TREA

Variable matrix decoder

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Information

  • Patent Grant
  • 4941177
  • Patent Number
    4,941,177
  • Date Filed
    Friday, July 22, 1988
    36 years ago
  • Date Issued
    Tuesday, July 10, 1990
    34 years ago
Information
Abstract
The decoder of this invention decodes at least two channel signals in a directional information system where at least four input signals containing directional information have been encoded into the two or more channel signals. The decoder generates a first control signal substantially proportional to the logarithm of the ratio of the amplitudes of two of the channel signals to detect, as between two of the channel signals, whether the amplitude of one signal dominates that of the other. The decoder also generates a second control signal substantially proportional to the logarithm of the ratio of the amplitudes of the sum and the difference between two of the channel signals to detect the dominant signal in terms of amplitude. The decoder includes a matrix means responsive to the two or more channel signals and the two control signals for generating a number of output signals according to an algorithm. The control signals generated are used to steer the directional information systems in such manner through the matrix means that the directional effects of the output signals are enhanced. Two decoders of the type described above may be used to decode the high frequency and low frequency portions of the channel signals where the high and low frequency portions are separated by means of two crossover filters. The crossover frequency of the two crossover filters is controlled so that it is approximately at the top end of the signal frequencies intended for the center loudspeaker. Very low frequency signal components are separately processed and evenly distributed among the left, center and right channels.
Description
Claims
  • 1. A decoder for decoding two or more channel signals in a directional information system wherein at least four input signals containing directional information are encoded into the two or more channel signals, said decoder comprising:
  • first means for generating at least a first dominance signal indicative of the ratio of the amplitudes of a pair of the channel signals;
  • second means for generating at least a second dominance signal indicative of the ratio of the amplitudes of the sum of and the difference between said pair of the channel signals; and
  • matrix means responsive to said two or more channel signals and to said at least two dominance signals for generating a plurality of output signals for which directional effects of the output signals are enhanced, wherein the first dominance signal D.sub.LR and the second dominance signal D.sub.CS are given by: ##EQU11## where L.sub.T, R.sub.T are two channel signals
  • P=L.sub.T +R.sub.T, M=L.sub.T -R.sub.T ;
  • and a, k are constants;
  • wherein said matrix means includes means for deriving three directional control signals E.sub.L, E.sub.R and E.sub.C.
  • 2. The decoder of claim 1, wherein said means for deriving three directional control signals E.sub.L, E.sub.R, E.sub.C derives the three directional control signals according to the following equations: ##EQU12##
  • 3. The decoder of claim 2, wherein the matrix means generates the three outputs signals L', R', C', so that the three output signals are defined by means of the equations below: ##EQU13## where V is a 1 by 4 matrix; G.sub.L, G.sub.R, G.sub.C, are 4 by 2 matrices of predetermined coefficients; b is a constant and
  • F.sub.L, F.sub.R, R.sub.C, are given by:
  • F.sub.L =a.sup.b.E.sub.L
  • F.sub.R =a.sup.b.E.sub.R
  • F.sub.C =a.sup.b.E.sub.C.
  • 4. The decoder of claim 3, wherein the value of b is about 0.839.
  • 5. The decoder of claim 3, wherein the G.sub.L, G.sub.R, G.sub.C matrices are as follows: ##EQU14##
  • 6. The decoder of claim 3, wherein the G.sub.L, G.sub.R, G.sub.C matrices are derived from four equations Q.times.G.sub.L =H.sub.L, Q.times.G.sub.R =H.sub.R, Q.times.G.sub.C =H.sub.C, where ##EQU15##
  • 7. The decoder of claim 3, wherein the matrix means further comprises:
  • means for generating 6 product signals wherein each of the product signals is the product of either L.sub.T lor R.sub.T with one of 3 signals F.sub.L, F.sub.C, F.sub.R ; and
  • means for adding weighted sums of the six product signals to obtain output signals L', C', R'.
  • 8. The decoder of claim 7, wherein said product signals generating means includes six voltage controlled amplifiers for generating the six product signals.
  • 9. The decoder of claim 3, wherein the matrix means further comprises:
  • means for generating the 3 signals F.sub.L, F.sub.C, F.sub.R from E.sub.L, E.sub.R l, E.sub.C l; and
  • means for performing the matrix multiplications V.times.G.sub.L, V.times.G.sub.R, V.times.G.sub.C.
