuit  44 . When there is no dominant signal present at any one of the inputs, there will be no output signal present from the difference amplifiers  43  and  47 . In this condition, the variable low-pass filters  48  and  49  will have a corner frequency at approximatley 1 Hz. The typical volt per decibel response at the output of the difference amplifiers  48  and  49  is on the order of 3 volts/decibel. When the output of either difference amplifier  43  or  47  exceeds 0.5 volts positive or negative, the filter control circuit  44  will start to increase the cut-off frequency of the variable low-pass filters  48  and  49 . As the output of either difference amplifier  43  or  47  increases positive or negative from 0.5 volts to 3 volts, the cut-off frequency of the variable filters  48  and  49  will change in a relatively linear response from 1 Hz to approximately 16 Hz. This provides the proper response time for the control voltage signals to provide fast response time for sudden changes in dominance. This will provide a slow response when there is little or no directional dominance and also avoid distortion of the audio in the steering control circuits. The output or the first variable filter  48  feeds a full-wave rectifier circuit  100 . The output of the rectifier circuit  100  is positive when there is dominance in either the left or the right input signal L or R. The output L/R of the full-wave rectifier  100  appears at the output  118  of the steering voltage generator  40 . 
     The output of the second difference amplifier  47  feeds both the input of the second variable low-pass filter  49  and the filter control circuit  44 . The operation of the second variable low-pass filter  49  and the filter control circuit with respect to the output signal of the second difference amplifier  47  is identical to that described above with reference to the first amplifier  43  and filter  48 . When there is a dominance in the left plus right input signal L+R, the output of the second difference amplifier  47  will be positive. Conversely, when there is a dominance in the left minus right input signal L−R, the output of the second difference amplifier  47  will be negative. The output volt per decibel response of the second difference amplifier  47  will be the same as the first amplifier  43 , 3 volts/decibel. The output of the second variable filter  49  feeds the input of a half wave rectifier  101 . The output of the rectifier  101  will be positive when there is a positive voltage at the output of the variable filter  49  and will be 0 volts when the output of the variable filter  49  goes negative. The output of the half-wave rectifier  101  feeds one input of an inverting summing amplifier  114 . The second input of the inverting summing amplifier  114  is tied to a negative reference voltage. The output of the inverting amplifier  114  feeds the center control voltge C that appears at another output  199  of the steering voltage generator  40 . When the output of the half-wave rectifier  101  is at 0 volts, the output of the inverting summing amplifier  114  will be positive due to the negative reference voltage. The quiescent output voltage will be approximately 4 volts. The significance of this positive offset will be described later with reference to the steering circuits. The output of the second variable filter  49  also connects to the input of an inverting amplifier  102 . The output of the inverting amplifier  102  connects to the input of a second half-wave rectifier  103 . The second half wave rectifier  103  operates the same as the first half wave rectifier  101  and will provide a positive output only when the input signal is positive and will produce no output when the input is negative. When the output of the second variable filter  49  is negative, the output of the second half-wave rectifier  103  will be positive. The output of the second half-wave rectifier  103  feeds the surround output voltage S that appears at another output  120  of the steering voltage generator  40 . In operation, the left plus right output L/R will go positive when there is a dominance in either the left or right input, L or R, the center output C will go positive when there is a dominance of left plus right L+R or center information in the input signals L and R, and the S output will become positive when there is a dominance of left minus right L−R or difference information L−R in the input signals L and R. The left plus right and left minus right input signals L+R and L−R also connect to the input of high-pass filters  104  and  107 , respectively. The outputs of the high pass filters  104  and  107  feed fifth and sixth logging circuits  105  and  108  that feed fifth and sixth full-wave rectifiers  106  and  109 , respectively. The output of the fifth rectifier  106  connects to the positive input of a third difference amplifier  110 . The output of the sixth rectifier  109  connects to the negative input of the third difference amplifier  110 . The operations of the fifth and sixth log converters  105  and  108 , fifth and sixth full wave rectifiers  106  and  109  and the third difference amplifier  110  are identical to that described above. The high-pass filters  104  and  107  have a 12 db/octave response so as to provide an increasing sensitivity at high frequencies at the input to the fifth and sixth log converters  105  and  108 . The result is that, when there is an increasing left plus right L+R or center frequency signal at the input  116  of the steering voltage generator  40 , the output of the third difference amplifier  110  will produce an increasing output voltage. The