EP0571455B1 - Systeme de reproduction sonore - Google Patents

Systeme de reproduction sonore Download PDF

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Publication number
EP0571455B1
EP0571455B1 EP92904564A EP92904564A EP0571455B1 EP 0571455 B1 EP0571455 B1 EP 0571455B1 EP 92904564 A EP92904564 A EP 92904564A EP 92904564 A EP92904564 A EP 92904564A EP 0571455 B1 EP0571455 B1 EP 0571455B1
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Prior art keywords
matrix
signals
reproduction
loudspeakers
decoder
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German (de)
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EP0571455A1 (fr
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Michael Anthony Gerzon
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Trifield Productions Ltd
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Trifield Productions Ltd
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    • 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 
    • H04S2420/00—Techniques used stereophonic systems covered by H04S but not provided for in its groups
    • H04S2420/11—Application of ambisonics in stereophonic audio systems
    • 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

  • This invention relates to the reproduction and transmission of sound using more than two loudspeakers.
  • the reproduction of stereophonic sound using two loudspeakers has long been known to give an imperfect illusion of phantom illusory sound images lying between the locations of the two loudspeakers.
  • the high frequencies of phantom illusory images are displaced further from the point midway between the two loudspeakers than are the low frequencies, resulting in imperfect image sharpness.
  • the illusory sound images are all displaced towards the nearer of the two loudspeakers.
  • the illusory images also rotate in position to a lesser extent.
  • loudspeakers distributed across the stereophonic sound stage.
  • These loudspeakers can either be fed with independent transmission channel signals, one for each loudspeaker system, conveying an improved stereophonic illusion, or they can be fed with signals derived from a smaller number of transmission channel signals using a mixing or matrixing process.
  • This invention relates to the use of an improved matrixing process to obtain improved illusory phantom images.
  • signals L and R are normally used to feed the respective left and right loudspeakers of a two-loudspeaker stereophonic system, then these can be supplemented by an additional central loudspeaker fed with the signal 1 ⁇ 2k(L+R), where k is a predetermined amplitude gain.
  • k is a predetermined amplitude gain.
  • the total reproduced energy at a moment fed into the listening room is proportional to the sum of the squares of the outputs of the loudspeakers in the room, and the total energy L 2 +R 2 of the two-speaker stereophonic signal described in the above example does not equal, and is not proportional to, the total reproduced energy L 2 + R 2 + [1 ⁇ 2k(L+R)] 2 emitted by the three loudspeakers of the bridged centre loudspeaker method.
  • This proposal which has been termed the Hughes SRS method, gives very stable reproduction of monophonic central images, which are only emitted by the centre loudspeaker, and also gives a reasonable impression of left/right directionality for a listener positioned at an ideal stereo seat position by means of the process known as acoustic matrixing, whereby the sounds travelling from different loudspeakers to the two ears reinforce and cancel each other in such a manner as to recreate interaural phase relationships characteristic of left/right positioning.
  • the Hughes SRS method has the defect that all sounds are reproduced with equal energy from both the left and the right loudspeakers, so that any illusion of directionality is created purely by phase relationships between the loudspeaker outputs. Under these conditions, acoustic matrixing creates an impression of left/right directionality only over a narrow listening area, and even at an ideal stereo seat position gives an illusion that gives poor reproduced width at higher frequencies, especially those above about 2 kHz.
  • acoustical pressure which is a scalar quantity
  • the acoustic velocity which is a vector quantity with direction
  • the acoustic energy which is a scalar quantity
  • the sound intensity which is a vector quantity describing the direction and magnitude of energy flow of the sound field.
  • the ratio of acoustic velocity to acoustic pressure provides a vector quantity that can be used, over any limited frequency band below a frequency of about 700 Hz, to predict the localisation of sounds according to theories of sound localisation based on interaural phase cues.
  • the ratio of sound intensity to acoustical energy can similarly be used to predict the localisation of sounds at higher frequencies, typically between 700 Hz and 5 kHz, but can also be used to predict localisation at lower frequencies when the sounds arriving from different loudspeakers are largely uncorrelated in phase, as is the case when loudspeakers are at different distances from the listener with path length differences of a number of wavelengths.
  • velocity vector localisation theories Sound localisation theories based on the ratio of acoustic velocity to acoustic pressure are termed velocity vector localisation theories, whereas those based on the ratio of sound intensity to acoustic energy are termed energy vector theories.
  • energy vector theories Sound localisation theories based on the ratio of acoustic velocity to acoustic pressure are termed energy vector theories.
  • All prior art methods of reproduction of a first plurality of signals intended for stereophonic reproduction via a first stereophonic arrangement of loudspeakers via a second larger plurality of loudspeakers suffer from one or more defects, which include an alteration of the recorded level-balance between sounds in a stereophonic recording, angular differences between the vector directions of acoustical velocity and of sound intensity, and an inadequate width of reproduction of the stereophonic sound stage.
  • matrix methods are not only used to feed a first plurality of loudspeaker feed signals into a second larger plurality of loudspeakers, but are also used to provide third pluralities of transmission channel signals, intended for use in storage, transmission or recording of the stereophonic effect, and for providing from such third pluralities of transmission channel signals loudspeaker feed signals intended for reproduction via a second plurality of loudspeakers.
  • the process of deriving the third plurality of transmission signals from the first plurality of loudspeaker feed signals is generally termed encoding
  • the process of deriving the second plurality of loudspeaker feed signals from the third plurality of transmission channel signals is generally termed decoding.
  • Such systems of matrix encoding and decoding have been widely used in connection with prior art quadraphonic, surround-sound and ambisonic systems.
  • Some such systems are hierarchical in the sense that they allow for a number of different possible values for the first plurality, a number of different values for the second plurality, and a number of different values for the third plurality, while ensuring the following desirable properties:
  • This kind of hierarchical system of encoding and decoding is operationally desirable in that the procedure for handling a plurality of loudspeaker feed signals does not depend on whether it was originated originally for another number of loudspeakers, nor on whether it has been passed through intermediate stages of encoding and decoding. It will be appreciated that there are various proposals for stereophonic sound using different numbers of loudspeaker feed signals, including possible pluralities two, three, four or five for covering a frontal stereophonic sector of directions.
  • different pluralities of loudspeaker feed signals may be operationally convenient or customary.
  • most sound broadcasting and recordings made for record or Compact Disc release have been prepared in a two-speaker format, although some recordings in the 1950's were prepared in a three-speaker format.
  • Many recordings made for standard Television use similarly use the two-speaker format, but many cinema soundtracks have been recorded in three or five-speaker formats for the front-stage stereophonic sound.
  • high definition Television HDTV
  • a hierarchical system of encoding and decoding stereophony would greatly ease the task of converting signals intended for one plurality of stereophonic loudspeakers for reproduction via another, and would allow each recording or broadcasting organisation to make their own choice of plurality while being able to make use of stereophonic material made by other organisations using a different plurality. Similarly, the final listener will also have the choice of which plurality of loudspeakers he or she uses.
  • the UMX system of surround-sound reproduction is a known prior-art hierarchical system, but is not optimised for frontal-stage stereophony.
  • the problem of designing an effective hierarchical system of stereophony has not hitherto been solved. This is because in the case of surround sound, one can make use of the rotational symmetry of the desired sound stage, whereas stereophony has a much lesser degree of mathematical symmetry, which makes the problem of finding hierarchical systems much harder to solve, especially if one takes the subjective quality of directional results into account, i.e. the requirement (iv) listed above in the requirements of a hierarchical system.
  • stereophonic loudspeaker arrangements do have at least an approximate left/right symmetry, i.e. for each speaker placed to the left of a forward direction, there is a second loudspeaker placed in a symmetrically disposed position to the right of the forward direction, and vice-versa. While in practice there are often departures from exact left/right symmetry, it is customary to design loudspeaker feed signals on the assumption of an exact such symmetry in the loudspeaker layout. It is found that with normal small departures from symmetry, the subjective results remain reasonably satisfactory.
  • references to “front”, “forward”, “left” and “right” directions in this document are purely a matter of convenience, and that the "front” or “forward” direction may in fact be any chosen convenient direction in space, and the “left” and “right” directions may be any chosen opposite directions orthogonal to that direction designated as “front” or”forward".
  • the matrix converter may, for example form part of a transmission encoder, or a reproduction decoder as later described. It may be implemented by software in an appropriate digital signal processor of the type well known in the art, or by a hard-wired network in the analogue domain.
  • the matrix converter is further such as substantially to preserve, to within a second constant of proportionality, the reproduced angular disposition of velocity vectors and is further such as substantially to preserve, to within a third constant of proportionality, the reproduced angular disposition of sound intensity vectors.
  • said matrix reproduction decoding means is preferably also left/right symmetrical, in the sense that if all left inputs and outputs were to be exchanged with their right counterparts, the results given by the matrix reproduction decoding means would remain substantially unchanged.
  • the angular dispositions of the reproduced velocity vectors at frequencies across several octaves of the audio frequency range is arranged to be substantially identical to the angular dispositions of the sound intensity vectors when said matrix means provides signals to be reproduced via said second stereophonic arrangement.
  • said third constant of proportionality is arranged to be greater within an audio frequency band above 5 kHz than within the audio band at frequencies between 700 Hz and 3 kHz. Said increased third constant of proportionality above 5 kHz is especially desirable when said first plurality equals two.
  • the ratio of said second constant of proportionality to said third constant of proportionality should lie within the range from one half to two.
  • a matrix converter for converting a first ambisonically encoded audio signal having components W, X and Y or linear combinations thereof into a second, stereophonically encoded signal for reproduction over n 2 loudspeakers, wherein n 2 is an integer ⁇ 3, the conversion matrix of the converter comprises a n 2 x 2 matrix converter for converting said first audio signal, and having an energy preserving matrix arranged substantially to preserve to within an overall constant of proportionality the total reproduced energy and the reproduced directional effect of the encoded audio signal, said matrix converter being arranged to receive at one input a first signal M dec formed from the sum of the omnidirectional component W and a first velocity component X and at the other input a signal S dec formed from the other velocity component Y and means for outputting a further signal component derived from the difference T dec of the said components W and X, the matrix converter being frequency dependent.
  • This aspect encompasses both the case where the sum and difference components are explicitly present and also a matrix arranged to carry out equivalent operations on pseudo-left/right signal.
  • the matrices referred may be split to form a functionally equivalent series of matrices or may be coalesced into a single equivalent matrix and it will be understood that all such arrangements fall within the scope of the invention.
  • the invention also provides the combination of a matrix converter of the first aspect for converting a first audio signal stereophonically encoded for reproduction over 2 speakers into a second audio signal for reproduction over 3 speakers and a second matrix for converting a third audio signal encoded for reproduction over 3 speakers into a fourth audio signal for reproduction over n 2 speakers, the second matrix being a frequency independent matrix.
  • said transmission channel signals are such that for each first plurality not greater than said third plurality, precisely a said first plurality of transmission channel signals may be substantially nonzero, and such that for any first said first plurality less than a second said first plurality.
  • the transmission channel inputs to said transmission matrix decoding means for which said transmission matrix channel signals are substantially nonzero for said first said first plurality is a subset of the transmission channel inputs for which the transmission channel signals are substantially nonzero for said second said first plurality.
  • a transmission matrix encoder means responsive to a plurality greater than two of signals representing loudspeaker feed signals intended to feed a said plurality of loudspeakers disposed in a stereophonic arrangement across a sector of directions producing a said plurality of transmission channel signals suitable for use with a signal transmission, recording or storage means, whereby the inverse of said transmission matrix encoder means constitutes a transmission matrix decoder means according to the invention in its third aspect.
  • the inverse transmission matrix decoder means according to the invention in its third aspect is in accordance with the preferred implementation of the invention in its third aspect, and the additional transmission matrix encoder means required to produce a smaller said first plurality greater than two of transmission channel signals that are substantially nonzero representing loudspeaker feed signals intended for reproduction via a said smaller said first plurality of loudspeakers is also a transmission matrix encoder means according to the invention in its fourth aspect.
  • This preferred form of the invention in its fourth aspect ensures that the different third pluralities of transmission channel signals provided in response to the different first pluralities of loudspeaker feed signals by encoding means, and the associated second pluralities of decoded loudspeaker feed signals derived from the different third pluralities of transmission channel signals derived by the inverse decoders constitutes a hierarchical system of encoding and decoding in the earlier-defined sense.
  • a matrix system for encoding a first plurality of signals representing loudspeaker feed signals intended for reproduction via a said first plurality of loudspeakers disposed in a first stereophonic arrangement across a first sector of directions into a third plurality of transmission channel signals and for decoding said third plurality of transmission channel signals into a second plurality of output signals representing loudspeaker feed signals intended for reproduction via a said second plurality of loudspeakers disposed in a second stereophonic arrangement across a second sector of directions, such that said transmission matrix encoding means used in conjunction with said transmission matrix decoding means constitutes a reproduction matrix decoding means in accordance with the invention in its first or second aspects.
