Patent Publication Number: US-6711266-B1

Title: Surround sound channel encoding and decoding

Description:
The present invention relates in general to surround sound encoding and decoding and more particularly concerns novel apparatus and techniques for encoding five major channels of a surround sound signal into two channels and decoding the encoded two channels to effectively retrieve the five major channels. 
     A typical surround sound signal includes at least left front, center front, right front, left rear and right rear signals. A typical prior art approach combines these signals into two signals that are typically decoded to recover a left front signal, a right front signal, a center front signal and a monophonic rear signal representative of the sum of the original left rear and right rear signals. 
     It is an important object of the invention to provide improved apparatus and techniques for encoding and decoding surround signals. 
     A feature of the invention resides in an adaptive matrix decode algorithm signal processor which allows for significantly improved steady-state adjacent channel separation, including a processor for generating true-stereo surround-sound signals with limited channel separation, and an additional center surround signal. This center surround signal can be decoded either from conventional matrix-encoded stereo signals or alternately furnished as an additional signal from discrete channel media. 
     Another feature resides in electroacoustically manipulating the front stage signals wherein the discretely panned left or right signal information can be “squeezed” inboard of the left and right channel loudspeakers. This feature facilitates reducing the perceived width of the front left/right sound stage image when listening to audio-for-video sound fields reproduced in concert with a video display device, thereby allowing conventional placement of the left/right channel loudspeakers spaced from the display device as in conventional stereophonic sound field reproduction without unnecessarily comprising the audio-for-video sound field reproduction. 
     Still another feature resides in means for encoding the original 5.1 channel source media into a conventional stereophonic signal, wherein the discrete left and right surround signals are monaurally encoded into a more conventional left total/right total signal format, herein referred to as LT, RT, but with much of the original directional concept preserved. 
    
    
     Other features, objects and advantages of the invention will become apparent from the following detailed description when read in connection with the accompanying drawings in which: 
     FIG. 1 is a block diagram illustrating the logical arrangement of a generalized standard matrix encoder; 
     FIG. 2 is a block diagram illustrating the logical arrangement of a system for input amplitude normalization; 
     FIGS. 3A and 3B are block diagrams illustrating the logical arrangement for generating a difference signal output for sum signal dominance and a sum signal output for difference signal dominance, respectively; 
     FIG. 4 is a schematic circuit diagram of circuitry for generating the left or right dominant control signal; 
     FIGS. 5A and 5B are block diagrams illustrating the logical arrangements for generating sum and difference adjacent channel signals, respectively; 
     FIG. 5C is a block diagram illustrating the logical arrangement of apparatus for removing the adjacent channel signal from the sum and difference signals; 
     FIG. 6 is a block diagram illustrating the logical arrangement for normalizing the matrix for quadrature encoded signals; 
     FIG. 7 is a block diagram illustrating the logical arrangement for generating left, center and right surround channel signals; 
     FIG. 8 is a block diagram illustrating the logical arrangement for generating left/right squeeze signals; 
     FIG. 9 is a block diagram illustrating the logical arrangement for matrixing the discrete 5.1 channel source media signal to derive the center surround channel signal and bass channel signal; 
     FIG. 10 is a block diagram illustrating the logical arrangement for modified matrix encoding with split surround channel signals; 
     FIG. 11 is a block diagram illustrating the logical arrangement of a broadband block decoder; 
     FIG. 12 is a modification of the block diagram of FIG. 11 illustrating the logical arrangement of a broadband block decoder with an enhanced sound imaging feature; 
     FIG. 13 is a block diagram illustrating the logical arrangement of another modification of the arrangement of FIG. 11 characterized by frequency division; 
     FIG. 14 is a block diagram illustrating the logical arrangement of a system for processing left and right transmitted signals to provide an output bass signal; 
     FIG. 15 is a block diagram illustrating the logical arrangement of another decoding system according to the invention that provides left, center and right output signal and a monophonic surround output signal; 
     FIG. 16 is another embodiment of a decoder according to the invention that provides a stereo surround signal; 
     FIG. 17 is a block diagram illustrating the logical arrangement of a decoding system according to the invention using a plurality of stereo decoders to provide left, right, center, left surround, center surround, right surround, left side surround and right side surround signals; and 
     FIG. 18 is a table illustrating the signals at the different terminals of the stereo decoders of FIG.  17 . 
    
