Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S3/00—Systems employing more than two channels, e.g. quadraphonic
  • H04S3/02—Systems employing more than two channels, e.g. quadraphonic of the matrix type, i.e. in which input signals are combined algebraically, e.g. after having been phase shifted with respect to each other
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S5/00—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation 
  • H04S5/005—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation  of the pseudo five- or more-channel type, e.g. virtual surround

Definitions

• the invention relates to audio signal processing.
• the invention relates to "multidirectional" (or “multichannel”) audio decoding using an "adaptive” (or
• active audio matrix method that derives three or more audio signal streams (or “signals” or “channels”) from a pair of audio input signal streams (or “signals” or
• the invention is useful for recovering audio signals in which each signal is associated with a direction and was combined into a fewer number of signals by an encoding matrix.
• each signal is associated with a direction and was combined into a fewer number of signals by an encoding matrix.
• an encoding matrix Although the invention is described in terms of such a deliberate matrix encoding, it should be understood that the invention need not be used with any particular matrix encoding and is also useful for generating pleasing directional effects from material originally recorded for two-channel reproduction.
• Audio matrix encoding and decoding is well known in the prior art.
• four source signals typically associated with four cardinal directions (such as, for example, left, center, right and surround or left front, right front, left back and right back) are amplitude- phase matrix encoded into two signals.
• the two signals are transmitted or stored and then decoded by an amplitude-phase matrix decoder in order to recover approximations of the original four source signals.
• the decoded signals are approximations because matrix decoders suffer the well-known disadvantage of crosstalk among the decoded audio signals.
• the decoded signals should be identical to the source signals, with infinite separation among the signals.
• the inherent crosstalk in matrix decoders results in only 3 dB separation between signals associated with adjacent directions.
• An audio matrix in which the matrix characteristics do not vary is known in the art as a "passive" matrix.
• the present invention is directed to methods and apparatus that recognize and employ heretofore unappreciated relationships among intermediate signals in adaptive matrix decoders. Exploitation of these relationships allows undesired crosstalk components to be cancelled easily, particularly by using automatic self- cancelling arrangements using negative feedback.
• the invention constitutes a method for deriving at least three audio output signals from two input audio signals, in which four audio signals are derived from the two input audio signals by a passive matrix that produces two pairs of audio signals in response to two audio signals: a first pair of derived audio signals representing directions lying on a first axis (such as “left” and “right” signals) and a second pair of derived audio signals representing directions lying on a second axis (such as “center” and “surround” signals), said first and second axes being substantially mutually orthogonal to each other.
• Each of the pairs of derived audio signals are processed to produce respective first and second pairs (the left/right and center/surround pairs, respectively) of intermediate audio signals such that the magnitudes of the relative amplitudes of the audio signals in each pair of intermediate audio signals are urged toward equality.
• a first output signal (such as the left output signal L out ) representing a first direction lying on the axis of the pair of derived audio signals (the left/right pair) from which the first pair (the left/right pair) of intermediate signals are produced, is produced at least by combining, with the same polarity, at least a component of each of the second pair (the center/ surround pair) of intermediate audio signals.
• a second output signal (such as the right output signal R ⁇ representing a second direction lying on the axis of the pair of derived audio signals (the left/right pair) from which the first pair (the left/right pair) of intermediate signals are produced, is produced at least by combining, with the opposite polarity, at least a component of each of the second pair (the center/surround pair) of intermediate audio signals.
• a third output signal (such as the center output signal C out or the surround output signal S tolerate_) representing a first direction lying on the axis of the pair (the center/surround pair) of derived audio signals from which the second pair (the center/surround pair) of intermediate signals are produced, is produced at least by combining, with the same polarity or the opposite polarity, at least a component of each of the first pair (the left/right pair) of intermediate audio signals.
• a fourth output signal (such as the surround output signal S ⁇ , if the third output signal is the enter output signal C ⁇ , or C out if the third output signal is S ⁇ ,) representing a second direction lying on the axis of the pair (center/surround) of derived audio signals from which the second pair (center/surround) of intermediate signals are produced, is produced at least by combining, with the opposite polarity, if the third output signal is produced by combining with the same polarity, or by combining with the same polarity, if the third output signal is produced by combining with the opposite polarity, at least a component of each of said first pair (the left/right pair) of intermediate audio signals.
• the heretofore unappreciated relationships among the decoded signals is that by urging toward equality the magnitudes of the intermediate audio signals in each pair of intermediate audio signals, undesired crosstalk components in the decoded output signals are substantially suppressed.
• the principle does not require complete equality in order to achieve substantial crosstalk cancellation.
• Such processing is readily and preferably implemented by the use of negative feedback arrangements that act to cause automatic cancellation of undesired crosstalk components.
• the invention includes embodiments having equivalent topologies.
• intermediate signals are derived from a passive matrix operating on a pair of input signals and those intermediate signals are urged toward equality.
• a cancellation component of the intermediate signals are combined with passive matrix signals (from the passive matrix operating on the input signals or otherwise) to produce output signals.
• pairs of the intermediate signals are combined to output signals.