  • 10. The decoder of claim 1, further comprising a threshold detection means for detecting the amplitudes of the dominance signals, an averaging means and a switch, wherein the threshold detection means causes an average value of the dominance signals over a preceding time period to be applied to the matrix means upon detecting that the amplitudes of the dominance signals are below a predetermined threshold, so that the directional enhancements are determined by the average value of the dominance signals.
  • 11. The decoder of claim 1, further comprising an averaging means for applying an average value of the dominance signals over a preceding time period to the matrix means, so that the directional enhancement of the output signals by the matrix means is in accordance with the average value.
  • 12. The decoder of claim 11, wherein the averaging means has two different time constants, one time constant being operative when at least one dominance signal has an amplitude greater than a threshold value and the other time constant being operative when neither of the dominance signals have an amplitude above said threshold value.
  • 13. The decoder of claim 12, wherein said averaging means comprises:
  • a variable resistor whose resistance varies inversely with the amplitudes of the dominance signals, said resistor connected between the first or second dominance signal generating means and the matrix means; and
  • an impedance means forming a low pass filter configuration with the resistor.
  • 14. The decoder of claim 12, wherein said impedance means comprises:
  • a first and a second capacitor means connected in series between ground and a first point in the connection between the variable resistor and the matrix means;
  • a first resistor means forming a charge path for the first capacitor means, the first capacitor means having a capacitance much smaller than that of the second capacitor means so that the voltage across the first capacitor means responds more quickly than that across the second capacitor means to changes in the amplitudes of the dominance signals, so that when dominance signals increase in amplitudes, the averaging means will respond mainly to the voltage across the first capacitor means, thereby enabling the decoder to use the dominance signals to steer the decoder.
  • 15. The decoder of claim 14, said decoder further comprising means for discharging the second capacitor means so that when the amplitudes of both dominance signals decrease to substantially zero, the first capacitor means is discharged through the first resistor means in a much shorter time than the second capacitor means, so that the directional enhancement of the output signals by the matrix means is substantially determined by the voltage across the second capacitor means before the second capacitor means has been discharged, and so that no directional enhancement is applied when the second capacitor means has been substantially discharged.
  • 16. The decoder of claim 13, wherein said variable resistor is a transconductance amplifier.
  • 17. A method for decoding two or more channel signals using a directional information system wherein at least four input signals containing directional information have been encoded into the two or more channel signals, said method comprising:
  • generating at least a first dominance signal indicative of the ratio of the amplitudes of a pair of the channel signals;
  • generating at least a second dominance signal indicative of the ratio of the amplitudes of the sum of and the difference between said pair of the channel signals; and
  • generating a plurality of output signals for which directional effects of the output signals are enhanced in response to said two or more channel signals and to said at least two dominance signals, wherein the first dominance signal D.sub.LR and the second dominance signals D.sub.CS are given by ##EQU16## where L.sub.T, R.sub.T are two channel signals
  • P=L.sub.T +R.sub.T, M=L.sub.T -R.sub.T ;
  • and a, k are constants.
  • wherein said output signals generating step includes deriving three directional control signals E.sub.L, E.sub.R, E.sub.C.
  • 18. The method of claim 17, wherein said output signals generating step derives the control signals E.sub.L, E.sub.R, E.sub.C according to the following equations: ##EQU17##
  • 19. The method of claim 18, wherein the output signals generating step generates the three output signals L', R', C', so that the three output signals are defined by means of the equations below: ##EQU18## where V is a 1 by 4 matrix; G.sub.L, G.sub.R, are 4 by 2 matrices of predetermined coefficients; b is a constant and
  • F.sub.L, F.sub.R, F.sub.C are given by: ##EQU19##
  • 20. The method of claim 19, wherein the G.sub.L, G.sub.R, G.sub.C matrices are as follows: ##EQU20##
  • 21. The method of claim 19, further comprising the step of deriving the G.sub.L, G.sub.R, G.sub.C matrices from four equations Q.times.G.sub.L =H.sub.L, Q.times.G.sub.R =H.sub.R, Q.times.G.sub.C =H.sub.C, where ##EQU21##
  • 22. The method of claim 19, wherein the output signals generating step further comprises:
  • generating 6 product signals wherein each of the product signals is the product of either L.sub.T or R.sub.T with one of 3 signals F.sub.L, F.sub.C, F.sub.R ; and
  • adding weighted sums of the six product signals to obtain output signals L', C', R'.
  • 23. The method of claim 19, wherein the output signals generating step further comprises:
  • generating the 3 signals F.sub.L, F.sub.C, F.sub.R from E.sub.L, E.sub.R, E.sub.C ; and
  • performing the matrix multiplications V.times.G.sub.L, V.times.G.sub.R ; V.times.G.sub.C.