    
     With reference now to the drawings and more particularly FIG. 1, there is shown the logical arrangement of a generalized standard matrix encoder left, center, right and surround input terminals  11 ,  12 ,  13  and  14  receive left, center, right and surround signals, respectively. Left-center adder  15  combines the signals on left and center input terminals  11  and  12  to provide a left-center signal to left center phase shift network  16 . Right-center summer  17  combines the signals on center and right terminals  12  and  13  to provide a right-center signal to right-center phase shift network  18 . Quadrature phase shifter  21  receives the surround signal on terminal  14  to provide a quadrature phase-shifted surround signal that is combined with the left-center phase-shifted signal provided by left-center phase shifter  16  to left output adder  22  to provide the left transmitted signal LT and with the right-center phase-shifted signal provided by right-center phase shifter  18  to right output adder  23  to provide the right-transmitted signal RT. 
     The surround channel and center channel signals are defined as equal amplitude out-of-phase and in-phase signals, respectively. Encoding a left and center channel signal simultaneously produces only the center channel output at the output of right output adder  23 , and the left channel signal plus the center channel signal at the output of left adder  22 . Thus, the left and center channel signals cannot be accurately retrieved without first normalizing the relative time-average magnitudes of the left and right transmitted signals LT and RT such that LT is equal to RT at the input terminals of the input amplitude normalization circuitry shown in FIG.  2 . 
     Referring to FIG. 2, the left and right transmitted signals LT and RT from left and right output adders  22  and  23 , respectively, on terminals  24  and  25 , respectively, are multiplied by right and left magnitude signals [R]/Y and [L]/Y, respectively, on input terminals  26  and  27 , respectively, of sum and difference multiplier  31  and  32 , respectively. The outputs of left and right multipliers  31  and  32  are cumulatively combined in adder  33  and differentially combined in subtractor  34  to provide on terminals  35  and  36  sum and difference levels, respectively. 
     One method of normalizing the relative magnitudes of LT and RT at the decoder input terminals involves deriving the time-averaged magnitude of LT, RT, and the time-averaged magnitude of whichever of the two is greater when [LT]≠[RT]. (herein referred to as Y). When the two magnitudes are equal, Y is either the time-averaged magnitude of [LT] or [RT]. Expressing these magnitudes in terms of Y produces two usable coefficients: 
     
       
           A   1 =[ LT]/Y   
       
     
     and 
     
       
           A   2 =[ RT]/Y.   
       
     
     For all LT dominant conditions, the coefficient A 1  has a value of one, and the coefficient A 2  is the ratio of the magnitudes of RT to LT. The opposite is true for all RT dominant input signal conditions. The domain of each of the two coefficients is from 0 to 1 inclusive. Multiplying LT by [RT]/Y and RT by [LT]/Y produces equal magnitude signals at the output of each of multipliers  31  and  32 . If the normalization function is the result of a broadband measurement of the spectrum at LT and RT, then summing the modified signal will not, in all cases, produce the encoded center channel or surround channel signal because the sum signal or difference signal may yet contain information for reproduction by the left (or right) channel signal. 
     For example, consider encoding the center and left channel signals as two sine waves of arbitrary frequency. If the left channel signal L is 5 kHz, and the center channel signal is a 1 kHz signal, each with a unit amplitude of 1 at the encoder output terminals, since the left channel signal is the greater of the left and right channel signals, the coefficient A 1  has a value of 1, and the coefficient of A 2  has a value 0.707. Thus, the output of sum multiplier  31  is 0.707 (5 kHz sine wave+1 kHz sine wave) and the output of right multiplier  32  is 1 (1 kHz sine wave). The sum and the difference signals obtained in this example both contain 0.3535 (5 kHz sine wave), which originated as a left channel signal and not as a center or surround channel signal. 
     Now compare the results of this example with those obtained when the two signals are of the same frequency and phase. In this example, A 1  has a value of 1, and the coefficient A 2  has a value of 0.5. The resulting outputs of left multiplier  31  and right multiplier  32  are equal, with a unit amplitude of 1. In this example, the sum of the signals and the absence of a difference signal are expected conditions and accurately represent the information in the signals originally encoded. 
     It has been discovered that the distinction between these and similar examples resides in the indication (or absence) of a difference signal. Stated in general terms, the difference signal obtained when the spectrum is sum signal and left (or right) channel dominant, contains some of the left (or right) channel signal. Similarly, the sum signal obtained when the spectrum is difference signal dominant and left (or right) channel dominant contains some of the left (or right) channel signal. The invention takes advantage of this property to remove the undesired signal from the resulting sum and difference signals furnished by sum and difference summers  33  and  34 . 
     Referring to FIGS. 3A and 3B, there is shown the logical arrangement of apparatus for generating a difference signal output for sum signal dominance and a sum signal output for difference signal dominance. 
     It is convenient to establish the condition of sum or difference signal dominance by deriving the time-averaged magnitude of each of these signal quantities, and the time-averaged magnitude of whichever of the two is greater (herein referred to as X) when [L−R]≠[L+R]. When [L+R]=[L−R], X is the time-averaged magnitude of either [L+R] or [L−R]. Expressing the sum and difference magnitudes in terms of X produces the coefficients [L+R]/X and [L−R]/X, respectively. For all sum signal dominant conditions, the coefficient [L+R]/X has a value of one, and the coefficient [L−R]/X is the ratio of the difference signal to sum signal magnitudes. The opposite is true for all difference signal dominant conditions. Rearranging these coefficients into a useful form produces: 
     
       
           A   3 =1 −[L+R]/X ,=( X−[L+R ])/ X   
       
     
     
       
           A   4 =1 −[L−R]/X ,=( X−[L−R ])/ X   
       
     
     The domains A 3  and A 4  are from zero to one, inclusive. For all sum signal dominant conditions, the coefficient A 3  is zero, and the coefficient A 4  is 1 minus the ratio of the difference signal magnitude to the sum signal magnitude. The opposite is true for all difference signal dominant conditions. Both A 3  and A 4  are zero when [L+R] is equal to [L−R]. Multiplying the sum signal and difference signal by A 3  and A 4 , respectively, produces only some of the difference signal when the spectrum is sum signal dominant, and only some of the sum signal when the spectrum is difference signal dominant. By multiplying the resulting output signals by the complement of each coefficient A 3 , A 4 , undesired signal components may be removed. It is convenient to designate these complementary coefficients as A 5  and A 6 . Thus: 
     
       
           A   5 = X/[[L+R]−X]+e   
       
     
     