output of the third difference amplifier  110  connects to the input of a third low-pass filter  111 . The corner frequency of the filter  111  is on the order of 100 Hz. This provides a much faster response at the output of the filter  111  than is available from the variable filters  48  and  49 . The output of the low-pass filter  111  feeds the input of a third half-wave rectifier  112 . When the output of this filter  111  is positive, the output of the third half wave rectifier  112  will be positive. When the output of the filter  111  is negative, the output of the third half-wave rectifier  112  will be 0 volts. The output of the third half-wave rectifier  112  connects to one input of a summing amplifier  113 . The second input of the summing amplifier  113  is connected to the output of the first half-wave rectifier  101 . The outputs of both the first and third half-wave rectifiers  101  and  112  produce a 3 volt/decibel response. When there is strong de-correlated input signal and, simultaneously, the presence of dominant center information that does not contain a large amount of high frequency information, the output of the third difference amplifier  110  will produce a negative signal, and the output of the second difference amplifier  47  will be positive as a result of the presence of dominant broadband center information. Under this condition, the output of the difference amplifier  113  will be slightly positive due to the positive output at the first half wave rectifier  101 . When there is a large amount of high frequency left plus right L+R or center information, the output of the third rectifier  112  will be strongly positive and, therefore, the output of the difference amplifier  113  will be strongly positive. The operation of the steering voltage generator  40  and the resulting control of the steering circuits will be further explained later, after a detailed description of the steering circuits. 
     Referring to  FIG. 3 , the left front steering circuit  50  will now be described. The left high-passed signal L H  is applied to the positive input of a difference amplifier  133 . The right high-passed input signal R H  is applied to the input of an inverting amplifier  130 . The output of the inverting amplifier  130  is connected to the input of a voltage controlled amplifier or VCA  131 . VCA&#39;s are commonly known in the art and, therefore, a detailed description of the VCA need not be included. In the current invention, it is desirable to use a VCA that has a linear volt per decibel response to the control signal. The VCA  131  will have a control law such that at 0 volts, the VCA  131  will be at unity gain and the gain will vary linearly to −60 db with a control voltage of approximately 3 volts. This provides a 0.5 volt per 10 db response. The control port of the VCA  131  receives the center control signal C from the second output  119  of the steering voltage generator  40 . The output of the VCA  131  feeds the input of a voltage controlled variable low-pass filter  132 . Voltage controlled low-pass filters are well known in the art and are described in great detail in U.S. Pat. No. 5,736,899. The filter  132  used in the preferred embodiment of the invention has a corner frequency of 1 kHz when the control voltage is at 0 volts. When the control voltage is at approximatley 6 volts, the corner frequency of the filter  132  will be above 20 kHz. The filter  132  will vary in a relatively linear response over the control voltage range of 0 volts to approximately 6 volts. The control port of the variable low-pass filter  132  receives the frequency control signal F from the fourth output  121  of steering voltage generator  40 . The output of the variable low-pass filter  132  connects to the negative input of a difference amplifier  133 . The output of the difference amplifier  133  feeds the input of a second VCA  134 . The output of the second VCA  134  feeds the left front steering output  56 . The second VCA  134  has a control law similar to that of the first VCA  131  where 0 volts equals unity gain and the gain will attenuate as the control voltage is increased positive. The control port of the second VCA  134  receives the surround control signal S from the third output  120  of steering voltage generator  40 . It becomes apparent that, when the gain of the first VCA  131  is at a minimum setting, no signal appears at the output of the first VCA  131 . As a result, there will also be no output signal at the output of the filter  132 . Thus, the output of the difference amp  133  will be equal to the left high band input L H . If the gain of the second VCA  134  equals 1, then the left high band input signal L H  will appear unaltered at the steering generator output  56 . If the corner frequency of the filter  132  is above 20 kHz and the gain of the first VCA  131  is at unity, then the output of the difference amp  133  will equal the left high band input signal minus the right high band input signal L H −R H . This will cancel all center or left plus right information L H +R H  from the output  56  between 200 Hz to 20 kHz. If the corner frequency of the filter  132  were reduced to 3 kHz, the output of the difference amplifier  133  would equal the left high band input signal minus the right high band input signal L H −R H  from 200 Hz to 3 kHz and would equal the left high band input signal L H  at frequencies above 3 kHz. It also becomes clear that if the steering voltage S at the control port of the second VCA  234  becomes positive, the signal level at the output  56  will be attenuated. Referring back to  FIG. 2 , the steering voltage C at the second output  119  will be at 4 volts when there is no dominant center channel signal present. This positive offset voltge will cause the fourth VCA  131  to attenuate to greater than −60 db. This attenuation will allow the left high band input signal L H  to pass unaltered to the output  56 . When the steering voltage C of the second output  119  of the steering generator  40  decreases, indicating an increase in center channel audio in the input, the gain of the first VCA  131  will increase. When the voltage C goes to 0 volts, the gain of the first VCA  131  will reach unity. The result is that the inverted right high band input signal will pass unaltered to the input of the filter  132 . 
