Abstract:
A matrix system encodes five discrete audio signals down to a two-channel stereo recording and decodes the recorded stereo signal into at least five stand alone, independent channels to allow placement of specific sounds at any one of 5 or more predetermined locations as individual, independent sound sources, thus producing a 5-2-5 matrix system. One embodiment of the system provides signals to left front, right front, center, left rear, and right rear speaker locations. The matrix system is compatible with all existing stereo materials and material encoded for use with other existing surround systems. Material specifically encoded for this system can be played back through any other existing decoding systems without producing undesirable results.

Description:
This application is a continuation of copending application number 08/769,452, Dec. 18, 1996; now U.S. Pat. No. 5,771,295, issued on Jun. 23, 1998, and claims the benefit of provisional application No. 60/009,229, Dec. 26, 1995. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to audio sound systems and more specifically to audio sound systems which can decode from two-channel stereo into multi-channel sound, commonly referred to as “surround” sound. 
     Since Peter Scheiber&#39;s U.S. Pat. No. 3,632,886 issued in the 1960s, many patents have been issued regarding multidimensional sound systems. These systems are commonly known as 4-2-4 matrix systems, where four discrete audio signals are encoded into a two channel stereo signal. This encoded stereo signal can then be played through a decoder, which extracts the four encoded signals and feeds them to their intended speaker locations. 
     4-2-4 matrix designs were originally applied to the quadraphonic sound systems of the 1970s, but in recent years have become enormously popular for cinematic applications and, even more recently, home theater applications. Following the demise of quadraphonic sound, companies such as Dolby Laboratories adapted the matrix scheme to cinematic applications in an attempt to provide additional realism to feature films. The aforementioned Scheiber patent, as well as his subsequent patents U.S. Pat. Nos. 3,746,792 and 3,959,590, are the patents cited by Dolby Laboratories for the Dolby Surround™ system. Popular surround systems for cinematic and home theater applications typically provide discrete audio signals to four speaker locations—front left, front right, front center and rear surround. The rear surround environment is typically configured with at least two speakers, located to the left and right, which are each fed the mono surround signal. 
     Subsequent patents on 4-2-4 matrix systems have attempted to improve on the performance of the matrix. For example, the original passive systems were only capable of 3 dB of separation between adjacent channels (i.e. left-center, center-right, right-surround and surround-left), therefore it was desirable to develop a steered system which incorporated gain control and steering logic to enhance the perceived separation between channels. 
     Many prior art surround systems have utilized a variable matrix for decoding a given signal into multi-channel outputs. Such a system is disclosed in U.S. Pat. No. 4,799,260, assigned to Dolby Laboratories, as well as in U.S. Pat. No. 5,172,415 from Fosgate. Each of these patents disclose a variable output matrix which provides the final outputs for the system. Other designs, such as that shown in U.S. Pat. No. 4,589,129 from David Blackmer, disclose a system which does not include a variable output matrix but instead includes individual steering blocks for left, center, right and surround. 
     The evolution of the surround sound system has seen the developers of such systems progressively attempt to develop the technology which would allow audio engineers the ability to place specific sounds at any desired location in the 360° soundfield surrounding the listener. A recent result of this can be seen with the development of Dolby Laboratories&#39; AC3 system, which provides five discreet channels of audio. However, there are at least two major drawbacks to such a system: (1) it is not backward-compatible with all existing material, and, (2) it requires digital data storage—not allowing for analog recording of data (i.e. audio tape, video tape, etc.). A Dolby AC3-encoded digital soundtrack can not be played back through a Dolby Pro Logic system. 
     The inventions described in my U.S. Pat. Nos. 5,319,713 and 5,333,201 are major improvements over what has become commercially known and available as Dolby Surround™ and Dolby Pro Logic™, primarily in that those patents cited describe a means of providing directional information to the rear channels—a feature which the Dolby systems do not provide. This feature is very desirable in exclusive audio applications, as well as in applications where audio is synched to video (A/V), and is fully described in the above-cited patents. However, although the inventions described in my above-cited patents greatly improve on the previous designs, none of the matrix-based systems disclosed to date have provided a means of achieving independent left and right rear channels when decoded. 