• Figure 1 is a functional and schematic diagram of a prior art passive decoding matrix useful in understanding the present invention.
• Figure 2 is a functional and schematic diagram of a prior art active matrix decoder useful in understanding the present invention in which variably scaled versions of a passive matrix' outputs are summed with the unaltered passive matrix' outputs in linear combiners.
• FIG 3 is a functional and schematic diagram of a feedback-derived control system according to the present invention for the left and right VCAs and the sum and difference VCAs of Figure 2 and for VCAs in other embodiments of the present invention.
• Figure 4 is a functional and schematic diagram showing an arrangement according to the present invention equivalent to the combination of Figures 2 and 3 in which the output combiners generate the passive matrix output signal components in response to the L t and R t input signals instead of receiving them from the passive matrix from which the cancellation components are derived.
• FIG. 5 is a functional and schematic diagram according to the present invention showing an arrangement equivalent to the combination of Figures 2 and 3 and Figure 4.
• the signals that are to be maintained equal are the signals applied to the output deriving combiners and to the feedback circuits for control of the VCAs; the outputs of the feedback circuits include the passive matrix components.
• Figure 6 is a functional and schematic diagram according to the present invention showing an arrangement equivalent to the arrangements of the combination of Figures 2 and 3, Figure 4 and Figure 5, in which the variable-gain-circuit gain (1-g) provided by a VCA and subtracter is replaced by a VCA whose gain varies in the opposite direction of the VCAs in the VCA and subtractor configurations.
• the passive matrix components are implicit. In the other embodiments, the passive matrix components are explicit.
• Figure 7 is an idealized graph, plotting the left and right VCA gains g, and g r of the L/R t feedback-derived control system (vertical axis) against the panning angle ⁇ (horizontal axis).
• Figure 8 is an idealized graph, plotting the sum and difference VCA gains g c and g ⁇ of the sum/difference feedback-derived control system (vertical axis) against the panning angle ⁇ (horizontal axis).
• Figure 9 is an idealized graph, plotting the left/right and the inverted sum/difference control voltages for a scaling in which the maximum and minimum values of control signals are + /-15 volts (vertical axis) against the panning angle ⁇ (horizontal axis).
• Figure 10 is an idealized graph, plotting the lesser of the curves in Figure 9 (vertical axis) against the panning angle ⁇ (horizontal axis).
• Figure 11 is an idealized graph, plotting the lesser of the curves in Figure 9 (vertical axis) against the panning angle ⁇ (horizontal axis) for the case in which the sum/difference voltage has been scaled by 0.8 prior to taking the lesser of the curves.
• Figure 12 is an idealized graph, plotting the left back and right back VCA gains g lb and g A of the left-back/right-back feedback-derived control system (vertical axis) against the panning angle ⁇ (horizontal axis).
• Figure 13 is a functional and schematic diagram of a portion of an active matrix decoder according to the present invention in which six outputs are obtained.
• Figure 14 is a functional and schematic diagram showing the derivation of six cancellation signals for use in a six output active matrix decoder such as that of Figure 13.
• Figure 15 is a schematic circuit diagram showing a practical circuit embodying aspects of the present invention. Best Mode for Carrying out the Invention
• a passive decoding matrix is shown functionally and schematically in Figure 1.
• the following equations relate the outputs to the inputs, L t and R, ("left total” and "right total”): L ⁇ L, (Eqn. 1)
• the center output is the sum of the inputs, and the surround output is the difference between the inputs. Both have, in addition, a scaling; this scaling is arbitrary, and is chosen to be 2 for the purpose of ease in explanation. Other scaling values are possible.
• the C out output is obtained by applying L t and R x with a scale factor of + V2 to a linear combiner 2.
• the S out output is obtained by applying L t and Rj with scale factors of +V2 and -V2, respectively, to a linear combiner 4.
• the passive matrix of Figure 1 thus produces two pairs of audio signals; the first pair is L out and R out ; the second pair is C out and S out .
• the cardinal directions of the passive matrix are designated "left,” “center,” “right,” and “surround.” Adjacent cardinal directions lie on mutually orthogonal axes, such that, for these direction labels, left is adjacent to center and surround; surround is adjacent to left and right, etc. It should be understood that the invention is applicable to any orthogonal 2:4 decoding matrix.
• a passive matrix decoder derives n audio signals from m audio signals, where n is greater than m, in accordance with an invariable relationship (for example, in Figure 1, C ⁇ t i s always 1 /2*(R out + L out )).
• an active matrix decoder derives n audio signals in accordance with a variable relationship.
• One way to configure an active matrix decoder is to combine signal-dependent signal components with the output signals of a passive matrix.
• VCAs voltage-controlled amplifiers
• the VCAs have their inputs derived from the left, right, center and surround outputs of the passive matrix, respectively, their gains may be designated gggi g r , g c , and g tone (all positive).
• the VCA output signals constitute cancellation signals and are combined with passively derived outputs having crosstalk from the directions from which the cancellation signals are derived in order to enhance the matrix decoder's directional performance by suppressing crosstalk.