     Referring now to  FIG. 4 , the right front steering circuit  57  is described. The last digits of the reference designators used are the same as those of the left front steering circuit  50 . The operation of the right front steering circuit  51  is identical to the left front steering circuit  50 . The only difference is that the left and right high band input signals L H  and R H  are swapped. The positive input of the difference amplifier  143  receives the high-passed right input signal R H  and the input of the inverting amplifier  140  receives the high-passed left input signal L H . The final output of the VCA  144  drives the right output  57  of the first steering circuit  52 . 
     Referring now to  FIGS. 6 and 7 , the left surround and right surround steering circuits  54  and  55  will be described. The last digits of the reference designators used are the same as those used in the front steering circuits  50  and  52  to depict similar operation. The left surround steering circuit  54  is similar to the front steering circuit  50 , including the left and right high band input signals L H  and R H . The operation of the left surround steering circuit  54  is similar to that explained above with reference to the left front steering circuit  50 . The only difference between these two circuits is that the left surround steering circuit VCA  164  receives its control signal L/R from the first output  118  of the steering voltage generator  40 . This means that, when there is a dominant left or right input signal L or R, the VCA  164  will attenuate, thereby reducing the level of the output S L  at the left surround channel output  84 . The right surround steering circuit  55  is similar to the right front steering circuit  52  including the left and right high band input signals L H  and R H . The only difference between these two circuits is that the right surround steering circuit VCA  174  receives its control signal L/R first output  118  of the steering voltage generator  40 . This means that when there is a dominant left or right input signal L or R, the VCA  174  will attenuate, thereby reducing the level of the output S R  at the right surround channel output  85 . 
     Referring now to  FIG. 5 , the center front steering circuit  51  will be described. The high-passed summed left plus right signal L H +R H  is applied to the input of a variable low-pass filter  150 . The filter  150  is similar to the filter  132  described above with reference to  FIG. 3 . The control port of the variable filter  150  received the control signal F from the fourth output  121  of the steering voltage generator  40  in  FIG. 1 . The output of the variable filter  150  is fed to the input of a VCA  151 . The output C F  of the VCA  151  feeds the center front channel output  81  in  FIG. 1 . Referring again to  FIG. 5 , the VCA  151  has two positive control ports which produce unity gain when the control voltage is at 0 volts and will provide attenuation when the control signal L/R or S on either port is positive. One control port receives the left/right control voltage L/R from the first output  118  of the steering voltage generator and the second control port receives the surround control voltage S from the third output  120  of the steering voltage generator  40 . The filter  150  is designed to have a quiescent corner frequency of 3 kHz. Thus, in the absence of any dominant left plus right signal L+R or center channel information at the input to the system, the output bandwidth of the filter  150  will be 200 Hz to 3 kHz. This is sufficient bandwidth to cover voice band audio information but will attenuate higher frequency information that may be present in the stereo left and right input signals L and R. This will noticeably increase the impact of the stereo information at the left and right front channels outputs  80  and  82 . When there is either an increase in dominant left plus right signal L+R or front center information at the input to the system, the center control voltage C at the second output  119  of the steering voltage generator  40  will increase and will cause the bandwidth of the filter  150  to increase. With an increase in the high band left plus right L H +R H  frequency spectrum and/or a strong increase in the dominant left plus right signal L+R information, the bandwidth of the filter  150  will increase to over 20 kHz. Center channel audio is attenuated when there is an increase in dominant left or right audio at the inputs of the system. The first control voltage L/R at the first steering voltage generator output  118  will increase, thereby causing the VCA  151  to attenuate. The VCA  151  will also produce increasing attenuation as the third control voltage S at the third steering voltage generator output  120  increases. This attenuation helps to reduce cross talk from surround channels into the front center channel when there is stereo surround encoded signal present. 