  • a transmission matrix decoding means responsive to a third plurality of transmission channel signals and providing a second plurality of output signals representing loudspeaker feed signals intended for reproduction via a said second plurality of loudspeakers disposed in a second stereophonic arrangement across a second sector of directions intended for use with transmission channel signals provided via a transmission matrix encoding means, such that the resulting system constitutes a matrix encoding and decoding system in accordance with the invention in its fifth aspect.
  • a transmission matrix encoding means responsive to one or more first pluralities of signals representing loudspeaker feed signals intended for reproduction via a said first plurality of loudspeakers disposed in a first stereophonic arrangement across a first sector of directions and providing a third plurality greater than two and not less than said said first plurality of transmission channel signals intended for use with a transmission matrix decoding means such that the resulting system constitutes a matrix encoding and decoding system in accordance with the invention in its fifth aspect.
  • matrix decoding means according to the invention in its first, second, third or sixth aspects intended for use with loudspeakers (or loudspeaker systems) some of which have a more limited bass reproduction capability than the other loudspeakers, whereby said matrix decoding means is modified at low frequencies so as to provide less bass to said loudspeakers or loudspeaker systems which have a more limited bass reproduction capability than to said other loudspeakers.
  • a matrix decoding means according to the invention in its first, second, third, sixth or eighth aspects, also incorporating or used in association with delay compensation means for output signals intended for feeding to reproduction loudspeakers not all disposed at identical distances from a preferred listening position, whereby said delay compensation means ensures that signals from all loudspeakers arrive at said listening position at a substantially identical time.
  • the intended stereophonic arrangement of the reproduction loudspeakers is substantially left/right symmetric and said preferred listening position is disposed on the axis of left/right symmetry.
  • transmission encoding means for encoding a first plurality of signals representing loudspeaker feed signals intended for reproduction via a said first plurality of loudspeakers disposed in a stereophonic arrangement across a sector of directions into a larger third plurality of transmission channel signals, said encoding means providing results equivalent to a reproduction matrix decoding means according to the invention in its first, second, third, or sixth aspects responsive to said first plurality of signals providing a fourth plurality, not greater than said third plurality and larger than said first plurality, of signals representing loudspeaker feed signals intended for reproduction via a said fourth plurality of loudspeakers disposed in a stereophonic arrangement across a fourth sector of directions, followed by an encoding means according to the invention in its fourth or seventh aspects responsive to said fourth plurality of signals and providing said third plurality of transmission channel signals.
  • reproduction matrix decoder means responsive to a first plurality of signals proportional to signals intended for reproduction via a said first plurality of loudspeakers disposed in a first left/right symmetric stereophonic arrangement across a first sector of directions and providing a second greater plurality of signals proportional to signals intended for reproduction via a said second plurality of loudspeakers disposed in a second left/right symmetric stereophonic arrangement across a second sector of directions
  • said matrix decoder means comprising an input sum and difference matrix means for each pair of signals intended for a left/right symmetrically disposed pair of loudspeakers in said first arrangement, a first linear or matrix means responsive to all said sum signals and to any of said first plurality of signals proportional to a central loudspeaker feed signal for said first arrangement providing a first number not less than the number of signals into said first linear or matrix means of first output signals, a second linear or matrix means responsive to all said difference signals providing a second number not less than the number of said difference signals of
  • FIG. 1a shows a typical monophonic loudspeaker C 1 in front of a listener (4), such as might be used for monophonic reproduction of a stereophonic signal.
  • Figure 1b shows a typical two-speaker arrangement with respective left and right loudspeakers L 2 and R 2 .
  • Figure 1c shows a typical three-speaker arrangement with respective left, centre and right loudpeakers L 3 , C 3 and R 3 .
  • Figure 1d shows a typical four-speaker arrangement with respective loudspeakers L 4 , L 5 , R 5 and R 4 from left to right in front of the listener (4).
  • Figure 1e shows a typical five-speaker arrangement with respective loudspeakers L 6 , L 7 , C 5 , R 7 , and R 6 from left to right in front of the listener (4).
  • C p is used to indicate a central loudspeaker in a (notional) frontal direction (5) with respect to an ideally situated listener (4)
  • L p is used to indicate a loudspeaker placed in a direction at an angle ⁇ p towards the (notional) left (6) of due front (5) at the listener (4), measured in an anticlockwise direction
  • R p is used to indicate a loudspeaker placed symmetrically to the right in a direction at an angle ⁇ p to the (notional) right of due front (5).
  • all loudspeakers are placed at equal distances from the ideal listener position (4) and face towards the position of the listener (4).
  • figure 1f shows an alternative preferred three-speaker arrangement with respective left, centre and right loudspeakers L 3 , C 3 and R 3 in which the three loudspeakers are at an equal distance from the listener (4), but where the two outer loudspeakers are angled in such that their axes (10) cross in front of the listener (4) as shown.
  • Figure 1g shows another alternative three-speaker arrangement in which the outer loudspeakers L 3 and R 3 are angled in as before, but where the centre loudspeaker C 3 lies at the centre of a line joining L 3 and R 3 , and so is closer to the listener (4).
  • the angles ⁇ p subtended by the loudspeakers may be chosen across a broad range of values according to convenience or the desired stage width of stereophonic presentation. However, it is generally found that if the angle subtended at the listener (4) between adjacent loudspeakers is too large, then the quality of phantom illusory images becomes poor. There is no sharp delineation between angular widths that give a totally satisfactory and a totally unsatisfactory image quality, but as an indication, it is found that for two-speaker stereo, ⁇ 2 greater than 35° (giving a total angular width of the reproduced sector (3) of directions of more than 70°) gives poor image quality.
  • ⁇ 3 is not more than 45° (giving a reproduced sector (3) angle of 90°); whereas wider stage widths covering sectors (3) of 120° or more can be used with four or more loudspeakers with satisfactory results.
  • the sector (3) of reproduced directions using four or more loudspeakers will not exceed 180°, although in some cases a slightly larger angular coverage, for example 210° or 225°, may be used.
  • the included angle to the rear of the listener (4) between the outermost loudspeakers is so large that stable imaging to the rear of the listener is not possible.
  • the invention is applicable only to stereophonic arrangements covering a sector (3) of directions not including stable imaging of excluded angular positions, and is not applicable to loudspeaker arrangements capable of covering a 360° surround-sound stage.
  • stereophonic signals capable of producing a desired directional illusion across the available sector (3) of directions via any specific stereophonic loudspeaker arrangement, such as those illustrated in figures 1b to 1f, can be created, recorded, stored or transmitted.
  • An object of this invention is to substantially retain or improve this desired stereophonic effect via an arrangement with a larger number of loudspeakers, such as one of those shown in figures 1c to 1e.
  • FIG 2 The general method of doing this according to the invention is illustrated in figure 2, whereby an original first plurality (20) n 1 of signals from a stereophonic signal source (1), which may for example be a stereophonic microphone arrangement, the outputs from a mixing desk, the outputs from a tape or disc reproducer, a broadcast receiver or a telecommunications link, said signals representing loudspeaker feed signals suitable for a first stereophonic arrangement are fed into a reproduction matrix decoding means (2) to produce a second greater plurality (40) n 2 of signals representingloudspeaker feed signals suitable for a second stereophonic arrangement (50).
  • a stereophonic signal source (1) which may for example be a stereophonic microphone arrangement
  • the outputs from a mixing desk the outputs from a tape or disc reproducer, a broadcast receiver or a telecommunications link
  • said signals representing loudspeaker feed signals suitable for a first stereophonic arrangement are fed into a reproduction matrix decoding means (2) to produce a second greater plurality (40) n
  • this second plurality of signals is shown as being fed into loudspeakers (50) direct from the matrix means (2), it will be understood that generally such feeds to loudspeakers may involve necessary or desirable intermediate stages evident to those skilled in the art, such as amplification and gain adjustment stages, overall volume and tone control adjustments, equalisers for loudspeaker and room characteristics, time delays for adjusting the time of arrivals at a listener from individual loudspeakers, connecting means such as cables or infra-red links, and the like.
  • the n 2 ⁇ n 1 reproduction matrix decoding means (2) causes each of the n 2 output signals to be linear combinations of the n 1 input signals (20).
  • the n 2 ⁇ n 1 coefficients of these linear combinations are referred to as "matrix coefficients". These linear combinations may be independent of frequency, or may alternatively be frequency-dependent. If the linear combinations are frequency-dependent, then the matrix coefficients will be complex gains that are a function of frequency. In preferred forms of the invention in the case when the matrix coefficients are frequency-dependent, the matrix coefficients will be approximately real and frequency-independent across two or three relatively broad audio frequency bands, and will vary significantly only in the transition frequency regions between these frequency bands.
  • MS sum-and-difference
  • a matrix means implementing this above-described MS or sum-and-difference process will be termed an "MS matrix" means.
  • Sum-and-difference techniques have been used in the stereo art since UK patent 394,325 in 1931, and are widely knovn, for example, in connection with the MS stereo microphone technique and the Zenith/GE system of FM stereo multiplex broadcasting.
  • signals of the form M p or C p as “sum” signals and of the form S p as “difference” signals. It is often convenient to represent two-speaker stereophonic signals L 2 and R 2 in MS form as M 2 and S 2 ; to represent three-speaker signals L 3 , C 3 and R 3 in MS form as M 3 , C 3 and S 3 ; to represent four-speaker signals L 4 , L 5 , R 5 and R 4 in MS form by M 4 , M 5 , S 4 and S 5 ; and to represent five-speaker signals L 6 , L 7 , C 5 , R 7 and R 6 in MS form by M 6 , M 7 , C 5 , S 6 and S 7 .
  • reproduction matrix decoding means (2) It is sometimes convenient to describe the reproduction matrix decoding means (2) in terms of what it does to signals in MS form. By using the above MS matrix equations, such a description is easily converted into one describing the action of matrix means (2) on signals in left/right form.
  • the invention is applicable to matrix means (2) that accept signals (20) in either or both left/right or MS forms, and that produce signals (40) in either or both left/right or MS forms. If outputs (40) are produced in MS form, it will be understood that the connection of the output signals (40) to the reproduction loudspeakers (50) will involve a necessary further MS matrix stage.
  • the reproduction matrix decoding means (2) should substantially preserve the total energy of the input signals (20) fed to the intended first stereophonic arrangement when the matrix means (2) output signals (40) are reproduced via the second stereophonic arrangement (50) of loudspeakers.
  • the matrix means (2) output signals (40) are reproduced via the second stereophonic arrangement (50) of loudspeakers.
  • all loudspeakers have identical characteristics and a flat frequency response, so that the signals fed to the loudspeakers are identical, to within a constant of gain, to the signals emitted by the loudspeakers into the room.
  • a reproduction matrix decoding means (2) should be substantially energy preserving, although it will be understood that overall adjustments of gain or tonal quality affecting all component stereo signals equally are permissable for reasons of convenience or desired effect. It is preferred that the gain variations at any frequency produced by use of the reproduction matrix decoder means (2) between different components of the stereo signal should not exceed about 3 dB, and it is desirable that such variations of gain should be less than 2 dB, and ideally be less than 1 dB for high quality results.
  • R 3 (1 ⁇ 2sin ⁇ )(L 2 + R 2 ) - 1 ⁇ 2w(L 2 -R 2 )
  • C 3 (2 -1 ⁇ 2 cos ⁇ )(L 2 + R 2 ) from the input signals (20) L 2 and R 2
  • Figure 4 shows a schematic of a 3 ⁇ 2 reproduction matrix decoding means (2) satisfying the above 3 ⁇ 2 matrix means decoding means equations.
  • An initial MS matrix means (31) receives the input signals L 2 (21) and R 2 (22) to produce signals M 2 and S 2 ; the difference signal S 2 is given an optional width gain adjustment (32) w to provide any desired adjustment of reproduced stage width, producing a signal S 3 ; the sum signal M 2 is passed into a network (33) such as a constant-power pair of gain adjustments or a sine/cosine potentiometer or gain adjustment producing two outputs with respective gains cos ⁇ and sin ⁇ whose squares add up to one.
  • a network such as a constant-power pair of gain adjustments or a sine/cosine potentiometer or gain adjustment producing two outputs with respective gains cos ⁇ and sin ⁇ whose squares add up to one.
  • This network (33) may consist of a fixed pair of gain adjustment stages or a fixed resistor network for a fixed value of the parameter ⁇ , or can comprise an adjustable network giving constant power output.