• each output is the combination of the respective passive matrix output plus the output of two VCAs.
• the VCA outputs are selected and scaled to provide the desired crosstalk cancellation for the respective passive matrix output, taking into consideration that crosstalk components occur in outputs representing adjacent cardinal directions. For example, a center signal has crosstalk in the passively decoded left and right signals and a surround signal has crosstalk in the passively decoded left and right signals. Accordingly, the left signal output should be combined with cancellation signal components derived from the passively decoded center and surround signals, and similarly for the other four outputs.
• the manner in which the signals are scaled, polarized, and combined in Figure 2 provides the desired crosstalk suppression. By varying the respective VCA gain in the range of zero to one (for the scaling example of Figure 2), undesired crosstalk components in the passively decoded outputs may be suppressed.
• the arrangement of Figure 2 is the same as the passive matrix apart from a constant scaling. For example, if all VCAs had gains of 0.1 :
• the VCAs can be controlled so that the one corresponding to the desired cardinal direction has a gain of 1 and the remaining ones are much less than 1, then at all outputs except the desired one, the VCA signals will cancel the unwanted outputs.
• the VCA outputs act to cancel crosstalk components in the adjacent cardinal directions (into which the passive matrix has crosstalk).
• each output is the combination of two signals.
• L out and R ⁇ both involve the sum and difference of the input signals and the gains of the sum and difference VCAs (the VCAs whose inputs are derived from the center and surround directions, the pair of directions orthogonal to the left and right directions).
• C ⁇ , and S ⁇ both involve the actual input signals and the gains of the left and right VCAs (the VCAs whose respective inputs are derived from the left and right directions, the pair of directions orthogonal to the center and surround directions).
• Equations 15 and 16 are the same as those of Equations 13 and 14 but with the scaling omitted.
• the polarity with which the signals are combined and their scaling may be taken care of when the respective outputs are obtained as with the combiners 14, 16, 18 and 20 of Figure 2.
• the invention is based on the discovery of these heretofore unappreciated equal amplitude magnitude relationships, and, preferably, as described below, the use of self- acting feedback control to maintain these relationships.
• the maximum gain for a VCA should be unity. Under quiescent, undefined, or "unsteered” conditions, the VCAs should adopt a small gain, providing effectively the passive matrix. When the gain of one VCA of a pair needs to rise from its quiescent value towards unity, the other of the pair may remain at the quiescent gain or may move in the opposite direction. One convenient and practical relationship is to keep the product of the gains of the pair constant.
• VCAs whose gain in dB is a linear function of their control voltage, this happens automatically if a control voltage is applied equally (but with effective opposite polarity) to the two of a pair.
• Another alternative is to keep the sum of the gains of the pair constant.
• the invention may be implemented digitally or in software rather than by using analog components.
• a typical value for "a" might lie in the range 10 to 20.
• Figure 3 shows, functionally and schematically, a feedback-derived control system for the left and right VCAs (6 and 12, respectively) of Figure 2. It receives the L j and R, input signals, processes them to derive intermediate L t *(l-g,) and R,*(l-g r ) signals, compares the magnitude of the intermediate signals, and generates an error signal in response to any difference in magnitude, the error signal causing the VCAs to reduce the difference in magnitude.
• One way to achieve such a result is to rectify the intermediate signals to derive their magnitudes and apply the two magnitude signals to a comparator whose output controls the gains of the VCAs with such a polarity that, for example, an increase in the L t signal increases g, and decreases g r . Circuit values (or their equivalents in digital or software implementations) are chosen so that when the comparator output is zero, the quiescent amplifier gain is less than unity (e.g. , 1/a).
• the L, input is applied to the "left" VCA 6 and to one input of a linear combiner 22 where it is applied with a scaling of + 1.
• the left VCA 6 output is applied to the combiner 22 with a scaling of -1 (thus forming a subtractor) and the output of combiner 22 is applied to a full-wave rectifier 24.
• the R, input is applied to the right VCA 12 and to one input of a linear combiner 26 where it is applied with a scaling of + 1.
• the right VCA 12 output is applied to the combiner 26 with a scaling of -1 (thus forming a subtractor) and the output of combiner 26 is applied to a full-wave rectifier 28.
• the rectifier 24 and 28 outputs are applied, respectively, to non-inverting and inverting inputs of an operational amplifier 30, operating as a differential amplifier.
• the amplifier 30 output provides a control signal in the nature of an error signal that is applied without inversion to the gain controlling input of VCA 6 and with polarity inversion to the gain controlling input of VCA 12.
• the error signal indicates that the two signals, whose magnitudes are to be equalized, differ in magnitude. This error signal is used to "steer" the VCAs in the correct direction to reduce the difference in magnitude of the intermediate signals.
• the outputs to the combiners 16 and 18 are taken from the VCA 6 and VCA 12 outputs. Thus, only a component of each intermediate signal is applied to the output combiners, namely, - L-g.
• the difference in magnitude may be reduced to a negligible amount by providing enough loop gain.
