WO1999012386A9 - Systeme de codage et de decodage a matrice 5-2-5 - Google Patents

Systeme de codage et de decodage a matrice 5-2-5

Info

Publication number
WO1999012386A9
WO1999012386A9 PCT/US1998/018390 US9818390W WO9912386A9 WO 1999012386 A9 WO1999012386 A9 WO 1999012386A9 US 9818390 W US9818390 W US 9818390W WO 9912386 A9 WO9912386 A9 WO 9912386A9
Authority
WO
WIPO (PCT)
Prior art keywords
center
decoder
signals
surround
output
Prior art date
Application number
PCT/US1998/018390
Other languages
English (en)
Other versions
WO1999012386A1 (fr
Inventor
David H Griesinger
Original Assignee
Lexicon
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Lexicon filed Critical Lexicon
Priority to CA002301547A priority Critical patent/CA2301547C/fr
Priority to AU93026/98A priority patent/AU750877C/en
Priority to JP2000509252A priority patent/JP2004507904A/ja
Priority to EP98945881A priority patent/EP1013140B1/fr
Priority to BR9812432-3A priority patent/BR9812432A/pt
Priority claimed from US09/146,442 external-priority patent/US6697491B1/en
Publication of WO1999012386A1 publication Critical patent/WO1999012386A1/fr
Publication of WO1999012386A9 publication Critical patent/WO1999012386A9/fr

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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

Definitions

  • This invention relates to sound reproduction systems involving the decoding of a stereophonic pair of input audio signals into a multiplicity of output signals for reproduction after suitable amplification through a like plurality of loudspeakers arranged to surround a listener, as well as the encoding of multichannel material into two channels.
  • the present invention concerns an improved set of design criteria and their solution to create a decoding matrix having optimum psychoacoustic PATENT Attorney Docket No. 21647.1
  • this invention concerns an improved set of design criteria and their solution to create an encoding circuit for the encoding of ulti- channel sound onto two channels for reproduction in standard two channel receivers and by decoders according to the invention.
  • the present invention is part of a continuing effort to refine means of encoding multichannel audio signals into two separate channels, and then means for separating the resulting two channels back into the multichannel signals from which they were derived.
  • One of the goals of this encode/decode process is to recreate the original signals as perceptually identical to the originals as possible.
  • Another important goal of the decoder is to extract five or more separate channels from a two channel source that was not encoded from a five channel original. The resulting five channel presentation must be at least as musically tasteful and enjoyable as the original two channel presentation.
  • the present invention is concerned with improvements to the derivation of suitable variable matrix coefficients
  • this disclosure makes reference to Griesinger s U.S. Patent No. 4.862.502 ( 1989) which will be referred to as the ' 89 patent; U. S. Patent No. 5, 136,650 (1992), which will be referred to as the '92 patent; the July 1996
  • the present invention is concerned with the realization of an active matrix having r rtain properties that maximize its psychoacoustic performance.
  • it discloses frequency dependent modification of certain of the outputs from the active matrix.
  • Lix a further aspect, it provides active circuitry that encodes five input channels into a two channel output that will perform optimally both with standard two channel equipment, with decoders according to the invention, and with industry standard "Dolby Pro- Logic" decoders.
  • the invention is in part an active matrix decoder having matrix elements that vary depending on the directional component of the incoming signals.
  • the matrix elements vary in such a way as to reduce the loudness of directionally encoded signals in outputs that are not involved in the intended direction, while enhancing the loudness of these signals in directions that are involved in reproducing the intended direction, while. preserving at all times the left/right separation of other signals that may be present at the same time at the inputs.
  • matrix elements in accordance with the invention restore the left/right separation of decorrelated two channel material that has been directionally encoded by increasing or decreasing the blend between the two inputs — for example with a stereo width control.
  • matrix elements in accordance with the invention are designed to preserve as much as possible the energy * balance between various components of the input signal, such that the balance between vocals and accompaniment is preserved in the decoder outputs.
  • matrix elements in accordance with the invention preserve both the loudness of non directionally encoded elements of the input sound and the left/right separation of these elements.
  • decoders in accordance with the invention include PATENT Attorney Docket No 21647 15
  • frequency dependent circuits that improve the compatibility of the decoder outputs when standard two channel material is played, that convert the surround outputs from two for a five channel decoder to four for a seven channel decoder, and that modify the spectrum of the rear channels in a five channel decoder so that the sound direction appears to be more like the sound direction from a seven channel decoder.
  • Encoders in accordance with the invention mix five (or five full-range plus one low frequency) input channels into two output channels stich that when the input level of a particular input is strong the energy present at that input is preserved in the output; such that the direction of a strong input is encoded in the phase/amplitude ratio of the output signals: such that strong signals can be panned between any two inputs of the encoder and the output will be correctly directionally encoded.
  • decorrelated material applied to the two rear inputs of the encoder will be encoded to two channels m such a fashion that the left/right separation of the inputs will be preserved when the encoder output is decoded by decoders according to the invention: such that ln-pha ⁇ e inputs to the two rear channels of the encoder will produce a two channel output that will decode to the rear channels of decoders according the invention, and decoders according the Dolby standard, and such that anti-phase inputs to the two rear channels of the encoder will produce outputs that will correspond to a non-directional signal for decoders according to the invention and to decoders according to the Dolby standard, and also such that low level reverberant signals applied to the two rear inputs of the encoder will be encoded with a 3dB reduction in level in the two channel output.