     My currently pending U.S. patent application Ser. No. 08/426,055 discloses a means of providing additional discrete signals through the practice of embedding one or more signaling tones at the upper edge of the audio spectrum during the encode process. These tones can then be detected during the decode process to re-configure the system such that front left, center and front right channels become disabled—thus allowing for signals panned left, center and right to be fed exclusively to the rear left, overhead and rear right locations, respectively. The detection of an additional signaling tone can then reset the system configuration, if desired. Although this system provides a means of producing additional channels and is an improvement to existing systems, it does introduce drawbacks. For example, the practice of embedding tones within the audio spectrum introduces the possibility of them becoming audible to the listener, which is unacceptable. In addition, such a system could only be applicable to a limited number of recording mediums, due to the inherent limitations of mediums such as cassette tape and the optical soundtrack for 35 mm film. 
     It is desirable, therefore, to be able to encode five discrete audio signals down to a two-channel stereo recording and then have the ability to place specific sounds at any one of 5 or more predetermined locations as individual, independent sound sources when decoded—thus producing a 5-2-5 matrix system. A typical implementation of such a system might provide signals to left front, right front, center, left rear, and right rear speaker locations. There are numerous other embodiments of the invention with many other possible channel configurations, as will be apparent to those skilled in the art. It is, therefore, a primary object of the present invention to provide a matrix system which would decode a stereo signal into at least five stand-alone, independent channels. It is also an object of the present invention to achieve a matrix system which is compatible with all existing stereo material. Another object of this invention is to provide a matrix system which is compatible with material encoded for use with other existing surround systems. Yet another object of this invention is to provide a matrix system such that material specifically encoded for this system can be played back through any other existing decoding systems without producing undesirable results. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a matrix system is provided to encode five discrete audio signals down to a two-channel stereo recording and to decode the recorded stereo signal into at least five stand alone, independent channels to allow placement of specific sounds at any one of 5 or more predetermined locations as individual, independent sound sources, thus producing a 5-2-5 matrix system. One embodiment of the system provides signals to left front, right front, center, left rear, and right rear speaker locations. The matrix system is compatible with all existing stereo materials and material encoded for use with other existing surround systems. Material specifically encoded for this system can be played back through any other existing decoding systems without producing undesirable results. 
    
    
     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 block diagram of a preferred embodiment of the present invention; 
     FIG. 2 is a partial block-partial schematic diagram of Steering Voltage Generator of FIG. 1; 
     FIG. 3 is a block diagram of a prior art encoding method; 
     FIG. 4 is a phase vs. frequency graph of the outputs of the all-pass networks of FIG. 3; 
     FIG. 5 is a block diagram of the encoding method implemented for the present invention; 
     FIG. 6L is a partial block/partial schematic diagram of Left Steering Circuit of FIG. 2; 
     FIG. 6R is a partial block/partial schematic diagram of Right Steering Circuit of FIG. 2; 
     FIG. 7 is a partial block/partial schematic diagram of Center Steering Circuit of FIG. 2; and 
     FIG. 8 is a partial block/partial schematic diagram of Surround Steering Circuit of FIG.  2 . 
    
    
     While the invention will be described in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. 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, a fully implemented surround system is shown in which a left input signal is applied to an input node  9 L. This input signal is buffered by an amplifier  10 L and fed to a Left Steering Circuit  40  which provides the left front output L O , as well as to a summing amplifier  20 , a difference amplifier  30  and a Steering Voltage Generator  80 . A right input signal is fed to input node  9 R which is buffered by an amplifier 10R and fed to a Right Steering Circuit  60  which provides the right front output R O , and to a summing amplifier  20 , a difference amplifier  30  and a Steering Voltage Generator  80 . The signal output from the summing amplifier  20  is fed to a Center Steering Circuit  120 , which then provides the center channel output C O , while the signal output from the difference amplifier  30  is fed to the Surround Steering Circuit  130  which then provides the left and right rear outputs L RO  and R RO . Each of the steering circuits  40 ,  60 ,  120  and  130  are controlled by the Steering Voltage Generator  80 . 
