WO2001082651A1 - Multi-channel surround sound mastering and reproduction techniques that preserve spatial harmonics in three dimensions - Google Patents
Multi-channel surround sound mastering and reproduction techniques that preserve spatial harmonics in three dimensions Download PDFInfo
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- WO2001082651A1 WO2001082651A1 PCT/US2000/027851 US0027851W WO0182651A1 WO 2001082651 A1 WO2001082651 A1 WO 2001082651A1 US 0027851 W US0027851 W US 0027851W WO 0182651 A1 WO0182651 A1 WO 0182651A1
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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
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04S—STEREOPHONIC SYSTEMS
- H04S2400/00—Details of stereophonic systems covered by H04S but not provided for in its groups
- H04S2400/15—Aspects of sound capture and related signal processing for recording or reproduction
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- 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
Definitions
- This invention relates generally to the art of electronic sound transmission, recording and reproduction, and, more specifically, to improvements in surround sound techniques.
- Stereo (two channel) recording and playback through spatially separated loud speakers significantly improved the realism of the reproduced sound, when compared to earlier monaural (one channel) sound reproduction.
- the audio signals have been encoded in the two channels in a manner to drive four or more loud speakers positioned to surround the listener. This surround sound has further added to the realism of the reproduced sound.
- Multi-channel (three or more channel) recording is used for the sound tracks of most movies, which provides some spectacular audio effects in theaters that are suitably equipped with a sound system that includes loud speakers positioned around its walls to surround the audience.
- an audio field is acquired and reproduced by multiple signals through four or more loud speakers positioned to surround a listening area, the signals being processed in a manner that reproduces substantially exactly a specified number of spatial harmonics of the acquired audio field with practically any specific arrangement of the speakers around the listening area.
- whatever speaker locations that exist are used as parameters in the electronic encoding and/or decoding of the multiple channel sound signals to bring about this favorable result in a particular reproduction layout. If one or more of the speakers is moved, these parameters are changed to preserve the spatial harmonics in the reproduced sound.
- Use of five channels and five speakers are described below to illustrate the various aspects of the present invention.
- individual monaural sounds are mixed together by use of a matrix that, when making a recording or forming a sound transmission, angularly positions them, when reproduced through an assumed speaker arrangement around the listener, with improved realism.
- a matrix that, when making a recording or forming a sound transmission, angularly positions them, when reproduced through an assumed speaker arrangement around the listener, with improved realism.
- all of the channels are potentially involved in order to reproduce the sound with the desired spatial harmonics.
- An example application is in the mastering of a recording of several musicians playing together. The sound of each instrument is first recorded separately and then mixed in a manner to position the sound around the listening area upon reproduction. By using all the channels to maintain spatial harmonics, the reproduced sound field is closer to that which exists in the room where the musicians are playing.
- the multi-channel sound may be rematrixed at the home, theater or other location where being reproduced, in order to accommodate a different arrangement of speakers than was assumed when originally mastered.
- the desired spatial harmonics are accurately reproduced with the different actual arrangement of speakers. This allows freedom of speaker placement, particularly important in the home which often imposes constraints on speaker placement, without losing the improved realism of the sound.
- a sound field is initially acquired with directional information by a use of multiple directional microphones.
- Either the microphone outputs, or spatial harmonic signals resulting from an initial partial matrixing of the microphone outputs, are recorded or transmitted to the listening location by separate channels.
- the transmitted signals are then matrixed in the home or other listening location in a manner that takes into account the actual speaker locations, in order to reproduce the recorded sound field with some number of spatial harmonics that are matched to those of the recording location.
- these various aspects may use spatial harmonics in either two or three dimensions.
- the audio wave front is reproduced by an arrangement of loud speakers that is largely coplanar, whether the initial recordings were based on two dimensional spatial harmonics or through projecting three dimensional harmonics on to the plane of the speakers.
- a three dimensional reproduction one or more of the speakers is placed at a different elevation than this two dimensional plane.
- the three dimensional sound field is acquired by a non-coplanar arrangement of the multiple directional microphones.
- Figure 1 is a plan view of the placement of multiple loud speakers surrounding a listening area
- Figures 2A-D illustrate acoustic spatial frequencies of the sound reproduction arrangement of Figure 1;
- Figure 3 is a block diagram of a matrixing system for placing the locations of monaural sounds;
- Figure 4 is a block diagram for re-matrixed the signals matrixed in
- Figures 5 and 6 are block diagrams that show alternate arrangements for acquiring and reproducing sounds from multiple directional microphones; Figure 7 provides more detail of the microphone matrix block in
- Figure 8 shows an arrangement of three microphones as the source of the audio signals to the systems of Figures 5 and 6.