       
           A   6 = X/[[L−R]−X]+e   
       
     
     The quantity e is a small quantity added for theoretical consideration to avoid division by zero. Multiplying the sum signal by A 3 ×A 5  and the difference signal by A 4 ×A 6  produces only the difference signal (when one is present) when the spectrum is sum signal dominant, and only the sum signal (when one is present) when the spectrum is difference signal dominant. 
     FIG. 3A implements this process with input multiplier  36  multiplying the signal on line  35  by coefficient A 3  to provide a first product signal that is multiplied in output multiplier  37  by coefficient A 5  to provide no output on line  38  unless [L−R]&gt;[L+R]. 
     Similarly, in FIG. 3B input multiplier  41  multiplies the signal on line  34  by a signal related to coefficient A 4  to provide a first product signal that is multiplied in output multiplier  42  by a coefficient signal A 6  to provide no output on line  43  unless [L+R]&gt;[L−R]. 
     To remove the undesired signals present in the sum and difference signals, LT and RT must be unequal. Thus, the invention includes means for disabling signal removal when LT=RT. 
     Referring to FIG. 4, there is shown a schematic circuit diagram of apparatus for generating the left or right dominant control signal. Left and right comparators  44  and  45  receive signals on inverting inputs  44 A and  45 A representative of the negative magnitude of LT and RT, respectively. Resistor R couples the negative Y signal on terminal  46  to the noninverting inputs of comparators  44  and  45 . The attenuating network formed by these resistors of value R 2  and R lightly attenuate the magnitude of the inverted variable signal Y, thereby providing a reliable dead band such that the output of both comparators  44  and  45  are logical 1 when the magnitudes of LT and RT are equal. 
     The output of left comparator  44  is at logical 0 when the spectrum is RT dominant. The output of right comparator  45  is at logical 0 when the spectrum is LT dominant. The outputs of comparators  44  and  45  gate multipliers (FIG. 5) from full on to cutoff. 
     Referring to FIG. 5A, there is shown a block diagram illustrating the logical arrangement for apparatus for generating the sum or difference adjacent channel signal. The voltage gain of the multiplier circuitry is responsive to the comparator circuitry of FIG. 4 such that all multipliers have a voltage gain of unity for the condition LT and RT are equal. For the condition [LT]&gt;[RT], the multipliers connected to the output of comparator  45  are cutoff leaving the multipliers connected to comparator  44  at a gain of unity. The opposite is true for the condition [RT]&gt;[LT]. Subtracting the input of the multipliers from the output of the multipliers produces the desired signals which are combined with the original sum and difference signals to produce modified sum and difference signals which are free of the left or right channel signals. 
     Thus, line  38  from FIG. 3A is coupled to the inputs of multipliers  51 ,  52  and the + input of combiners  53  and  54 , respectively. The outputs of multipliers  51  and  52  are connected to the − input of combiners  53  and  54 , respectively. The other inputs of multipliers  51  and  52  are coupled to the outputs of comparators  44  and  45 , respectively. Thus, combiner  53  produces no output unless both [L]&gt;[R] and [L−R]&gt;[L+R], and combiner  54  provides no output unless both [R]&gt;[L] and [L−R]&gt;[L+R]. 
     Similarly, output  43  from FIG. 3B is coupled to an input of multipliers  55  and  56  and the + inputs of combiners  57  and  58 , respectively. The outputs of multipliers  55  and  56  are coupled to the − inputs of combiners  57  and  58 , respectively. The other inputs of multipliers  55  and  56  are coupled to the outputs of comparators  44  and  45 , respectively. Thus, combiner  57  provides no output unless [L]&gt;[R] and [L+R]&gt;[L−R], and combiner  58  provides no output unless [R]&gt;[L] and [L+R]&gt;[L−R]. 
     Referring to FIG. 5B, there is shown added to the system of FIG. 5A, sum output combiner  61  and difference output combiner  62  each having five inputs added and subtracted as indicated to provide a modified sum signal on output  63  and a modified difference signal on output  64  free of the left or right channel signals. 
     Referring to FIG. 5C, there is shown a block diagram illustrating the logical arrangement of a system for removing the adjacent channel signal from the sum and difference signals. 
     Referring again to FIG. 1, consider the left transmitted signal LT and the right transmitted signal RT on lines  24  and  25 , respectively, when the encoded signals are applied as arbitrary sine waves to the center and surround terminals  12  and  14 , respectively. Consider the center channel signal being 1 unit of a 1 kHz sine wave as measured at the outputs  25  and  29 . Consider that the surround channel signal is 1 unit of a 5 kHz sine wave as measured at the output terminals  24  and  25 . In this example, the magnitude of LT is equal to RT and the sum signal magnitude is equal to the difference between signal magnitudes. The resulting sum and difference signals provided by the system of FIG. 2 accurately reflect the original signal that was encoded. 
     Now consider signal conditions at the center and surround input terminals  12  and  14  resulting in sine waves of the same frequency and amplitude on outputs  25  and  29 , such as 1 unit of 1 kHz sine waves. These output signals are in phase quadrature, the magnitudes of these signals are equal and the magnitude of their sum is equal to the magnitude of their difference. However, the sum and difference signal components contain some left and right channel information. 
     By further processing the sum and difference signals according to the invention, the correct amount of left and right channel information remains in the transmitted signals LT and RT after the sum and difference signal components (representing the center and surround signals) have been removed from LT and RT. 
     When the two output signals are in phase quadrature, the encoded signals processed by the decoder should appear at all output terminals of the decoder with equal amplitude at each output terminal; that is, left, right, center and surround. By adding to the left channel and right channel signals equal predetermined amounts of sum and difference signal, the correct amount of left and right channel information remains in the left and right channels. 
     Referring to FIG. 6, there is shown a block diagram of the circuitry of FIG. 5B with additional components added to assure proper decoding when the sine wave signals on output terminals  25  and  29  are in phase quadrature and of equal amplitude caused by signals applied to the center and surround inputs  12  and  14 , respectively. Center multiplier  65  has one input coupled to left output combiner  61  and the other input receives a signal related to the ratio of X to the sum of the magnitudes of the sum and difference signals to provide a product signal that is differentially combined with the output of left output combiner  61  to provide a center complement signal by combining complement center combiner  66  to provide the center complement signal {overscore (C)} on line  67  that is differentially combined with the signal on line  63  in combiner  68  to provide the center signal C on line  71 . 
     Similarly, surround multiplier  72  has one input coupled to the output  64  of right combiner  62  and the other input receives the same signal applied to the other input of center multiplier  65  to provide a product signal that is differentially combined with the signal on line  64  to complement surround combiner  73  to provide the surround complement signal {overscore (S)} on line  74  that is differentially combined with the signal on line  64  in surround combiner  75  to provide the surround signal S on line  76 . 
     Consider still another situation wherein the signal on output terminal  29  includes 1 unit of 5 kHz sine wave and the signal on output terminal  25  includes 1 unit of 1 kHz sine wave caused by a left channel signal on line  11  and right channel signal on right input terminal  13 . This third situation is indistinguishable from the previous two. For a broad spectral band it has been discovered that under these conditions it is desirable to maintain the relevant relationship of the sum and difference signals with respect to each other. Any manipulation of the sum and difference signals for subtracting these signal quantities from the transmitted LT and RT as the center and surround signals will result in a degradation of the separation from the left channel to the right channel and right channel to left channel if the relationship of the sum and difference signals (with respect to each other) are not carefully controlled according to the invention. 
     According to the invention, multipliers  65  and  72  multiply the processed sum and difference signals furnished by the system of FIG. 5A by the following common coefficient signals {overscore (C)} and {overscore (S)} corresponding to coefficients A 7  and A 8  related as follows: 
     