     Referring to FIG. 2, the Steering Voltage Generator  80  accepts the left and right input signals L and R which are fed through high pass filters  82 L and  82 R, respectively. These filters are shown and described in FIG. 4 of my U.S. Pat. No. 5,319,713, herein incorporated by reference. The filtered signals are then fed to level detectors  83 L and  83 R, which are the equivalent of those provided by the RSP 2060 IC available from Rocktron Corporation of Rochester Hills, Mich., All detectors shown in FIG. 2 are equivalent to those provided by the RSP 2060 IC, although other forms of level detection can be implemented, such as peak averaging, RMS detection, etc. The detected signals are buffered through buffer amplifiers  84 L and  84 R before being applied to a difference amplifier  85 . 
     Predominant right high band information detected will result in a positive-going output from the difference amplifier  85 . This positive-going output is fed through a VCA  118 A and a diode  87 R to a Time Constant Generator  88 R. A positive voltage applied to the Time Constant Generator  88 R will produce a positive voltage that is stored by a capacitor  88 B. Therefore, the attack time constant is extremely fast, as a positive voltage applied from the output of the amplifier  85  will produce an instantaneous charge current for the capacitor  88 B. The release characteristics of the Time Constant Generator  88 R are produced by the capacitor  88 B and a resistor  88 A. The resistor  88 A will be the only to discharge path for the capacitor  88 B. The voltage on the capacitor  88 B is buffered by an amplifier  88 C, which then provides the Right Rear High band Voltage output signal R RHV  fed to the Surround Steering Circuit  130  illustrated in greater detail in FIG.  7 . All Time Constant Generators shown in FIG. 2 operate identically to the Time Constant Generator  88 R above described. 
     Conversely, predominant left high band information will result in a negative-going output from the amplifier  85 . This negative-going output is fed through the VCA  118 A before being inverted by an inverting amplifier  86 , producing a positive-going output through a diode  87 L and a Time Constant Generator  88 L to provide the Left Rear High band Voltage output signal L RHV  fed to the Surround Steering Circuit  130 . 
     The L and R input signals applied to the Steering Voltage Generator  80  are also fed through low pass filters  90 L and  90 R, respectively, before level detection is derived by detectors  91 L and  91 R. The detected signals are buffered through operational amplifiers  92 L and  92 R before being applied to a difference amplifier  93 . Predominant right low band information detected will result in a positive-going output from the difference amplifier  93 . This positive-going output is then fed through a VCA  118 B and a diode  95 R to a Time Constant Generator  96 R, to provide the Right Rear Low band Voltage output signal R RLV  fed to the Surround Steering Circuit  130 . 
     Conversely, predominant left low band information will result in a negative-going output from the amplifier  93 . This negative-going output is fed through the VCA  118 B and inverted by an inverting amplifier  94 , producing a positive-going output through a diode  95 L and a Time Constant Generator  96 L to provide the Left Rear Low band Voltage output signal L RLV  fed to the Surround Steering Circuit  130 . 
     In addition, the L and R input signals applied to the Steering Voltage Generator  80  are broadband level detected through detectors  98 L and  98 R, respectively. The detected signals are then buffered through operational amplifiers  99 L and  99 R before being applied to a difference amplifier  100 . Predominant left information detected will cause the amplifier  100  to provide a negative-going signal which is fed to an inverting amplifier  101 . The positive output from amplifier the  101  is fed through a diode  102 L to a Time Constant Generator  103 L, which produces a positive-going voltage at the output of the Time Constant Generator  103 L. Conversely, if predominant right information is detected, the output of the difference amplifier  100  provides a positive-going signal which feeds a diode  102 R and a Time Constant Generator  103 R. The outputs of both Time Constant Generators  103 L and  103 R are fed to a summing amplifier  104  so that an output voltage L/R V  will be derived from either a predominant left or right signal. This output voltage L/R V  is then fed to the Surround Steering Circuit  130 , a Center Steering Circuit  120 , and an Overhead Steering Circuit  150 . 