  • FIG. 1 is a block schematic of a direction detection section and a two to five channel matrix section of a decoder according to the present invention, but not including aspects further shown in FIGs. 2 and 3;
  • FIG. 2 is a block schematic of a five-channel frequency-dependent active signal processor circuit which may be connected between the outp its of the matrix section of FIG. 1 and the decoder outputs:
  • FIG. 3 is a block schematic of a five-to-seven channel frequency-dependent active signal processor which may alternatively be connected between the outputs of the matrix section of FIG. 1 and the decoder outputs:
  • FIG. 4 is a block schematic of an active five-channel to two-channel encoder according to the invention.
  • FIG. 5 is a three-dimensional graphical representation of the prior art Left Front Left (LFL) matrix element from the '89 patent and Dolby Pro-Logic, scaled so the maximum value is one;
  • FIG. 6 is a three-dimensional graphical representation of the prior art Left Front Right (LFR) matrix element from the '89 patent and Dolby Pro- Logic, scaled by .71 so the rninimum value is -0.5 and the maximum value is +- 0.5;
  • FIG. 7 is a tnree-dimensional graphical representation of the square root of the sum of the squares of the prior art LFL and LFR from the '89 patent, PATENT Attorne-. Docket No 21647 15
  • FIG 8 is a three-dimensional graphical representation of the square root of the sum of the LFL and LFR matrix elements from application No 08/742 460 scaled so the max value is 1
  • FIG. 9 is a three-dimensional graphical representation of the Left Front
  • FIG. 10 is a three-dimensional graphical representation of the partially completed Left Front Left matrix element in the present invention.
  • FIG 11 is a graph showing the behavior of LFL and LFR m the present invention along the rear boundary between left and full rear
  • FIG 12 is a three-dimensional graphical representation of the full ⁇ completed Left Front Left (LFL) matrix element m the present ind ention as Mewed from the left rear,
  • FIG 13 is a three-dimensional graphical representation of the full ⁇ completed Left Front Right (LFR) matrix element in the present m * ention
  • FIG 14 is a three-dimensional graphical representation of the root mean squared sum of LFL and LFR according to the present im ention
  • FIG 15 is a three-dimensional graphical representation of the square root of the sum of the squares of LFL and LFR according to the present invention, including the correction to the rear level, viewed from the left rear
  • FIG 16 is a graph showing with the solid curve the center matr ix elements that should be used in a prior art Dolb ⁇ Pro-Logic decoder as a function of CS in dB, and with the dotted curve the actual value of the center matrix elements in the Dolby Pro-Logic decoder;
  • FIG. 17 is a graph showing with the solid curve the ideal value of the center matrix elements, and with the dotted curve the actual values of the PATENT Attorney Docket No. ' 21647.15
  • FIG. IS is a three-dimensional graphical representation of the square root of the stim of the squares of LRL and LRR, using the prior art elements of VI. 1 1
  • FIG. 20 is a three-dimensional graphical representation of the square root of the sum of the squares of LRL and ' LRR using the values for GR and GS according to the present invention
  • FIG. 21 is a three-dimensional graphical representation of the prior art Center Left (CD matrix element of the '89 patent four channel decoder (and the Dolby Pro-Logic decoder) which can also represent the Center Right (CR) matrix element with left and right interchanged;
  • FIG. 22 is a three-dimensional graphical representation of the Center
  • FIG. 23 is a graph showing with the solid curve the center output channel attenuation needed for the new LFL and LFR, and with the dotted curve the center attenuation for a standard prior art Dolby Pro-Logic decoder:
  • FIG. 24 is a graph showing with the solid curve the ideal center attenuation for the "film ' ' strategy, according to the present invention, with the dashed curve a value that works significantly better, and with the dotted curve the center attenuation for the standard Dolby decoder for comparison;
  • FIG. 25 shows the center attenuation used in the "music" strategy in the present invention
  • FIG. 26 is a graph showing with the solid curve the value of GF needed for constant energy ratios with the "music" center attenuation O 1""1 of the p resent invention, with the dashed curve the previous LFR element -U t ⁇ ' .' ⁇ rr 1. and v ⁇ ⁇ tn the dotted curve the value of smlcs ,
  • FIG. 27 is a l ⁇ uree-din ⁇ ensional graphical representation of the Left Front
  • FIG. 28 is a three-dimensional graphical representation of the Center Left (CL) matrix element with the new center boost function; and FIG. 29 is a graph which plots the output level from the left front output
  • the decoder in this application will be described as consisting of two separate parts.
  • the first part is a matrix that splits the two channels into five output channels, which are usually identified as center, left front right front, left rear and right rear.
  • the second part consists of a series of delays and filters that modify the spectrum and the levels of the two rear outputs
  • One of the functions of the second part is to derive an additional pair of outputs, left side and right side, when a seven channel version of the decoder is desired.
  • the second part was not explicit — the two additional channels were derived from an additional pair of matrix elements in the original matrix
  • FIG. 1 which is identical to FIG. 4 in U. S. Patent Application No. 08/742.460. shows a block diagram of the first part of the decoder, a two channel to five channel matrix 90.
  • the left half of FIG. 1 partitioned by a vertical dashed line shows a means for deriving the two steering voltages l/r and c/s These voltages represent the degree to which the input signals have an inherent or encoded directional component in the left/right or front/back directions respectively
  • This part of the FIG. will not be explicitly discussed in th s application as it has been fully described m the above patent application
  • the directional detection means of decoder 90 comprising elements 92 through 138 -is followed by a 5 x 2 matrix to the right of the vertical dashed line.