       
           A   7 =(1 −X/[L+R]+[L−R ])×1.414 ={overscore (C)}   
       
     
     
       
           A   8 =(1 −X/[L+R]+[L−R ])×1.414 ={overscore (S)}   
       
     
     The sum and difference signals at the output of center complement combiner  67  and surround complement combiner  73  are each added to LT ({overscore (C)}+{overscore (S)}) and are added with and subtracted from RT, ({overscore (C)}−{overscore (S)}) which restores to LT and RT some L and R, respectively. Similarly, some of the resulting signals {overscore (C)} and are removed from the sum and difference signals. If [L+R]=[L−R], the amount of signals added to LT and RT is 0.707L and 0.707R, respectively. 
     When the spectrum of LT and RT is purely in-phase monophonic spectral components, no signal is added to LT and RT. The same is true when the spectrum at LT and RT is purely out-of-phase monophonic components. To complete the basic decoding process, the final sum and difference signals are multiplied (post matrix) by 1.414 for basic adaptive matrix decoding with a singular surround channel. Performing the signal processing in each of the three previous illustrations in individual spectral bands recovers the signals originally encoded. 
     There has been described apparatus and techniques which overcomes a basic limitation of conventional decoding techniques when attempting to decode two adjacent channel signals simultaneously, and in particular, removed the center C and surround S components from LT and RT without significant degradation of the left/right separation. Furthermore, by processing in accordance with the invention in an adequate number of spectral bands, the invention accurately decodes the encoded signals. 
     Referring again to FIG.  2  and coefficients A 1 , A 2 , multiplying the decoded surround channel signal S by these coefficients effectively adds directional capability to the monaural surround signal S. It is possible to have a surround channel signal and a left and right channel signal simultaneously. 
     Consider encoding a monaural surround and left channel signal of equal amplitude as provided at the LT and RT terminals  24  and  25 , respectively. The LT output then contains 1 unit of left channel information, and the LT and RT output terminals  24  and  25 , respectively, each contain 1 unit of surround channel information. Since the relative amplitudes of the LT and RT signals differ by 6 dB, and the signal on the LT input terminal  24  is dominant, the coefficient A 1 =[LT]/Y is unity, and the coefficient A 2 =[RT]/Y is 0.5. The decoded difference signal then has a magnitude of 1 unit of surround channel information, which, when removed from LT and RT, leaves 1 unit of left information in the left channel. 
     Referring to FIG. 7, the left and right surround channels are respectively LS=S×[L]/Y and RS=S×[R]/Y. 
     Recall that the behavior of the coefficients is such that for all LT dominant conditions, A 1  =[L]/Y is unity and A 2 =[R]/Y is the ratio of the RT input to LT input signals. Thus, a 6 dB difference in input signal levels at the input terminals of the decoder produces a 6 dB difference in the left and right surround channel signals. The invention achieves this result, not by raising the relative level of the dominant surround channel, but by decreasing the level of the benign channel. This property prevents unnatural increases in surround channel signal level that would otherwise occur if the dominant surround signal level were increased. The resulting surround channel signals (from the preceding example) are 1 unit of surround channel information in the left surround channel, and 0.5 of surround channel information in the right surround channel. In FIG. 7, combiner  81  cumulative combines the sum of the signals from combiners  57  and  58  in FIG. 6 with the surround signal S on line  76  of FIG.  6  and combiner  82  differentially combines these signals. Multipliers  83  and  84  multiply the output signals of combiners  81  and  82  with coefficient signals A 1  and A 2 , respectively, to provide respective product signals differentially combined with the signals from combiners  81  and  82 , respectively, by combiners  85  and  86 , respectively, to provide the right channel matrix signal on line  87  and the left channel matrix signal on line  88 . Multipliers  83  and  84  furnish the left surround output and right surround output signals on lines  91  and  92 , respectively. Center combiner  93  combines the left and right surround output signals and the right channel matrix and left channel matrix signals to provide the center surround output signal on line  94 . 
     In the previous illustration, the surround channel signal is decidedly dominant. It is advantageous to have the left surround channel dominant over the left front channel. By performing the operations 1−LS =R and 1−RS=L, it is possible to remove from the dominant front channel the signal which appears as either L or R, and thereby improve the separation between the dominant front and rear channels. For the previous example, 1−LS is 0, and 1−RS furnishes 0.5 units of surround channel signal information. Subtracting this quantity from the left front channel signal decreases the left front channel signal to 0.5 units of left front channel information and effectively places the left rear channel in dominance by 6 dB over the left front channel and right surround channel, respectively. The process is symmetrical for a surround dominant and right channel signal combination. The illustration above is the asymptotic condition, (6 dB left to right surround channel separation with 6 dB dominant rear to dominant front channel separation) because any additional LT or RT dominance results in a diminished surround channel signal. 