     The Steering Voltage Generator  80  also accepts an L+R input signal as well as an L−R input signal. These input signals are level detected through detectors  107 F and  107 B, respectively, and buffered through amplifiers  108 F and  108 B. The buffered signals are then applied to a difference amplifier  109 . Predominant L+R information detected will produce a positive-going voltage at the output of the amplifier  109  to a Time Constant Generator  112 F. An operational amplifier  113  inverts this signal to a negative-going voltage which is then used to control the steering VCAs in the Left Steering Circuit  40 , shown in greater detail in FIG.  5 L and the Right Steering Circuit  60  shown in greater detail in FIG.  5 R. The amplifier  113  is configured as a unity gain inverting amplifier which has an additional resistor  115  applied between its “−” input and the negative supply voltage to provide a positive offset voltage at the output of another amplifier  113 . In a quiescent condition, in which no front L+R or L−R information is present, the amplifier  113  will always provide a specified positive offset voltage so that, when applied to the Left Steering Circuit  40  and the Right Steering Circuit  60 , it provides the proper voltage to attenuate the steering VCAs in those circuits. Therefore, a positive voltage is always applied at the F V  output unless front information is detected. When front L+R information is detected, the output of the amplifier  113  will begin going negative from the positive offset voltage that was present prior to detecting the presence of the front L+R information. A strong presence of L+R information will cause the output of the amplifier  113  to go negative enough to cross 0 volts. When the output of the amplifier  113  crosses 0 volts, a diode  117  becomes reverse biased and provides zero output voltage at the F V  output. Predominant L−R surround information detected will produce a negative-going voltage at the output of the difference amplifier  109 . This negative-going voltage is inverted by an inverting amplifier  110  and therefore produces a positive output from a Time Constant Generator  112 B to provide the B V  output which controls steering VCAs in the Left Steering Circuit  40  and the Right Steering Circuit  60 . 
     The signal B V  is also fed to a Threshold Detect circuit  119 , which feeds the control ports of the Voltage Controlled Amplifiers  118 A and  118 B. Under hard surround-panned conditions, the VCAs  118 A and  118 B dynamically increase the gain of the output of their input amplifiers  85  and  93 , respectively, up to a gain of 10. The VCAs  118 A and  118 B provide gain only when signals are panned exclusively to surround positions, and otherwise provide unity gain output under all other conditions. The Threshold Detect circuit  119  monitors the level of the signal B V  to determine when the VCAs  118 A and  118 B are active, and to what degree they increase the output of the amplifiers  85  and  93 . When a strong surround signal L−R is detected, the signal B V  will exceed 2 volts. As B V  exceeds 2 volts, the Threshold Detect circuit  119  applies a positive voltage to the control ports of the VCAs  118 A and  118 B, thus increasing the gain output from their import amplifiers  85  and  93 , respectively. When B V  is at 2 volts, the gain factor of the VCAs  118 A and  118 B is very low. However, as the B V  signal level increases, stronger L−R information being detected at the input and approaches 3 volts, the gains of the VCAs  118 A and  118 B increase proportionately. When the signal B V  reaches 3 volts, the gains of the VCAs  118 A and  118 B reach a maximum gain factor of 10. 
     The high and low band level detectors  83 L,  83 R,  91 L and  91 R provide a response of one volt per 10 dB change in input balance. For ease of explanation, the VCAs  139 ,  140 ,  141  and  142  all shown in FIG. 7, can also be configured to provide a 1 volt/10 dB response. Therefore, if a hard surround L−R signal is detected at the input with the L information at unity gain and the −R information at −3 dB, a 3 dB left dominance will be detected and the output of the high and low band amplifiers  85  and  93  will each be −0.3 volts. Because the input is panned hard-surround, causing the signal B V  to reach 3 volts, this −0.3 volts will be amplified by a factor of 10 by the VCAs  118 A and  118 B, thereby producing a L RHV  and L RLV  of 3 volts. These 3 volt signals are then applied to the VCAs  139  and  141 , shown in FIG. 7, respectively, which will steer the respective left rear output by 30 dB. 
     Referring to FIG. 3, a block diagram of a typical prior art encoding scheme is disclosed, wherein four discrete signals, left, right, center and surround, are encoded down to a two-channel stereo signal. A left input signal L is fed to a summing amplifier  31 , while a right input signal R is fed to another summing amplifier  32 . A center channel input C is fed equally to the summing amplifiers  31  and  32  at −3 dB. The output of the first amplifier  31  is fed to an all-pass network  33 , which provides a linear phase vs. frequency response. The output of the all-pass network  33  is then fed to a third summing amplifier  36 . The output of the second amplifier  32  is fed to another all-pass network  35 , which is similar to the first all-pass network  33  and also provides a linear phase vs. frequency response. The output of the second all-pass network  35  is then fed to a fourth summing amplifier  37 . A surround input signal S is fed directly to a third all-pass network  34 , which provides a 90° phase shift and a linear phase vs. frequency response. The output of the third all-pass network  34  is fed equally to the third and fourth summing amplifiers  36  and  37  at −3 dB. It also must be noted that the output of the third all pass network  34  is fed to the inverting input of the fourth summing amplifier  37 , so as to avoid any cancellation of the R T  signal. The third and fourth amplifiers  36  and  37  provide the left and right encoded outputs L T  and R T . 