  • the signal C 3 with gain cos ⁇ can be used as the centre loudspeaker feed signal (42), and the signal M 3 with gain sin ⁇ is fed with S 3 to a second MS matrix means (39) to provide signals L 3 (41) and R 3 (43) suitable for feeding the outer loudspeakers of a three-speaker stereophonic arrangement (50) such as shown in figures 3, 1c, 1f or 1g.
  • values of ⁇ within the range 15° to 75° give a generally satisfactory quality of reproduction at frequencies below around 700 Hz, and that values of ⁇ around 35° give a reasonable stability of images and width at frequencies up to around 4 kHz.
  • ⁇ typically around 55°.
  • the stability of central images is determined mainly by frequencies between around 300 Hz and 5 kHz, whereas the frequencies above 5 kHz are important for creating a sense of wide stage width.
  • FIG. 5 shows one realisation of a frequency-dependent matrix version of the invention.
  • MS versions M 2 and S 2 of the input signals L 2 (21) and R 2 (22) are produced by an MS matrix means (31), and the difference signal S 2 is passed via a direct connection (37) through an optional width gain adjustment (32) as before.
  • the sum signal M 2 is passed through a bandsplit filter (34) that divides the signal into two sets of frequency components; typically this may consist of a low-pass filter (34a) and a high-pass filter (34b) whose outputs sum to their input M 2 .
  • these filters may be complementary first-order or RC filters with a cross-over frequency at around 5 or 6 kHz, although a sharper transition rate can be achieved by using second-order or higher-order filters.
  • the high-pass signal component of M 2 from bandsplit means (34b) is fed to a constant-power gain adjustment means (33b) to produce gains cos ⁇ H and sin ⁇ H as shown, where ⁇ H is the desired high-frequency value (typically around 55°) of the angle parameter ⁇ , and the low-pass signal component from the bandsplit filter means (34a) is fed to another constant-power gain adjustment means (33a) to produce gains cos ⁇ M and sin ⁇ M as shown, where ⁇ M is the desired mid and low frequency value (typically around 35°) of the parameter ⁇ .
  • the sin ⁇ outputs of these gain adjustment means (33) are fed to summing means (36) to produce a signal M 3
  • the cos ⁇ outputs of these gain adjustment means (33) are fed to another summing means (35) to produce a signal C 3
  • the signals M 3 and S 3 are fed to a second MS matrix means (39) to produce left anu right signals L 3 and R 3 .
  • the three signals L 3 (41), C 3 (42) and R 3 (43) are loudspeaker feed signals suitable for use with the three-speaker arrangements of figures 3, 1c, 1f or 1g.
  • bandplitting filters (34) may be implemented subsequently to the constant-power gain adjustment stages (33) rather than preceeding them.
  • Bandsplitting filters (34) may be used whose outputs substantially sum to an all-pass response rather than to their input signal, in which case a parallel all-pass filter (37) with a substantially identical all-pass characteristic should be placed in series with the S 2 signal path, for example as shown in figure 5, in order that the phase relationships between the parallel signal paths remain substantially unaffected.
  • a particular desirable implementation uses filters (34a), (34b) and (37) that have identical phase characteristics in order that all interpath phase differences be eliminated. This may be achieved for example by using a first order all-pass network (37) with low-pass means (34a) comprising two cascaded first-order low-pass stages, and high-pass means (34b) comprising two cascaded first-order high-pass stages with a polarity inversion, all stages and filters having identical time constants.
  • the frequency-dependency of the invention may be extended to the case where the bandsplit network (34) comprises filter means giving three or more outputs that substantially sum to its input or to an all-pass response, which feed a corresponding number three or more of constant-power gain adjustment stages (33) whose sine gain outputs are fed to summing means (36) to produce a signal M 3 and whose cosine gain outputs are fed to another summing means (35) to produce a signal C 3 .
  • Such a version of the invention may be used to choose one value ⁇ L of the parameter ⁇ at low frequencies below say around 200 Hz, a second value ⁇ M of ⁇ between around 200 Hz and around 5 kHz, and a third high frequency value ⁇ H of ⁇ above around 5 kHz.
  • the value of ⁇ L may be adjusted in the range 0° to 90° to achieve a satisfactory result taking into account the performance of the three loudspeakers at bass frequencies.
  • very small loudspeakers have poor bass response extension, and for reasons of convenience, cost, space, appearance or physical size, it may be desired to use only one or two out of the three loudspeakers shown in figure 3 with an extended bass response. If the centre loudspeaker C 3 has a poorer bass response than L 3 or R 3 , then a value of ⁇ L near 90° may be used to minimise bass fed to the centre loudspeaker. If instead only the centre loudspeaker has an extended bass response, a value ⁇ L near 0° will minimise bass signals to the other two loudspeakers. Similarly, a system using three small loudspeakers plus a single "superwoofer" for bass frequencies will work best if used with ⁇ L near 0° and if the super-woofer feed is derived from the C 3 signal.
  • ⁇ L ⁇ M as shown in figure 5, or may alternatively use a value ⁇ L near 54.74°.
  • the latter value has the advantage that central bass sounds, which are typically the most powerful bass sounds in most stereo programmes, are reproduced with identical energy from all three loudspeakers, which maximises the bass power handling capacity of the loudspeaker arrangement and maximises subjective bass response.
  • phase-adjustment means at the outputs (41), (42) and (43) of the 3 ⁇ 2 reproduction matrix decoding means in order to compensate for phase response differences between the three loudspeakers.
  • the low-pass filter means (34a) in figure 5 may be replaced by a bandpass means for frequencies between around 200 Hz and 5 kHz, and the high-pass filter means (34b) may be replaced by a complementary bandstop filter means.
  • any of the above 3 ⁇ 2 matrix decoders according to the invention it is possible to make the gain w frequency-dependent if desired. This may be particularly advantageous at frequencies below 600 Hz, where an increased width, say by a factor 1.4, at lower frequencies is sometimes found to enhance the quality of spaciousness of arecording.
  • This system encodes two (21b, 22b) or three (21c, 22c, 23c) signals from a respective two- or three-speaker stereo source (1b or 1c respectively) via transmission matrix encoding means (7 or 7b) to produce transmission channel signals (60), which are transmitted via transmission channels (8) which may for example consist of wire, broadcast or telecommunications channels, tape or disc recording and playback channels, digital storage channels or the like, and which are then decoded using transmission matrix decoding means (9 or 9b) to produce two-speaker signals (41b and 42b) or three-speaker feed signals (41c, 42c and 43c).
  • Figure 6 shows a 3 ⁇ 3 transmission matrix encoding means (7) receiving three-speaker feed signals L 3 , C 3 and R 3 and producing transmission channel signals L, R and T transmitted via transmission means (8) and 3 ⁇ 3 transmission matrix decoding means (9) producing reconstructed three-speaker feed signals L 3 , C 3 and R 3 .
  • the 3 ⁇ 3 transmission decoding means (9) it is required according to the invention in its third aspect that the resulting 3 ⁇ 2 reproduction matrix decoding means (2) should be a 3 ⁇ 2 decoder according to the 3 ⁇ 2 matrix decoding equations described above. It is also required according to the invention that the 3 ⁇ 3 transmission matrix decoding means (9) should be inverse to the 3 ⁇ 3 transmission matrix encoding means (7), so that three-speaker feed signals are recovered substantially unaltered after 3-channel transmission.
  • the 3 ⁇ 3 transmission decoder acts as a 3 ⁇ 2 reproduction matrix decoder means (2) of the form described earlier, for example with reference to figures 3 and 4.
  • Figure 7 shows the schematic of the hierarchical transmission system when MS transmission channel signals are used.
  • a three-speaker stereophonic reproduction apparatus will receive the originally intended three-speaker effect for three-speaker transmitted signals, and will receive a transmitted two-speaker stereo signal in a manner decoded according to the 3 ⁇ 2 reproduction matrix decoder of the invention.
  • This allows material originated from two- and three-speaker stereophonic sources to be mixed together freely in programme creation, such as is shown via the adder means (70) in figures 6 and 7, without any need for listeners to change decoding apparatus (9).
  • a two-speaker stereo listener receiving just the two-channel signals L and R or M and S from material originating from three-speaker stereo sources (1c) will obtain a satisfactory two-speaker presentation for earlier-described preferred values of the parameters ⁇ ', ⁇ " and w'.
  • Central images remain central, and provided that, as is preferred, w' is less than cos ( ⁇ '- ⁇ ") / sin ⁇ ", extreme left and right source images are reproduced at positions marginally wider than the extreme left and right positions of the two-speaker stereo stage.
  • a disadvantage of using a fixed predetermined value of the angle parameter ⁇ ' for the above 3 ⁇ 3 transmission encoding and decoding equations is that the decoding of two channels via three loudspeakers does not have an optimum frequency-dependent form. While it is possible to use frequency-dependent encoding parameters, this has two disadvantages: (i) that the two-channel transmitted signal L and R is frequency-dependent and so not of optimum compatibility with two-speaker reproduction, and (ii) a standardisation of the frequency-dependence does not allow of any future modification that may improve subjective results further.
  • the transmission decoding matrix may be switched or adjustable to provide a decoder with a frequency-dependent value of the decoder parameter ⁇ .
  • the two-speaker stereo signals L 2 and R 2 may first converted to three-speaker form by means of a 3 ⁇ 2 matrix reproduction decoding means such as shown in figure 5, and then fed into the 3 ⁇ 3 encoding matrix (7) to produce three transmission signals.
  • the decoded signals L 3 , C 3 and R 3 obtained after transmission matrix decoding (9) will be the same as if a frequency-dependent matrix reproduction decoder such as that of figure 5 had been used by the final listener.
  • L 1 ⁇ 2(cos( ⁇ - ⁇ '))(L 2 + R 2 ) + 1 ⁇ 2(L 2 - R 2 )
  • R 1 ⁇ 2(cos( ⁇ - ⁇ '))(L 2 + R 2 ) - 1 ⁇ 2(L 2 - R 2 )
  • T (2 -1 ⁇ 2 sin( ⁇ - ⁇ '))(L 2 + R 2 ) ; so that if
  • input stereo signals L 2 and R 2 are passed into an MS matrix (31) and the difference signal S 2 is (optionally) passed through an optional width gain control (32) to provide an (optionally) modified difference signal S (62).
  • the sum signal M 2 from the MS matrix (31) is used to provide a signal M (61) and also passed to the filter means (38) discussed above to provide a third signal T (63).
  • the three signals M, S, T are three-channel transmission signals in MS form which may be used to feed a transmission system in accordance with the invention with signals derived from a two-speaker stereo source when psycho-acoustic frequency-dependence and (optional) width control is desired.
  • figure 8 constitutes an alternative frequency-dependent 3 ⁇ 2 reproduction matrix decoding means to that shown in figure 5 according to the invention.
  • the means shown in figure 8 may also incorporate switching (not shown) in the signal paths (61-63) to accept as inputs two- and 3-channel transmissions in MS form as an alternative to inputs (21,22) in L 2 ,R 2 form.
  • the T signal path (63) only may be switchable to accept a third channel signal T from a three-channel transmission source L,R,T as an alternative to the synthesised third channel signal at the output of the filter (38) derived from a two-channel input.
  • a frequency-dependent n ⁇ 2 reproduction matrix decoding means producing loudspeaker feeds for n greater than 3 loudspeakers according to the invention may be achieved by substituting in figure 8 an n ⁇ 3 transmission matrix decoder means of the type described subsequently for the 3 ⁇ 3 decoder means (9) shown in figure 8.
  • n 1 input signals (20) from a stereo source (1) are converted into n 3 signals (20a) by n 3 ⁇ n 1 matrix (2a) and then converted by n 2 ⁇ n 3 matrix (2b) into n 2 signals (40), constitutes an n 2 ⁇ n 1 reproduction matrix decoder (2) in accordance with the invention.
  • each component decoder (2a) and (2b) preserves the total energy of the pluralities of signals passing through them, then so does the composite decoder (2). If each of the component decoders (2a) and (2b) substantially preserves or improves the intended stereo effect, so does the composite decoder (2), and if each of the component decoders (2a) and (2b) substantially preserve, to within constants of proportionality, the angular dispositions of reproduced velocity vectors or of sound intensity vectors at ideal listening position, then so does the composite decoder (2).
  • a composite decoder based on two known decoders need not be implemented by physically implementing and connecting together the two known component decoders, but can alternatively be implemented as a single matrix circuit or means designed, by methods evident to those skilled in the art, to achieve the same end-result as a cascaded connection of the two known decoders.
  • the matrix coefficients of the n 3 ⁇ n 1 matrix decoder (2a) are represented by an n 3 ⁇ n 1 matrix R n3n1 and the matrix coefficients of the n 2 ⁇ n 3 matrix decoder (2b) are represented by the n 2 ⁇ n 3 matrix R n2n3 , then the matrix coefficients of the composite decoder (2) are represented by the n 2 ⁇ n 1 product matrix.