- Figure 9 illustrates the arrangement of the spherical coordinates.
- Figure 10 shows an angular alignment for a three dimensional array of four microphones.
- a person 11 is shown in Figure 1 to be at the middle of a listening area surrounded by loudspeakers SP1, SP2, SP3, SP4 and SP5 that are pointed to direct their sounds toward the center.
- a system of angular coordinates is established for the purpose of the descriptions in this application.
- the forward direction of the listener 11, facing a front speaker SP1 is taken to be positioned at ( ⁇ i ⁇ 0 ,0°) as a reference.
- the angular positions of the remaining speakers SP2 (front left), SP3 (rear left), SP4 (rear right) and SP5 (front right) are respectively ( ⁇ 2 , ⁇ 2 ), ( ⁇ 3 , ⁇ 3 ), (0 4 , 4 ), and ( ⁇ 5 , ⁇ 5 ) from that reference.
- each of ⁇ x - ⁇ 5 is then 90° and these ⁇ s will not be explicitly expressed for the time being and are omitted from Figure 1.
- the elevation of one or more of the speakers above one or more of the other speakers is not required but may be done in order to accommodate a restricted space. The case of one or more of the 0; ⁇ 90° is discussed below.
- the sounds of the individual instruments will be positioned at different angles ⁇ 0 around the listening area during the mastering process.
- the sound of each instrument is typically acquired by one or more microphones recorded monaurally on at least one separate channel. These monaural recordings serve as the sources of the sounds during the mastering process.
- the mastering may be performed in real time from the separate instrument microphones.
- Figures 2A-D are referenced to illustrate the concept of spatial frequencies.
- Figure 2A shows the space surrounding the listening area of Figure 1 in terms of angular position.
- the five locations of each of the speakers SPl, SP2, SP3, SP4 and SP5 are shown, as is the desired location of the sound source 13.
- the sound 13 may be viewed as a spatial impulse which in turn may be expressed as a Fourier expansion, as follows:
- a m is the coefficient of one component of each harmonic and b m is a coefficient of an orthogonal component of each harmonic.
- the value a 0 thus represents the value of the spatial function's zero order.
- the spatial zero order is shown in Figure 2B, having an equal magnitude around entire space that rises and falls with the magnitude of the spatial impulse sound source 13.
- Figure 2C shows a first order spatial function, being a maximum at the angle of the impulse 13 while having one complete cycle around the space.
- a second order spatial function as illustrated in Figure 2D, has two complete cycles around the space.
- the spatial impulse 13 is accurately represented by a large number of orders but the fact of only a few speakers being used places a limit upon the number of spatial harmonics that may be included in the reproduced sound field. If the number of speakers is equal to or greater than (1 + 2n), where n here is the number of harmonics desired to be reproduced, then spatial harmonics zero through n of the reproduced sound field may be reproduced substantially exactly as exist in the original sound field. Conversely, the spatial harmonics which can be reproduced exactly are harmonics zero through n, where n is the highest whole integer that is equal to or less than one-half of one less than the number of speakers positioned around a listening area. Alternately, fewer than this maximum number of possible spatial harmonics may be chosen to be reproduced as in a particular system.
- Figure 3 schematically shows certain functions of a sound console used to master multiple channel recordings.
- five signals SI, S2, S3, S4, and S5 are being recorded in five separate channels of a suitable recording medium such as tape, likely in digital form. Each of these signals is to drive an individual loud speaker.
- Two monaural sources 17 and 19 of sound are illustrated to be mixed into the recorded signals S1-S5.
- the sources 17 and 19 can be, for example, either live or recorded signals of different musical instruments that are being blended together.
- One or both of the sources 17 and 19 can also be synthetically generated or naturally recorded sound effects, voices and the like. In practice, there are usually far more than two such signals used to make a recording.
- the individual signals may be added to the recording tracks one at a time or mixed together for simultaneous recording.
- Figure 3 What is illustrated by Figure 3 is a technique of "positioning" the monaural sounds. That is, the apparent location of each of the sources 17 and 19 of sound when the recording is played back through a surround sound system, is set during the mastering process, as described above with respect to Figure 1.