     The directional capability of the surround channel signals is a significant improvement. Still another feature of the invention improves spatial realism of the left/right surround channels by the modified circuitry shown in FIG.  8  and by adding, in matrix fashion, sum signal components to the surround channel signal. 
     With reference to the coefficient A 3 , recall that this coefficient has a value of 0 for all sum signal dominant conditions, and is essentially 1 minus the ratio of the sum signal to the difference signal for all difference signal dominant conditions. In the limit, for a pure difference signal condition, there is no sum signal content in the spectrum. It is thus inconsequential to matrix the sum signal with the difference signal then. When the spectrum is sum signal dominant, the output of the multiplier is zero, and again, there is no sum signal component to matrix with the difference signal component. This property is highly advantageous because there is no sum signal matrix with the difference signal when the signals LT and RT are primarily monaural or dialog dominant typically occurring for voices originating from the stream of a video display. As the spectrum becomes difference signal dominant, there is less sum signal content, and it is advantageous to matrix increasing amounts of sum signal spectrum with the increasing dominant difference signal spectrum. In FIG. 8, multipliers  101  and  102  multiply the LT and RT signals respectively by the coefficient signals A 2  and A 1 , respectively, to provide product signals differentially combined by combiners  103  and  104 , respectively, with the LT and RT signals, respectively, to provide the squeeze left to center and squeeze right to center signals respectively on lines  105  and  106 , respectively, through potentiometers  107  and  108 , respectively. 
     Combiner  111  cumulatively combines the product signals provided by multipliers  101  and  102 , and combiner  112  differentially combines these signals to provide the indicated output signals. 
     Combiner  113  cumulative combines the center channel signal on line  71  with the squeeze left to center and squeeze right to center on lines  105  and  106 , respectively, to provide the center channel output signal on line  114 . 
     The left and right surround channels are out-of-phase. If A 1  equals A 2 , the matrix sum signal appears common mode at the output of the left and right surround outputs on lines  91  and  92  when the left and right surround channels are subtracted from each other. This property is an advantageous characteristic of the center surround channel because the signal is predominantly monaural and unique relative to the left and right surround channels. The circuit arrangement maintains the output amplitude of the center surround channel always equal to the output amplitude of the lesser surround channel signal (left or right). The output amplitude of the center surround channel signal is equal in amplitude to the left and right surround channel signals when A 1  equals A 2 . In the limit, the output of the center surround channel is zero for an exclusive LT or RT signal input although there is no surround channel signal for either of these conditions. 
     These considerations make the derived center surround channel according to the invention very suitable for use with 5.1 channel discrete source material. The original 5.1 channels are matrixed as indicated in the block diagram of FIG. 9 to form the transmitted signals LT and RT and may be applied to the decoding circuitry. When the decoder circuitry is used to decode these signals, only the center surround channel and derived bass signals are used as actual output signals from the decoder. The originating left, right, center, left surround and right surround signals are used in place of the output signals from the matrix decoder, augmented by the center surround channel signal and the bass signal outputs of the decoder. 
     In FIG. 9, left input combiner  115  cumulatively combines the left signal, the left surround signal, 0.707 of the low frequency effects (LFE) signal and 0.707 of the center channel signal to provide the left transmitted signal LT on line  123 . Right signal combiner  122  cumulatively combines the right signal, 0.707 of the center channel signal, 0.707 of the LFE signal and differentially combines the sum of these signals with the right surround channel signal to provide the right transmitted signal RT on line  124 . 
     Referring to FIG. 10, there is shown a block diagram illustrating the logical arrangement of a modified matrix encoder with split surround channels. Surround input combiner  131  cumulatively combines the left surround and right surround signals LS and RS to provide a sum signal that is applied to multipliers  132  and  133  multiplied by the ratio of the time-averaged magnitudes of the left and right surround signals, respectively, to the sum of these time-averaged magnitudes to provide product signals that are differentially combined by combiner  134 . Combiner  135  cumulatively combines . 33  of the output signal of combiner  134  with the signal from combiner  131 , and combiner  136  differentially combines . 33  of the output signal of combiner  134  with the signal from combiner  131 . Left output combiner  137  cumulatively combines the left channel signal, the output signal of combiner  135 , 0.707 of the LFE signal and 0.707 of the center channel signal to provide the left transmitted signal LT on line  138 . Combiner  139  cumulatively combines 0.707, the center channel signal, 0.707 of the LFE signal, and the right channel signal differentially with the output of combiner  136  to provide the right transmitted signal RT on line  140 . 