  • R n2n1 R n2n3 R n3n1 .
  • an n 2 ⁇ n 1 reproduction matrix decoder according to the invention can be designed so long as one knows for each plurality n how to design an (n+1) ⁇ n reproduction matrix decoder according to the invention, by series connection for increasing n such as shown in the schematic of figure 10.
  • This shows successive signals sources (1a to 1e) intended to feed the respective loudspeaker layouts shown in figures 1a to 1e. (We have included the monophonic case for completeness).
  • figure 10 only shows up to five-speaker stereo, it is evident that further matrices, e.g. the 6 ⁇ 5 and 7 ⁇ 6 cases, may extend this schematic to any number of loudspeakers. In most practical reproduction matrix decoders, most or all parts of the schematic of figure 10 will not be explicitly implemented, but such a decoder may nevertheless have an overall effect equivalent to that of specific signal paths within figure 10.
  • Figure 11 shows the schematic of a general system for encoding n 1 signals (20) from an n 1 -speaker stereo source (1) into m transmission channel signals (60a) by an m ⁇ n 1 transmission matrix encoder means (7) described by an m ⁇ n 1 matrix E mn1 , which are then conveyed by a chosen transmission medium (8) to be received as m signals (60b) fed into a n 2 ⁇ m transmission matrix decoding means (9) described by an n 2 ⁇ m matrix D n2m to produce n 2 signals (40) representing feed signals for n 2 loudspeakers in a stereophonic arrangement (50) spread across a sector (3) of directions at a listener (4).
  • the overall encoding/ transmission/decoding signal path (2) constitutes an n 2 ⁇ n 1 reproduction matrix decoding means for the source signals (20).
  • n 3 ⁇ n 1 reproduction matrix decoder (2g) according to the invention described by an n 3 ⁇ n 1 matrix R n3n1 by an m ⁇ n 3 transmission matrix encoder (7g) described by an m ⁇ n 3 matrix E mn3 according to the invention should, for n 3 greater than n 1 also constitute a transmission matrix encoder (7) according to the invention.
  • figure 13 shows how a composite transmission matrix decoder (9) in accordance with the invention may be constructed by a series connection of another transmission matrix decoder (9h) with a reproduction matrix decoder (2h) in accordance with the invention.
  • n-channel loudpeaker signals may be encoded into n transmission channels for every first plurality n, and that the n+1 transmission channels required for (n+1)-speaker stereo transmission should be such that they constitute the n channels used for transmitting n-speaker stereo plus one additional transmission channel denoted T n+1 .
  • Figure 15 illustrates the corresponding inverse decoder hierarchy, where the respective n ⁇ n decoders (9b) to (9e) described by n ⁇ n matrices D nn derive n signals representative of loudspeaker feeds for n-speaker stereo from the transmission channel signals M, S, T, T 4 and T 5 .
  • E 22 and D 22 is given by the conventional left/right or MS matrix encoding and decoding methods used in the prior art to transmit two-speaker stereo.
  • the (n+1) ⁇ (n+1) decoder matrix D n+1 n+1 may be devised as follows.
  • the first n columns of D n+1 n+1 representing the response to the first n transmission channels T 1 to T n form the (n+1) ⁇ n matrix R n+1 n D nn , and the last column is chosen to be any convenient nonzero column vector that is not a linear combination of the first n columns.
  • E n+1 n+1 is then computed as the inverse (D n+1 n+1 ) -1 of the decoding matrix.
  • the choice of the last column of D n+1 n+1 in the above design procedure is largely arbitrary, but is conveniently restricted further in preferred implementations.
  • the matrices all have real frequency-independent entries, as is generally preferred, one can use the fact that, because preferred reproduction decoder matrices R n+1 n preserve total signal energy, their columns are unit-length orthogonal vectors, and one can ensure that the matrices D nn are orthogonal matrices at each stage simply by constructing the last column of D n+1 n+1 at each stage to be that unit-length vector orthogonal to the other n columns, e.g. using the process of Gram-Schmidt orthogonalisation found in textbooks on matrix algebra.
  • the last column of D n+1 n+1 can be chosen to meet the requirements of left/right symmetry, by ensuring that T n+1 for odd n is a linear combination of signals only of the form S p in MS form, and that T n+1 for even n is a linear combination of signals only of the form M p or C p in MS form.
  • n 2 ⁇ n 1 reproduction matrix decoders falls into two main parts: first imposing an objective requirement that the decoder should substantially preserve the total energy of stereo signals passing through them, apart from a possible overal gain and equalisation change affecting all signal components equally, and a second more subjective or psychoacoustic requirement that requires a substantially preserved or improved stereo directional effect. It is convenient first to deal with the energy preservation requirement.
  • the n 2 ⁇ n 1 matrix R n2n1 describing the reproduction matrix decoder preserves energy if and only if its n 1 columns are of unit length (i.e. the sum of the squares of the absolute values of the matrix coefficients in that column equals one) and the columns are pairwise orthogonal (i.e. the sum of the products of entries of one column with the complex conjugate of the corresponding entries of another is zero).
  • R n2n1 is the first n 1 columns of an n 2 ⁇ n 2 unitary matrix, or, if all entries have real values, of an n 2 ⁇ n 2 orthogonal matrix.
  • n ⁇ n orthogonal matrices The general form of n ⁇ n orthogonal matrices is known to mathematicians, and there is a 1 ⁇ 2(n-1)n - parameter family of such n ⁇ n orthogonal matrices describing rotations in n-dimensional space; all other orthogonal n ⁇ n matrices ae obtained from these by reversing the sign of the entries of the last column.
  • the product of any two orthogonal matrices is also orthogonal.
  • all 2 ⁇ 2 orthogonal matrices have the explicit form for an angle parameter ⁇
  • the energy preserving matrices have an especially simple form when expressed in MS form, since sum signals (i.e. those of the form M p or C p ) must be converted into sum signals and difference signals (i.e. those of the form S p ) must be converted into difference signals by the reproduction matrix.
  • an energy-preserving left/right symmetric 4 ⁇ 3 reproduction decoder must satisfy equations of the form and where ⁇ 3 and ⁇ D are angle parameters.
  • figure 4 showed the form of an energy-preserving 3 ⁇ 2 reproduction decoder according to the equation given above.
  • This form can be generalised to other pluralities of inputs and outputs.
  • figure 17 shows a 4 ⁇ 2 reproduction matrix decoding means in accordance with the invention and the above equations.
  • Two-speaker stereo signals L 2 and R 2 are converted by input MS matrix means (31) into signals M 2 and S 2 ;
  • S 2 may be passed through an optional width gain adjustment means (32); each of M 2 and S 2 is then passed into constant power or sine/cosine gain adjustment means, respectively (33c) and (33d).
  • One output from each of these means (33) is passed to a first output MS matrix means (39c) to produce output signals L 4 and R 4
  • the other outputs from each of the means (33) is passed to a second output MS matrix means (39d) to produce output signals L 5 and R 5 .
  • These output signals L 4 , L 5 , R 5 , R 4 may be used to feed a four-speaker stereo loudspeaker arrangement such as that of figure 1d, via appropriate gain, equalisation, preamplification and amplification means.
  • the angle parameters ⁇ 42 and ⁇ D associated with the respective sine/cosine gain adjustment means (33c) and (33d) may be made frequency-dependent by the methods already discussed in connection with means (33) of figure 5 in the 3 ⁇ 2 case.
  • Figure 18 shows a 4 ⁇ 3 reproduction matrix decoding means in accordance with the above equations and the invention.
  • Input signals L 3 ,C 3 and R 3 intended for three-speaker stereo reproduction are accepted as inputs; L 3 and R 3 are fed to an input MS matrix means (31) to derive signals M 3 and S 3 ; S 3 is passed into a constant-power or sine/cosine gain adjustment means (33e) to produce two output difference signals S 4 and S 5 ; M 3 and the input C 3 are passed into a 2 ⁇ 2 orthogonal rotation matrix means (33f) producing outputs M 4 and M 5 ; M 4 and S 4 are passed through a first output MS matrix means (39e) to produce signals L 4 and R 4 , and M 5 and S 5 are passed through a second output MS matrix means (39f) to produce otput signals L 5 and R 5 .
  • the signals L 4 , L 5 , R 5 and R 4 are suitable for providing feed signals for a four-speaker stereo arrangement such as that of figure 1d.
  • bandsplitting filter means can be used in association with means (33e) and (33f) to provide frequency-dependent values of the angle parameters ⁇ 3 and ⁇ D if these are desired.
  • Figure 19 shows one generic form of an energy-preserving left/right symmetric reproduction matrix decoding means according to the invention, generalising the special cases shown in figures 4, 17 and 18.
  • the sum signals (28) are passed into a matrix A means (33g) giving a plurality n 2 " of output signals (48) whose total energy may substantially equal that of signals (28), and the difference signals (29) are passed into a matrix B means (33h) giving a number n 2 ' (which equals n 2 " or n 2 "-1) of output signals (49) whose total energy may substantially equal that of signals (29).
  • the matrix A and B means (33g) and (33h) may be frequency-dependent if desired by means similar to that used in connection with means (33) of figure 5 or by other means.
  • Other implementations of energy-preserving left/right symmetric n 2 ⁇ n 1 reproduction matrix decoders according to the invention not shown in figure 19 are possible, for example by separating and recombining the functions of the matrix means (31) (33g), (33h) and (39) in ways evident to those skilled in the art.
  • n 2 ⁇ n 1 reproduction matrix encoding equations in MS form can be given, which specify equations for the matrix A means (33g) and for the matrix B means (33h).
  • the pressure gain at the listener (4) is proportional to and the energy gain at the listener (4) is proportional to
  • the (notional) forward direction (5) at the listener (4) be the x-axis and the (notional) left direction (6) be the y-axis of rectangular coordinates, and let directions around the listener (4) be measured as angles ⁇ and measured anticlockwise (i.e. towards the y-axis) from the x-axis, as shown in figure 20.
  • ⁇ E represents the apparent sound direction when a listener faces the apparent sound source, especially at frequencies between around 700 Hz and 5 kHz, where localisation is largely determined by interaural intensity ratios.
  • This direction is the direction along which the sound intensity gain vector points.
  • the quantity r E termed the energy vector magnitude, equals 1 for natural sound sources, but is less than 1 for sounds emerging from more than one loudspeaker, and is useful for describing the stability of the illusory sound image as a listener changes position.
  • r E be as close to the ideal value 1 as possible.
  • the velocity vector magnitude r V should have a value close to one, with values much larger than or much smaller than one resulting in image instability when the listener's head is rotated.
  • the direction ⁇ V is often known as the "Makita localisation” direction, named after an author who introduced this localisation parameter.
  • the Makita direction ⁇ V describes the apparent localisation at low frequencies below around 700 Hz according to interaural phase localisation theories if the listener faces the apparent sound source.
  • the Makita direction ⁇ V should be similar to the energy vector direction ⁇ E for sharp images.
  • the imaginary part (Im(v x /P), Im(v y /P)) of the velocity ratio vector, termed the "phasiness vector” mainly affects the subjective quality of an image, rather than its apparent direction, imparting a generally unpleasant quality often termed "phasiness", which also manifests itself in image broadening.
  • the magnitude of the phasiness vector should be kept as small as possible, preferably having a length less than 0.2.
  • the relative values of matrix coefficients normally depart from real values only by small amount, and such departures are largely confined to transition frequency bands, so that phasiness effects for an ideally situated listener are usually adequately small and may be ignored.
  • the Makita direction ⁇ V substantially coincides with the direction in which the velocity gain vector of a signal is pointing, so that these two directions may be used interchangeably.
  • a computation of the four localisation parameters r V , r E . ⁇ V and ⁇ E can be performed using the above equations for any predetermined loudspeaker arrangement all equidistant from an ideal listening position (4) for any predetermined loudspeaker signal feeds, including those derived from a decoder matrix.
  • a reproduction matrix decoding means accepting a first plurality n 1 of loudspeaker feed signals intended for a first stereophonic arrangement of n 1 loudspeakers across a sector of directions should give a larger plurality n 2 of output signals intended to feed n 2 loudspeakers in a second stereophonic arrangement across a second sector of directions in such a manner that the four localisation parameters are either substantially preserved in value or "improved", by, for example covering a different sector of directions (providing typically a wider image) or greater image stability in those directions for which image stability was poor in the original intended stereo reproduction.
  • the values of r V and r E should either be maintained or made closer to 1 by the matrix decoder reproduction, and the values of the reproduced image directions ⁇ V and ⁇ E should be substantially preserved.