  • the elements of this matrix 140 through 158 determine the amount of each input chan el that is linearly combined with the other input channel to form each output channel.
  • These matrix elements are assumed to be real. (The case of complex matrix elements was described in L0 S. Patent Application No. 08/742,460 and will not be discussed here.)
  • the matrix elements are functions of the two steering voltages l/r and c/s.
  • U. S. Patent Application No. 08/742,460 presented mathematical formulae for these PATENT Attorney Docket N . 21647 15
  • the steering voltages c/s and l/r are derived from the logarithm of the ratio of the left input amplitude at terminal 92 to the right input amplitude at terminal 94, and the logarithm of the ratio of the stun amplitude to the difference amplitude.
  • l/r and c/s are derived from the logarithm of the ratio of the left input amplitude at terminal 92 to the right input amplitude at terminal 94, and the logarithm of the ratio of the stun amplitude to the difference amplitude.
  • Ir 90 - arctani 10 ⁇ ((//r)/20)) .
  • cs 90 - arctan( 10 ((c/s)/20)) . .( lb)
  • the angles Ir and cs determine the degree to which the input signals have a directional component. For example, when the inputs to the decoder are decorrelated. both Ir and cs are zero. For a signal that comes from the center only, Ir is zero, and cs has the value 45 degrees. For a signal that comes from the rear, Ir is zero, and cs is -45 degrees.
  • a signal that comes from the left has an Ir value of 45 degrees and a cs value of zero
  • a signal from the right has an Ir value of -45 degrees, and a cs value of zero.
  • the matrix elements shown in FIG. 1 are real and thus frequency independent. All signals in the inputs will be directed to the outputs depending on the derived angles Ir and cs. (In the current art low frequencies and very high frequencies are attenuated in the derivation of Ir and cs from the input signals by filters not shown in FIG. 1. However, the matrix itself is broadband.)
  • FIG 1 - has been described in L S Patent Application No -S 742 -ruC and ⁇ _ nut oe fur th discussed here
  • FIG. 2 shows a five channel version of the additional frequency dependent circuits. These circuits do not have fixed parameters. The frequency and level behavior is dependent on the steering values Ir and cs The circuits accomplish several purposes.
  • the modified left surround and right surround signals appear at terminals 198 and 200, respectively. Additional details of this circuit will be given in a later section of this disclosure.
  • FIG. 3 shows the seven channel version of the frequency dependent elements.
  • the first set of filters 182, 184, 188, 190 attenuate the upper frequencies of the side and rear outputs when the steering is neutral or forward, again controlled by the background control signal 186. This attenuation also results in a more forward sound image, and can be adjusted to the listener's taste.
  • additional circiiits 202, 204. 206. 208 act to differentiate the side outputs from the rear outputs.
  • the attenuation mentioned above in the side speakers is first removed by elements 204 and 206 to produce a side oriented sound.
  • FIG 4 shows a block diagram of an encoder designed to a utomat ⁇ all . mix five input channels into two output channels.
  • the architecture is quite i erent rom the encoder described in L " . S. Patent Apphcation No. 08/742,460.
  • the object of the new design is to preserve the musical balances of the five channel original, while providing phase/amplitude cues that allow the original five channels to be extracted by the decoder.
  • the previous encoder had similar goals, but there have been improvements in the methods used to achieve these goals.
  • the preservation of musical balances is highly important in the encoder.
  • One of the primary purposes of the encoder is to automatically create a two channel mix of a five channel recording, that will play in an ordinary two channel system with the same artistic quality as the five channel original
  • the new encoder design includes active elements to ensure that musical balance is preserved. Unlike the encoder of the November 97 application the new design allows mput signals to be panned between any of the five inputs of the encoder For example a sound may be panned from the left front inp ut to the rmht rear- input. When the resulting two channel signal is decoded by the decoder- described in this apphcation the result will be quite close to the original sound. Decoding through an earlier surround decoder will also be similar to the original.
  • the invention is a surround sound decoder having variable matrix values so constructed as to reduce dir tiuna lly encoded audio components in outputs which are not directly involved in reproducing them in the int n ⁇ ed direction, enhance directionally encoded audio components in the outputs which are directly involved in reproducing them in the intended direction so as to maintain constant total power for such signals; while preserving high separation between the left and right channel components of non-directional signals regardless of the steering signals; and maintaini g the loudness defined as the total audio power level of non- directional signals effectively constant whether or not directionally encoded signals are present and regardless of their intended direction if present.
  • 08/742.460 is retained.
  • One of the most important of the previous goals is the explicit maintenance of high separation between the left and right channels of the decoder under all conditions. All previous four channel decoders are unable to maintain separation in the rear, because they provide only a single rear channel. Five channel decoders from other manufacturers compromise separation in many ways. The decoder described in this application meets this goal in a similar way as that of VI.11 - but it meets additional goals as well.
  • the front input signals L C and R are applied to input terminals 50 52 and 54 respectively L and R go directly to the adders 278 md 282 es e .-tr ⁇ factor fen m attenuator 372 before being applied to inputs of both adders 278 and 282.
  • the low frequency effects signal LFE is passed through a gain of 2.0 in element 374 and then applied to both adders 278 and 282.
