WAVE FIELD SYNTHESIS A PROMISING SPATIAL AUDIO RENDERING CONCEPT. Günther Theile. Institut für Rundfunktechnik (IRT) Munich, Germany

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1 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Italy, Octber 5-8, 5-8, DAFx Keynte WAVE FIELD SYNTHESIS A PROMISING SPATIAL AUDIO RENDERING CONCEPT Günther Theile Institut für Rundfunktechnik (IRT) Munich, Germany theile@irt.de ABSTRACT Mdern cnvlutin technlgies ffer pssibilities t vercme principle shrtcmings f ludspeaker sterephny by expliting the Wave Field Synthesis (WFS) cncept fr rendering virtual spatial characteristics f sund events. Based n the Huygens principle ludspeaker arrays are reprducing a synthetic sund field arund the listener, whereby the dry audi signal is cmbined with measured r mdelled infrmatin abut the rm and the surce s psitin t enable the accurate reprductin f the surce within its acustical envirnment. Nt surprisingly, basic and practical cnstraints f WFS systems limit the rendering accurateness and the perceived spatial audi quality t a certain degree, dependent n characteristic features and technical parameters f the sund field synthesis. Hwever, recent develpments have shwn already that a number f applicatins culd be pssible in the near future. An attractive example is the synthesis f WFS and sterephny ffering enhanced freedm in sund design as well as imprved quality and mre flexibility in practical playback situatins fr multichannel sund mixes. 1. INTRODUCTION Three psychacustic fundamentally different spatial audi imaging methds shuld be distinguished: (Multichannel) ludspeaker sterephny Binaural recnstructin f the ear input signals Syntheses f the sund field arund the listener All knwn spatial sund systems can be traced back t ne f these methds r can cntain mixed frms theref, whereby certain advantages f the methds are being explited, respectively its disadvantages are avided, dependent n the intended applicatin area Ludspeaker sterephny This is in principle based n the characteristics f lcalizatin in the superimpsed sund field, generated by tw ludspeakers [1]. Directinal imaging is dne in the imaging area between tw adjacent ludspeakers [2]. In the case f 3/2 sterephny, with the assistance f surrund channels the imaging area between the frnt ludspeakers can be extended. Therefre pssibilities are ffered fr the reprductin f early lateral sund fr imaging f spatial depth as well as reverberatin, in rder t prduce the spatial impressin and the envelpment. Details are described in [3] Binaural recnstructin f the ear input signals The riginal emplyment f this methd is the knwn dummy head sterephny. It is nt intended t reprduce a suitable sund field at the reprductin lcatin. Instead, the effective ear signals in the recrding lcatin are recrded with the assistance f a dummy head and replayed in principle via headphnes. Under ideal circumstances, the reprduced binaural signals are identical t the riginal ear signals that the listener received in the recrding lcatin. In practice it is pssible t reprduce auditry events with excellent realism regarding spatial characteristics and sund clr Synthesis f the sund field arund the listener The third apprach has been pursued within the framewrk f the Eurpean Research Prject CARROUSO [4]. It is based n the cncept f Wave Field Synthesis (WFS, develped at the Technical University Delft, refer e.g. [5], [6]), i.e. the representatin f a virtual surce and a virtual rm is achieved by rendering an acustically crrect sund field. The principle f WFS is based upn the assistance f ludspeaker arrays, when a cmplete sund field is generated in the listening zne which is identical t an apprpriate real sund event (see Sectin 2). This acustical cunterpart t the ptical hlgraphy is als described as hlphny. The binaural ear input signals that are active fr the auditry event thus arise in a natural way within the sund field, cntrary t dummy head sterephny Cmbining sterephny and WFS Further develpments f spatial audi systems are based n useful cmbinatins f basic methds, using sphisticated real time cnvlutin algrithm. This paper cncentrates n vercming certain practical drawbacks f multichannel sund n the ne hand and f WFS n the ther. S called Virtual Panning Spts (VPS) are intrduced t imprve the WFS rendering quality f large cmplex surces (e.g. chir ), t reduce the number f WFS transmissin channels and t ensure cmpatibility and scalability. Useful cmbinatins f VPS and cnventinal r sphisticated sterephnic panning and mixing techniques will prvide advanced facilities fr spatial sund design. A special VPS applicatin allws play back f cnventinal multichannel mixes in a virtual high quality listening rm rendered by means f WFS technlgies, ffering full backwards cmpatibility with usual ludspeaker sterephny, ptimum multichannel frmat flexibility, as well as attractive practical benefits in the hme, in the cinema, r in ther applicatins. DAFX DAFx'04 Prceedings 125

