EP0959644A2 - Méthode pour modifier un filtre pour l'implémentation d'une fonction de transfert se rapportant à une tête artificielle - Google Patents

Méthode pour modifier un filtre pour l'implémentation d'une fonction de transfert se rapportant à une tête artificielle Download PDF

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Publication number
EP0959644A2
EP0959644A2 EP19990303966 EP99303966A EP0959644A2 EP 0959644 A2 EP0959644 A2 EP 0959644A2 EP 19990303966 EP19990303966 EP 19990303966 EP 99303966 A EP99303966 A EP 99303966A EP 0959644 A2 EP0959644 A2 EP 0959644A2
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EP
European Patent Office
Prior art keywords
transfer function
filter
ear
ear transfer
amplitude
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Withdrawn
Application number
EP19990303966
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German (de)
English (en)
Inventor
Alastair Sibbald
Fawad Nackvi
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Creative Technology Ltd
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Central Research Laboratories Ltd
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    • 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
    • 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
    • H04S2420/01—Enhancing the perception of the sound image or of the spatial distribution using head related transfer functions [HRTF's] or equivalents thereof, e.g. interaural time difference [ITD] or interaural level difference [ILD]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S3/00—Systems employing more than two channels, e.g. quadraphonic

Definitions

  • This invention relates to a method of modifying a filter for implementing a head-related transfer function (HRTF) for use in the reproduction of three-dimensional (3D) sound.
  • HRTF head-related transfer function
  • Transaural crosstalk from each of the loudspeakers may be cancelled by creating appropriate crosstalk cancellation signals from the opposite loudspeaker.
  • Crosstalk cancellation signals are equal in magnitude and inverted (opposite in polarity) with respect to the transaural crosstalk signals.
  • a system for performing transaural crosstalk cancellation is discussed in the published International Patent Application No. WO-A1-9515069.
  • the direct sound When listening to a real sound source in an ordinary environment (e.g. a living room), the first sound that the listener hears is termed the "direct" sound (so called because it travels directly to the ears).
  • the direct sound is soon followed by the first reflections from the floor, ceiling and walls, some milliseconds later (or tens of milliseconds, depending on the dimensions of the room).
  • the first reflections are themselves reflected back again to the listener from other boundaries, and these sound waves are termed secondary reflections, or second-order reflections. This process continues until the sound energy has been totally absorbed by the boundaries of the environment, and by the air itself.
  • the reflections which follow the first few reflections soon begin to overlap each other, becoming complex and scattered, and are termed the reverberant sound.
  • the placing of a virtual sound source using HRTF filters uses a considerable amount of computational effort, it is common to simulate only the direct sound, and not the reflections. Consequently, the resulting virtual sound is anechoic, that is, it lacks the reflected components. This can be a disadvantage, as such reflected components can help the brain determine distance and reinforce spatial effects.
  • a further limitation in conventional 3D sound reproduction is that when reproducing virtual sounds via loudspeakers, the sounds originating from the loudspeakers themselves may be reflected from surfaces such as walls, floor, ceiling, and furniture. These sound reflections may conflict with the virtual sound image, especially if the virtual sound image is placed behind the listener. This is because sound reflections from room boundaries close to the loudspeaker "overwhelm" the 3D cue arising from spectral shaping by the outer ear, and so the inter-aural time delay (ITD) cue predominates. This causes the virtual sound source to flip from the required rearward position to a position in front of the listener which shares the same ITD value.
  • ITD inter-aural time delay
  • FIG. 4 An example illustrating this point is the virtualisation of rear surround speakers for the Dolby AC-3 5.1 system.
  • Dolby and AC-3 are trademarks of Dolby Laboratories Inc.
  • An audio system incorporating the AC-3 compression standard provides for multi-channel digital surround sound.
  • AC-3 5.1 gives separate audio channels for left, right, and centre speakers in front of the listening position, two rear surround speakers, and a sub-woofer positioned according to the listener's preference.
  • a typical loudspeaker configuration for the AC-3 system is shown in Figure 4.
  • Figures 1 and 2 show a co-ordinate system used for the following description.
  • the convention chosen here for referring to azimuth angles is that they are measured from the frontal pole P towards the rear pole P' , with positive values of azimuth on the right-hand side of the listener and negative values on the left-hand side.