  • the surround input signals LS and RS are applied through two input terminals 62 and 64 respectively each to two separate paths' for the LS signal the path through attenuator 378 has gain fs ⁇ l,ls) and the RS signal passes through a corresponding attenuator 380 with gain fs(r,rs)
  • the outputs of these are passed into cross-coupling elements 384 and 386 having a gain factor of -crx where crx is nominally 0 383
  • the cross-coupled signals from these elements are fed to summers 392 and 394 which also receive the attenuated LS and RS signals from 0 91 attenuators 388 and 392
  • the other signal branch passes the LS and RS signals through attenuators 376 with gain fc ⁇ l,ls) and 382 with gain fc ⁇ r,rs) respectively, and then through a similar arrangement of cross-coupling elements 396 398 402 404 406 and 408 the summers 406 and 408 having outputs representing left rear and right rear inputs at 45 degrees left and right of center rear, as before.
  • these signals now each pass through phase shifter elements 234 and 246 respectively, while the left and right signals from adders 278 and 282 pass PATENT Attorney Docket No. 21647.15
  • phase shifter elements 286 and 288 are phase shifter elements 286 and 288 respectively.
  • Each of these phase shifter elements is an ail-pass filter the phase response being ⁇ f) for elements 286 and 288. and ⁇ (f)-9Q " for elements 234 and 246. Calculation of the component values required in these filters is well know n in the art. and will not be discussed further here. The result is that the outputs of summers 406 and
  • the gain functions fs and fc are designed to allow strong surround signals to be presented in phase with the other sounds while weak surround signals pass through the 90 degree phase-shifted path to retain constant power for decorrelated "music " signals
  • the value of crx can also change and varies the angle from whicn the surround signals are heard
  • both the left decoder input and the right decoder input will be reproduced by the center speaker, and sounds that were originally only in the left (or right) channel will also be reproduced from the center. The result must be that the apparent positions of these sounds will be drawn to the middle of the room. The degree to which this occurs depends on the loudness of the center channel. " . S. Patents Nos. 4,862,502 and 5, 136,650 used matrix elements that had a minimum value of 3dB compared to the left and right channels.
  • U. S. Patent Application No. 08/742,460 used center matrix elements that had a minimum value 4.5dB less than the earher values. This mmimum value was chosen on the basis of listening tests. This attenuation causes a pleasing spread to the front image when the input material is uncorrelated - as with orchestral music. The front image is not seriously narrowed.
  • U. S. Patent Apphcation No. 08/742,460 as the steering moved forward these matrix elements increased, ultimately reaching the values used in the Dolby matrix.
  • the VI.11 decoder showed that although the reduction in center channel loudness solved the spatial problem, the power balance in the PATENT Attorney Docket No. 21647.1
  • This new strategy allowing the center channel component to come from all three front speakers, and lirniting the steering action when the center is 6dB louder than the front left and right, turns out to be excellent for all types of music.
  • Both encoded five-channel mixes and ordinary two-channel mixes decode with a stable center, and adequate separation between the center channel and the left and right. Note that unlike previous decoders the separation between center and left and right is deliberately not complete. A signal intended to come from the left is eliminated from the center channel, but not the other way around. For music the high lateral separation and stable front image that this strategy offers outweighs this lack of complete separation. Listening tests with this setting on films reveal that although there is some dialog coming from the left and right front speakers the stability of the rest ⁇ ting sound image is quite good. The result is pleasant and not distracting.
  • the left or right front output had an output 9dB less than the corresponding rear output.
  • the front matrix elements are modified to eliminate sound from the front when steering is anywhere between left rear and right rear.
  • the math used to describe the matrix elements is not based on continuous functions of the variables cs and Ir. In general there are conditionals, absolute values, and other non-linear modifications to the formulae. For this reason we will describe the matrix elements using a programming language.
  • the Matlab language provides a simple method of checking the formulation graphically.
  • tu en The anable x includes all ten values It is described as ⁇ v ector a 1 by 10 matrix
  • An individual number within each vector can be accessed or manipulated.
  • x(4) 4; will set the fourth member of the vector x to the value 4.
  • Variables can also represent a two dimensional matrix. Individual elements in the matrix can be assigned in a similar way:
  • Reference [2] includes the 'tv matrix ' feature This feature reduces the lev l o t e rear outputs when the steering is forw ai or neutral In this application this function is achiev ed through the frequenc ⁇ dependent circuits that follow the matrix Therefore w e have left out the v matrix ' correction.
  • the section on the center matrix elements has been modified to include references to the "film'' strategy, the "music” strategy, and a strategy that limits the action of the "music" setting Reference [2] described onh the music" setting, without the limit
  • Matrix decoders in equations and graphics lh reference [1] we presented the design of a matrix decoder that can be described by the elements of a n x 2 matrix, where n is the number of output P-ATENT ⁇ ttornev Docket No 21647 15
  • Each output can be seen as a linear combination of the two inputs r' here the - er ⁇ cients of the Linear combination are gr en b ⁇ the elements ui the matrix In this paper the elements are identified bv a simple combination ot letters Reference [1J described a fi e-channel and a sev en-channel decoder The conversion from five channels to seven channels is now done in the frequency dependent part of the decoder, so here we will describe a five- channel decoder only.