- usual panning techniques of mastering consoles direct a monaural sound into only two of the recorded signals S1-S5 that feed the speakers on either side of the location desired for the sound, with relative amplitudes that determines the apparent position to the listener of the source of the sound. But this lacks certain realism.
- each source of sound is fed into each of the five channels with relative gains being set to construct a set of signals that have a certain number of spatial harmonics, at least the zero and first harmonics, of a sound field emanating from that location.
- One or more of the channels may still receive no portion of a particular signal but now because it is a result of preserving a given number of spatial harmonics, not because the signal is being artificially limited to only two of the channels.
- the relative contributions of the source 17 signal to the five separate channels S1-S5 is indicated by respective variable gain amplifiers 21, 22, 23, 24 and 25.
- Respective gains g l5 g ⁇ g 3 , g 4 and g 5 of these amplifiers are set by control signals in circuits 27 from a control processor 29.
- the sound signal of the source 19 is directed into each of the channels S1-S5 through respective amplifiers 31, 32, 33, 34 and 35.
- Respective gains g x , g 2 ', g 3 ', g 4 ' and g 5 ' of the amplifiers 31-35 are also set by the control processor 29 through circuits 37. These sets of gains are calculated by the control processor 29 from inputs from a sound engineer through a control panel 45. These inputs include angles ⁇ ( Figure 1) of the desired placement of the sounds from the sources 17 and 19 and an assumed set of speaker placement angles ⁇ x - ⁇ s . Calculated parameters may optionally also be provided through circuits 47 to be recorded.
- Respective individual outputs of the amplifiers 21-25 are combined with those of the amplifiers 31-35 by respective summing nodes 39, 40, 41, 42 and 43 to provide the five channel signals S1-S5. These signals S1-S5 are eventually reproduced through respective ones of the speakers SP1-SP5.
- the control processor 29 includes a DSP (Digital Signal Processor) operating to solve simultaneous equations from the inputted information to calculate a set of relative gains for each of the monaural sound sources.
- DSP Digital Signal Processor
- ⁇ 0 represents the angle of the desired apparent position of the sound
- ⁇ and ⁇ ⁇ represent the angular positions that correspond to placement of the loudspeakers for the individual channels with each of i and j having values of integers from 1 to the number of channels
- m represents spatial harmonics that extend from 0 the number of harmonics being matched upon reproduction with those of the original sound field
- N is the total number of channels
- g £ represents the relative gains of the individual channels with i extending from 1 to the number of channels. It is this set of relative gains for which the equations are solved.
- Use of the i and j subscripts follows the usual mathematical notation for a matrix, where i is a row number and j a column number of the terms of the matrix.
- the definition of the velocity vector direction is on the left of the equal sign and that of the power vector on the right.
- For the power vector, taking the square of the gain terms is an approximation of a model of the way the human ear responds to the higher frequency range, so can vary somewhat between individuals.
- the resulting signals S1-S5 can be played back from the recording 15 and individually drive one of the speakers SP1-SP5. If the speakers are located exactly in the angular positions ⁇ ⁇ - ⁇ s around the listener 11 that were assumed when calculating the relative gains of each sound source, or very close to those positions, then the locations of all the sound sources will appear to the listener to be exactly where the sound engineer intended them to be located. The zero, first and any higher order spatial harmonics included in these calculations will be faithfully reproduced.
- the signals S1-S5 are rematrixed by the listener's sound system in a manner illustrated in Figure 4.
- the sound channels S1-S5 played back from the recording 15 are, in a specific implementation, initially converted to spatial harmonic signals & $ (zero harmonic), a j and b x (first harmonic) by a harmonic matrix 51.
- the first harmonic signals a x and b x are orthogonal to each other.
- the processor 59 calculates these gains from the mastering parameters that have been recorded and played back with the sound tracks, primarily the assumed speaker angles ⁇ ⁇ 2 , ⁇ 3 , 4 , and ⁇ 5 , and corresponding actual speaker angles ⁇ x , ⁇ 2 , ⁇ 3 ⁇ 4 and ⁇ 5 that are provided to the control processor by the listener through a control panel 61.
- the algorithm of the harmonic matrix 51 is illustrated by use of 15 variable gain amplifiers arranged in five sets of three each. Three of the amplifiers are connected to receive each of the sound signals S1-S5 being played back from the recording. Amplifiers 63, 64 and 65 receive the SI signal, amplifiers 67, 68 and 69 the S2 signal, and so on. An output from one amplifier of each of these five groups is connected with a summing node 81, having the ⁇ output signal, an output from another amplifier of each of these five groups is connected with a summing node 83, having the a x output signal, and an output from the third amplifier of each group is connected to a third summing node 85, whose output is the b x signal.