     FIG. 4 is a phase vs. frequency graph which illustrates the relationship between the outputs of the first and third all-pass networks  33  and  34  over the entire audio spectrum. It can be seen that, at any given frequency, the output of the third all-pass network  34  is always approximately 90° out of phase with the output of the first all-pass network  33 . 
     FIG. 5 discloses a system which accepts five discrete signals and encodes them down to a two-channel stereo signal. A left input signal L is fed to a summing amplifier  150 , while a right input signal R is fed to a second summing amplifier  151 . A center channel input C is fed equally to the summing amplifiers  150  and  151  at −3 dB. The output of the first amplifier  150  is fed to an all-pass network  152 , which provides a linear phase vs. frequency response. The output of the all-pass network  152  is then fed to a third summing amplifier  160 . The output of the second summing amplifier  151  is fed to a second all-pass network  155 , which is similar to the first all-pass network  152  and also provides a linear phase vs. frequency response. The output of the second all-pass network  155  is then fed to a fourth summing amplifier  161 . A left surround input signal S L  is fed directly to a third all-pass network  153 , which provides a 90° phase shift and a linear phase vs. frequency response. The output of the third all-pass network  153  is fed to the third summing amplifier  160  at −3 dB and a VCA  157 , which feeds the fourth amplifier  161 . A right surround input signal S R  is fed directly to a fourth all-pass network  154 , which provides a 90° phase shift and a linear phase vs. frequency response. The output of the fourth all-pass network  154  is fed to the fourth summing amplifier  161  at −3 dB and another VCA  156 , which feeds the third amplifier  160 . The left surround input signal S L  is also fed to a level detection circuit  162 . Likewise, the right surround input S R  is also fed to another level detection circuit  163 . The outputs of the detectors  162  and  163  are summed at a fifth amplifier  164 . The output of the fifth amplifier  164  feeds a diode  159  before being applied to the control port of another first VCA  157 . The output of the fifth amplifier  164  is also inverted by a sixth amplifier  165  before feeding another diode  158  and being applied to the control port of the second VCA  156 . In a quiescent condition the VCAs  156  and  157  each provide an output of −3 dB. The third and fourth amplifiers  160  and  161  provide the left and right encoded outputs L T  and R T . 
     In this configuration, a strong left surround signal S L  will be detected by the first detector  162  and inverted through the fifth amplifier  164 . The negative-going output from the fifth amplifier  164  is applied to the first VCA  157 , causing it to attenuate the output of the first VCA  157  an additional 3 dB. The negative-going output from the fifth amplifier  164  is also inverted through the sixth amplifier  165 . Due to reverse-biased second diode  158 , no voltage is applied to the control port of the second VCA  156 . Therefore, the output of the second VCA  156  remains −3 dB, and the left surround signal S L  is encoded 3 dB higher than the right surround signal S R . Conversely, a strong right surround signal SR detected by the second detector  163  will produce a positive-going output from the fifth amplifier  164 . This positive-going output is inverted through the sixth amplifier  165 , and fed through the second diode  158  to the control port of the second VCA  156  to attenuate the output of the second VCA  156  an additional 3 dB. Due to reverse-biased first diode  159 , the positive-going voltage is not applied to the control port of the first VCA  157 . Therefore, the output of the first VCA  157  remains −3 dB, and the right surround signal S R  is encoded 3 dB higher than the left surround signal S L . This technique allows for the encoding of a L−R signal where L is slightly hotter than −R, and can intentionally be steered specifically to the left rear with all of the other channels steered down. Likewise, an independent right surround signal can be realized by encoding the −R signal at unity gain while encoding the L signal at −3 dB. Thus, a 5-2-5 matrixing system can be achieved which allows any encoded signal can be fed exclusively to the front left, front right, center, rear left or rear right channels. 