     Referring to FIG. 11, there is shown a broadband block decoder according to the invention that includes an assembly of apparatus described above. Corresponding elements in FIG.  11  and the other figures are identified by corresponding reference symbols. The additional components not described above furnish the bass signal on line  141  at the output of combiner  142 . Combiner  142  cumulatively combines the decoded left, right and center channel signals with the output of combiner  143  that differentially combines the output of multiplier  144  with the product signal furnished by multiplier  145  that multiplies the latter product signal with the signal indicating that the magnitude of the right channel signal is greater than that of the left channel signal. Multiplier  144  provides a product signal that is the product of the coefficient A 3  signal with the output of combining network  112  (see FIG.  8 ). 
     Left output combiner  152  differentially combines the left squeeze signal from the arm of potentiometer  107 , cumulatively combines the LT signal, the signal from multipliers  52  and  56 , differentially combines the signals from combiner  66  and  63  and center surround output combiner  152  to provide the L signal on output  152 . Right output combiner  154  differentially combines the right squeeze signal from the arm of potentiometer  108 , cumulatively combines the RT signal, differentially combines the outputs of multiplier  55  and combiners  63  and  66  and cumulatively combines the outputs of multiplier  51 , combiner  62 , combiner  66  and center surround output combiner  152  to provide the right output signal on line  155 . 
     Left input surround combiner  161  cumulatively combines the signals from combiner  75  and multiplier  37  to provide a sum signal that is multiplied by the coefficient A 1  in multiplier  162  and differentially combined in left output combiner  163  with the output product signal from multiplier  162  to provide a left surround sum signal that is differentially combined in center surround output combiner  152 . The output of multiplier  162  is the left surround signal LS on line  164 . 
     Right input surround combiner  165  differentially combines the signal from multiplier  37  with the signal from combiner  75  to provide a difference signal that is multiplied by the factor A 2  in multiplier  166  and differentially combined with the output of multiplier  166  that is the right surround signal RS on line  167  in right surround output combiner  168  to provide a difference signal that cumulatively combined in center surround output combiner  152  that also differentially combines the right surround signal and cumulatively combines the left surround signal on lines  167  and  164 , respectively, to furnish the center surround signal as an output on line  168 . 
     Referring to FIG. 8, for all signal conditions where the time averaged magnitude of LT is equal to the time averaged magnitude of RT, the coefficients A 1  and A 2  are equal and have a value of unity. Thus, LT×1−A 2 =0 and RT×1−A 1 =0. 
     It follows that there is no squeezable contribution of the left total input signal or right total input signal to the decoded center channel output, and that there is no corresponding reduction in the decoded left or right channel output signals. However, when the time averaged magnitude of LT is greater than the time averaged magnitude of RT, such as occurs with the signal present in LT exclusively, the resulting signals: LT×(1−A 2 )=LT and RT×(1−A 1 )=0 are produced. For all LT dominant conditions, the expression RT×(1−A 1 ) is always 0. The opposite is true for all RT dominant conditions. 
     The outputs of the multiplier cells are fed to independently variable or ganged variable resistors, such as  107  and  108 . The variable resistors facilitate adjusting the relative amount of exclusive left/right total input signal information for subtraction from the decoded left and right channel output signals and added to the decoded center channel output signal. For example, placing equal amounts of left channel information in the center and left channel loudspeakers produces a virtual loudspeaker midway between the center and left channel loudspeakers, thereby placing the exclusive left channel apparent speaker location closer to the video display device. Varying the relative amount of exclusive left channel information removed from the decoded left output channel and added to the decoded center channel output channel serves to vary the apparent location of the virtual loudspeaker. The same condition exists for the exclusive right channel information. In this way, it is possible to place the virtual loudspeakers closer to the display device, such as a television screen, and thus maintain a reasonable relationship between the visual and acoustic images. This technique is advantageous for home theater applications where the left and right channel loudspeakers are placed typically well to the left and right of an attending display device and may be asymmetrically placed with respect to the display device. 
     Returning to FIG. 11, the bass channel output signal is the sum of the decoded left channel, right channel and center channel output signals. In addition, the normalized difference signal obtained from the output of the system of FIG. 2 is applied as one input of a multiplier who second input is the coefficient A 3 . Thus, 
     