- the matrix 51 calculates the intermediate signals a 0 , a, and b x from only the audio signals S1-S5 being played back from the recording 15 and the speaker angles ⁇ ⁇ ⁇ , ⁇ 3 , ⁇ , and ⁇ 5 , assumed during mastering, as follows:
- bi SI sin x + S2 sin ⁇ 2 + S3 sin 3 + S4 sin 4 + S5 sin ⁇ 5
- the amplifiers 63, 67, 70, 73 and 76 have unity gain
- the amplifiers 64, 68, 71, 74 and 77 have gains less than one that are cosine functions of the assumed speaker angles
- amplifiers 65, 69, 72, 75 and 78 have gains less than one that are sine functions of the assumed speaker angles.
- the matrix 53 takes these signals and provides new signals ST, S2', S3', S4' and S5' to drive the speakers having unique positions surrounding a listening area.
- the representation of the processing shown in Figure 4 includes 15 variable gain amplifiers 87-103 grouped with five amplifiers 87-91 receiving the signal ao, five amplifiers 92-97 receiving the signal a l3 and five amplifiers 98-103 receiving the signal b
- the output of a unique one of the amplifiers of each of these three groups provides an input to a summing node 105, the output of another of each of these groups provides an input to a summing node 107, and other amplifiers have their outputs connected to nodes 109, 111 and 113 in a similar manner, as shown.
- the relative gains of the amplifiers 87-103 are set to satisfy the following set of simultaneous equations that depend upon the actual speaker angles ⁇ :
- N 5 in this example, resulting in i and j having values of 1, 2, 3, 4 and 5.
- the result is the ability for the home, theater or other user to "dial in” the particular angles taken by the positions of the loud speakers, which can even be changed from time to time, to maintain the improved spatial performance that the mastering technique provides.
- the values of relative gains of the amplifiers 87-103 are chosen to implement the resulting coefficients of ao, a, and b x that result from solving the above matrix for the output signals Sl'-S5' of the circuit matrix 53 with a given set of actual speaker position angles ⁇ ⁇ - ⁇ 5 .
- FIG. 3 The description with respect to Figures 3 and 4 has been directed primarily to mastering a three-dimensional sound field, or at least contribute to one, from individual monaural sound sources.
- FIG 5 a technique is illustrated for mastering a recording or sound transmission from signals that represent a sound field in three dimensions.
- Three microphones 121, 123 and 125 are of a type and positioned with respect to the sound field to produce audio signals m l3 m 2 and m 3 that contain information of the sound field that allows it to be reproduced in a set of surround sound speakers. Positioning such microphones in a symphony hall, for example, produces signals from which the acoustic effect may be reconstructed with realistic directionality.
- the reproduction system includes a microphone matrix circuit 129 and a speaker matrix circuit 131 operated by a control processor 133 through respective circuits 135 and 137. This allows the microphone signals to be controlled and processed at the listening location in a way that optimizes, in order to accurately reproduce the original sound field with a specific unique arrangement of loud speakers around a listening area, the signals S1-S5 that are fed to the speakers.
- the matrix 129 develops the zero and first spatial harmonic signals ao, a, and b x from the microphone signals m l9 m 2 and m 3 .
- the speaker matrix 131 takes these signals and generates the individual speaker signals S1-S5 with the same algorithm as described for the matrix 53 of Figure 4.
- a control panel 139 allows the user at the listening location to specify the exact speaker locations for use by the matrix 131, and any other parameters required.
- the arrangement of Figure 6 is very similar to that of Figure 5, except that it differs in the signals that are recorded or transmitted. Instead of recording or transmitting the microphone signals at 127 (Figure 5), the microphone matrixing 129 is performed at the sound originating location ( Figure 6) and the resulting spatial harmonics ao, a, and b x of the sound field are recorded or transmitted at 127'.
- a control processor 141 and control panel 143 are used at the mastering location.
- a control processor 145 and control panel 147 are used at the listening location.
- Each of the three microphone signals m l3 m 2 and m 3 is an input to a bank of three variable gain amplifiers.
- the signal m x is applied to amplifiers 151- 153, the signal m 2 to amplifiers 154-156, and the signal m 3 to amplifiers 157-159.