     Now referring to FIG. 6L, L and R input signals are applied to the Left Steering Circuit  40 . The input signal L is inverted through an amplifier  42  and fed to a summing network  46 . The R input signal is fed through a VCA  43  before being fed to the summing network  46 . VCAs are commonly known and used in the art, and any skilled artisan will understand how to implement a Voltage Controlled Amplifier which will provide the proper functions for all of the Voltage Controlled Amplifiers demonstrated in the present invention. The VCA  43  is controlled by the signal F V  applied at its control port. The output of the VCA  43  is fed to the input of an 18 dB/octave inverting low pass filter  45 . Anyone skilled in the art will understand how to design and implement such a filter network. The output of the filter  45  is also fed to the summing network  46 . When the output of the filter  45  is summed with the output of the VCA  43 , all of the low band information below the corner frequency of the filter  45  is subtracted. In practice, this corner frequency is typically 200 Hz. When the outputs of the amplifier  42 , the VCA  43  and the low pass filter  45  are summed at the summing network  46 , the output of the summing network  46  will contain the difference between the left and right inputs. However, the low band information below the corner frequency of the low pass filter  45  is not affected, and therefore appears at the output. This process allows for the removal of center channel information from the left output L O  signal. As the signal FV applied to the control port of the VCA  43  goes positive, the output of the VCA  43  attenuates and less cancellation of the center signal L+R occurs. Therefore, it can be seen that, in a quiescent condition, the signal F V  applied at the control port of the VCA  43  is positive and no attenuation takes place. As center channel information L+R is detected by the Steering Voltage Generator  80 , the signal F V  will go negative, eventually reaching 0 volts, and will result in the total removal of the center channel signal from the left output L O . 
     The output of the summing amplifier  46  is then fed to a second VCA  50  which provides the left output signal L O . The second VCA  50  is controlled by the signal B V  derived in FIG.  2 . L−R information detected at the input will produce a positive-going voltage which will result in attenuation in the second VCA  50 . This allows strong surround information L−R to be attenuated in the left front output signal L O  such that a hard surround signal applied during the encoding process is totally eliminated in the left front and will only appear at the respective rear surround channel. 
     FIG. 6R discloses the Right Steering Circuit  60 . The Right Steering Circuit  60  operates identically to the Left Steering Circuit  40  to provide the Right output signal R O  with the exception that the input signals L and R are reversed. 
     Referring to FIG. 7, a Left+Right signal (L+R) is input to the Center Steering Circuit  120 . This input signal is fed through a VCA  122  to provide the center channel output C O  of the Center Steering Circuit  120 . The VCA  122  is controlled by the L/R V  signal from the Steering Voltage Generator  80 . It becomes apparent that left or right broadband panning will cause the VCA  122  to attenuate the center output C O , as broadband left or right panning will produce a positive-going URv signal into the control port of the VCA  122 . 
     Referring to FIG. 8, the Surround Steering Circuit  130  accepts the L−R signal at its input and applies it to the input of a VCA  132 , which is controlled by the L/R V  signal from the Steering Voltage Generator  80 . The system is configured such that only extreme hard left or hard right broadband panning causes the VCA  132  to attenuate, so that full left/right directional information remains present under typical stereo conditions. The output of the VCA  132  is applied to a high pass filter  137 , which produces high band output to two drive steering VCAs  139  and  140 . The output of the VCA  132  is also applied to a low pass filter  138 , which produces a low band output to two more drive steering VCAs  141  and  142 . The filters  137  and  138  are clearly disclosed and described in my previously cited &#39;713 patent as High Pass Filter  31  and Low Pass Filter  32 . The high band output from the first steering VCA  139  is summed with low band output from the third steering VCA  141  at a summing amplifier  147 . The summation of these two signals provides the Left Rear Output signal L RO  applied to the left rear channel. Similarly, the high band output from the second steering VCA  140  is summed with the low band output from the fourth steering VCA  142  to provide the Right Rear Output signal R RO  fed to the right rear channel. Steering voltages L RHV , R RHV , L RLV  and R RLV  applied to the control ports of the steering VCAs  139 ,  140 ,  141  and  142 , respectively, control the left and right rear or surround steering. The basic operation of multiband steering is described in my U.S. Pat. No. 5,319,713. 
     Thus, it is apparent that there has been provided, in accordance with the invention, a 5-2-5 matrix 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.