  • phase/amphtude decoders determine the apparent direction of the input bv comparing the ratio of the amplitudes of the input signals For example the degree of steernig in the right/left direction is determined from the ratio of the left input channel amphtude to the right input channel amplitude In a s ular way, the degree of steering in the front/back direction is determined from the ratio of the amplitudes of the sum and the difference of the input channels.
  • the degree of steernig in the right/left direction is determined from the ratio of the left input channel amphtude to the right input channel amplitude
  • the degree of steering in the front/back direction is determined from the ratio of the amplitudes of the sum and the difference of the input channels.
  • the elements presented here are not always correctly scaled. In general they are presented so that the unsteered value of the of the non-zero matrix elements are for any given channel is one. In practice the elements are usually scaled so the maximum value of each element is one or less. In any case, in a final product the scaling of the elements is additionally varied in the cahbration procedure.
  • the matrix elements presented here should be assumed to be scalable by appropriate constants.
  • cs and Ir are the steering directions in degrees in the center/surround and left/right axis respectively.
  • LFR -0.5*G( cs) ...(30 In the right rear quadrant: PATENT Attorney Docket No. 21647.15
  • G(x) was determined experimentally in the 1989 patent. and is specified mathematically in the ' 91 patent. It varies from 0 to 1 as x varies from 0 to 45 degrees.
  • G(x) can be shown to be equal to 1—
  • 1 11 are the right and left input amplitudes.
  • G(x) can also be described in terms of the steering angles using various formulae. One of these is given in the '91 patent, and another will be given later in this doctiment. See FIG. 5 and FIG. 6 for graphical representations of the LFL and LFR matrix elements plotted three-dimensionally against the ir and cs axes.
  • FIG. 7 shows the sum of the squares of these elements, demonstrating that the above matrix elements do not meet the requirement of constant loudness. In FIG 7, notice that the value is constant at .71 along the axis from unsteered to right. The unsteered to left rises 3dB to the value one.
  • LFL cos(- s) - 0.41*G(Z ) , . ⁇ 4e)
  • LFR si ⁇ (-cs) ... ⁇ 4f)
  • LFL (cos( s) - 0.41*G( r))*boost l(cs) ...(5a)
  • LFR (-sin(c5))*boostl(cs) ...(5b)
  • LFL (cos(- s) ⁇ 0.41*G( r))/boost(cs) ...(5e>
  • the performance of the March 1997 circuit can be improved.
  • the first problem is in the behavior of the steering along the boundaries between left and center, and between right and center. As a strong single signal pans from the left to the center, it can be seen in FIG. 9 that the value of the LFL matrix element increases to a maximum half-way between left and center. This increase in value is an unintended consequence of the deliberate increase in level for the left and right main outputs as a center signal is added to stereo music.
  • LFL and LFR in the front right quadrant are similar, but without the +0.41*G term.
  • These new definitions lead to the matrix element shown graphically in FIG. 10. In FIG 10, notice that the new element has the correct amplitude along the left to center boundary, as well as along the center to right boundary.
  • the steering in the rear quadrant is not optimal either.
  • the above matrix elements are given by:
  • LFL cos_tbl_minus(-cs) + 0.41*G(-cs) ...(8a)
  • LFR sin_tbl_minus(-cs) ...(8b) PATENT Attorney Docket No. 21647.15
  • LFR (sin(t)*cos(4*t)-cos(t)*sin(4*t) ...(9d) See FIG 11 for the plot of these coefficients LFL (solid curve) and LFR PATENT Attorney Docket No. 21647.15
  • LFL cos( s)/(cos(cs)+sin( s)) — front_boundary_tbl(b ; + 0.41*G(t>)
  • FIGs. 12 and 13 for a graphical display of these values.
  • FIG 12 which views the coefficient graph from left rear, note the large correction along the left-rear boundary. This causes the front left output to go to zero when steering goes from left to left rear. The output remains zero as the 1999/012386
  • the loudness in any given output of unsteered material presented to the inputs of the decoder should be constant, regardless of the direction of a steered signal which is present at the same time.
  • this requirement must be altered when there is strong steering in the direction of the output in question. That is. if we are looking at the left front output, the sum of the squares o the matrix elements must increase by 3dB when the steering goes full left.
  • This peak is a result of the dehberate boost of the left and right outputs during half-front steering. Note that in the other quadrants the rms sum is very close to one. as was the design intent. The value in the rear left quadrant is not quite equal to one. as the method used to produce the elements is an approximation, but the match is pretty good.
  • the unsteered (middle) to right axis has the value one
  • the center vertex has the value 0.71
  • the rear vertex has the value 0.5
  • the left vertex has the value 1.41. Note the peak along the middle to center axis.
  • the Dolby elements are similar to our ' 89 patent elements, but without the boost dependent on cs in the rear. This difference is in fact quite important, as after the standard cahbration procedure the elements have quite different values for unsteered signals.
  • our description of the matrix elements does not consider the cahbration procedure for these decoders. We derive all the matrix elements with a relatively arbitrary scaling. In most cases the elements are presented as if they had a maximum value of 1.41. In fact, for technical reasons the matrix elements are all eventually scaled so they have a maximum value of less than one. In addition, when the decoder is finally put to use. the gain of each output to the loudspeaker is adjusted.
  • the 3dB difference in the elements in the forward steered or unsteered condition is not trivial.
  • the elements from the '89 patent have the value 0.71, and the sum of the squares of the elements has the value of one. This is not true of the Dolby rear elements when calibrated.