  • a stereophonic recording originally intended to cover a first sector of directions of angular width ⁇ I via a second sector of directions covering a different angular width ⁇ O at the listener.
  • a simple proportional widening of the angular dispositions of stereo sound localisation directions is often desired or acceptable.
  • the angular width is widened by a factor k, then the value of 1 - r E is typically increased by a factor k 2 , and similarly for k less than one.
  • n 2 ⁇ n 1 reproduction matrix decoder There are two main aims one can design an n 2 ⁇ n 1 reproduction matrix decoder to satisfy from a stereo localisation point of view. On the one hand, one can aim to preserve the angular dispositions of the velocity and sound intensity vectors originally intended, to within a single overall constant of proportionality to take account of altered stage width.
  • a decoder of this type will be termed a "preservation decoder", and will also tend to preserve other localisation qualities indicated by r V and r E .
  • the ⁇ 50.36° 3 ⁇ 2 decoder is a preservation decoder in this sense, and also preserves all the defects of two-speaker stereo.
  • the other, less well defined, aim is to improve the reproduced illusion.
  • a reproduction decoder that increases the value of r E for directions for which it is particularly different from one, perhaps at the expense of decreasing r E somewhat for other directions, as shown for example in figure 24; such an "improvement decoder" might, for example, be designed to ensure that r E is roughly constant for all directions.
  • the design of preservation decoders is extremely laborious, since it involves calculating the localisation parameters for a large variety of n 1 -speaker stereo feed signals, and then for each possible value of the energy-preserving n 2 ⁇ n 1 decoder matrix parameters, to compute the localisatiom parameters of the resulting signals.
  • Such a search is not difficult for 3 ⁇ 2 decoders involving only the one free parameter ⁇ , but becomes difficult in more complicated cases, and the search needs to be done again for each possible first and second stereophonic arrangement of loudspeakers.
  • the localisation parameters r V , r E , ⁇ V and ⁇ E are highly nonlinear functions of the decoder matrix, and there are also many possible speaker gains G i for the n 1 -speaker stereo signals that might be used to create a stereo directional effect.
  • the gain coefficients G i of a stereo sound via n 1 loudspeakers form a vector (G 1 ,...,G n1 ) in an n 1 -dimensional space, and the possible set of such gains representing stereo signals covers a region in this n 1 -dimensional space.
  • these decoders satisfy the following matrix equations: for the 3 ⁇ 2 reproduction matrix decoder of a "preservation decoder” according to the invention, for the 4 ⁇ 3 reproduction matrix decoder of a "preservation decoder” according to the invention, and for the 5 ⁇ 4 reproduction matrix decoder of a "preservation decoder” according to the invention.
  • the 5 ⁇ 3 "preservation decoder” obtained by forming the composite decoder as in figure 9 from the above 4 ⁇ 3 and 5 ⁇ 4 "preservation decoder” matrices satisfies the matrix equations and similar composite decoder equations can be formed from the above equations for the 4 ⁇ 2 and 5 ⁇ 2 cases by multiplying the appropriate matrices; however as we have seen, for 2-speaker stereo signal sources, preserving the original effect is rarely the most desirable thing to do in view of the substantial defects of 2-speaker stereo.
  • Table 1 shows the computed localisation parameters via the 3 ⁇ 2 preservation decoder as compared to the original 2-speaker values for various input signal gains.
  • Table 2 shows the computed localisation parameters via the above 4 ⁇ 3 preservation decoder as compared to the original 3-speaker values for various input signal gains.
  • Table 3 shows the computed localisation parameters via the above 5 ⁇ 4 preservation decoder as compared to the original 4-speaker values for various input signal gains.
  • Table 4 shows the computed localisation parameters via the above 5 ⁇ 3 preservation decoder as compared to the original 3-speaker values for various input signal gains.
  • angular rotations of the matrices i.e. multiplication by an orthogonal matrix producing angular rotations
  • the decoder angle parameters may vary by up to 6° from the values given, and the direction of the (a,b,c) vector may also vary by 6° without substantial effect.
  • (n 1 +1) ⁇ n 1 reproduction preservation decoders can be designed by the above stereo test signal methods for other (n 1 +1)-speaker arrangements.
  • Table 5 lists the parameters ⁇ 3 and ⁇ D for 4 ⁇ 3 preservation decoders for various values of the angles ⁇ 4 and ⁇ 5 in figure 1d.
  • ⁇ 4 ⁇ 5 ⁇ 3 ⁇ D 45 9 9.07 33.39 45 15 10.40 28.32 50 10 9.08 32.72 50 16 2 / 3 10.57 28.64 60 12 9.16 31.64 60 15 9.89 30.75 60 20 10.98 29.42 60 24 11.73 28.37 60 30 12.65 26.70 75 15 9.49 30.92 75 25 11.76 31.01
  • the values of the decoder parameters do not vary greatly with the precise angular dispositions of the reproduction loudspeakers for a preservation decoder. Also, for all these decoders, the reproduced velocity and sound intensity vector directions ⁇ V and ⁇ E are substantially proportional to those intended via the original 3-speaker layout.
  • a preservation decoder according to the invention may, if desired, incorporate means of adjusting decoder parameters according to the angular disposition of the loudspeakers used, or may use fixed typical parameters.
  • the decoder according to the invention may be used to improve the reproduction via more loudspeakers. This may be achieved by altering the decoder parameters from their preservation decoder values computed above.
  • n 2 ⁇ 2 improvement decoders may be achieved by forming a composite decoder, as in figure 9, comprising a 3 ⁇ 2 improvement decoder followed by an n 2 ⁇ 3 preservation decoder, or by using a decoder having the same overall effect as such a composite decoder.
  • a 4 ⁇ 2 improvement decoder may have the angle parameter ⁇ D substantially as shown in table 5 for the 4-speaker arrangement shown in figure 1d, being typically around 281 ⁇ 2°, and the angle parameter ⁇ 42 may substantially equal 35°- ⁇ 3 (typically around 25°) at frequencies between around 400 Hz and around 5 kHz, and may substantially equal 55° - ⁇ 3 (typically around 45°) at frequencies above about 5 kHz, where ⁇ 3 is as given in table 5.
  • a frequency-dependent 4 ⁇ 2 improvement decoder of this kind may be implemented as in figure 17, but making the ⁇ 42 sine/cosine gain adjustment means (33c) frequency-dependent using associated bandsplitting means (34) such as shown in figure 5.
  • the ⁇ D sine/cosine means (33d) in figure 17 may similarly be made frequency-dependent if desired.
  • bass energy may be preferentially fed to loudspeakers L 4 and R 4 by making ⁇ 42 near 90° and ⁇ D near 0° at low bass frequencies, and to loudspeakers L 5 and R 5 by making ⁇ 42 near 0° and ⁇ D near 90° at low bass frequencies.
  • n 1 3- or more channel inputs.
  • acceptable small alterations of the decoder parameters are found to modify the values of the respective constants k V and k E of proportionality of the angular dispositions of velocity and sound intensity vectors, and improvement decoders will generally be designed to reduce the value of k E somewhat, for example to about 0.8k V or 0.9k V in the middle frequency range in order to increase r E somewhat, and to increase k E somewhat above 5 kHz while leaving k V largely unaltered. This strategy retains a maximum sense of width above 5 kHz, while improving image stability with listener movement at middle frequencies.
  • Composite improvement decoders can be implemented by cascading two improvement decoders, or by following an improvement decoder by a preservation decoder; such composite decoders may be implemented as a single decoder designed so as to achieve the same result as the cascaded decoders by methods known to those skilled in the art.
  • All subjectively desirable matrix reproduction decoders according to the invention have overall matrix coefficients expressed in left/right or direct loudspeaker-feed formats such that some matrix coefficients have substantially the opposite polarity to other larger predominant matrix coefficients, at least across several octaves which may include the middle frequency region from say 500 Hz to 3 kHz.
  • Such opposite-polarity subsiduary matrix coefficients have the effect of helping to stabilise images and of rendering the results of different auditory localisation mechanisms more consistent.
  • the coefficients that have substantially opposite polarities will have a magnitude of under two-fifths of that of the predominant matrix coefficients.
  • the parameters ⁇ and ⁇ 42 preferably lie within 25° and the parameters ⁇ 3 , ⁇ D , ⁇ 4 , ⁇ 5 and the vector (a,b,c) preferably lie within 15° of their preservation decoder values given earlier.
  • ⁇ ' 45° for the 3 ⁇ 2 reproduction matrix decoders.
  • the vector (a,b,c) is of unit length and in a direction within 6° of (0.6164,0.6558,0.4359).
  • the transmission signals M, S, T, T 4 and T 5 may be given arbitrary predetermined respective nonzero amplitude gains k 1 ', k 2 ', k 3 ', k 4 ' and k 5 ' in order that the amplitude levels of signals in each transmission channel should match the peak level and noise characteristics of that channel.
  • Such additional amplitude gains may be applied at the encoding matrix stages, and the inverse gains k i ' -1 applied to the respective channel signals at the decoding stage.
  • the gains k i ' may be positive or negative, or may have complex values, which may be frequency dependent in the case that equalisation is desired of a transmission channel.
  • the transmission channel signals M, S, T, T 4 and T 5 are of progressively decreasing average signal energy, so that the magnitudes of the associated channel gains k i ' may be chosen to be progressively of increasing value.
  • a transmission matrix decoder may be employed based on modified values of the parameters ⁇ ' (for 3-speaker reproduction), ⁇ 3 and ⁇ D (for 4-speaker reproduction), or ⁇ 4 , ⁇ 5 and (a,b,c) (for 5-speaker reproduction) matched to the loudspeaker arrangement actually used rather than to the encoding parameters. Since such modified transmission decoding matrices are orthogonal, such decoders still give overall energy-preserving results.
  • the transmission matrix decoder with the same parameters as the encoder can be followed by a reproduction matrix preservation or improvement decoder according to the invention; the transmission matrix decoder and a following reproduction matrix decoder may be combined into a single matrix means in ways evident to those skilled in the art.
  • the frequency-independent m ⁇ m transmission matrix encoder may be fed with the outputs of a frequency-dependent m ⁇ n reproduction matrix improvement decoder. or equivalent signals may be provided by a frequency-dependent matrix encoding means achieving the effects of such a composite encoder.
  • the explicit 4 ⁇ 3 or 5 ⁇ 3 transmission matrix decoding equations obtained from the above 4 ⁇ 4 or 5 ⁇ 5 matrix equations when T 4 and T 5 are set to zero may be used for the output means (9) of the decoder shown in figure 8 when a 4 ⁇ 2 or 5 ⁇ 2 frequency-dependent improvement reproduction decoder is required.
  • the design theory given above assumed loudspeaker arrangements in which all loudspeakers are at the same distance from an ideally situated listener.
  • the invention can be used with loudspeaker arrangements for which this equal-distance requirement does not hold, such as the arrangement of figure 1g or n -speaker arrangements lying, for example, along a straight line or along a non-circular path or along a circular path whose centre does not lie in the preferred listening area.
  • the results in such a case are generally less satisfactory than with an equal distance loudspeaker arrangement, but still usually acceptable.
  • a matrix decoder may be provided with or incorporate or be used in association with time delay means for all loudspeakers or for all but those loudspeakers most distant from the preferred listening position, the time delays provided for all loudspeaker feed signals being such as to ensure that the time of arrival at the preferred listening position of an impulse passing through the decoder is substantially identical for all of the loudspeakers.
  • Such delay compensation means may be provided using any available time-delay technology, including analog charge-coupled delay lines and digital delay technology.
  • the provision of digital delay compensation is particularly simple for matrix decoder means implemented using digital signal processing technology.
  • the intended loudspeaker arrangement is substantially left/right symmetric and the preferred listening position is on the axis of symmetry.
  • the delay compensation is not intended to provide compensation for listener positions away from the preferred position, but is purely intended as a compensation for the actual loudspeaker arrangement used and its general relationship to the broad listening area over which listeners may be placed.
  • the ears are less sensitive to localisation at low frequencies below about 200 Hz, and particularly below 100 Hz, than at higher frequencies.
  • matrix decoders according to the invention may depart from the strict requirements of the invention at such low frequencies.
  • decoders When used with loudspeakers some of which have limited bass reproduction capabilities, decoders according to the invention may incorporate modified matrix decoding parameters such as ⁇ ' or ⁇ , ⁇ 3 , ⁇ D , ⁇ 4 , ⁇ 5 and (a,b,c) at low frequencies in order to redistribute bass energy among the various loudspeakers, in the manner already described for 3 ⁇ 2 reproduction matrix decoders. Such decoders may also incorporate or be used in association with phase compensation means intended to compensate for differences in the bass phase responses of different loudspeakers, so that as much as possible of the remaining low-frequency localisation cues are retained.