       
         ( LT×[R]/Y−RT×[L]/Y )× X−[L+R]/X   
       
     
     produces an output signal only when the time averaged magnitude of the normalized sum signal is less than that of the normalized difference signal. Under these conditions, the spectrum would contain a dominant surround channel signal, and it is desirable to reproduce a bass signal which contains the dominant surround channel signal. The resulting signal obtained under these conditions, however, is further processed prior to adding it to the sum of the decoded left, right and center channel output signals if the spectrum is simultaneously difference signal dominant and left or right channel dominant. When the spectrum is difference signal dominant and left channel dominant, the processed difference signal is taken as shown in FIG.  11  and added directly to the decoded left, right and center channel output signals. When the spectrum is difference signal dominant and right channel dominant, the processed difference signal is inverted and added to the decoded left, right and center channel output signals. This arrangement excludes destructive summation of the processed difference signal with the decoded (dominant) right channel output signal, and permits reproducing the surround dominant bass signal in the presence of the dominant left or right channel output signal. 
     Referring to FIG. 12, there is shown a modification of the broadband block decoder according to the invention shown in FIG. 11 that includes modifications at the input end that avoids sound image collapse to the center under certain conditions that might occur with the embodiment of FIG.  11 . This circuitry includes left and right multipliers  101 ′ and  102 ′ for providing a product signal to potentiometers  107  and  108 , respectively, representative of the product of the left transmitted signal LT with coefficient signal A 9  and the product of the right transmitted signal RT with the coefficient signal A 10 , respectively. The circuitry also includes left signal combiner  103 ′ for cumulatively combining the left and right transmitted signals LT and RT and subtractively combining the product signals at the outputs of multipliers  101 ′ and  102 ′ to provide a signal representative of the magnitude of the sum of the left and right transmitted signals to multiplier  35  and representative of the magnitude of the difference therebetween to multiplier  41 . 
     The coefficient signals A 9  and A 10  are defined as follows: 
     
       
           A   9 =( Y−|{overscore (R)} |)/ Y   
       
     
     
       
           A   10 =( Y−|{overscore (L)} |)/ Y   
       
     
     An advantage of this arrangement is that the apparent location of the sound image is initially on the center surround axis that extends between the rear and front of the listening area as distinguished from being on the left-right axis at the front of the listening area. A sudden change in the position of the sound image is significantly less distracting to the listener than an initial sound image on the left-right axis. 
     Referring to FIG. 13, there is shown another embodiment of the invention representing a modification of the system of FIG. 12 constructed and arranged to couple the transmitted signals LT and RT to respective multipliers  101 A . . .  101 N and  102 A . . .  102 N, respectively, through filters  201 A . . .  201 N and  202 A . . .  202 N, respectively, the filters embracing contiguous frequency bands in the audio frequency range to transmit corresponding spectral components of the left and right transmitted signals LT and RT. The other input of each of these multipliers receive a coefficient signal A 21  . . . A 2 N and A 11  . . . A 1 N, respectively. The output product signals of multipliers  101 A . . .  101 N energize left combiner  111 ′ to cumulatively combine these signals. The output product signals of multipliers  102 A . . .  102 N energize respective inputs of right combiner  112 ′ to cumulatively combine these signals. The output of left signal combiner  111 ′ energizes one input of signal combiner  41 ′ that differentially combines this signal with the output of right combiner  112 ′ to provide an output signal to multiplier  42 . This signal also energizes one input of combiner  35 ′ whose other input receives the signal from right combiner  112 ′ to cumulatively combine these signals and furnish them to multiplier  37 . 
     This embodiment of the invention also includes circuitry constructed and arranged to include a signal representative of the left output signal on line  153  forming the left surround signal on line  164  coupled through signal combiner  204  which cumulatively combines the product signal from left surround output multiplier  203  with the product output signal of multiplier  162 . Left surround multiplier  203  furnishes a product signal related to the product of the left output signal on line  153  with the (|L−R|)/X coefficient signal at the other input. Similarly, there is circuitry constructed and arranged to include in the right surround signal on line  167  a component related to the right output signal on line  155  provided by output right surround multiplier  205  providing a product signal related to the product of the right output signal on line  155  with a (|L−R|)/X coefficient signal to provide a product signal cumulatively combined with the output of multiplier  166  in combiner  206 . Injecting right signal and left signal into right surround and left surround signal enhances the stereo image perceived by a listener. 
     Referring to FIG. 14, there is shown an alternative arrangement for providing a bass output signal on line  141 ′. A left input combiner  211  cumulatively combines the left transmitted signal LT and the right transmitted signal RT to provide a left combined signal multiplied by an A 11  coefficient signal to provide a left product signal by multiplier  212 . 
     Right combiner  213  differentially combines the left transmitted signal LT with the right transmitted signal RT to provide a right combined signal that is multiplied by the A 12  coefficient signal in first multiplier  214  to provide a first product signal that is multiplied by the A 13  coefficient signal in second multiplier  215  to provide a second product signal that is cumulatively combined with the product signal provided by multiplier  212  to provide a sum signal that is multiplied by the A 14  coefficient signal in bass output multiplier  216  to provide the bass output signal on line  141 ′. 
     The A 11  coefficient signal= 
     
       
         {10 ([ {overscore (L+R)}]−X )}+[ {overscore (L+R)}]X   
       
     
     The A 12  coefficient signal=1−A 11 . 
     The A 13  coefficient signals is a user selection to establish the surround bass volume to provide a voltage gain of 1 to 3 corresponding to a range in loudness of 0 to 10 db, usually preferred. 
     The A 14  coefficient= 
     
       
         