  • Rear pole P' is at an azimuth of +180° (and -180°). Angles of elevation are measured directly upwards (or downwards, for negative angles) from the origin at the centre of the head of the listener relative to the horizontal plane.
  • the preferred positions of the rear surround speakers in the AC-3 system are ⁇ 120° azimuth and 0° elevation. Therefore, the use of a +120°, and a -120°, HRTF is required.
  • the characteristics of the +120° and -120° HRTF are very similar to those of the +60° and -60° HRTF: the inter-aural time delays for both HRTFs are identical (522 ⁇ s). Consequently, when attempts are made to create a virtual sound source at +120° (or - 120°), the presence of unwanted reflections from room boundaries adjacent the loudspeakers, in addition to the absence of virtual reflections from the virtual sound source, causes the image to flip to the +60° (or -60°) position. Thus sounds placed at an azimuth of +120° (or -120°) appear to be in front of the listener at +60° (or -60°), and the illusion of the surround sound effect is disturbed.
  • An aim of the present invention is to provide more effective virtual sound source placement in three dimensions, particularly, but not exclusively, for virtual sound sources placed behind a listener, by modification of the characteristics of a filter for implementing a head-related transfer function.
  • a method of modifying the characteristics of a filter for implementing a head-related transfer function (HRTF), the HRTF including a near-ear transfer function and a far-ear transfer function comprising increasing the magnitude of the amplitude of the near-ear transfer function and/or far-ear transfer function over a range of frequencies to give an exaggerated near-ear transfer function and/or an exaggerated far-ear transfer function, the amount of the increase at a given frequency being a function of the amplitude of the corresponding transfer function or functions at the given frequency, thereby forming a filter which implements an HRTF having an exaggerated near-ear transfer function and/or an exaggerated far-ear transfer function.
  • HRTF head-related transfer function
  • the magnitude of the amplitude of the near-ear transfer function, and/or the far-ear transfer function is increased by convolving the transfer function with itself.
  • the amplitude of the exaggerated near-ear transfer function and/or the amplitude of the exaggerated far-ear transfer function may be limited over a range of frequencies above a threshold value.
  • the threshold value may be, for example, 6 kHz.
  • the amplitude of the exaggerated near-ear transfer function and/or the amplitude of the exaggerated far-ear transfer function may be adjusted so that the amplitude of the exaggerated near-ear transfer function and the amplitude of the exaggerated far-ear transfer function tend to the same value at frequencies below, for example, 100 Hz.
  • a filter modified using the aforedescribed method is provided.
  • the modified filter is used for implementing an HRTF, the HRTF having an amplitude response characteristic curve substantially as shown in plot B of Figure 8.
  • the filter may also include crosstalk cancellation means.
  • the filter may be used in a multi-channel surround sound system, or a multi-channel encoding system.
  • the modified filter for implementing an HRTF places a virtual sound source at positions behind a listener.
  • the virtual sound sources are placed at azimuths of ⁇ 120° and elevations of 0° relative to a listener.
  • the virtual sound source is placed at an elevation of ⁇ 90° relative to a listener.
  • the modified filter is a finite impulse response filter.
  • a sound recording or transmission made using a modified filter implementing an HRTF.
  • a signal processed using a modified filter implementing an HRTF is provided.
  • a filter implementing an HRTF (12), shown in Figure 3 is modified to provide improved positioning of a virtual sound source.
  • an HRTF (12) placing a virtual sound source at an azimuth of +120° and elevation of 0° is described.
  • an HRTF of azimuth angle 60° and elevation 0° will be referred to as a 60° HRTF.
  • the method described may also be applied to the -120°, or indeed any, HRTF.
  • Figure 5 shows the near-ear amplitude response (16a) of a 120° HRTF, and the far-ear amplitude response (16b) of the same function.
  • near-ear corresponds to the ear of a listener which is nearest to the virtual sound source
  • far-ear is the ear furthest away from the virtual sound source.
  • the HRTF (12) therefore comprises a near-ear transfer function (16a), a far-ear transfer function (16b), and an inter-aural time delay.
  • Figure 6 shows the near-ear amplitude response (18a), and the far-ear amplitude response (18b), of a 60° HRTF. It can be seen that the general form of the far-ear data (16a and 18b) for both plots is similar. However, the near-ear data (16a) of Figure 5 exhibits some differences from the near-ear data (18a) of Figure 6. It should be noted that, in this example, differences in the far-ear responses (16b, 18b) are not as obvious to the brain as differences in the near-ear responses (16a, 18a). This is because the far-ear response (16b, 18b) is generally associated with less energy than the near-ear response (16a, 18a).
  • the prime difference between the 120° HRTF and the 60° HRTF appears to be the near-ear amplitude responses (16a, 18a).