  • phase compensation means intended to compensate for differences in the bass phase responses of different loudspeakers, so that as much as possible of the remaining low-frequency localisation cues are retained.
  • a popular form of stereophonic apparatus is one-piece portable apparatus incorporating signal sources (1) such as cassette tape reproducers,radio reception means and compact disc players, amplification and control means and loudspeakers within a single unit, termed colloquially a "ghetto blaster".
  • signal sources (1) such as cassette tape reproducers,radio reception means and compact disc players, amplification and control means and loudspeakers within a single unit, termed colloquially a "ghetto blaster”.
  • Apparatus of this kind is sometimes equipped with a pair of demountable attached loudspeaker units, so that the apparatus may be used for reproduction either with the loudspeakers attached or with the loudspeakers separated from the main housing unit and from each other in order to provide a wider stereo effect.
  • the invention may be applied to a portable or transportable system for stereophonic reproduction using at least three loudspeaker systems each covering an audio frequency range including the range 400 Hz to 5 kHz, said system being capable of carried as a single unit, responsive to stereophonic source signals and incorporating a matrix decoding means for said source signals and providing feed signals for said loudspeaker systems, whereby at least one of said loudspeaker systems is securely attached to or integrated into the main housing unit of said portable or trasportable system, and whereby two of the additional said loudspeaker systems provided are attachable inclose proximity to said main housing unit and are also movable or demountable with respect to said main housing unit so as to capable of being used spaced apart from each other and from said main housing unit.
  • the system is so arranged that it may also be used for stereophonic reproduction when said two of the additional said loudspeaker systems are attached to said main housing unit. It is also preferred that said matrix decoder means be in accordance with the invention as described previously.
  • Apparatus of this kind preferably incorporates a 3 ⁇ 2 or 4 ⁇ 2 matrix decoding means of the type described earlier, with optional width adjustment means, responsive to 2-channel stereo source signals, which may be provided by a signal source or reception means incorporated into said main housing unit.
  • FIG. 27 shows, by way of example, a portable apparatus for multispeaker stereo reproduction of the above kind.
  • a main housing unit (81) incorporates a signal source such as a cassette player (1i), radio receiver (1j) and/or a compact disc player (1k), control means (82) such as volume, equalisation, width and source selection controls, and a centre loudspeaker (52) preferably placed at the front centre of said housing unit (81), and also incorporates within the main housing unit (81) 3 ⁇ 2 or 3 ⁇ 3 matrix decoder means (2) or (9) (not shown) responsive to stereo signal sources which feeds via amplification means (not shown) incorporated within said main housing unit (81) or within loudspeaker enclosures (85) the centre loudspeaker (52) and left (51) and right (53) loudspeakers all of which cover a frequency range including the primary frequency range 400 Hz to 5 kHz.
  • a signal source such as a cassette player (1i), radio receiver (1j) and/or a compact disc player (1k
  • control means (82) such
  • the loudspeaker enclosures (85) for left (51) and right (53) loudspeakers are shown attached to said main housing unit (81), but may be removed and spaced apart (85b) from said main housing unit (81) and each other, while remaining connected by audio signal cables (84) or by other audio signal communications means such as audio infra-red links.
  • the left and right loudspeaker enclosures (85) may be attachable to and removable from said main housing unit (81) by means of catches (83), clips, hooks, Velcro or other fastening or attachment means.
  • the enclosures (85) may be attached to the main housing unit (81) by means of movable arms or links (not shown) that slide or are otherwise movable (for example by a rotation or pantograph action) that allow the left and right loudspeaker enclosures (85) to be moved away from immediate proximity to said main housing unit (81) while still being physically connected to it by means of said arms or links.
  • An advantage of using a movable arm or link means of removing attachable loudspeaker enclosures (85) from immediate proximity to the main housing unit (81) is that this means provides exact control of the relative positions of the loudspeaker units (51-53) to ensure the best stereophonic effect, whereas unskilled users might place entirely removable loudspeaker enclosures (85b) in undesirable locations.
  • Moving arms or links also permit the entire unit to be carried by means of a single carrying handle (86) or shoulder strap attached to the main housing unit (81) while the loudspeaker enclosures (85) are removed from immediate proximity to said main housing unit (81).
  • four such systems may be provided for use in conjunction with 4 ⁇ 2, 4 ⁇ 3 or 4 ⁇ 4 matrix decoding means, with the outer pair in the movable loudspeaker enclosures (85) and the inner pair enclosed within the main housing unit (81).
  • the different loudspeaker systems (51,53) removable from the main housing unit (81) may have different frequency and/or phase response characteristics to those incorporated into the main housing unit (81), and equaliser compensation means may be incorporated into the apparatus for use in connection with said matrix decoding means to compensate for said differences of loudspeaker characteristics.
  • said matrix decoder means may use frequency-dependent matrix parameters so as to minimise the bass energy fed to those loudspeaker systems with limited bass capability.
  • the centre loudspeaker system (52) may have more bass power output than the movable loudspeaker systems (51, 53), and a 3 ⁇ 2 matrix decoder according to the invention may use a decoder parameter ⁇ that decreases to a value near 0° at low bass frequencies.
  • a 4 ⁇ 2 matrix decoder according to the invention may use a decoder parameter ⁇ 42 that decreases to a value near 0° at low bass frequencies.
  • the invention may be applied to reproduction with associated visual images where it is required to match the directions of sounds with those of associated visual images for listeners across a broad listening and viewing area. While applicable to situations where the visual image is that of physically present objects, such as in theatrical or live music performances, the invention is particularly applicable to reproduced images derived, for example, from Television broadcasts, video recordings, film projection or images generated by digital signal storage or processing means such as computer graphics or electronic games machines.
  • a visual reproduction means such as a display screen or projection means in a main housing unit, said housing unit also incorporating or being securely attached to at least one loudspeaker system covering at least a primary audio frequency range of 400 Hz to 5 kHz, and used with at least two loudspeaker systems each covering at least said primary frequency range capable of being moved so as to be spaced apart from and disposed to the two sides of said main housing unit, and a matrix decoding means according to earlier descriptions of the invention responsive to stereophonic source signals associated with the visual image and providing signals intended for reproduction via said loudspeaker systems.
  • Said movable loudspeakers may, if desired be attachable to and removable from said main housing unit by attachment or fastening means, and/or may be connected physically to said main housing unit by means of arm or link means, which may by sliding, rotation, pantograph or other action allow movement of said movable loudspeaker systems such that they may be used either in close proximity to said main housing unit or spaced apart and disposed to either side of said main housing unit.
  • FIGS. 28 and 29 show two examples of audiovisual apparatus which may benefit from the invention.
  • a main housing unit (81) incorporates a display screen (87) or other visual display or projection means, and is used with two loudspeaker enclosures (85), one placed to either side of the main housing (81) and spaced apart from it, each containing loudspeaker means covering at least said primary frequency range, said main housing unit (81) also containing one or two loudspeaker systems (52) covering at least said primary frequency range.
  • the main housing unit (81) incorporates, or is used in association with, matrix decoding means (not shown) responsive to stereo signals and providing signals suitable, after such processing and amplification means as may be neccessary or desirable, for feeding said loudspeaker systems (52) and (85), according to descriptions of the invention given earlier.
  • Figure 28 shows the case where a single centre loudspeaker (52) is used, in association with a 3 ⁇ 2 or 3 ⁇ 3 matrix decoder means (not shown); said loudspeaker is preferably placed centrally below or above said display screen (87) or display means in order to ensure correct localisation of central sound images with respect to the visual image.
  • Figure 29 shows the case where two loudspeaker systems (52) are incorporated into or immediately attached to said main housing unit (81) to either side of said display screen (87), for use with a 4 ⁇ 2, 4 ⁇ 3 or 4 ⁇ 4 matrix decoder means (not shown).
  • matrix decoders When used with matrix decoders according to the invention, the quality of stereophonic images is largely independent of the ratio of the spacing between the outer loudspeaker systems (85) to the spacing between the inner loudspeaker systems (52), over a range of values of said ratio between about 2 and 5.
  • a wider or narrower spacing of the outer loudspeaker enclosures (85) has little effect on the acceptability of stereophonic imaging over a wide range of placements.
  • the matrix decoder means may, if desired, incorporate electronic width adjustment means in order to provide a desired width of stereophonic sound stage with any given placement of said outer loudspeaker enclosures (85).
  • the audiovisual apparatus may incorporate equalisation means for compensating for any differences in frequency or phase response between inner (52) and outer (85) loudspeaker systems, and said matrix decoder means may additionally or instead have modified decoding matrix parameters at low frequencies so as to redistribute bass energy among the loudspeakers so as to take account of any differences in their bass reproduction capability.
  • loudspeaker units will generally be physically separate from each other and from preamplifier control means, which may incorporate matrix decoder means according to the invention or which may be used in association with physically separate matrix decoder means apparatus.
  • a preamplifier control means apparatus responsive to stereo source signals (1) incorporating matrix decoder means as earlier described according to the invention, said apparatus providing output signals intended, after subsequent amplification means (92) which may, if desired, be integrated with said apparatus for feeding to a stereophonic loudspeaker arrangement (50) comprising at least three loudspeaker systems disposed across a sector (3) of directions in front of a preferred listening position (4).
  • said preamplifier control means apparatus (91) also incorporates visual signal control means for receiving, selecting and/or modifying associated visual images intended to match reproduced sound images in direction.
  • another form of the invention provides a matrix decoder means apparatus (2) according to the invention responsive to signal outputs (20) of a preamplifier control means apparatus (91) and providing outputs (40) for feeding to amplification means (92) feeding a stereophonic loudspeaker arrangement (50) comprising at least three loudspeaker systems or units disposed across a sector (3) of directions in front of a preferred listening position (4).
  • PA apparatus intended to provide stereophonic reproduction with improved image stability for an audience of larger size than normally encountered in domestic applications.
  • PA apparatus may be used in cinema or film auditoria, for live amplified music, and in audiovisual and theatrical applications, among other applications.
  • clusters of loudspeakers in relatively close physical proximity be used instead of single loudspeaker systems sharing a single enclosure, in order to increase power output capability or to provide broader directional coverage of an audience area.
  • clusters constitute single “loudspeakers” as far as applications of the invention are concerned, and terms such as “loudspeaker” or “loudspeaker system” in this document may be interpreted to include such a cluster of loudspeakers.
  • different loudspeakers in a given cluster may handle different frequency ranges. Where the cluster of loudspeakers is mounted vertically on top of one another, such clusters are often termed “stacks" of loudspeakers.
  • Conventional stereophonic live music and theatrical PA apparatus usually uses a pair of stacks or clusters to either side of a stage or performance area, and occasionally a third central cluster is used placed over or behind the centre of the performance area.
  • Such clusters or stacks are fed by amplification apparatus which in turn is fed with stereophonic signals derived from a stereophonic mixing desk or apparatus which allows control of the level and stereo position of a number of separate sound sources, such as prerecorded sounds, sounds from various performers or their instruments picked up by microphones or electrical means, and sounds derived from effects devices such as synthetic echo or reverberation units.
  • such a stereophonic mixing apparatus (1) may incorporate or may feed a matrix decoding means (2) according to previous descriptions of the invention, and said matrix decoding means (2) may feed, via amplification means (92) three or more loudspeaker systems, clusters or stacks (50) in a stereophonic arrangement across, above or around the performance or visual display area (87) covering a sector of directions in front of a main audience area.
  • amplification means (92) three or more loudspeaker systems, clusters or stacks (50) in a stereophonic arrangement across, above or around the performance or visual display area (87) covering a sector of directions in front of a main audience area.
  • Figure 32 illustrates an example in which two loudspeaker stacks (51) and (53) are disposed at the respective left and right sides of a performance area (87) and a central loudspeaker system or cluster (52) is suspended over the front of said performance area (87) in order to avoid visual obstruction of the performance area.
  • any input and output sockets or connection means should meet professional standards for heavy-duty use, for example by the use of XLR-type or quarter-inch (6.3mm) jack connectors, and that adjustment means be provided to cope with typical operational problems.
  • the matrix decoder means should preferably incorporate or be used in association with delay compensation means to compensate for the positioning in distance of central or inner loudspeaker systems or clusters.
  • suspended central loudspeaker systems or clusters may have more limited bass capability than the outer stacks or clusters, since large bass units are too heavy or large for suspension without visual obstruction of the performance area.
  • the matrix decoder means (2) should thus preferably incorporate means of adjusting the low-frequency decoder matrix parameters so as to minimise the bass fed to such central loudspeakers, for example by putting ⁇ or ⁇ 42 close to 90° at low frequencies.
  • the bass transition frequency at which such parameter modifications take effect should be adjustable to match different bass deficiencies.