• a loop gain sufficient to reduce the dB difference by a factor of 10 results, theoretically, in worst-case crosstalk better than 30 dB down.
• time constants in the feedback control arrangement should be chosen to urge the magnitudes toward equality in a way that is essentially inaudible at least for most signal conditions. Details of the choice of time constants in the various configurations described are beyond the scope of the invention.
• circuit parameters are chosen to provide about 20 dB of negative feedback and so that the VCA gains cannot rise above unity.
• the VCA gains may vary from some small value (for example, 1/a 2 , much less than unity) up to, but not exceeding, unity for the scaling examples described herein in connection with the arrangements of Figures 2, 4 and 5. Due to the negative feedback, the arrangement of Figure 3 will act to hold the signals entering the rectifiers approximately equal.
• Figure 2 is substantially identical to the arrangement of Figure 3, as described, but receiving not L t and R, but their sum and difference and applying its outputs from VCA 6 and VCA 12 (constituting a component of the respective intermediate signal) to combiners 14 and 20.
• VCA 6 and VCA 12 constituting a component of the respective intermediate signal
• the feedback-derived control system operates to process pairs of audio signals from the passive matrix such that the magnitudes of the relative amplitudes of the intermediate audio signals in each pair of intermediate audio signals are urged toward equality.
• the feedback-derived control system shown in Figure 3 controls the gains of the two VCAs 6 and 12 inversely to urge the inputs to the rectifiers 24 and 28 towards equality.
• the degree to which these two terms are urged towards equality depends on the characteristics of the rectifiers, the comparator 30 following them and of the gain/control relationships of the VCAs. The greater the loop-gain, the closer the equality, but an urging towards equality will occur irrespective of the characteristics of these elements (provided of course the polarities of the signals are such as to reduce the level differences).
• the comparator may not have infinite gain but may be realized as a subtractor with finite gain.
• the comparator or subtractor output is a function of the signal voltage or current difference. If instead the rectifiers respond to the logarithm of their input magnitudes, that is to the level expressed in dB, a subtraction performed at the comparator input is equivalent to taking the ratio of the input levels. This is beneficial in that the result is then independent of the absolute signal level but depends only on the difference in signal expressed in dB. Considering the source signal levels expressed in dB to reflect more nearly human perception, this means that other things being equal the loop-gain is independent of loudness, and hence that the degree of urging towards equality is also independent of absolute loudness.
• the VCAs 6 and 12 may have gains that are directly or inversely proportional to their control voltages (that is, multipliers or dividers). This would have the effect that when the gains were small, small absolute changes in control voltage would cause large changes in gain expressed in dB.
• V c is near its maximum, a 100 mV (millivolt) change from say 9900 to 10000 mV delivers a gain change of 20*log(10000/9900) or about 0.09 dB.
• VCAs whose gain in dB is proportional to the control voltage, or expressed differently, whose voltage or current gain is dependent upon the exponent or antilog of the control voltage.
• a small change in control voltage such as 100 mV will then give the same dB change in gain wherever the control voltage is within its range.
• Such devices are readily available as analog ICs, and the characteristic, or an approximation to it, is easily achieved in digital implementations.
• the preferred embodiment therefore employs logarithmic rectifiers and exponentially controlled variable gain amplification, delivering more nearly uniform urging towards equality (considered in dB) over a wide range of input levels and of ratios of the two input signals.
• the rectifiers 24 and 28 in Figure 3 are preceded by filters derived empirically, providing a response that attenuates low frequencies and very high frequencies and provides a gently rising response over the middle of the audible range. Note that these filters do not alter the frequency response of the output signals, they merely alter the control signals and VCA gains in the feedback-derived control systems.
• FIG. 4 An arrangement equivalent to the combination of Figures 2 and 3 is shown functionally and schematically in Figure 4. It differs from the combination of Figures 2 and 3 in that the output combiners generate passive matrix output signal components in response to the L t and R, input signals instead of receiving them from the passive matrix from which the cancellation components are derived.
• the arrangement provides the same results as does the combination of Figures 2 and 3 provided that the summing coefficients are essentially the same in the passive matrices.
• Figure 4 incorporates the feedback arrangements described in connection with Figure 3.
• the L t and R t inputs are applied first to a passive matrix that includes combiners 2 and 4 as in the Figure 1 passive matrix configuration.
• the L, input which is also the passive matrix "left” output, is applied to the "left” VCA 32 and to one input of a linear combiner 34 with a scaling of + 1.
• the left VCA 32 output is applied to a combiner 34 with a scaling of -1 (thus forming a subtractor).
• the R t input which is also the passive matrix "right” output, is applied to the "right” VCA 44 and to one input of a linear combiner 46 with a scaling of + 1.
• the right VCA 44 output is applied to the combiner 46 with a scaling of -1 (thus forming a subtractor).
• the outputs of combiners 34 and 46 are the signals L ⁇ l-g,) and R t *(l-g r ), respectively, and it is desired to keep the magnitude of those signals equal or to urge them toward equality.
• those signals preferably are applied to a feedback circuit such as shown in Figure 3 and described in connection therewith.