     Referring now to  FIG. 8 , the center surround steering circuit  53  will be described. The center surround steering circuit  53  receives an audio input from the difference amplifier  30  in  FIG. 1 . The high-passed left minus right input signal L H −R H  is applied to the input of a variable filter  180 . The control port of the variable filter  180  is connected to the third control voltage S at the third output  120  of the steering voltage generator  40 . The output of the variable filter  180  feeds the input of a VCA  181 . The control port of the VCA  181  is connected to the first control voltage L/R at the first output  118  of the steering voltage generator  40 . The output C S  of the VCA  181  feeds the center surround channel output  83  of the system. The quiescent corner frequency of the variable filter  180  is set to 3 kHz. This reduces the center surround channel bandwidth to a maximum of 3 kHz in the absence of any dominant surround information. The output of the VCA  181  will also attenuate when there is any dominant left or right input L or R to the system. The bandwidth of the center surround output  83  will increase to over 20 kHz only when there is a dominant left minus right signal L−R or surround signal present at the input. As previously mentioned, the center surround channel steering circuit  53  can be omitted in applications that do not benefit from this additional channel such as PC sound and automotive applications. 
     Looking now at the operation of all of the components together, it can be seen that, in the absence of any dominant directional signal at the input of the system, all the output control L/R, C, S and F voltages of the steering voltage generator  40  will be at 0 volts. Under this condition, the left front and left surround signals F L  and S L  at the channel outputs  80  and  84  of the system will be the same as the left input signal L. Conversely, the right front and right surround signals F R  and S R  at the channel outputs  82  and  85  will be the same as the right front input signal R. If the input signals L and R contain a dominant amount of center or left plus right L+R information in the spectral region from 200 Hz to 3 kHz and simultaneously contain stereo de-correlated high frequency information, the first control voltage will be 0 volts, the second control voltage C will be strongly positive, the third control voltage S will be 0 volts and the fourth control voltage F will be only slightly positive. This will cause the first VCA&#39;s  131 ,  141 ,  161  and  171  of the left front, right front, left surround and right surround circuits  50 ,  52 ,  54  and  55 , respectively, to provide unity gain. At the same time, the corner frequency of the variable filters  132 ,  142 ,  162  and  172  of these circuits will be at approximately 3 kHz. The gain of the second VCA&#39;s  134 ,  144 ,  164  and  174  will be at unity. Thus, the signals F L  and S L  at the left front and second channel outputs  80  and  84  will be left minus right L−R from 200 Hz to 3 kHz and left L at frequencies above 3 kHz. The signals F R  and S R  at the right front and surround channel outputs  82  and  85  will be right minus left R−L from 200 Hz to 3 kHz and right R at frequencies above 3 kHz. This will provide a cancellation of the center channel voice band audio from the left and right channels while still providing true stereo operation in the spectrum above 3 kHz. This provides a tremendous improvement of the perceived left/right stereo separation. In the absence of any higher frequency stereo information, there would be noticeable leakage of center channel audio in the four left and right channels. This 3 Kz bandwidth is, however, sufficient in the presence of the higher frequency stereo information due to the fact that the higher frequency harmonic content of the center channel audio is subjectively masked by the higher frequency de-correlated stereo information present in the left and right output channels. The increase in the stereo separation of the system is a far grater benefit than a complete cancellation of all masked center channel harmonics still present in the left and right output channels. This is certainly a performance advantage when the system is used to decode non-encoded stereo music source material. Continuing with the complete system operation, when the input audio signal contains an increasing amount of center channel high frequency information, the voltage at the output of the third rectifier  112  in the steering voltage generator  40  will increase. This will produce an increasing control voltage F at the fourth