  • LRL has the unsteered value of one, and the sum of the squares is 2, or 3dB higher than the '89 outputs.
  • the cahbration procedure results in a matrix that does not correspond to a "Dolby Surround" passive matrix when the matrix is unsteered.
  • the Dolby Surround passive matrix specifies that the rear output should have the value of .71*(A m - B ⁇ , and the Pro-Logic matrix does not meet this specification.
  • the sound power in the room will be proportional to L ⁇ + RV 2 + CV 2 . If all three components have roughly equal amplitudes, the power ratio of the center component to the left plus right component will be 1:2. We would like our decoder to reprodtice sound power in the room with approximately the same power ratio as stereo, regardless of the power ratio of C ⁇ to .- J , and R ⁇ . We can express this mathematically. Essentially the equal power ratio requirement will specify the functional form of the center matrix elements along the cs axis, if all the other matrix elements are taken as given.
  • these matrix elements are very similar to the 89 elements, but with the addition of a G(lr) term in LRR, and a GS term in LRL. G(lr) was included to add signals from the B input channel of the decoder to the PATENT Attorney Docket No 21647 15
  • the two representations can be shown to be identical.
  • FIG. 20 notice that in the front left quadrant there is a 3dB dip along the line from the middle to the left vertex, and nearly a 3dB boost in the level along the boundary between left and center
  • the mountain range m the rear quadrant w ill be discussed later the tv matrix correction in VI 11 has been removed, to allow better comparison to the present invention m
  • FIG. 20
  • FIG. 18 shows several problems with the sound power.
  • This dip exists because of the functional shape of G(l ) in LRR is not optimal.
  • the choice of G ⁇ lr) was arbitrary — m the earlier design this function already existed in the decoder and its implementation in analog circuitry is easy.
  • Equations ( 18) and ( 19) result in a messy quadratic equation for GR and GS. which is solved numerically and graphed in FIG. 19.
  • % note that xymin varies from zero to 22.5 degrees.
  • FIG 20 in this application shows the matrix elements ithout the 'tv matri ' correction In this application this correction is handled by the frequency dependent circuitry that follows the matrix and wiU be described later.
  • the sum of the squares is close to one and continuous except for the deliberate rise in level in the rear.
  • the first interpolation fixes discontinuities along the cs — 0 boundary for LRL.
  • the interpolation causes the value to match the value of GS(t>) when cs is zero, and allows the value to rise smoothly to the value given by the previous math as cs increases negatively toward the rear.
  • Ir can vary from 0 to -45 degrees
  • cs can vary fr m 0 to —22.5 degrees.
  • the matrix elements given in the '91 patent achieve this goal. They are essentially the same as the rear matrix elements in the 4 channel decoder, with the addition of the sin(cs)4- cos (cs) correction for the unsteered loudness. When this is done the matrix elements are simple sines and cosines: 1999/012386
  • Rboost(cs> is defined in reference [ 1] and application No. 08/742.460. It is closely equivalent to the function 0.41*G(cs) in the earlier matrix elements, except that rboost(cs) is zero for 0 > cs > —22.5, and varies from zero to 0.41 as cs varies from —22.5 degrees to -45 degrees. Its exact functional shape is determined by the desire to keep the loudness of the rear output constant as sound is panned from left rear to full rear.
  • LRL is computed with interpolation, just as for LRR.
  • LRL (sra(lr) - sri(cs) - rboost( s))
  • LRR -srac(lr) - sric(cs_bounded)
  • the input to the decoder consists of uncorrelated left and right channels to which an unrelated center channel has been added, i.e.
  • FIG. 23 shows the center gain, needed (sohd curve) if the energy of the center component of the input signal is to be preserved in the front three channels as steering increases toward the front.
  • the needed rise in the level of the center channel is quite steep — the rise is man ⁇ ' dB of amplitude per dB of steering value.
  • the gain in a standard decoder (dotted curve). . " - ⁇ 0
  • FIGs. 23 and 24 is derived assuming the matrix elements previously given for LFL and LFR. What if we used different elements? Specifically, do we need to be aggressive about removing the center component from the left and right front outputs 0
  • the function (0.42 + GC(cs)) is plotted in FIG. 25. Note the quick rise from the value 0.42 (4.5dB lower than Dolby surround) , followed by a gentle rise, followed finally by a steep rise to the value 1..
  • PlVR (GP 2 +GF 2 )*(L ⁇ 2 -!-Ri Il 2 ) + (GP-GF) 2 *C ; ⁇ x 2 ... (29a)
  • Power from the center is:
  • RATIO (((GP(cs)-GF(cs)) 2 + 2 * (GC(cs) ⁇ +0.5 * (cos(cs)-sin(cs)) ' -"))
  • RATIO (C ⁇ / I *((GP(cs)-GF(cs)) 2 + 2*(GC(cs) '2 ) + 0.5*(cos(cs) - sin(cs)) '2 ) / (2*(GP(cs) 2 + GC(cs) '2 + GF(cs) '2 ) + 1) ...(31b)
  • the solid curve is the graph of GF needed for constant energy ratios with the new "music" center attenuation GC.
  • the dashed curve is the LFR element of March '97. sin(cs)*corrl.
  • the dotted curve is sin(cs) . the LFR element without the correction term corrl . Note that GF is close to zero until cs reaches 30 degrees, and then increases sharply. We have found in practice it is best to limit the value of cs at about 33 degrees. In practice LFR derived from these curves has a negative sign.