  • 24. A decoder for decoding two or more channel signals in a directional information system wherein at least four input signals containing directional information are encoded into the two or more channel signals, said decoder comprising:
  • first means for generating at least a first dominance signal indicative of the ratio of the amplitudes of the sum of and the difference between said pair of the channel signals; and
  • matrix means responsible to said two or more channel signals and to said at least two dominance signals for generating a plurality of output signals for which directional effects of the output signals are enhanced, wherein the first dominance signal D.sub.LR and the second dominance signal D.sub.CS are given by: ##EQU22## where L.sub.T, R.sub.T are two channel signals
  • P=L.sub.T +R.sub.T, M=L.sub.T -R.sub.T ;
  • and a is a constant,
  • and wherein said matrix means includes means for deriving four directional control signals E.sub.L, E.sub.R, E.sub.C, E.sub.S according to the following equations: ##EQU23##
  • 25. The decoder of claim 24, further comprising a threshold detection means for detecting the amplitudes of the dominance signals, an averaging means and a switch, wherein the threshold detection means causes an average value of the dominance signals over a preceding time period to be applied to the matrix means upon detecting that the amplitudes of the dominance signals are below a predetermined threshold, so that the directional enhancements are determined by the average value of the doninance signals.
  • 26. The decoder of claim 24, wherein the directional control signals deriving means derives the four output signals L', R', C', S', so that the four signals are defined by means of the equations below: ##EQU24## where V is a 1 by 5 matrix;
  • G.sub.L, G.sub.R, G.sub.C, G.sub.S are 5 by 2 matrices of predetermined coefficients; b is a constant and
  • F.sub.L, F.sub.R, F.sub.C, F.sub.S are given by: ##EQU25##
  • 27. The decoder of claim 26, wherein the value of b is about 0.839.
  • 28. The decoder of claim 26, wherein the G.sub.L, G.sub.R, G.sub.C, G.sub.S matrices are as follows: ##EQU26##
  • 29. The decoder of claim 26, wherein the G.sub.L, G.sub.R, G.sub.C, G.sub.S matrices are derived from four equations Q.times.G.sub.L =H.sub.L, Q.times.G.sub.R =H.sub.R, Q.times.G.sub.C =H.sub.C, Q.times.G.sub.S =H.sub.S, where ##EQU27##
  • 30. The decoder of claim 26, wherein the matrix means further comprises:
  • means for generating 8 product signals wherein each of the signals is the product of either L.sub.T or R.sub.T with one of 4 signals F.sub.L, F.sub.C, F.sub.R, F.sub.S ; and
  • means for adding weighted sums of the eight product signals to obtain output signals L', C', R', S'.
  • 31. The decoder of claim 30, wherein said product signals generating means includes eight voltage controlled amplifiers for generating the eight product signals.
  • 32. The decoder of claim 26, wherein the matrix means further comprises:
  • means for generating the 4 signals F.sub.L, F.sub.C, F.sub.R, F.sub.S from E.sub.L, E.sub.R, E.sub.C, E.sub.S ; and
  • means for performing the matrix multiplications V.times.G.sub.L, V.times.G.sub.R, V.times.G.sub.C, V.times.G.sub.S.
  • 33. The decoder of claim 24, further comprising an averaging means for applying an average value of the dominance signals over a preceding time period to the matrix means, so that the directional enhancement of the output signals by the matrix means is in accordance with the average value.
  • 34. The decoder of claim 33, wherein the averaging means has two different time constants, one time constant being operative when at least one dominance signal has an amplitude greater than a threshold value and the other time constant being operative when neither of the dominance signals have an amplitude above said threshold value.
  • 35. The decoder of claim 34, wherein said averaging means comprises:
  • a variable resistor whose resistance varies inversely with the amplitudes of the dominance signals, said resistor connected between the first or second dominance signal generating means and the matrix means; and
  • an impedance means forming a low pass filter configuration with the resistor.
  • 36. The decoder of claim 34, wherein said impedance means comprises:
  • a first and a second capacitor means connected in series between ground and a first point in the connection between the variable resistor and the matrix means;
  • a first resistor means forming a charge path for the first capacitor means, the first capacitor means having a capacitance much smaller than that of the second capacitor means so that the voltage across the first capacitor means responds more quickly than that across the second capacitor means to changes in the amplitudes of the dominance signals, so that when dominance signals increase in amplitudes, the averaging means will respond mainly to the voltage across the first capacitor means, thereby enabling the decoder to use the dominance signals to steer the decoder.
