EP0975201B1 - Procedé de traitement d'un signal audio à plussieurs canaux - Google Patents

Procedé de traitement d'un signal audio à plussieurs canaux Download PDF

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Publication number
EP0975201B1
EP0975201B1 EP99305562.3A EP99305562A EP0975201B1 EP 0975201 B1 EP0975201 B1 EP 0975201B1 EP 99305562 A EP99305562 A EP 99305562A EP 0975201 B1 EP0975201 B1 EP 0975201B1
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distance
head
loudspeaker
ear
chosen
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German (de)
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EP0975201A3 (fr
EP0975201A2 (fr
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Alastair Sibbald
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Creative Technology Ltd
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Creative Technology Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S1/00—Two-channel systems
    • H04S1/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution

Definitions

  • This invention relates to a method of processing a plural channel audio signal including left and right channels, the information in the channels representing a three dimensional sound-field for generation by respective left and right loudspeakers arranged at a given distance from the preferred position of a listener in use.
  • the fundamental Head Response Transfer Function (HRTF) characteristics which are required to implement a transaural crosstalk cancellation scheme are the left- and right-ear transfer functions associated with the azimuth angle at which the loudspeakers are situated ( Figure 1 ). For most applications, this is commonly accepted to be ⁇ 30°.
  • the near-ear function is sometimes referred to as the "same” side function (or “ S " function), and the far-ear function as the “alternate” (or " A ”) function.
  • S the near-ear function
  • A the far-ear function
  • Transaural crosstalk cancellation is described in more detail in WO 95/15069 .
  • HRTFs measured by the prior art methods do not contain LF information, although, of course, the LF response is present in reality.
  • the results of a typical HRTF measurement are shown in Figure 3 , depicting the A and S functions at 30° azimuth, measured from a commercial artificial head.
  • the uncertainty in the non-valid data, below several hundred Hz, is apparent. Accordingly, the missing LF properties must be replaced in order to create valid HRTFs, and this is conveniently done by extrapolating the amplitude data at the lowest valid frequency (200 Hz) back to 0 Hz (or in practise, back to the lowest practical frequency, say 10 Hz).
  • Prior art transaural crosstalk cancellation methods have always used A and S functions which tend to the same value at low frequencies (see for example, Atal and Schroeder, US 3,236,949 ). Using such functions, the anticipated crosstalk signal at the far ear is equal to the primary signal at the near ear at low frequencies, hence the ratio of crosstalk signal to primary signal is always 1:1 at low frequencies.
  • transaural crosstalk is defined to be the intensity ratio of the far ear signal with respect to the near ear signal. As these two functions have a different frequency dependence, this ratio will in general be a function of frequency. However, in the prior art the ratio approaches unity at low frequencies because A and S are forced to the same value below about 200 Hz. That is, the transaural crosstalk signal (far ear signal) is equal in magnitude to the primary signal (near ear signal) for such low frequencies.
  • the transaural crosstalk signal is substantially equal to (100% of) the primary signal-at low frequencies, regardless of loudspeaker distance and/or angle. Consequently, all the prior art methods of transaural crosstalk cancellation have not been optimal for the arrangements/distances of loudspeakers used in practice.
  • the invention provides a means for creating optimal transaural crosstalk cancellation particularly, though not exclusively, for users of Personal Computer (PC) - based multimedia systems, in which the loudspeakers are relatively close to the listener and might be at a variety of differing angles and distances, depending on the individual user's set-up configuration and preferences.
  • the amount of transaural crosstalk which occurs is also influenced by the angle of the loudspeakers. (Note that this is not to be confused with the use of the appropriate azimuth angle A and S functions, which is well known: i.e. use 30° A and S functions for speakers at 30°;15° A and S functions for speakers at 15°, and so on).
  • the present invention is a transaural crosstalk cancellation means based on "standard", 1 metre A and S functions.
  • the method employs an algorithm which controls the intensity of the transaural crosstalk cancellation signal relative to the near-ear intensity, using a crosstalk cancellation factor which is a function of loudspeaker proximity and spatial position.
  • the invention is based on the observation that when a sound source moves relatively closely towards the head (say, from a distance of several metres), then the individual far- and near-ear properties of the HRTF do not change a great deal in terms of their spectral properties, but their amplitudes, and the amplitude difference between them, do change substantially, caused by a distance ratio effect.
  • loudspeaker position angles lie in the range ⁇ 10° (for notebook PCs) to ⁇ 30° (for desktop PCs), and the distances (loudspeaker to ear) range from about 0.2 metres to 1 metre respectively. These ranges will be used here for illustrative purposes, but of course the invention is not restricted to these parameters.