• the feedback circuit then controls the gain of VCAs 32 and 44.
• the "center" output of the passive matrix from combiner 2 is applied to the "center" VCA 36 and to one input of a linear combiner 38 with a scaling of + 1.
• the center VCA 36 output is applied to the combiner 38 with a scaling of -1 (thus forming a subtractor).
• the "surround" output of the passive matrix from combiner 4 is applied to the "surround” VCA 40 and to one input of a linear combiner 42 with a scaling of + 1.
• the surround VCA 40 output is applied to the combiner 42 with a scaling of -1 (thus forming a subtractor).
• the outputs of combiners 38 and 42 are the signals V_.*(L t +R 1 )*(l-g c ) and 1 A* L t - ' R ⁇ *(l-g ⁇ , respectively, and it is desired to keep the magnitude of those signals equal or to urge them toward equality.
• those signals preferably are applied to a feedback circuit such as shown in Figure 3 and described in connection therewith. The feedback circuit then controls the gain of VCAs 38 and 42.
• the output signals L ⁇ ,, C out , S out , and R ⁇ are produced by combiners 48, 50, 52 and 54.
• Each combiner receives the output of two VCAs (the VCA outputs constituting a component of the intermediate signals whose magnitudes are sought to be kept equal) to provide cancellation signal components and either or both input signals so as to provide passive matrix signal components. More specifically, the input signal L t is applied with a scaling of + 1 to the L ⁇ , combiner 48, with a scaling of +V2 to the C ⁇ combiner 50, and with a scaling of +V2 to the S ⁇ , combiner 52.
• the input signal R is applied with a scaling of + 1 to the R, ⁇ combiner 54, with a scaling of +V2 to C out combiner 50, and with a scaling of -V2 to S out combiner 52.
• the left VCA 32 output is applied with a scaling of -V2 to C ⁇ , combiner 50 and also with a scaling of -V2 to S ⁇ combiner 52.
• the right VCA 44 output is applied with a scaling of -V2 to C ⁇ ,, combiner 50 and with a scaling of +V2 to S out combiner 52.
• the center VCA 36 output is applied with a scaling of -1 to L out combiner 48 and with a scaling of -1 to R ⁇ , combiner 54.
• the surround VCA 40 output is applied with a scaling of -1 to L out VCA 48 and with a scaling of + 1 to R ⁇ , VCA 54.
• Figure 4 is shown functionally and schematically in Figure 5 .
• the signals that are to be maintained equal are the signals applied to the output deriving combiners and to the feedback circuits for control of the VCAs. These signals include passive matrix output signal components.
• the signals applied to the output combiners from the feedback circuits are the VCA output signals and exclude the passive matrix components.
• passive matrix components must be explicitly combined with the outputs of the feedback circuits, whereas in Figure 5 the outputs of the feedback circuits include the passive matrix components and are sufficient in themselves.
• the connections among combiners 2 and 4, VCAs 32, 36, 40, and 44, and combiners 34, 38, 42 and 46 are the same as in the arrangement of Figure 4.
• the outputs of the combiners 34, 38, 42 and 46 preferably are applied to two feedback control circuits (the outputs of combiners 34 and 46 to a first such circuit in order to generate control signals for VCAs 32 and 44 and the outputs of combiners 38 and 42 to a second such circuit in order to generate control signals for VCAs 36 and 40).
• the output of combiner 34, the L t *(l-g,) signal is applied with a scaling of + 1 to the C ⁇ combiner 58 and with a scaling of + 1 to the S out combiner 60.
• the output of combiner 46, the R t *(l-g r ) signal is applied with a scaling of + 1 to the C ⁇ , combiner 58 and with a scaling of -1 to the S out combiner 60.
• the output of combiner 38, the 1 /2*(L t +R t )*(l-g c ) signal is applied to the L out combiner 56 with a scaling of + 1 and to the R ⁇ , combiner 62 with a scaling of + 1.
• the output of the combiner 42 is applied to the L out combiner 56 with a + 1 scaling and to the R ⁇ , combiner 62 with a -1 scaling.
• the invention preferably employs a closed-loop control in which the magnitudes of the signals providing the outputs are measured and fed back to provide the adaptation.
• the desired cancellation of unwanted signals for non-cardinal directions does not depend on an accurate matching of characteristics of the signal and control paths, and the closed-loop configurations greatly reduce the need for precision in the circuitry.
• each variable gain circuit incorporating a VCA is a subtractive arrangement in the form (1-g).
• Each VCA gain can vary from a small value up to but not exceeding unity.
• the variable-gain-circuit gain (1-g) can vary from very nearly unity down to zero.
• Figure 5 can be redrawn as Figure 6, where every VCA and associated subtractor has been replaced by a VCA alone, whose gain varies in the opposite direction to that of the VCAs in Figure 5.
• variable-gain-circuit gain (1-g) (implemented, for example by a VCA having a gain “g” whose output is subtracted from a passive matrix output as in Figures 2/3, 4 and 5) is replaced by a corresponding variable-gain-circuit gain "h” (implemented, for example by a standalone VCA having a gain "h” acting on a passive matrix output).