  • 37. The decoder of claim 36, said decoder further comprising means for discharging the second capacitor means so that when the amplitudes of both dominance signals decrease to substantially zero, the first capacitor means is discharged through the first resistor means in a much shorter time than the second capacitor means, so that the directional enhancement of the output signals by the matrix means is substantially determined by the voltage across the second capacitor means before the second capacitor means has been discharged, and so that no directional enhancement is applied when the second capacitor means has been substantially discharged.
  • 38. The decoder of claim 35, wherein said variable resistor is a transconductance amplifier.
Cross Reference to Related Application

This application is a continuation of Ser. No. 833,120, filed Feb. 26, 1986, now U.S. Pat. No. 4,799,260 which is a continuation in part application of Ser. No. 708,982, entitled "Variable Matrix Decoder" by Douglas Evan Mandell and Craig C. Todd, filed Mar. 7, 1985, now abandoned. This invention relates to a directional information system where a number of input signals are encoded for recording or transmission on a medium into two or more channel signals and where the channel signals are decoded into a number of output signals corresponding to the directional information input signals. The decoder of this invention decodes the two or more channel signals so that directional effects are enhanced. In quadraphony the loudspeakers are spaced horizontally around the listeners in four locations, to create an impression of the original program in full horizontal surround sound. In some quadraphonic systems the loudspeakers are placed at the four corners of the room. In other quadraphonic systems, such as those used in motion picture theaters, loudspeakers are not all placed at corners. Instead they may be placed at the left and right front corners of the theater, at the center of the front stage and dispersed around the back wall of the theater. The loudspeakers placed at the front left and right corners are still known as the left and right speakers; the on as placed at the center of the front stage known as the center speakers; and those at the back wall as the surround speakers. In order for the recording played back through the loudspeakers to recreate a realistic impression of the original program, the recording must contain directional information. In some quadraphonic systems four discrete input channels are actually recorded; this is known as the 4-4-4 format. The other general approach, termed 4-2-4, uses some kind of matrix encoding of the four audio input channels into two channels such as two conventional stereo-recorded channels, which are decoded back to four audio output channels during playback. In the 4-2-4 sound systems, since the four directional audio input signals are transformed into two channel signals by the encoder, some directional information will be lost so that it is impossible for the decoder to reproduce signals perfectly identical to the original directional audio input signals. As a result, the cross-talk between adjacent channels and the reproduced sound signal may greatly reduce the directional effect of the quadraphonic system. Numerous attempts have been made to enhance the directional effects of quadraphonic 4-2-4 systems. In one approach known as gain riding, the net sound level of each of the four loudspeakers is adjusted without adjusting the relative contributions of the two channel signals to reduce cross-talk. In another approach known as the variable matrix approach, the four output signals fed to the four loudspeakers are derived by certain mathematical computations performed on the two channel signals to vary the relative contributions of the two channel signals in order to reduce the effect of cross-talk. Ito et al., in U.S. Pat. No. 3,825,684, disclosed a variable matrix decoder for enhancing the directional effects of a four channel playback system with loudspeakers placed at the four corners of the room. The decoder has a control unit which detects the phase difference between the two channel signals and produces two control signals, one for controlling the separation of the two front outputs and the second control signal for controlling the separation of the two rear outputs. The two control signals are also used to control the level of the front output signals relative to the rear output signals. In reference to FIG. 10 of U.S. Pat. No. 3,825,684, for example, the separation between the two front outputs is controlled by the gain f applied by variable amplifier 122 and appears to vary inversely with the magnitude of the phase difference between the two channel signals L and R. The separation between the two rear outputs is controlled by the gain b of variable amplifier 127 and appears to vary directly with the magnitude of the phase between L and R. In U.S. Pat. No. 3,944,735, Willcocks discloses a directional enhancement system used together with existing matrix decoders and for enhancing the directional effects of output signals from these decoders. It does not include a 2-4 matrix decoder as such. Instead the system modifies the four output signals obtained from a preceding quadraphonic matrix decoder to enhance the directional content of the signals before presenting them to the loudspeakers. The system comprises a detector which generates 6, 8 or 10 directional control signals by comparing envelopes of certain signals derived by fixed matrices from the channel signals. The detector generates these control signals using automatic gain control to avoid dependence on signal levels. Willcocks employs a processor which generates from the control signals the coefficients of a modifying matrix, and employs a matrix modifier which modifies the four output signals of the preceding matrix decoder by the modifying matrix. In many quadraphonic sound applications, such as in motion picture theaters, it may be desirable to enhance the directional effects only of sound within certain frequency ranges, such as the frequency range of speech. In a wide band quadraphonic system, if the low frequency information such as speech comes from a particular direction, and if the high frequency background sound such as wind appears in all directions, the high frequency background as well as the low frequency speech signals