  • the distance ratio (far-ear to sound source vs. near-ear to sound source) becomes greater.
  • the intensity of a sound source diminishes with distance as the energy of the propagating wave is spread over an increasing area.
  • the wavefront is similar to an expanding bubble, and hence the energy density is related to the surface area of the propagating wavefront, which is related by a square law to the distance travelled (the radius of the bubble). This is described in the Appendix.
  • the intensity ratios of left and right channels are related to the ratio of the squares of the distances.
  • the intensity ratios for the above examples at distances of 1 m, 0.5 m and 0.2 m are approximately 0.80, 0.62 and 0.35 respectively. In dB units, these ratios are -0.97 dB, -2.08 dB and -4.56 dB respectively.
  • Figure 5 shows a diagram of the near space around the listener, together with the reference planes and axes which will be referred to during the following descriptions, in which P-P' represents the front-back axis in the horizontal plane, intercepting the centre of the listener's head, and with Q-Q' representing the corresponding lateral axis from left to right.
  • the near-ear distance can be determined, for example, by the following calculation.
  • Figure 6 shows a plan view of the listener's head, together with the near area surrounding it.
  • Figure 7 we are interested in the front-right quadrant in order to derive an expression for the source to near-ear distance.
  • the situation is trivial to resolve, as shown in Figure 7 , if the "true" source-to-ear paths for the close frontal positions (such as path "A") are assumed to be similar to the direct distance (indicated by "B"). This simplifies the situation, as is shown on the left diagram of Figure 7 , indicating a sound source S in the front-right quadrant, at an azimuth angle of ⁇ degrees with respect to the listener.
  • the angle subtended by S-head_centre-Q' is (90° - ⁇ ).
  • the far-ear distance can be determined, for example, by the following calculation.
  • Figure 8 shows a plan view of the listener's head, together with the near-field area surrounding it. Once again, we are particularly interested in the front-right quadrant. However, the path between the sound source and the far-ear comprises two serial elements, as is shown dearly in the right hand detail of Figure 8 .
  • First there is a direct path from the source, S, tangentially to the head, labelled q
  • second there is a circumferential path around the head, C, from the tangent point to the far-ear.
  • the distance from the sound source to the centre of the head is d, and the head radius is r.
  • the angle subtended by the tangent point and the head centre at the source is angle R.
  • the crosstalk factor which is the ratio of (far-ear/near-ear) intensities, as a fraction or percentage of this limiting, 100% value.
  • This would define how much attenuation should be applied to the crossfeed path in a transaural crosstalk cancellation system ("C" in Figure 2 ) based on conventional "infinitely distant" A and S functions.
  • the crosstalk cancellation factor, X could be converted into dB units of sound intensity, X(dB) and used to define the LF asymptote difference of an A and S function pair, as shown in Figure 9 , which could then be used in a conventional crosstalk cancellation scheme (for example Figure 2 , corresponding to Atal and Schroeder, US 3,236,949 ) to the same effect.
  • the A function LF asymptote would be set so as to lie X(dB) below the S asymptote (because the far ( A ) ear is always more distant).
  • the crosstalk factor X is the far-ear LF intensity (I F ) expressed as a fraction of the near-ear LF intensity (I N ).
  • the intensities are related to the distances from the source to far-ear (D F ) and near-ear (D N ) by the square law relationship (see Appendix), as follows.
  • the transaural crosstalk cancellation factor X is incorporated into the filter design procedure, thus allowing a range of different transaural crosstalk cancellation filters to be created from standard low frequency convergent A and S functions, but with differing values of X, for a range of speaker configurations, such that the end user can select the most appropriate one for their particular speaker configuration.
  • a range of filters for X values in the range say, 0.5 to 1.0 in 0.05 increments.
  • a further disadvantage of this alternative approach is that it would require many measurements at different distances and angles, and would result in quantised-distance effects: an optimum value could not be calculated and easily be provided for all loudspeaker configurations.
  • the present invention allows both distance and angle parameters to be used to calculate a single crosstalk cancellation factor, from which an associated filter is selected, based on accurate, 1 metre measurement.
  • the pressure fluctuations propagate away from the source in a spherical manner - the wavefront is just like an expanding "bubble".
  • the wavefront sphere increases in size, and hence its energy is spread over a larger surface area. Consequently, the energy density - and intensity - of the expanding wavefront diminishes.
  • the expanding sphere is relatively small, having radius r, such that I, represents the energy received per second from sound source s.
  • the wavefront has expanded to a larger sphere having radius r 2 , and intensity I 2 at the surface.
  • I 1 I 2 r 2 2 r 1 2