• gain "(1-g)” is the same as gain "h” and if the feedback circuits act to maintain equality between the magnitude of the requisite pairs of signals
• the Figure 6 configuration is equivalent to the Figure 5 configuration and will deliver the same outputs. Indeed, all of the disclosed configurations, the configurations of Figures 2/3, 4, 5, and 6, are equivalent to each other.
• the "right” output of the passive matrix which is also the same as the input signal R, is applied to a "right” VCA 70 having a gain h r to produce the intermediate signal R t *h r .
• the "center” output of the passive matrix from combiner 2 is applied to a "center” VCA 66 having a gain h c to produce an intermediate signal 1 /2*(L t +R t )*h c .
• the “surround” output of the passive matrix from combiner 4 is applied to a "surround” VCA 68 having a gain h ⁇ to produce an intermediate signal 1 /2*(L t -R 1 )*h_.
• the VCA gains h operate inversely to the VCA gains g, so that the h gain characteristics are the same as the (1-g) gain characteristics.
• control voltages An analysis of the control signals developed in connection with the embodiments described thus far is useful in better understanding the present invention and in explaining how the teachings of the present invention may be applied to deriving five or more audio signal streams, each associated with a direction, from a pair of audio input signal streams.
• the left and right front loudspeakers are generally placed further forward than +/- 90 degrees relative to the center (for example, +/- 30 to 45 degrees), so ⁇ does not actually represent the angle with respect to the listener but is an arbitrary parameter to illustrate panning.
• the precise position of the maximum can be moved by offsetting (adding or subtracting a constant to) or scaling one or both of the left/right and sum/difference control signals so that their curves cross at preferred values of ⁇ , before taking the more-positive or more-negative function.
• a second new control signal can be derived whose maximum occurs in a predetermined position corresponding to the right back of the listener, at a desired and predetermined ⁇ (for instance, 360-31 or 329 degrees, 31 degrees the other side of zero, symmetrical with the left back). It is a left/right reversal of Figure 11.
• Figure 12 shows the effect of applying these derived control signals to VCAs in such a manner that the most positive value gives a gain of unity.
• the modification of the main control signals to move their crossing point before taking the greater or lesser may alternatively consist of a non-linear operation instead of or in addition to an offset or a scaling. It will be apparent that the modification allows the generation of further control voltages whose maxima lie at almost any desired ratio of the magnitudes and relative polarities of L t and R t (the input signals).
• the initial passive matrix is the same as that of the four-output system described above (a direct L, input, the combination of L, plus R, scaled by one-half and applied to a linear combiner 80 to yield center front, the combination of L, minus R, scaled by one-half and applied to a linear combiner 82 to yield center back, and a direct R t input).
• the output linear combiners receive multiple active cancellation terms (on lines 100, 102, 104, 106, 108, 110, 112, 114, 116, 118, 120 and 122) as required to cancel the passive matrix outputs.
• active cancellation terms consist of the inputs and/or combinations of the inputs multiplied by the gains of VCAs (not shown) or combinations of the inputs and the inputs multiplied by the gains of VCAs.
• the VCAs are controlled so that their gains rise to unity for a cardinal input condition and are substantially smaller for other conditions.
• the configuration of Figure 13 has six cardinal directions, provided by inputs I_ t and R t in defined relative magnitudes and polarities, each of which should result in signals from the appropriate output only, with substantial cancellation of signals in the other five outputs.
• the outputs corresponding to those cardinal directions should deliver signals but the remaining outputs should deliver little or nothing.
• there will be several cancellation terms in practice, more than the two shown in Figure 13), each corresponding to the undesired output for an input corresponding to each of the other cardinal directions.
• the arrangement of Figure 13 may be modified to eliminate the center back S benefit_ output (thus eliminating combiners 82 and 94) so that center back is merely a pan half-way between left back and right back rather than a sixth cardinal direction.
• center back S benefit_ output for either the six-output system of Figure 13 or its five-output alternative there are six possible cancellation signals: the four derived via the two pairs of VCAs that are parts of the left/right and sum/difference feedback-derived control systems and two more derived via left back and right back VCAs controlled as described above (see also the embodiment of Figure 14, described below).
• the gains of the six VCAs are in accordance with Figure 7 (g, left and g r right), Figure 8 (g c sum and g 8 difference) and Figure 12 (g, b left back and g ⁇ right back).
• the cancellation signals are summed with the passive matrix terms using coefficients calculated or otherwise chosen to minimize unwanted crosstalk, as described below.
• the coefficient x can be derived empirically or from a consideration of the precise VCA gains when a source is in the region of the left back cardinal direction.
• the term [LJ is the passive matrix term.
• the terms 1 /2*g c *(L t +R t ), and 1 /2*x*g Ib *((g 1 *L t +g ⁇ * I / *(L t -R ⁇ ) represent cancellation terms (see Figure 14) that may be combined with L j in linear combiner 88 ( Figure 13) in order to derive the output audio signal L out .