may all be steered in the direction of speech. This creates sound impressions which deviate from the original program and is undesirable. It is therefore desirable to provide a splitband system through which the above difficulty is alleviated. None of the above directional enhancement systems for 4-2-4 quadraphonic decoders are entirely satisfactory. It is therefore desirable to provide systems with better directional enhancement capabilities and with simpler circuitry. The decoder of this invention decodes at least two channel signals in a directional information system where at least four input signals containing directional information have been encoded into the two or more channel signals. The decoder includes a first means for generating at least a first dominance signal indicative of the ratio of the amplitudes of a pair of the channel signals. In the preferred embodiment, the first dominance signal is substantially proportional to the logarithm of the ratio of the amplitudes of a pair of the channel signals. The first generating means of the decoder thus detects, as between the pair of channel signals, whether the amplitude of one signal dominates that of the other. The decoder also includes a second means for generating at least a second dominance signal indicative of the ratio of the amplitudes of the sum of and difference between the pair of channel signals. In the preferred embodiment, the second dominance signal is substantially proportional to the logarithm of the ratio of the amplitudes of the sum and the difference between said pair of channel signals. The second generating means detects, as between two signals, one being equal to the sum of the pair of channel signals and the other being equal to the difference between them, whether the amplitude of one signal dominates the other. The decoder further includes a matrix means responsive to the two or more channel signals and the at least two dominance signals from the two generating means for generating a number of output signals. Thus, if the first generating means or the second generating means detects the dominance of one channel signal over another or the dominance of the amplitude of the sum of these channel signals over their difference, or vice versa, the dominance signals generated are used to steer the directional information systems in such manner through the matrix means that the directional effects of the output signals are enhanced. By detecting the dominance between pairs of channel signals and between the sum of and the difference between the two signals in each of these pairs as ratios between their amplitudes, the detection capability of the decoder is not tied to a set reference level; hence, the decoder is capable of detecting the directional information in the two or more channel signals as described above even at very low signal levels. By detecting the dominance between pairs of signals in the form of the logarithms of the amplitude ratios, such dominance can be conveniently expressed in decibels. If all channel signals are such that no significant dominance is detected between them or between the sum of and the difference between pairs of channel signals, an averaging circuit in the decoder having a large time constant is enabled to maintain the previous steering pattern. The particular algorithm of the matrix means used in the decoder of this invention is effective in reducing cross-talk and creating a realistic impression that the directional information is coming at accurate angular positions. In another aspect of the invention, the channel signals are each separated by a separating means into a high frequency portion having frequency components above a separation frequency and a low frequency portion having frequency components below the separation frequency. The high frequency portions of the channel signals are decoded by a first decoder and the low frequency portions of the channel signals decoded by a second decoder. The corresponding output signals of the two decoders are then added to give the total output signals. The frequency range of the signals destined for a particular output channel is detected. The separation frequency is then altered, where necessary, so that it coincides with the top end of such frequency range. This allows the signal components in the frequency range of the particular output channel and below to be steered differently than signal components at higher frequencies. In such manner, speech signals and background sound in the speech frequency range can be steered apart from high frequency background sound. The above aspect of the invention is implemented in the preferred embodiment as follows. The amplitudes of the high frequency portions of the two or more channel signals are compared by a comparing means to generate a first dominance signal indicating whether the signals intended for the particular output channel dominates those of signals intended for the other channels at frequencies above the separation frequency. The comparing means also compares the low frequency portions of the channel signals and generates a second dominance signal indicating whether the signals intended for the particular output channel dominates those of signals intended for the other channels at frequencies below the separation frequency. The two dominance signals are compared by a second comparing means to provide an output signal for controlling the separating means, so that the separation frequency of the separating means varies in such manner that the amplitude of the second dominance signal bears a substantially constant large ratio to that of the first dominance signal. Yet another aspect of the invention is based on the observation that the very low frequency components of the channel signals may be simply evenly distributed among two or more of the decoder outputs, for example, the left, center and right output channels. For this purpose the very low frequency components of the channel signals are added and then passed by a low pass filter means in a separate path in parallel with the variable matrix decoder and portions of it are then simply added equally to the outputs of the decoder.

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Entry
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Continuations (1)
Number Date Country
Parent 833120 Feb 1986
Continuation in Parts (1)
Number Date Country
Parent 708982 Mar 1985