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Stereophonic System (AREA)
  • Circuit For Audible Band Transducer (AREA)
  • Stereophonic Arrangements (AREA)

Claims (8)

  1. Procédé de traitement d'un signal audio à canaux binauraux comprenant des canaux gauche et droit, les informations dans les canaux représentant un champ sonore tridimensionnel destiné à être généré par des haut-parleurs gauche et droit respectifs agencés à une certaine distance de la position préférentielle d'une personne qui écoute lors de l'utilisation, le procédé comprenant :
    le choix d'une distance entre lesdits haut-parleurs et ladite position préférentielle ;
    la détermination, pour chacun des canaux gauche et droit, d'une fonction de transfert près de l'oreille, S, et d'une fonction de transfert loin de l'oreille, A, lesquelles sont égales à des fréquences inférieures à 200 Hz ; la détermination, à partir de la grandeur de cette distance choisie, d'un facteur de compensation de diaphonie transaurale, X, lequel est une fonction de la distance choisie ; et
    l'ajustement des fonctions de transfert près et/ou loin de l'oreille, de sorte qu'elles se rapprochent de différentes valeurs respectives à des fréquences inférieures à 200 Hz dictées par le facteur de compensation de diaphonie transaurale.
  2. Procédé selon la revendication 1, comprenant en outre le choix d'un angle entre le haut-parleur de canal gauche et le haut-parleur de canal droit, tel que vu à partir de ladite position préférentielle, et la détermination, à partir dudit angle choisi et de ladite distance choisie, du facteur de compensation de diaphonie transaurale, ledit facteur de compensation de diaphonie transaurale étant une fonction de l'angle choisi et de la distance choisie.
  3. Procédé selon la revendication 2, comprenant en outre la détermination d'une pluralité de facteurs de diaphonie transaurale à une pluralité de distances choisies et d'angles choisis, et la sélection du facteur le plus approprié pour une configuration de haut-parleur spécifique.
  4. Procédé selon l'une quelconque des revendications précédentes, comprenant en outre la conversion du facteur de diaphonie en dB, et dans lequel l'application du facteur de diaphonie comprend l'utilisation de la valeur en dB dans un schéma de suppression de diaphonie conventionnel.
  5. Procédé selon l'une quelconque des revendications précédentes, comprenant en outre la détermination d'une distance entre une oreille lointaine et les haut-parleurs gauche et droit comprenant deux éléments en série, un premier élément comprenant un chemin direct allant du haut-parleur tangentiellement à la tête de la personne qui écoute, et un second élément comprenant le chemin circonférentiel autour de la tête.
  6. Procédé selon la revendication 5, dans lequel la distance entre une oreille lointaine et les haut-parleurs gauche et droit est déterminée par l'équation ci-dessous : d 2 - 7.5 2 + 2 ⁢ π ⁢ r θ + sin - 1 7.5 d 360
    Figure imgb0022
    où d est la distance entre le haut-parleur et la tête d'une personne qui écoute, r est le rayon de la tête et θ est l'angle azimutal de la tête au haut-parleur.
  7. Procédé selon l'une quelconque des revendications précédentes, dans lequel le facteur de compensation de diaphonie transaurale, en fonction de la distance et/ou de l'angle, est tel que défini dans
    l'équation ci-dessous : X = d 2 + | 7.5 | 2 - 15 ⁢ d . sin θ ⁢ | θ = 0 θ = 90 d 2 - 7.5 2 + 15 ⁢ π θ ⁢ | θ = 0 θ = 90 + sin - 1 7.5 d 360
    Figure imgb0023
    où d est la distance du haut-parleur à la tête, et θ est l'angle azimutal de la tête au haut-parleur.
  8. Moyen de filtre de diaphonie transaurale construit et agencé de manière à mettre en oeuvre le procédé selon l'une quelconque des revendications précédentes.
EP99305562.3A 1998-07-24 1999-07-12 Procedé de traitement d'un signal audio à plussieurs canaux Expired - Lifetime EP0975201B1 (fr)