- One output of each bank of amplifiers is connected to a summing node that results in the zero spatial harmonic signal a Q .
- another one of the amplifier outputs of each bank is connected to a summing node 163, resulting in the first spatial harmonic signal a x .
- outputs of the third amplifier of each bank are connected together in a summing node 165, providing first harmonic signal b ⁇
- the gains of the amplifiers 151-159 are individually set by the control processor 133 or 141 ( Figures 5 or 6) through circuits 135. These gains define the transfer function of the microphone matrix 129.
- the transfer function that is necessary depends upon the type and arrangement of the microphones 121, 123 and 125 being used.
- Figure 8 illustrates one specific arrangement of microphones. They can be identical but need not be. No more than one of the microphones can be omni-directional. As a specific example, each is a pressure gradient type of microphone having a cardioid pattern. They are arranged in a Y-pattern with axes of their major sensitivities being directed outward in the directions of the arrows. The directions of the microphones 121 and 125 are positioned at an angle on opposite sides of the directional axis of the other microphone 123.
- the microphone signals can be expressed as follows, where vis an angle of the sound source with respect to the directional axis of the microphone 123:
- the gains of the amplifiers 151-159 are the coefficients of each of the m ls m 2 and m 3 terms of these equations.
- the matrices are formed with parameters that include either expected or actual speaker locations. Few constraints are placed upon these speaker locations. Whatever they are, they are taken into account as parameters in the various algorithms. Improved realism is obtained without requiring specific speaker locations suggested by others to be necessary, such as use of diametrically opposed speaker pairs, speakers positioned at floor and ceiling corners of a rectangular room, other specific rectilinear arrangements, and the like. Rather, the processing of the present invention allows the speakers to first be placed where desired around a listening area, and those positions are then used as parameters in the signal processing to obtain signals that reproduce sound through those speakers with a specified number of spatial harmonics that are substantially exactly the same as those of the original audio wavefront.
- the spatial harmonics being faithfully reproduced in the examples given above are the zero and first harmonics but higher harmonics may also be reproduced if there are enough speakers being used to do so. Further, the signal processing is the same for all frequencies being reproduced, a high quality system extending from a low of a few tens of Hertz to 20,000 Hz. or more. Separate processing of the signals in two frequency bands is not required.
- the spherical harmonics are functions of two coordinates on the sphere, the angles ⁇ and ⁇ . These are shown in Figure 9 where a point on the surface of the sphere is represented by the pair ( ⁇ , ⁇ ). ⁇ is azimuth. Zero degrees is straight ahead. 90° is to the left. 180° is directly behind, ⁇ is declination (up and down). Zero degrees is directly overhead. 90° is the horizontal plane, and 180° is straight down. Note that the range of ⁇ is zero to 180°, whereas the range of ⁇ is zero to 360° (or -180° to 180°). In the discussion in two dimensions, the angular variable ⁇ has been suppressed and taken as equal to 90°. More generally, both angle are included.
- Figures 1 and 8 can be considered either as a coplanar arrangement of the shown elements or a projection of the three dimensional situation onto a particular planar subspace.
- the gains to each of the speakers, g vide are sought so that the resulting sound field around a point at the center corresponds to the desired sound field (f Q ( ⁇ , ⁇ ) above) as well as possible. These gains may be obtained by requiring the integrated square difference between the resulting sound field and the desired sound field be as small as possible.
- the result of this optimization is the following matrix equation that generalizes equation (2) with the right and left hand sides switched:
- G is a column vector of the speaker gains
- the components of the matrix B may be computed as follows:
- equation (19) is similar to the expansion in equation (16) for the unit impulse in a certain direction but for the term (- 1)"'.
- the rank of the matrix B depends on how many terms of the expansion are retained. If the 0 th and 1 st terms are retained, the rank of B will be 4. If one more term is taken, the rank will be 9. The rank of B also determines the minimum number of speakers required to match that many terms of the expansion.
- any number of speakers may be used, but the system of equations will be under-determined if the number of speakers is not the perfect square number (T+l) 2 corresponding to the T ih order harmonics.
- T+l perfect square number
- One way is to solve the system using the pseudo- inverse of the matrix B. This is equivalent to choosing the minimum-norm solution, and provides a perfectly acceptable solution.