           {square root over (|LT| 2 +|RT| 2 )}/X 
         
       
     
     which is approximately equal to 
     
       
           Y+[Y −{([ {overscore (LT)}]−[{overscore (RT)} ])×0.5 }]/X   
       
     
     The circuitry is constructed and arranged so that there is a vector combination of bass components. If the phase angle between surround and main bass components is less than 90°, these components are cumulatively combined. If the phase angle is greater than 90°, these components are differentially combined. 
     Referring to FIG. 15, there is shown the logical arrangement of another decoding system according to the invention having advantageous properties in a system that provides left, center and right output signals and a monophonic surround output signal. Center signal combiner  211 ′ cumulatively combines the left transmitted signal LT with the right transmitted signal RT to provide an output signal to one input of center combiner  223  for differential combining with the left and right output signals, respectively, from left signal combiner  221  and right signal combiner  222 , respectively. 
     The left transmitted signal also energizes one input of left multiplier  212 ′ energized by the A 1  coefficient signal to provide a left product signal that is differentially combined with the left transmitted signal by left output combiner  221  to provide the left output signal. 
     Right input combiner  213 ′ differentially combines the left transmitted signal LT and right transmitted signal RT to provide an output signal that is applied to one input of surround output combiner  224  for cumulative combination with the right output signal provided by right output combiner  222  and differential combination with the output of left output combiner  221  to provide the surround output signal. 
     The right transmitted signal also energizes one input of right multiplier  214 ′ for multiplication by the A 2  coefficient signal applied to the other input to provide a right product signal that is differentially combined with the right transmitted signal in right output combiner  222  to provide the right output signal. 
     The following table indicates the values of X and Y for the indicated conditions determined by the magnitude detectors that compare the magnitudes of L and R and the magnitudes of L+R and L−R. 
     
       
           X=[L+R ] for [ L+R]&gt;[L−R]   
       
     
     
       
           X=[L−R ] for [ L+R]&lt;[L−R]   
       
     
     
       
           X=[L+R ] for [ L+R]=[L−R]   
       
     
     
       
           Y=[|L ] for [ L]&gt;[R]   
       
     
     
       
           Y=[|R ] for [ L]&lt;[R]   
       
     
     
       
           Y=[|L ] for [ L]=[R]   
       
     
     Referring to FIG. 16, there is shown another embodiment of a decoder according to the invention relatively free from complexity that provides a stereo surround signal. This embodiment is a modification of the embodiment of FIG.  16  and includes additional elements to provide the right and left surround output. The output of left output combiner  221  is delivered to one input of left output multiplier  231  whose other input receives the A 3  coefficient signal to provide the left output signal that is differentially combined with the output of left output combiner  221  in left input surround combiner  233  to provide a product signal that is cumulatively combined with the output of surround output combiner  224 ′ by right surround output combiner  235  to provide the right surround output signal. 
     The output of right output combiner  232  energizes one input of right output multiplier  232  energized at its other input by the A 3  coefficient signal to provide the right output signal that is differentially combined with the output of right output combiner  222  in right input surround combiner  234  to provide a signal that is differentially combined with the output of right surround output combiner  224 ′ to provide the left surround output. 
     Referring to FIG. 17, there is shown a block diagram illustrating the logical arrangement of a multiple axis decoding system that uses a number of stereo decoders, each of which may be a conventional stereo decoder or a decoder described above capable of responding to a left transmitted signal Lt and a right transmitted signal Rt typically having a L left signal output, a C center signal output, a R right signal output with the first also having at least an S surround signal output to provide a left output signal, a right output signal, a center output signal, a left surround output signal, a center surround output signal, a right surround output signal, a left side surround output output and a right side surround output signal. 
     Input decoder  301  receives the left transmitted signal Lt on line  24  and the right transmitted signal Rt on line  25  and provides on its L output  301 L a signal that is applied to the Lt input  302 Lt of left decoder  302  and on its right output  301 R a signal delivered to the Rt input  303 Rt of right decoder  303 . 
     Input decoder  301  provides on the surround S output  301 S a signal that is delivered to the Rt input  302 Rt of left decoder  302  and to the Lt input  303 Lt of right decoder  303  and provides the center output signal on its C output  301 C. 
     Left decoder  302  provides the left output signal on its L output  302 L, the left side surround output signal LS S  on its C output  302 C and a signal on its R output  302 R that is delivered to the Lt input  304 Lt of surround output decoder  304  that provides the left surround output signal Ls on the L output  304 L. 
     Right decoder  303  provides the right output signal on the L output  303 L, the right side surround output signal RS, on the C output  303 C and a signal on the R output  303 R delivered to the Rt input  304 Rt of surround decoder  304  that provides the right surround output signal Rs on its R output  304 R and the center surround output signal Cs on its C output  304 C. 
     Referring to FIG. 18, there is shown a table helpful in understanding the signals from and to the four decoders. It is convenient to identify the input decoder  301  as decoder  1 , the left decoder  302  as decoder  2 , the right decoder  303  as decoder  3  and the surround decoder  304  as decoder  4 . Designating the two inputs of each decoder as Lt in and Rt in and the outputs of each decoder as L out, R out, C out and S out, the table shows the signals at each of these terminals that results in furnishing left, center and right output signals L, C and R, respectively, normally reproduced by left front, center front and right front speakers, left and right side surround output signals L S  and R S  signals, respectively, normally reproduced by left and right side speakers, respectively, and left surround, center surround and right surround output signals, Ls, Cs and Rs, respectively, normally reproduced by left, center and right rear speakers, respectively. 
     Other embodiments are within the claims.