Applications Claiming Priority (2)

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GB9816059 1998-07-24
GB9816059A GB2340005B (en) 1998-07-24 1998-07-24 A method of processing a plural channel audio signal

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EP0975201A2 EP0975201A2 (fr) 2000-01-26
EP0975201A3 EP0975201A3 (fr) 2005-06-08
EP0975201B1 true EP0975201B1 (fr) 2013-04-10

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Cited By (1)

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CN111587582A (zh) * 2017-10-18 2020-08-25 Dts公司 用于3d音频虚拟化的音频信号预调节

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JP4835185B2 (ja) * 2006-02-08 2011-12-14 ソニー株式会社 オーディオ信号出力装置および音漏れ低減方法
US9094771B2 (en) 2011-04-18 2015-07-28 Dolby Laboratories Licensing Corporation Method and system for upmixing audio to generate 3D audio
KR101687493B1 (ko) * 2015-08-12 2016-12-16 연세대학교 산학협력단 Ftn 시스템에서 신호 전송 방법 및 장치
WO2020107192A1 (fr) * 2018-11-27 2020-06-04 深圳市欢太科技有限公司 Appareil et procédé de lecture stéréophonique, support d'informations, et dispositif électronique

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0730812A1 (fr) * 1993-11-25 1996-09-11 Central Research Laboratories Limited Appareil destine au traitement de signaux binauraux
WO1998020707A1 (fr) * 1996-11-01 1998-05-14 Central Research Laboratories Limited Expanseur pour chaine stereophonique

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JPS5832840B2 (ja) * 1977-09-10 1983-07-15 日本ビクター株式会社 立体音場拡大装置
JPH07105999B2 (ja) * 1990-10-11 1995-11-13 ヤマハ株式会社 音像定位装置
JPH10108300A (ja) * 1996-09-27 1998-04-24 Yamaha Corp 音場再生装置
GB2334867A (en) * 1998-02-25 1999-09-01 Steels Elizabeth Anne Spatial localisation of sound

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0730812A1 (fr) * 1993-11-25 1996-09-11 Central Research Laboratories Limited Appareil destine au traitement de signaux binauraux
WO1998020707A1 (fr) * 1996-11-01 1998-05-14 Central Research Laboratories Limited Expanseur pour chaine stereophonique

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111587582A (zh) * 2017-10-18 2020-08-25 Dts公司 用于3d音频虚拟化的音频信号预调节
CN111587582B (zh) * 2017-10-18 2022-09-02 Dts公司 用于3d音频虚拟化的音频信号预调节的系统、方法、以及存储介质

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GB9816059D0 (en) 1998-09-23
EP0975201A3 (fr) 2005-06-08
GB2340005B (en) 2003-03-19
EP0975201A2 (fr) 2000-01-26
GB2340005A (en) 2000-02-09
JP2000059892A (ja) 2000-02-25

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