  • this difference is not large enough for the brain to be able to distinguish the 120° near-ear response (16a) from the 60° near-ear response (18a) in the presence of real reflections, and the absence of virtual reflections.
  • the invention overcomes this deficiency by exaggerating the spectral features of the near-ear amplitude response (16a, 18a) to provide more spectral information to the listener's brain.
  • the first embodiment of the present invention provides a method of creating more pronounced spectral data by increasing the magnitude of the amplitude of the near-ear function (16a, 18a) over a range of frequencies.
  • the amount of the increase at a given frequency is a function of the amplitude of the near-ear function (16a,18a) at the given frequency.
  • the near-ear function (16a) is convolved with itself. This results in an exaggerated near-ear function (26a), as shown in Figure 7, with an increase in the magnitude of peaks and troughs, at all frequencies.
  • the magnitude of the trough at 4 kHz in the unmodified function has been increased.
  • a filter may then be designed to implement an HRTF having an exaggerated near-ear function (26a).
  • an exaggerated near-ear function 26a
  • a near-ear function and a far-ear function which have undergone any one of a number of processing steps according to the method described herein, are known as exaggerated near-ear and far-ear functions, respectively.
  • Figure 7 shows the near-ear transfer function (16a) of the 120° HRTF (12a), convolved with itself (26a), and its overall gain adjusted for low frequency alignment of the modified and unmodified functions.
  • the virtual sound source appears to be located at +120°, and not at +60° as can occur with the unmodified filter which implements the original 120° HRTF.
  • the size of the increase in magnitude of the amplitude of the near-ear function may be varied. For example, if the near-ear transfer function is convolved with itself, the amplitude values of the transfer function are squared at a given frequency. If, however, the amplitudes of the transfer function are raised to the power 3, the resulting modified function will have more exaggerated features, and the 3D effects will be enhanced further. This may be appropriate for use in computer games, for example. Alternatively, the amplitude values of the transfer function may be raised to the power 1.5. This results in more subtle effects, and may be used advantageously, for example, for classical music recordings.
  • the high-frequency components of the exaggerated near-ear function can be limited, typically by appropriate design of the filters used for the signal processing. In this example, frequencies of more than 10 kHz are limited. This is shown in Figure 8, plot B. However, the point at which the high frequencies are limited may vary from 10 kHz. For example, it may be desirable to reduce high frequency components above 6 kHz, or above 20 kHz.
  • Modified filters which implement the exaggerated HRTFs may be used in many applications. Examples of these applications will now be described.
  • the AC-3 surround sound listening format there is provision for 6 loudspeakers: front left, centre, front right, surround left (rear), surround right (rear), and a non-directional sub-woofer.
  • a sound engineer can "pan" sounds from one position to another by varying the relative loudness of the sound being fed to the various loudspeakers. For example, a sound source may be panned from the front right speaker to the rear left speaker, and the sound would appear to the listener to move from the front right speaker to the rear left speaker through him or herself. However, it may be required for some applications that a sound is panned over the head of the listener, or underneath the listener.
  • HRTF may be produced via the method described in the first embodiment of the invention, and used as a "height" filter for surround sound mastering (or encoding) applications. This would enable panning from the front of a listener, to behind the listener, passing over the top of the listener's head.
  • HRTF may also be produced to make a "depression” filter, and could be used to enable panning from a position in front of a listener, passing underneath the listener, to a position behind the listener. This approach enables the conventional sound format to extend into the third dimension without any changes in the user's hardware, and without any change in format, bandwidth and the like.
  • the method of the invention may also be used in conjunction with vertical balance adjustment.
  • Vertical balance adjustment is described in published International Patent Application, No. WO-A1-9517799.
  • a set of digital filters may be produced which implement an entire exaggerated HRTF library. This may be appropriate for applications such as PC games, where 3D effects with great spectral impact are more important than optimal tonal quality.
  • a sound recording or a transmission such as, for example, via wire based or wireless telegraphy, may be made by using modified filters which implement the exaggerated HRTFs.
  • the method of the invention may be applied to the far-ear transfer function (16b,18b), or to both the near-ear transfer function (16a,18a) and the far-ear transfer function (16b,18b).