output  121  of the steering voltage generator  40 , and will result in an increase in the corner cut off frequency of the filters  132 ,  142 ,  162  and  172 . The result is that the cancellation of center channel information in the left and right channels will increase in bandwidth, thus avoiding any un-masked leakage of high band center information in the left and right channels. If the input signal contains only voice band, center channel audio without any de-correlated, or stereo, information, then the output of the first rectifier  101  in the steering generator  40  will be sufficiently positive so as to cause the corner frequency of the variable filters  132 ,  142 ,  162  and  172  to increase above 20 kHz. This will ensure that there is no leakage of center channel audio information into the left and right output channels. Due to the fact that the system does not revert to L−R and R−L across the entire spectrum in the left and right front and left and right surround channels F L , F R , S L  and S R , there is a decrease in the amount of difference information when compared to other matrix decoding systems. The result is that the output of the described invention more closely replicates the balance of L+R to L−R information in the original recording. This will reduce the objectionable increase of reverberant information typical when decoding stereo source material with other matrix decoding systems. 
     Continuing with the system operation, when there is a center channel voice band signal and a strong stereo de-correlated signal, the system will work as described above. When there is a sudden but short increase in center channel high frequency information, such a sharp sibilance in a lead vocal, the time constant of the high frequency weighted summed and difference signals L H +R H  and L H −R H  at the output of the third rectifier  112  of the steering voltage generator  40  will be sufficiently fast. This will allow the steering circuit filters to respond quickly so as to avoid any side effects, such as spitting in the left and right channels. This time constant can be considerably faster than that of the VCA steering voltages without any concern of audible distortion in the audio. When there is a dominant increase in the left channel input L, the control voltage C at the second output  4 L of the steering generator  40  will become 4 volts. The control voltage F at the fourth output  121  will be at 0 volts. There will be a positive control voltage L/R at the first steering voltage output  118 . The positive 4 volts control voltage C at the second output  4 L will cause the VCA&#39;s  131 ,  141 ,  161  and  171  to attenuate to greater than 60 db. This will return the system to true stereo operation in the left and right channels. The positive control voltage L/R at the first steering voltage output will cause all three surround channels S L , S R  and S C  and the center front channel F C  to attenuate. This will allow dominant left channel information to be output only in the left front channel  80 . Conversely, when the right input channel becomes dominant, the control voltage C at the second steering voltage generator output  119  will be at 4 volts and the left/right control voltage L/R at the first steering generator output  118  will again be positive. This will allow dominant right channel input signals to output only in the right front channel  82 . When there is a dominant L−R or surround signal, the control voltage C at the second steering voltage output  119  will be at 4 volts. The L/R control voltage L/r at the first steering generator output  118  will be at 0 volts. The control voltage F at the fourth steering generator output  121  will be at 0 volts. The control voltage S at the third steering generator output  120  will be positive. Since the second steering voltage C is at 4 volts, any stereo difference or surround information will appear in the left and right surround channels  84  and  85 . Since the third control voltage S will be positive, the VCA&#39;s  134  and  144  in the left and right front steering circuits  50  and  52  will attenuate and the output signal will only be present in the surround channels. The positive steering voltages at the third steering generator output  120  will also increase the corner frequency of the variable filter  180  of the center surround steering circuit  53 . This will provide an increased bandwidth signal in the output of the center surround channel  83 . A full 20 kHz response will only be present in the center surround channel  83  when there is a dominant center surround signal L−R present in the input. 