- Another way is to augment the system with equations that force some number of higher harmonics to zero. This involves taking the minimum number of rows of B that preserves it rank, then adding rows of the following form:
- Figures 3 and 4 illustrated the mastering and reconstruction process for a coplanar example of two monaural sources mixed into five signals which are then converted into the spatial harmonics through first order and finally matrixed into a modified set of signals.
- any of these specific choices could be taken differently, although the choices of five signals being recording and five modified signals resulting as the output are convenient as a common multichannel arrangement is the 5.1 format of movie and home cinema soundtracks.
- Alternative multichannel recording and reproduction methods for example that described in the co-pending U.S. patent application Ser. No. 09/505,556, filed February 17, 2000, by James A. Moorer, entitled “CD Playback Augmentation” which is hereby incorporated herein by this reference..
- one convenient choice for the three dimensional, non- coplanar case is to use six signals S1-S6 and also a modified set of six signals Sl'-S6'.
- non-coplanar speakers are required for the spherical harmonics just as at least three non-collinear speakers are required in the 2D case, since at least four non-coplanar points are needed to define a sphere and three non-collinear points define a circle in a plane.
- the reason six speakers is a convenient choice is that it allows for four or five of the recorded or transmitted tracks on medium 15 to be mixed for a coplanar arrangement, with the remaining two or one tracks for speakers placed off the plane.
- each of the six signals S1-S6 would feed four amplifiers in matrix 51, one for each of the four summing nodes corresponding to A 0 , A h A n , and B n (or, more generally, four independent linear combinations of these) to produce theses four output in this example using the 0 th and 1 st order harmonics.
- Matrix 53 now has six amplifiers for each of these four harmonics to produce the set of six modified signals Sl-S6'. Again, the declination as well as the azimuthal location of the actual speaker placements is now used. More generally, control panel 61 could also supply control processor 59 with radial information on any speakers not on the same spherical surface as the other speakers. The control processor 59 could then use this information matrix 53 to produce corresponding modified signals to compensate for any differing radii by introducing delay, compensation for wave front spreading, or both.
- a 0 SI + S2 + S3 + S4 + S5 + S6
- A SI cos ⁇ i + S2 cos6> 2 + S3 cos ⁇ 9 3 + S4 cos ⁇ 9 4 + S5 cos ⁇ 9 5 + S6 cos6> 6
- a n SI cos ⁇ sinf.? ! + S2 cos ⁇ 2 sin# 2 + S3 cos 3 sin ⁇ 9 3 (&)
- a standard directional microphone has a pickup pattern that can be expressed as the 0 th and 1 st spatial spherical harmonics.
- the equation for the pattern of a standard pressure-gradient microphone is the following:
- the constant C is called the "directionality" of the microphone and is determined by the type of microphone. C is one for an omni-directional microphone and is zero for a "figure-eight" microphone.
- This equation corresponds to the 2D 0 th and 1 st spatial harmonics of equation (10).
- the spatial harmonic coefficients on the left side of the equations are sometimes called W, Y, Zand in commercial sound-field microphones. Representation of the 3-dimensional sound field by these four coefficients is sometimes referred to as "B- format,” (The nomenclature is just to distinguish it from the direct microphone feeds, which are sometimes called "A-format").
- m x , ..., m M refer to M pressure-gradient microphones with principal axes at the angles ( ⁇ X , ⁇ X ), ..., (6 ⁇ , ⁇ M )-
- the matrix D may be defined by its inverse as follows:
- Each row of this matrix is just the directional pattern of one of the microphones.
- Four microphones unambiguously determine all the coefficients for the 0 th and 1 st order terms of the spherical harmonic expansion.
- the angles of the microphones should be distinct (there should not be two microphones pointing in the same direction) and non-coplanar (since that would provide information only in one angular dimension and not two). In these cases, the matrix is well-conditioned and has an inverse.
- Another alternative is to place the microphones with two rearward facing microphones as shown in Figure 10, with m x 121 at (90°, a), m 2 123 at (90°+ ⁇ 5,180°), m 3 125 at (90°,- ), and m 4 126 at (90°- ⁇ 5,180°).
- one of the microphones may be placed at a different radius for practical reasons, in which case some delay or advance of the corresponding signal should be introduced. For example, if the rear-facing microphone m 2 of Figure 8 were displaced a ways to the rear, the recording advanced about 1ms for each foot of displacement to compensate for the difference in propagation time.