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  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
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EP19990303966 1998-05-22 1999-05-21 Méthode pour modifier un filtre pour l'implémentation d'une fonction de transfert se rapportant à une tête artificielle Withdrawn EP0959644A2 (fr)

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GB9811054A GB2337676B (en) 1998-05-22 1998-05-22 Method of modifying a filter for implementing a head-related transfer function

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1542502A2 (fr) * 2003-12-10 2005-06-15 Ultrasone AG Casque d'écoute à effet spatial
US20120008789A1 (en) * 2010-07-07 2012-01-12 Korea Advanced Institute Of Science And Technology 3d sound reproducing method and apparatus
US9204236B2 (en) 2011-07-01 2015-12-01 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US10142761B2 (en) 2014-03-06 2018-11-27 Dolby Laboratories Licensing Corporation Structural modeling of the head related impulse response
CN112188358A (zh) * 2019-07-04 2021-01-05 歌拉利旺株式会社 音频信号处理装置、音频信号处理方法及非易失性计算机可读记录介质

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2366975A (en) * 2000-09-19 2002-03-20 Central Research Lab Ltd A method of audio signal processing for a loudspeaker located close to an ear
US6738479B1 (en) 2000-11-13 2004-05-18 Creative Technology Ltd. Method of audio signal processing for a loudspeaker located close to an ear
US6741711B1 (en) 2000-11-14 2004-05-25 Creative Technology Ltd. Method of synthesizing an approximate impulse response function

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA2158451A1 (fr) * 1993-03-18 1994-09-29 Alastair Sibbald Traitement de signaux audio provenant de canaux multiples

Cited By (19)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1542502A2 (fr) * 2003-12-10 2005-06-15 Ultrasone AG Casque d'écoute à effet spatial
EP1542502A3 (fr) * 2003-12-10 2009-04-22 Ultrasone AG Casque d'écoute à effet spatial
US20120008789A1 (en) * 2010-07-07 2012-01-12 Korea Advanced Institute Of Science And Technology 3d sound reproducing method and apparatus
KR20230019809A (ko) * 2010-07-07 2023-02-09 삼성전자주식회사 입체 음향 재생 방법 및 장치
US10531215B2 (en) * 2010-07-07 2020-01-07 Samsung Electronics Co., Ltd. 3D sound reproducing method and apparatus
AU2018211314B2 (en) * 2010-07-07 2019-08-22 Korea Advanced Institute Of Science And Technology 3d sound reproducing method and apparatus
AU2017200552B2 (en) * 2010-07-07 2018-05-10 Korea Advanced Institute Of Science And Technology 3d sound reproducing method and apparatus
US10244343B2 (en) 2011-07-01 2019-03-26 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US9838826B2 (en) 2011-07-01 2017-12-05 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US9549275B2 (en) 2011-07-01 2017-01-17 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US10609506B2 (en) 2011-07-01 2020-03-31 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US11057731B2 (en) 2011-07-01 2021-07-06 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US9204236B2 (en) 2011-07-01 2015-12-01 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US11641562B2 (en) 2011-07-01 2023-05-02 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US12047768B2 (en) 2011-07-01 2024-07-23 Dolby Laboratories Licensing Corporation System and tools for enhanced 3D audio authoring and rendering
US10142761B2 (en) 2014-03-06 2018-11-27 Dolby Laboratories Licensing Corporation Structural modeling of the head related impulse response
CN112188358A (zh) * 2019-07-04 2021-01-05 歌拉利旺株式会社 音频信号处理装置、音频信号处理方法及非易失性计算机可读记录介质
EP3761674A1 (fr) * 2019-07-04 2021-01-06 Clarion Co., Ltd. Appareil, procédé et programme de traitement du signal audio
JP2021013063A (ja) * 2019-07-04 2021-02-04 クラリオン株式会社 オーディオ信号処理装置、オーディオ信号処理方法及びオーディオ信号処理プログラム

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GB2337676B (en) 2003-02-26
GB9811054D0 (en) 1998-07-22
GB2337676A (en) 1999-11-24

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