  • Such a matrix decoder may also provide user preset adjustment of the values of the matrix decoder parameters within one or more frequency range, so that the decoded effect may be optimised for each PA installation.
  • n 2 of loudspeaker systems or clusters may be used for each frequency range for which distinct loudspeaker types are provided, with a separate decoder provided for each plurality n 2 used.
  • n 2 5 for treble loudspeaker systems
  • n 2 3 or 4 for mid-frequency loudspeaker units
  • the inputs of said separate decoders may be derived using electronic cross-over filter networks of the kind normally used to provide feed signals for PA loudspeaker units covering a partial frequency range.
  • the invention provides a solution to particular problems associated with stereo systems used in vehicles, particularly cars, i.e. automobiles, and the like.
  • stereo reproduction conventionally gives a particularly poor directional illusion because of necessary limitations on the positioning both of loudspeakers and of listeners.
  • drivers are generally positioned to one side and towards the front of the listening area, and stereo loudspeakers have generally to be installed either to each side of the front of the interior of the vehicle or within the doors to either side of the front-seat area.
  • Such arrangements are far from the ideal disposition for good stereo images.
  • a third central loudspeaker (52) is provided supplementing the typical left and right loudspeakers (51) and (53) conventionally provided, said centre loudspeaker typically being mounted at, above or below the centre of the vehicle dashboard.
  • the left (51) and right (53) loudspeakers may be mounted at the two sides of the dashboard or in the respective front doors of the vehicle.
  • the invention may also be used with two or three additional loudspeakers between the conventional pair, using an n 2 ⁇ n 1 matrix decoder according to the invention.
  • Equalisation means associated with each of the loudspeaker systems may be incorporated or added to compensate both for different frequency responses of different loudspeaker systems and for typical absorption or diffraction characteristics to which the sound from each loudspeaker is subjected on its passage to the listener.
  • the invention may be used with any stereophonic arrangement of more than two loudspeakers disposed to the front and possibly sides of the front seating area of the vehicle, responsive to two or more stereo source signals, and delay compensation means may also be used in association with each or some of the loudspeaker feed signals for said stereophonic arrangement.
  • a second stereophonic arrangement disposed either to the front or the rear of a rear seating area, may also or additionally be provided according to the invention to serve listeners in said rear seating area.
  • any matrix means described may have component means rearranged, combined, split apart and recombined; gains and polarity inversions may be inserted and addition means replaced by subtraction means at different points while preserving the overall matrix means performance, and all-pass means affecting all parallel signal paths identically may be incorporated.
  • Means responsive to signals in left/right form may be made responsive to signals in MS form by the addition or deletion, as appropriate, of MS matrix means, and conversely for means responsive to signals in MS form.
  • means producing signals in one of left/right or MS forms may produce signals in the other form by the addition or deletion, as appropriate, of MS matrix means.
  • any means satisfying known matrix equations may be replaced by any other means producing results satisfying the same matrix equations designed by methods known to those skilled in the art.
  • any matrix means comprising two cascaded matrix means may be replaced by a single matrix means described by the matrix coefficients of the product of the matrices describing the input/output behaviour of the component matrix means.
  • loudspeakers or loudspeaker systems are referred to, clusters of loudspeaker units or systems placed relatively close to one another so as substantially to act as a single loudspeaker may equally be used.
  • different pluralities of loudspeakers may be used for reproduction of each component frequency range fed by an appropriate decoder according to the invention for that frequency range.
  • the invention may equally be applied to stereophonic loudspeakers covering other sectors of directions, such as for example, a sector behind a listener, to one side of a listener or above or below a listener, or to a vertical sector.
  • the invention may also be applied to a stereophonic arrangement of loudspeakers covering a sector of directions used in conjuction with other loudspeakers in other directions, such as rear loudspeakers covering a rear sector of directions also in accordance with the invention, or fed with delayed or reverberated versions of the signals fed to the front loudspeakers.
  • some stereophonic signals are processed in accordance with the invention to provide loudspeaker signals for a component stereophonic arrangement of a larger arrangement of loudspeakers, any additional loudspeakers or additional signals from other sources fed to the loudspeakers do not affect the scope of the invention.
  • additional "surround" signals may be transmitted and reproduced to supplement the front-stage stereo effect produced by the invention.
  • Transmission channel signals may be transmitted and received in either left/right or MS form; this may also include the possible use of a left/right form of transmission signals T 2n-1 and T 2n of the form 2 -1 ⁇ 2 (T 2n-1 + T 2n ) and 2 -1 ⁇ 2 (T 2n-1 - T 2n ).
  • the invention is also applicable to stereophonic arrangements of loudspeakers covering a sector of directions in front of a listener wherein different loudspeakers within the arrangement may lie at different heights or angles of elevation or declination.
  • matrix reproduction decoders have mainly been such that they exactly preserve the total energy of signals passing through them (to within a constant of proportionality that may be dependent on frequency), a limited but not substantial degree of departure from such exact energy preservation is permissible.
  • the permissible degree of departure that substantially retains the psychoacoustic advantages of the invention is such that, at any frequency, the gain of any two stereophonic signal components passing through a reproduction decoder according to the invention differs by not more than 3 dB, and preferably by less than 2 dB, and highly preferably by less than 1 dB, and such that, expressed in terms of the effect on direct loudspeaker feed signals, some matrix coefficients of said decoder are, across several octaves of the audio frequency range, substantially of opposite polarity to and of magnitude less than two fifths of the dominant or largest matrix coefficients.
  • Such small departures from exact energy preservation to within a constant of proportionality may typically be implemented by small departures from exact energy preservation of the matrix A means (33g) and the matrix B means (33h) of figure 19.
  • the matrix A means and the matrix B means may be adjustable, for example for the purposes of electronic width control or other desired effects, such that, at each frequency, different signal components of the signals (28) or (29) passing through matrix A means (33g) or matrix B means (33h) to produce signals (48) or (49) have a difference in relative total energy gain of not more than 3 dB, and preferably less than 2 dB, and highly preferably less than 1 dB.
  • the matrix A means (33g) may be energy preserving and the matrix B means (33h) may be energy preserving with an added overall gain of between -3 dB and + 3 dB, or the signals (28) and (29) may be given possibly differing gains within 3 dB of one another, or the signals (48) and (49) may be given possibly differing gains within 3 dB of one another when matrix A means (33g) and matrix B means (33h) are energy preserving, providing that these gain modifications are such as to retain the substantially opposite polarity of some matrix coefficients relative to the dominant matrix coefficients of the overall matrix reproduction decoder of figure 19, and that said substantially opposite polarity coefficients have a magnitude of less than two-fifths of said dominant matrix coefficients.
  • the decoder will remain according to the invention.
  • the present invention can also be applied to the provision of, e.g., a 3-speaker stereo feed from an ambisonically encoded signal.
  • Ambisonic techniques are described and claimed in patents GB2073556,GB1550627, GB1494752, GB1494751 all assigned to NRDC and in the present inventor's paper "Ambisonics in Multichannel Broadcasting and Video” pp 859-871, J. Audio Eng. soc. Vol 33 no. 11 (1985 Nov.). This aspect is not limited to B format but may also apply to other ambisonic formats.
  • the matrixing of equ. (21) is energy preserving as ⁇ varies, and the matrixing of equ. (22) shown as (9) in figure 8 is orthogonal, and so also preserves total energy.
  • the optimal 3 ⁇ 2 decoder was designed to handle central sounds best below 5 kHz and edge-of-stage sounds best above 5 kHz
  • the B-format 3-speaker decoder of equ. (23) which is frequency independent, manages to achieve both these optimum behaviours across the whole frequency range for front-stage azimuths, perhaps at the expense of rather unpleasant-sounding stereo effects for rear-azimuth sounds.
  • the decoder algorithm shown in the Figure may be replaced by any frequency-dependent matrix algorithm whose matrix coefficients equal those given by the Figure.
  • the decoder described above for 3-speaker stereo can be generalised to an n-speaker decoder, as shown in Figure 34b.
  • the final matrix D n,3 is an n ⁇ 3 transmission decoding matrix of the form also described above.
  • signals emerging from the input matrix (equ. 23) are called M dec , S dec , T dec and signals entering the output matrix are denoted M, S, T.
  • the matrix may be implemented by bandsplitting in a manner analoguous to Figure 5.
  • the rotation matrix is implemented in a fashion analogous to that described with respect to Figure 8 above.
  • the matrix is shown in Figure 34c.
  • the function of the filter means 38 and all pass 38a and gain 38b is the same as in Figure 8, and the element referenced 38 is identical to 38e, 38c is identical to 38a and 38d is identical to 38b. It can be shown that this is a close approximation to the ideal rotation matrix for values of phi near 45 °, e.g. 35 or 55°.
  • this aspect can also be used with any signals incorporating directionally encoded 360° surround sound signals that are linear combinations of an omnidirectional signal W, a signal X with gain proportional to cosine of direction and a signal Y with gain proportional to sine of direction.
  • those elements may have been transformed by, e.g., a forward dominance transformation.
  • Lorentz transformation termed by the inventor the "forward dominance" transformation, and defined in detail below, has the effect of increasing front sound gain by a factor ⁇ while altering the rear sound gain by an inverse factor 1/ ⁇ .
  • ⁇ is a real parameter having any desired positive value.
  • the transformed components still satisfy the characteristic relationships between B format signals W, X, Y, but with gains and azimuthal orientations different from those of the raw components.
  • Figure 16 shows a design process for a transmission hierarchy.
  • the last column of D nn can be chosen at will subject to linear independence of the other columns and this choice then determines the corresponding coefficients of the encoding matrix.
  • the encoder values may vary moment by moment provided that the choice is transmitted to the decoder as a side chain signal so that decoder can perform the inverse of the encoding function at any given moment.
  • Error noise artefacts such as those introduced by data compression can be subjectively minimised by adaptively modifying the encoding equation to match the instantaneous distribution of signal energies among the speaker feed signals, and using an inverse equation for the decoder.
  • a preferred strategy adjusts the coefficients so the transmission channels more nearly diagonalise the signal correlation matrix than would a fixed encoding function.
  • the appendix below lists a cascadable hierarchy of conversion matrices between any numbers n 1 and n 2 speaker feed signals between 1 and 5 for multi-speaker stereo based on the results of encoding from n 1 speakers and decoding into n 2 speakers using the orthogonal transmission systems designed using the flow diagram of figure 16 with the earlier highly preferred values of ⁇ ', ⁇ 3 , ⁇ D , ⁇ 4 , ⁇ 5 and the vector (a,b,c) parameters, whose transmission encoding and decoding matrices were given earlier.
  • the conversion matrices from a smaller to a larger number of loudspeakers in the hierarchy listed in the appendix are preferred matrix reproduction decoders as described earlier, and that the conversion matrices from a larger number of loudspeakers to a smaller number of loudspeakers have matrices that are the matrix transposes of the matrices from the smaller to the larger number with any frequency-dependent all-pass component deleted.
  • the following pages describe how a more general hierarchy may be constructed for conversion between formats having different numbers n of channels.
  • a cascadable hierarchy transmission system constructed following the method of Figure 16 when the input to a given conversion stage has a smaller number of channels than the output from the stage then an upconversion matrix as previously described for n-speaker stereo is used. Where a smaller number of channels are output then a downconversion matrix is used.
  • the transmission decoding and encoding matrices D nn and E nn in the construction of fig. 16 are orthogonal it can be shown that the downconversion matrices are the matrix transpose of the upconversion matrices.
  • R ji an "upconversion" matrix, and write A i ⁇ A j , if and only if R ji takes linearly independent signals in the system A i into linearly independent signals in the system A j (which requires that n i ⁇ n j ).
  • Cascadable hierarchies are desirable because not only do they allow sounds encoded for any one system A i to be converted by a matrix means R ji for reproduction from any other system A j in the hierarchy with satisfactory results, but also ensures that the results of repeated conversions between different systems, such as may take place in a long broadcast chain or when material intended for one systems is converted and then reconverted several times before reaching the final user, are satisfactory also, never sounding any worse than the results obtained by a single conversion down to the "maximum" system of which all the systems in the cascaded chain are upconversions, followed by a single upconversic to the final system.
  • Noncascadable hierarchies such as have been proposed in the prior art, lead to a continuing degradation of the reproduced directional effect as repeated conversions occur.
  • the transmission decoding matrix D jj for a system A j should chosen such that for all systems A i ⁇ A j , the n i columns of D jj corresponding to those transmission channels in Z i which are also transmission channels in Z j must be chosen to be equal to the n j ⁇ n i matrix R ji D ii , and the remaining columns chosen to be linearly independent of each other and of the other n i columns.