     Referring now to  FIGS. 9 and 10 , the left surround steering circuit  54  and the right surround steering circuit  56  are shown in a system which further includes additional variable filters in the final outputs. The left surround steering circuit  54  includes an additional variable low-pass filter  165  which operates with the same response as that of the filter  180  in  FIG. 8 . The right surround steering circuit  55  also includes an additional variable low-pass filter  175 . The quiescent corner frequency of the added variable filters  165  and  175  is set to 3 kHz. This reduces the surround channel bandwidth to a maximum of 3 kHz in the absence of any dominant surround information. The control ports of the added variable filter  165  and  175  connect to the control voltage S at the third output  120  of the steering voltage generator  40 . The control response of the added variable filters  165  and  175  will provide a 3 kHz corner frequency of 0 volts and will linearly increase to over 20 kHz at 6 volts. In operation, when the input of the system includes any dominance in front, left or right, the bandwidth of the surround channels will not exceed 3 kHz. This is actually sonically closer to the perceived natural reflections present in an acoustical environment. This will improve the perceived separation between the front and surround channels while simultaneously providing a more close approximation of a real acoustic environment. The bandwidth of the surround channels will slightly increase above 3 kHz if the input contains a large amount of stereo de-correlated information. When there is an encoded dominant surround signal in the input, which is intended to produce directional impact in the surround channels, the control voltage S at the third output  120  of the steering voltage generator  40  will increase. This will cause the corner frequency of the added filters  165  and  175  to increase, allowing the dominant surround signal to be reproduced at full bandwidth. The operation of the additional elements of left and right surround steering circuits  54  and  55  are the same as described above in reference to  FIGS. 6 and 7 . 
     Referring now to  FIGS. 11–13 , a lower cost embodiment of the invention will be described. There are applications for the invention where a lower cost alternative with slightly reduced performance will be desirable.  FIG. 11  illustrates a simplified steering voltage generator  240  where the weighted high-passed L+R/L−R difference circuit and all of the associated frequency controlling elements are omitted. The designations used in  FIG. 11  are the same as those used in  FIG. 2  to indicate identical functions. The operation of the steering voltage generator  240  described in  FIG. 11  is identical to that described in  FIG. 2  except with the removal of the high frequency weighted L+R/L−R detection path. Referring now to  FIGS. 12 and 13 , the left front and left surround steering circuits  250  and  254  are shown. The right front and right surround steering circuits functions are the same as described below except for the required change in inputs and outputs signals. The numbers used in  FIGS. 12 and 13  are the same as those used in  FIG. 3  and  FIG. 6  to indicate identical functions. However, the variable filters  132  and  162  in  FIGS. 3 and 6  respectively are replaced by fixed filters  232  and  262  in  FIGS. 12 and 13 . The fixed filters  232  and  262  are single pole 6 db per octave filters having a 3 db or corner frequency at approximately 6 kHz. When the control voltage C at the second output  119  of the steering generator  40  is at 0 volts, the VCA&#39;s  131  and  161  will be at unity gain. The output  56  of the left steering circuit  250  will be L−R at frequencies below 6 kHz and L at frequencies above 6 kHz. This will provide cancellation of the center channel information at frequencies below 6 kHz. The output of the left surround steering circuit  254  will be L−R at frequencies below 6 kHz and L at frequencies above 6 kHz. This will provide cancellation of the center channel information at frequencies below 6 kHz as described above. The result is that, at frequencies above 6 kHz, the system will maintain true stereo performance. This allows the higher frequency information where most of the stereo cues are present to produce true stereo performance. All other operation of the system is identical to that previously described with reference to the previous drawings. This novel approach to providing surround channels by canceling front center information in the surround channels only at the point in time that center information is present and providing full bandwidth, true stereo in the absence of any dominant center signal is a major improvement over other surround decoding systems. 
     The teachings regarding the use of all pass phase-shift circuits contained in U.S. Pat. No. 5,319,713 can also be applied to this disclosure. 
     Thus, it is apparent that there has been provided, in accordance with the invention, a dynamic spectral matrix surround system that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art and in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit of the appended claims.