  • gf_diff cancels the value of sin(cs) , reducing the value of the element to zero along the first part of the Ir - 0 axis. See FIG. 27.
  • the value then falls off to match the previous value along the boundary from left to center and from right to center.
  • FIG 29 is a graph of the left front (dotted curve a d center (sohd curve) outputs, note that center steering is at the left of the plot, and full left is at the right.
  • cs the value of cs to about 33 degrees, (about 13 on the axis as labeled) where the center is about
  • the Logic 7 encoder There are two major goals of the Logic 7 encoder. Firstly, it should be able to encode a 5.1 channel tape in a way that allows the encoded version to be decoded by a Logic 7 decoder with minimal subjective change. Secondly, the encoded output should be stereo compatible - that is, it should sound as close as possible to a manual two channel mix of the same material. One factor in this stereo compatibility should be that the output of the encoder, when played on a standard stereo system, should give identical perceived loudness for each sound source in an original 5 channel mix. The apparent position of the sound source in stereo should also be as close as possible to the apparent position in the 5 channel original.
  • the active circuitry corrects several small errors inherent in the design. However even without the active correction the performance is better than the previous encoder.
  • the new encoder handles the left, center, and right signals identically to the previous design and identically to the Dolby encoder, providing the center attenuation function fen is equal to 0.71, or -3dB.
  • the surround channels look more complicated than they are.
  • the functions fcQ and fsQ direct the surround channels either to a path with a 90 degree phase shift relative to the front channels, or to a path with no phase shift.
  • fc In the basic operation of the encoder fc is one, and fs is zero - that is, only the path which uses the 90 degree phase shifts is active.
  • the value crx is typically 0.38. It controls the amount of negative cross feed for each surround channel. As in the previous encoder, when there is only an input to one of the surround channels the A and B outputs have an amplitude ratio of -.38, .91, which results in a steering angle of 22.5 degrees to the rear. ⁇ s usual, the total power in the two output channels is unity - that is the sum of the squares of .91 and .38 is one.
  • a major function of the function fc in the surround channels is to reduce the level of the surround channels in the output mix by 3dB when the surround channels are much softer than the front channels. Circuitry is provided to compare the front and rear levels, and when the rear is less by 3dB, the value of fc is reduced to a maximum of 3dB. The maximum attenuation is reached when the rear channels are 8dB less strong than the front channels.
  • This active circuit appears to work well. It makes the new encoder compatible with the European standard encoder for classical music. The action of the active circuits causes instruments which are intended to be strong in the rear channels to be encoded with full level. PATENT Attorney Docket No. 21647.15
  • Level detecting circuits look at the phase relationship between the center channel and the front left and right. Some popular music recordings that use five channels mix the vocals into all three front channels. When there is a strong signal in all three inputs the encoder output will have excessive vocal povver. since the three front channels will add together in phase. When this occurs, active circuits increase the attenuation in the center channel by 3clB to restore the power balance in the encoder output.
  • FIG. 2 shows a block diagram of the frequency dependent circuits that follow the matrix in a five channel version of the decoder. There are three sections: a variable low pass filter, a variable shelf filter, and a HRTF (Head
  • the HRTF filter changes its characteristics depending on the value of the rear steering voltage c/s.
  • the first two filters change their characteristics in response to a signal that is intended to represent the average direction of the input signals to the decoder during pauses between strongly steered signals. This signal is called the background control signal.
  • One of the major goals of the current decoder is to be able to optimally create a five channel surround signal from an ordinary two channel stereo signal. It is also highly desirable that the decoder should recreate a five channel PATENT Attorney Docket No. 21647.15
  • surround recording that was encoded into two channels by the encoder described a s part of this application. These two apphcations differ in the way the surround channels are perceived. With an ordinary stereo input the majority of the sound needs to be in front of the hstener. The surround speakers should contribute a pleasant sense of envelopment and ambience, but should not draw attention to themselves. An encoded surround recording needs the surround speakers to be stronger and more aggressive.
  • the background control signal is designed to make this mscrirnination.
  • the background control signal (BCS) is similar to and derived from the rear steering signal cs.
  • BCS represents the negative peak value of cs. That is. when cs more negative than BCS, then BCS is made to equal cs. When cs is more positive than BCS the value of BCS slowly decays. However the decay of BCS involves a further calculation.
  • Music of many types consists of a series of strong foreground notes — or in the case of a song, sung words. In between the foreground notes there is a background.
  • the background may consist of other instruments playing other- notes, or it may consist of reverberation.
  • the circuit that derives the BCS signal keeps track of the peak level of the foreground notes. When the current level is ⁇ 7dB less than the peak level of the foreground, the level of cs is measured. The value of cs during these gaps between foreground peaks is used to control the decay of BCS. If the material in the gaps between notes is reverberation, it may tend to have a net rearward bias in a recording that was made by encoding a five channel original. This is because the reverberation on the rear channels of the original will be encoded with a rearward bias.
  • BCS derived in this way tends to reflect the type of recording Any time there is significant rear steered material BCS will always be strongly negative. However BCS can be negative even in the absence of strong steering to the rear if there the reverberation in the recording has a net rearward bias.
  • the second filter is a variable shelf filter.
  • the low frequency section (the pole) of this filter is fixed, at 500Hz.
  • the high frequency section (the zero) varies depending on user adjustment and on BCS.