• the equation for R ⁇ is derived similarly, or by symmetry:
• R out [RJ-V2*g c *(L t +R t )+ y2*g ⁇ *(L t -R 1 )- 1 /2*x*g rb *((g r *R t -g B *(L t -R ) (Eqn. 22)
• RJ is the passive matrix term.
• the terms - 1 /2*g c *(L t +R t ), 1 A*g *(L t - R,), and - 1 /2*x*g rb *((g r *R t -g ⁇ *(L t -RJ) represent cancellation terms (see Figure 14) that may be combined with R, in linear combiner 98 ( Figure 13) in order to derive the output audio signal R ⁇ ,,.
• the center front output, C ⁇ contains the passive matrix term 1 /2*(L t +R t ), plus the left and right cancellation terms as for the four-output system, - 1 /2*g,*L, and - ⁇ g.*! ⁇ :
• the starting passive matrix is L, - b*R,.
• the required cancellation term is therefore -g ⁇ *L f
• the cancellation term is +b*g r *R t .
• the right back cancellation term is -g rb *(g r *R t - 1 /2*g ⁇ *(L,-R t )), the same as the term used for R ⁇ , with an optimized coefficient y, which may again be arrived at empirically or calculated from the VCA gains in the left or right back conditions.
• the term [Lt-b*RJ is the passive matrix term and the terms -g,*! ⁇ , +b*g f *Rdon - 1 /2*(l-b)*g c *(L t +R t ) and -y*grb*((gr*Rt-gs* x /2*(Lt-Rt)) represent cancellation terms (see Figure 14) that may be combined with L,-bR t in linear combiner 92 ( Figure 13) in order to derive the output audio signal LB out .
• the [R,-b*LJ is the passive matrix term and the components -g r *Rt, b*L t *gggi - 1 /2*(l-b)*g c *(L t +R0, and -y*g l (g ⁇ *L t +g_*t ⁇ *(L r RO) represent cancellation terms (see Figure 14) that may be combined with R t -b*L, in linear combiner 96 ( Figure 13) in order to derive the output audio signal RB out .
• Additional control signals can be derived by further application of the scaling, offsetting or non-linear processing of the two main control signals from the left/right and sum/difference feedback portions of the feedback-derived control systems, permitting the generation of additional cancellation signals via VCAs whose gains rise to maxima at other desired predetermined values of ⁇ .
• input signals Lt and Rt are applied to a passive matrix 130 that produces a left matrix signal output from the L t input, a right matrix signal output from the R t input, a center output from a linear combiner 132 whose input is L, and R t , each with a scale factor of +V_., and a surround output from a linear combiner 134 whose input is L t and R, with scale factors of +V2 and -V2, respectively.
• the cardinal directions of the passive matrix are designated "left,” “center,” “right,” and “surround.” Adjacent cardinal directions lie on mutually orthogonal axes, such that, for these direction labels, left is adjacent to center and surround; surround is adjacent to left and right, etc.
• the left and right passive matrix signals are applied to a first pair of variable gain circuits 136 and 138 and associated feedback-derived control system 140.
• the center and surround passive matrix signals are applied to a second pair of variable gain circuits 142 and 144 and associated feedback-derived control system 146.
• the "left" variable gain circuit 136 includes a voltage controlled amplifier (VCA) 148 having a gain g, and a linear combiner 150.
• VCA voltage controlled amplifier
• the VCA output is subtracted from the left passive matrix signal in combiner 150 so that the overall gain of the variable gain circuit is (1-g,) and the output of the variable gain circuit at the combiner output, constituting an intermediate signal, is (l-g ⁇ L,.
• the VCA 148 output signal, constituting a cancellation signal, is g,*! ⁇
• the "right" variable gain circuit 138 includes a voltage controlled amplifier (VCA) 152 having a gain g r and a linear combiner 154.
• VCA voltage controlled amplifier
• the VCA output is subtracted from the right passive matrix signal in combiner 154 so that the overall gain of the variable gain circuit is (l-g r ) and the output of the variable gain circuit at the combiner output, constituting an intermediate signal, is (l-g r )*R t .
• the VCA 152 output signal g r *R t constitutes a cancellation signal.
• the (l-g r )*R t and (l-g,)*L, intermediate signals constitute a first pair of intermediate signals. It is desired that the relative magnitudes of this first pair of intermediate signals be urged toward equality. This is accomplished by the associated feedback-derived control system 140, described below.
• the "center" variable gain circuit 142 includes a voltage controlled amplifier (VCA) 156 having a gain g c and a linear combiner 158.
• VCA voltage controlled amplifier
• the VCA output is subtracted from the center passive matrix signal in combiner 158 so that the overall gain of the variable gain circuit is (l-g ⁇ and the output of the variable gain circuit at the combiner output, constituting an intermediate signal, is 1 ⁇ *(l-g c )*(L t +R t ).
• the VCA 156 output signal I /2*g c *(L,-r-R t ) constitutes a cancellation signal.
• the "surround" variable gain circuit 144 includes a voltage controlled amplifier (VCA) 160 having a gain g r and a linear combiner 162.