  • figure 16 provided such a cascadable hierarchy in the special case of frontal stage stereo signals, where A i may be the signals intended to feed i-speaker stereo speakers.
  • the BEF-format signals provide additional information permitting sound reproduction with improved frontal-stage image stability and improved front/rear stage separation as compared to reproduction from B-format.
  • the BE-format signals provide only improved frontal image stability, and the BF-format signals provide only improved front/rear stage separation.
  • a cascadable hierarchy may be formed from the ten directional encoding systems just described using five transmission channels M T , S T , T T , B T , F T giving satisfactory subjective results when one encoding is reproduced via reproduction from any other, when a transmission system using encoding matrices E ii with matrix coefficients similar to those indicated below is constructed:
  • E ii are as given earlier as the preferred encoding matrices for the 3-speaker stereo hierarchy described in connection with figures 6 and 7.
  • frontal stereo stage signals for 2:1, 3:1 and 3:2 stereo are also encoded into the M T , S T and T T transmission channels in the same way as frontal-only stereo signals, but that rear-stage stereo signals are encoded into these three transmission channels at a reduced gain, because it has been found that frontal stereo reproduction of "surround sound" material sounds best if the rear stage sounds are reproduced around 3 to 6 dB down.
  • the B T transmission channel is intended to convey predominantly rear stage material, and F T corresponds to the difference signal across a frontal stage minus a difference signal across a rear stage.
  • the decoding matrices D ii of this transmission hierarchy are simply given by the matrix inverse E ii -1 of E ii , which may be computed from the above matrices using any matrix inverse program on a computer or calculator.
  • the resulting conversion matrices on the ten systems create a cascadable hierarchy satisfying conditions (1) to (5) earlier, in which R ji is an upconversion matrix whenever the transmission signals of A i are also transmission signals for A j , which can be determined by inspection of figure X3.
  • the conversion matrices R ij thus obtained which give satisfactory reproduction of signals intended or converted for system A i via reproduction for system A j , may be used directly for conversion between different directional encoding modes, for example in a sound reproducer arranged for reproduction for one mode when receiving signals intended for another.
  • Such direct conversion can also be used in a professional or studio environment for conversion of available programme sources in one mode for recording, reproduction or subsequent transmission in another, without fear of the possibility of excessive degradtion of directional quality due to possible previous conversions.
  • such conversion can be achieved by using intermediate transmission channel signals via encoding and decoding matrices, which may be, but need not be, of the form of the signals M T , S T , T T , B T and F T described above.
  • the transmission signals may be encoded with an additional nonzero gain, and decoded with the inverse ofsaid gain, said gain possibly being different for each transmission signal, or desired independent linear combinations of M T , S T , B T , T T and F T may be used as intermediate transmission signals.
  • Depleted BE-format is similarly described as comprising the four signals W', X', Y and E from depleted BEF-format.
  • any output signals may be subjected to predetermined nonzero gains, including possibly polarity inversion, so as to achieve output signals having levels and/or polarities suitable for use with available signal channels or recording or transmission channels.
  • Some of the prior art surround sound systems for directional encoding of 360° azimuthal sound including all systems in the prior art UMX hierarchy and the B-format encoding, have mathematical rotational symmetry in the sense that, for every angle of rotation of the whole 360° sound stage, there exists a corresponding n ⁇ n matrix on the n channel signals of the directional encoding such that the application of this matrix to the original encoded signals produces signals encoded for the same encoding system, but with all encoded sound source positions rotated by said angle of rotation within the 360° stage.
  • the following upconversion matrices are subjectively exceptionally good gerformers, giving substantially optimal preservation of the originally intended stereo effect via a larger number of speakers.
  • the above conversion matrices are optimised according to the specific values of decoder parameters ⁇ , ⁇ ', ⁇ 3 , ⁇ D , ⁇ 4 , ⁇ 5 , (a,b,c). Slightly different values, associated with different speaker layouts, will give marginally different equations above, but in all cases, coefficients will differ only a little from those given here.

Claims (19)

  1. Un convertisseur matriciel Rn2,n1 pour convertir un premier signal audio (20) à codage stéréophonique pour reproduction par n1 haut-parleurs en un deuxième signal audio (40) à codage stéréophonique pour reproduction par n2 haut-parleurs (52) où n1 ≥ 2 et n2 est un entier > n1, où le convertisseur matriciel R inclut une matrice (31, 39) de préservation d'énergie qui dépend de la fréquence agencée sensiblement de manière à préserver à l'intérieur d'une constante globale de proportionnalité l'énergie totale reproduite et l'effet directionnel reproduit du signal audio codé.
  2. Un convertisseur matriciel selon la revendication 1, qui est en outre apte à préserver sensiblement, à l'intérieur d'une deuxième constante de proportionnalité, la disposition angulaire reproduite de vecteurs de vitesse et est apte en outre à préserver sensiblement, à l'intérieur d'une troisième constante de proportionnalité, la disposition angulaire reproduite de vecteurs d'intensité sonore. .
  3. Un convertisseur matriciel selon la revendication 2, dans lequel ladite troisième constante de proportionnalité dépend de la fréquence.
  4. Un convertisseur matriciel selon la revendication 3, dans lequel ladite troisième constante de proportionnalité est agencée de manière à être supérieure, à l'intérieur d'une bande de fréquence audio supérieure à 5 kHz, à sa valeur à l'intérieur de la bande de fréquences audio comprises entre 700 Hz et 3 kHz.
  5. Un convertisseur matriciel selon la revendication 1, dans lequel est agencé un moyen de modification de la largeur reproduite, qui exerce l'effet de modifier le gain de la composante de signal qui représente la composante de différence du premier signal audio.
  6. Un convertisseur matriciel selon la revendication 1, qui répond à des signaux L2 et R2 prévus pour des haut-parleurs respectifs gauche et droit d'un agencement stéréophonique à deux haut-parleurs et produit des signaux L3, C3 et R3 prévus pour une reproduction par des haut-parleurs respectifs gauche, central et droit d'un agencement stéréo à trois haut-parleurs dans lequel
    Figure 01810001
    et S3 = wS2    à l'intérieur d'une constante globale de proportionnalité de gain qui peut varier en fonction de la fréquence où Mp = 2-1/2 (Lp + Rp) et Sp = 2-1/2 (Lp - Rp) pour p = 2 et 3, où  est un paramètre qui dépend de la fréquence entre 15° et 75° et où w est un gain de largeur dépassant sin qui peut aussi dépendre de la fréquence.
  7. Un convertisseur matriciel selon la revendication 6, dans lequel  prend des valeurs proches de 0° ou de 90° à des fréquences basses de sons graves.
  8. Un convertisseur matriciel selon la revendication 1, qui répond à des signaux L2 et R2 prévus pour des haut-parleurs respectifs gauche et droit d'un agencement stéréophonique à deux haut-parleurs et produit des signaux L4, L5, R5 et R4 prévus pour une reproduction par des haut-parleurs respectifs extérieur gauche, intérieur gauche, intérieur droit et extérieur droit d'un agencement stéréo à quatre haut-parleurs où sensiblement
    Figure 01820001
    Figure 01820002
       à l'intérieur d'une constante globale de proportionnalité de gain qui peut varier avec la fréquence où Mp = 2-1/2 (Lp + Rp) et Sp = 2-1/2 (Lp - Rp) pour p = 2, 4 et 5 où 42 et D et w sont des paramètres qui peuvent varier avec la fréquence, où 42 s'écarte au plus de 25° de sa valeur de décodeur de préservation de 39,79° = 50,36° - 10,57° et où D s'écarte au plus de 15° de sa valeur de "décodeur de préservation" de 28,64°, où 42 et D peuvent être compris entre 0° et 90° à des basses fréquences de sons graves.
  9. Un codeur matriciel de transmission qui inclut un convertisseur matriciel selon la revendication 1, le codeur de transmission incluant une entrée agencée pour recevoir le premier signal audio et une sortie pour sortir le deuxième signal audio sur un support de transmission d'enregistrement.
  10. Un convertisseur matriciel selon la revendication 1, dans lequel le premier signal audio entré dans le convertisseur est un signal de transmission.
  11. Un convertisseur matriciel Rn1,n2 pour convertir un premier signal audio à codage stéréophonique pour reproduction par n2 haut-parleurs en un deuxième signal audio à codage stéréophonique pour reproduction par n1 haut-parleurs où la matrice est la transposée matricielle des coefficients de la matrice d'un convertisseur matriciel Rn2,n1 selon la revendication 1.
  12. Un convertisseur matriciel pour convertir un premier signal audio à codage ambisonique à composants W, X et Y ou à combinaisons linéaires de ceux-ci en un deuxième signal à codage stéréophonique pour reproduction par n2 haut-parleurs, où n2 est un entier ≥ 3 et le convertisseur matriciel comprend une matrice de conversion n2 × 2 pour convertir ledit premier signal audio, et inclut une matrice de préservation d'énergie agencée sensiblement de manière à préserver à l'intérieur d'une constante globale de proportionnalité l'énergie totale reproduite et l'effet directionnel reproduit du signal audio codé, ledit convertisseur matriciel étant agencé pour recevoir à une entrée un premier signal Mdec formé de la somme de la composante omnidirectionnelle W et d'une première composante X de vitesse et à l'autre entrée un signal Sdec formé de l'autre composante de vitesse Y et un moyen de sortie d'une composante additionnelle de signal dérivée de la différente Tdec desdites composants W et X, ledit convertisseur matriciel dépendant de la fréquence.
  13. Un convertisseur matriciel selon la revendication 12, qui comprend en outre une matrice de rotation agencée pour appliquer une rotation d'un angle ( - 45°) qui dépend de la fréquence aux composantes de somme et de différence Mdec et Tdec pour conférer la dépendance du convertisseur par rapport à la fréquence.
  14. Un convertisseur matriciel selon la revendication 13, dans lequel  varie d'une valeur inférieure comprise dans une plage sensiblement incluse entre 25 et 45° en dessous sensiblement de 5 kHz à une valeur supérieure comprise dans une plage de sensiblement 45° à sensiblement 65° au-dessus de 5kHz.
  15. Un convertisseur matriciel selon la revendication 12, qui comporte en outre un moyen d'application d'une atténuation variable à la composante de différence Tdec.
  16. Un convertisseur matriciel selon la revendication 1 pour convertir un premier signal audio à codage stéréophonique pour reproduction par 2 haut-parleurs en un deuxième signal audio pour reproduction par n2 haut-parleurs, où n2 est un entier > 3, dans lequel le convertisseur comprend une première matrice pour convertir un premier signal audio à codage stéréophonique pour reproduction par 2 haut-parleurs en un deuxième signal audio pour reproduction par 3 haut-parleurs et une deuxième matrice pour convertir un troisième signal audio codé pour reproduction par 3 haut-parleurs en un quatrième signal audio pour reproduction par n2 haut-parleurs, la première matrice étant une matrice qui dépend de la fréquence et la deuxième matrice étant une matrice indépendante de la fréquence.
  17. Un convertisseur selon la revendication 7, dans lequel les termes de la matrice sont combinés en une matrice unique de conversion n2 x n1.
  18. Un système de transmission/reproduction audio qui inclut comme première entrée un signal audio (21b, 22b) codé pour reproduction par 2 haut-parleurs et une deuxième entrée (21c, 23c, 22c) codée pour, reproduction par 3 haut-parleurs, le système comprenant un premier convertisseur matriciel (7b) conforme à la revendication 1 pour convertir la première entrée en un premier signal audio modifié à codage stéréophonique pour reproduction par 3 haut-parleurs, un deuxième convertisseur matriciel (7) pour convertir la deuxième entrée en un deuxième signal modifié, codé pour reproduction par 3 haut-parleurs, un moyen (70) de combinaison du premier et du deuxième signaux modifiés, et un moyen de transmission (8).
  19. Une reproduction matricielle qui inclut un convertisseur matriciel selon la revendication 1, le décodeur de reproduction incluant une entrée agencée pour recevoir le premier signal audio à partir d'un support de transmission ou d'enregistrement et un moyen d'envoi de signaux correspondant aux n2 signaux d'alimentation de haut-parleurs.
EP92904564A 1991-02-15 1992-02-14 Systeme de reproduction sonore Expired - Lifetime EP0571455B1 (fr)

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GB919103207A GB9103207D0 (en) 1991-02-15 1991-02-15 Stereophonic sound reproduction system
GB9103207 1991-02-15
PCT/GB1992/000267 WO1992015180A1 (fr) 1991-02-15 1992-02-14 Systeme de reproduction sonore

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EP0571455A1 (fr) 1993-12-01
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GB9103207D0 (en) 1991-04-03
DE69232327T2 (de) 2002-08-22
WO1992015180A1 (fr) 1992-09-03

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