- Equation (23) is valid for any set of four microphones, again assuming no more than one of them is omni-directional. By looking at this equation for two different sets of microphones, the directional pattern of the pickup can be changed by matrixing these four signals.
- the starting point is equations (23) and (24) for two different sets of microphones and their corresponding matrix D.
- the actual microphones and matrix will be indicated by the letters m and D, with the rematrixed, "virtual" quantities indicated by a tilde.
- these microphone feeds may be transformed into the set of "virtual" microphone feeds as follows:
- the matrix D represents the directionality and angles of the "virtual" microphones. The result of this will be the sound that would have been recorded if the virtual microphones had been present at the recording instead of the ones that were used.
- This allows recordings to be made using a "generic" sound-field microphone and then later matrix them into any set of microphones. For instance, we might pick just the first two virtual microphones, m x and m 2 , and use them as a stereo pair for a standard CD recording. m 3 could then be added in for the sort of planar surround sound recording described above, with m 4 used for the full three dimensional realization.
- any non-degenerate transformation of these four microphone feeds can be used to create any other set of microphone feeds, or can be used to generate speaker feeds for any number of speakers (greater than 4) that can recreate exactly the 0 th and 1 st spatial harmonics of the original sound field.
- the sound field microphone technique can be used to adjust the directional characteristics and angles of the microphones after the recording has been completed.
- the microphones can be revised through simple matrix operations. Whether the material is intended to be released in multi-channel format or not, the recording of the third, rear-facing channel allows increased freedom in a stereo release, with the recording of a fourth, non-coplanar channel increasing freedom in both stereo and planar surround-sound.
- the matrix, R x is simply the 0 th and 1 st order spherical harmonics evaluated at the speaker positions.
- the three or four channels of (preferably uncompressed) audio material respectively corresponding to the 2D and 3D sound field may be stored on the disk or other medium, and then rematrixed to stereo or surround in a simple manner.
- equation (25) or its 2D reduction
- two channels could store a suitable stereo mix
- the third store a channel for a 2D surround mix
- the matrix D or its inverse is also stored on the medium.
- the player simply ignores the third and fourth channels of audio and plays the other two as the left and right feeds.
- the inverse of the matrix D is used to derive the 0-th and first 2D spatial harmonics from the first three channels.
- a matrix such as equation (8) or the planar projection of equation (17) is formed and the speaker feeds calculated.
- the 3D harmonics are derived from D using all four channels to form the matrix of equation (17) and calculate the speaker feeds.
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Priority Applications (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2001578151A JP4861593B2 (en) | 2000-04-19 | 2000-10-06 | Multi-channel surround sound mastering and playback method for preserving 3D spatial harmonics |
EP00970687A EP1275272B1 (en) | 2000-04-19 | 2000-10-06 | Multi-channel surround sound mastering and reproduction techniques that preserve spatial harmonics in three dimensions |
CA002406926A CA2406926A1 (en) | 2000-04-19 | 2000-10-06 | Multi-channel surround sound mastering and reproduction techniques that preserve spatial harmonics in three dimensions |
AU2000280030A AU2000280030A1 (en) | 2000-04-19 | 2000-10-06 | Multi-channel surround sound mastering and reproduction techniques that preservespatial harmonics in three dimensions |
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US09/552,378 US6904152B1 (en) | 1997-09-24 | 2000-04-19 | Multi-channel surround sound mastering and reproduction techniques that preserve spatial harmonics in three dimensions |
US09/552,378 | 2000-04-19 |
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EP (1) | EP1275272B1 (en) |
JP (1) | JP4861593B2 (en) |
CN (1) | CN1452851A (en) |
AU (1) | AU2000280030A1 (en) |
CA (1) | CA2406926A1 (en) |
WO (1) | WO2001082651A1 (en) |
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- 2000-10-06 CA CA002406926A patent/CA2406926A1/en not_active Abandoned
- 2000-10-06 AU AU2000280030A patent/AU2000280030A1/en not_active Abandoned
- 2000-10-06 EP EP00970687A patent/EP1275272B1/en not_active Expired - Lifetime
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Also Published As
Publication number | Publication date |
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EP1275272B1 (en) | 2012-11-21 |
JP4861593B2 (en) | 2012-01-25 |
CA2406926A1 (en) | 2001-11-01 |
CN1452851A (en) | 2003-10-29 |
EP1275272A1 (en) | 2003-01-15 |
AU2000280030A1 (en) | 2001-11-07 |
JP2003531555A (en) | 2003-10-21 |
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