• VCA voltage controlled amplifier
• the VCA output is subtracted from the surround passive matrix signal in combiner 162 so that the overall gain of the variable gain circuit is (1-gJ and the output of the variable gain circuit at the combiner output, constituting an intermediate signal, is 1 /2*(l-g ⁇ )*(L,-RJ.
• the VCA 160 output signal /2*g ⁇ )*(L,-R constitutes a cancellation signal.
• the 1 /2*(l-g c )*(L,+R t ) and V4*(l- g_)*(L,-R intermediate signals constitute a second pair of intermediate signals. It is also desired that the relative magnitudes of this second pair of intermediate signals be urged toward equality. This is accomplished by the associated feedback-derived control system 146, described below.
• the feedback-derived control system 140 associated with the first pair of intermediate signals includes filters 164 and 166 receiving the outputs of combiners 150 and 154, respectively.
• the respective filter outputs are applied to log rectifiers 168 and 170 that rectify and produce the logarithm of their inputs.
• the rectified and logged outputs are applied with opposite polarities to a linear combiner 172 whose output, constituting a subtraction of its inputs, is applied to a non-inverting amplifier 174 (devices 172 and 174 correspond to the magnitude comparator 30 of Figure 3).
• VCAs 148 and 152 are of the type that inherently take the antilog of their control inputs, thus taking the antilog of the control output of the logarithmically-based comparator.
• the output of amplifier 174 constitutes a control signal for VCAs 148 and 152.
• the filters 164 and 166 may be derived empirically, providing a response that attenuates low frequencies and very high frequencies and provides a gently rising response over the middle of the audible range. These filters do not alter the frequency response of the output signals, they merely alter the control signals and VCA gains in the feedback-derived control systems.
• the feedback-derived control system 146 associated with the second pair of intermediate signals includes filters 176 and 178 receiving the outputs of VCAs 158 and 162, respectively.
• the respective filter outputs are applied to log rectifiers 180 and 182 that rectify and produce the logarithm of their inputs.
• the rectified and logged outputs are applied with opposite polarities to a linear combiner 184 whose output, constituting a subtraction of its inputs, is applied to a non-inverting amplifier 186 (devices 184 and 186 correspond to the magnitude comparator 30 of Figure 3).
• the feedback-derived control system 146 operates in the same manner as control system 140.
• the output of amplifier 186 constitutes a control signal for VCAs 158 and 162.
• Additional control signals are derived from the control signals of feedback- derived control systems 140 and 146.
• the control signal of control system 140 is applied to first and second scaling, offset, inversion, etc. functions 188 and 190.
• the control signal of control system 146 is applied to first and second scaling, offset, inversion, etc. functions 192 and 194.
• Functions 188, 190, 192 and 194 may include one or more of the polarity inverting, amplitude offsetting, amplitude scaling and/or non-linearly processing described above.
• the lesser or the greater of the outputs of functions 188 and 192 and of functions 190 and 194 are taken in by lesser or greater functions 196 and 198, respectively, in order to produce additional control signals that are applied to a left back VCA 200 and a right back VCA 202, respectively.
• the additional control signals are derived in the manner described above in order to provide control signals suitable for generating a left back cancellation signal and a right back cancellation signal.
• the input to left back VCA 200 is obtained by additively combining the left and surround cancellation signals in a linear combiner 204.
• the input to right back VCA 202 is obtained by subtractively combining the right and surround cancellation signals in a linear combiner 204.
• the inputs to the VCAs 200 and 202 may be derived from the left and surround passive matrix outputs and from the right and surround passive matrix output, respectively.
• the output of left back VCA 200 is the left back cancellation signal g B ,* 1 /2*((g 1 *L,+g 8 (L t -Rt)).
• the output of right back VCA 202 is the right back cancellation signal g lb * 1 /2*((g r *R,+g 8 (L,-Rt)).
• Figure 15 is a schematic circuit diagram showing a practical circuit embodying aspects of the present invention. Resistor values shown are in ohms. Where not indicated, capacitor values are in microfarads.
• T074 is a Texas Instruments' quad low-noise JFET-input (high input impedance) general purpose operational amplifier intended for high-fidelity and audio preamplifier applications. Details of the device are widely available in published literature. A data sheet may be found on the Internet at ⁇ ⁇ http://www.ti.com/sc/docs/products/analog/tl074.html> > .
• SSM-2120 in Figure 15 is a monolithic integrated circuit intended for audio applications. It includes two VCAs and two level detectors, allowing logarithmic control of the gain or attenuation of signals presented to the level detectors depending on their magnitudes. Details of the device are widely available in published literature. A data sheet may be found on the Internet at ⁇ ⁇ http://www.analog.com pdf/i788_c.pdf> > .
• the labels on the wires going to the output matrix resistors are intended to convey the functions of the signals, not their sources.
• the top few wires leading to the left front output are as follows:
• the present invention may be implemented using analog, hybrid analog/digital and/or digital signal processing in which functions are performed in software and/or firmware.
• Analog terms such as VCA, rectifier etc. are intended to include their digital equivalents.
• a VCA is realized by multiplication or division.

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