  • This filter implements the "soundstage" control in the current decoder. In the apphcation No. 08/742,460 'soundstage" is implemented through the matrix elements, using the "tv matrix” correction.
  • the earher decoders based on this work reduced the overall level of the rear channels when the steering was neutral or forward. In the new decoder presented here the matrix elements do not include the "tv matrix” correction.
  • the high frequency section of the shelf filter is set equal to the low frequency section — in other words the shelf has no attenuation, and the filter has flat response.
  • the third filter is controlled by c/s and not by BCS.
  • This filter is designed to emulate the frequency responses of the human head and pinnae when a sound source is approximately 150 degrees in azimuth from the front of the listener. This type of frequency response curve is called a "Head Related Transfer Function ' ' or HRTF. These frequency response functions have been measured for many angles for many different people. In general when a sound source is about 150 degrees from the front there is a strong notch in the frequency response at about 5kHz. A similar notch exists when a sound source PATENT Attorney Docket No. 21647.15
  • notch is at about 8kHz. Sound sources to the side of the listener do not produce these notches.
  • the current standard for five channel sound reproduction recommends that the two rear speakers be placed slightly behind the hstener, at +/— 110 or 120 degrees from the front. This speaker position supplies good envelopment at low frequencies.
  • a sound from the side a hstener does not produce the same level of excitement as a sound that is fully behind a hstener. Very often a film director wants a sound effect to come from behind the hstener. and not from the side.
  • the HRTF filter in the decoder adds the frequency notches of a rear sound source, so that a listener hears the sound as further behind that the actual position of the loudspeaker.
  • the filter is designed to vary with cs. When cs is positive or zero, the filter is maximum. This causes ambient sounds and reverberation to seem to be more behind the hstener. As cs becomes negative the filter is reduced. When cs is approximately —15 degrees the filter is completely removed, and the sound source appears to come fully from the side.
  • FIG 3 shows the frequency dependent circuits in the seven channel version of the decoder. These are shown as consisting of three sections — although in an actual implementation the second two sections can be combined into one circuit.
  • the first two sections are identical to the two sections in the five channel decoder, and perform the same function.
  • the third section is unique to the seven channel decoder.
  • VI.11 and apphcation No. 08/742,460 the side and the rear channels had separate matrix elements. The action of the elements was such that when cs was positive or neutral the side and the rear outputs were identical except for delay. The two outputs stayed identical until cs was more negative than 22 degrees. As the steering moved further to the rear the side outputs were attenuated by 6dB, and the rear outputs were boosted by 2dB. This caused the sound to appear to move from the sides of the hstener to the rear of the listener.
  • the differentiation between the side output and the rear output is achieved by a variable shelf filter in the side output.
  • the third shelf filter in FIG. 3 has no attenuation when cs is forward or zero. When cs becomes more negative than 22 degrees the zero in the shelf filter moves rapidly toward l lOOHz, resulting in an attenuation of the high frequencies of about 7dB.
  • this shelf filter has been described as a separate filter from the shelf filter that provides the "soundstage" function, the action of the two shelf filters can be combined into a single shelf through suitable control circuitry.

Abstract

L'invention concerne un système de reproduction sonore permettant de convertir en signaux destinés à plusieurs canaux de sortie, les signaux stéréo de deux canaux d'entrée (92, 94), le codage et la corrélation de l'une des composantes du signal au moins étant directionnels et le codage de l'une des composantes du signal au moins étant non directionnel et non corrélé. Ledit système comprend un décodeur (90) destiné à renforcer la composante de corrélation des signaux d'entrée dans le sens voulu et à réduire la puissance de tels signaux dans les canaux qui ne sont pas associés au sens de codage. Toutefois, on préserve la séparation entre les canaux de sortie respectivement gauche et droit (172, 176) et l'énergie totale de la composante non corrélée des canaux d'entrée dans chacun des canaux de sortie. Il en résulte que les instruments enregistrés sur le canal d'entrée droit restent du côté droit des canaux de sortie et que les instruments enregistrés sur le canal d'entrée gauche restent du côté gauche, et que la sonie apparente de tous les instruments de tous les canaux de sortie ne change pas quel que soit le sens de la composante des signaux d'entrée à codage directionnel. Le système comprend également des organes de codage destinés à coder cinq canaux d'entrée, si bien que ces derniers feront le codage en respectant les conditions de sens et de niveau dans les décodeurs de l'invention, ainsi que dans les décodeurs conformes aux standards de film courants.
PCT/US1998/018390 1997-09-05 1998-09-03 Systeme de codage et de decodage a matrice 5-2-5 WO1999012386A1 (fr)

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CA002301547A CA2301547C (fr) 1997-09-05 1998-09-03 Systeme de codage et de decodage a matrice 5-2-5
AU93026/98A AU750877C (en) 1997-09-05 1998-09-03 5-2-5 matrix encoder and decoder system
JP2000509252A JP2004507904A (ja) 1997-09-05 1998-09-03 ５−２−５マトリックス・エンコーダおよびデコーダ・システム
EP98945881A EP1013140B1 (fr) 1997-09-05 1998-09-03 Systeme de decodage a matrice 5-2-5
BR9812432-3A BR9812432A (pt) 1997-09-05 1998-09-03 Sistema codificador e decodificador de matriz 5-2-5

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US60/058,169 1997-09-05
US09/146,442 US6697491B1 (en) 1996-07-19 1998-09-03 5-2-5 matrix encoder and decoder system

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