2 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Italy, Octber 5-8, 5-8, WFS PRINCIPLES AND PROPERTIES 2.1. The Huygens principle If frm a pint S f a hmgeneus istrpic medium a spherical wave is emitted, ne can imagine the prcedure f the individual wave reprductin in that a particle brught int scillatin by external frces, transfers its mvements t its neighbring particles. This prcedure then cntinues symmetrically in all directins and in this way gives cause t a spherical wave... [7]. If a sund surce S (the s-called primary surce ) emits spherical wave frnts, ne can imagine in accrdance with the Huygens Principle each emitted wave frnt (refer t Figure 1a blue), thrugh the additin f all participating secndary surces (which als emit spherical waves) n the surface O. Due t the knwledge f the wave frnt n surface O (Figure 1a red) the state f the scillatin can be determined at an arbitrary pint P f the sund field. The wave frnt thrugh pint P is cnstructed thrugh the summatin f all participating secndary surce signals. surce S, (which emits a sine impulse and is lcated in an infinitely large plane withut demarcatin f walls), a wave frnt results as illustrated in Figure 2a). If ne nw places an array f n micrphnes (M) in this primary sund field and ne reprduces the recrded micrphne signals via an equally arranged array f n ludspeakers (L) special equalizatin has t be included accrding t the relevant physical basics in a reprductin rm (Figure 2b), ne btains the synthesized wave frnt in the (red dtted) listening area. At any place in the listening area the listener perceives a virtual sund surce S, as he can mve arund freely, whilst the virtual sund surce remains crrectly lcalized in terms f its directin (see [5] r [8]). (a) (a) (b) (b) Figure 2: Principle f WFS: (a) ideal surce respnse and (b) typical utput f a finite WFS array Special Prperties f WFS Figure 1: The Huygens Principle: (a) Theretical Mdel; (b) Applicatin WFS. In principle, in the case f the WFS, ne replaces the secndary pint surces by ludspeakers and in this way again prduces a spherical wave (refer t Figure 1b). The sund surce S is virtual; the listener in pint P receives the same wave frnt which is transmitted by the sund surce S WFS the applicatin f the Huygens Principle This applies crrespndingly fr a circular arrangement f the ludspeakers n a tw dimensinal level. Cncerning a sund Lcalizatin f virtual surces Thrugh WFS the sund engineer has a pwerful tl t design a sund scene. One f the mst imprtant (with respect t cnventinal techniques) nvel prperties is its utstanding capability f prviding a realistic lcalizatin f virtual surces. Typical prblems and cnstraints f a sterephnic image vanish in a WFS sund scene. In cntrast t sterephny WFS is able t: prduce virtual surces that are lcalized n the same psitin thrughut the entire listening area, refer Figure 5: The red (dashed) and pink (dtted) arrws indicate the directins f the auditry events when the red and pink virtual pint surces are reprduced. prduce plane waves that are lcalized in the same di- DAFX DAFx'04 Prceedings 126

3 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Italy, Octber 5-8, 5-8, (a) (b) Listening Area Figure 4: Wave frnts f virtual surces [10]: (a) behind the array; (b) in frnt f the array. Figure 3: WFS is capable f reprducing bth the stable psitins f pint surces (red and pink, dashed and dtted) and the stable directin f a plane wave (blue, slid). rectin thrughut the entire listening area, refer Figure 5: The blue (slid) arrws indicate the directin f the auditry event when the blue plane wave is reprduced. enhance the lcalizatin f virtual surces and the sense f presence and envelpment thrugh a realistic reprductin f the amplitude distributin f a virtual surce. In ther wrds, when the listener is appraching the lcatin f a virtual surce the amplitude increases in a realistic way. Accrdingly, the amplitude f a plane wave - which can be seen as a surce in infinite distance - changes least n different listener psitins. These prperties enable the synthesis f cmplex sund scenes which can be experienced by the listener while mving arund within the listening area. Figure 3 illustrates the way in which the sund image changes at different listening psitins. This feature can be made use f deliberately by the sund engineer t realize new spatial sund design ideas. Mrever, it has been shwn that the enhanced reslutin f the lcalizatin cmpared with sterephny [9] enables the listener t easily distinguish between different virtual surces, which makes the sund scene significantly mre transparent Virtual Sund Surces in Frnt f the Ludspeaker Array ( Fcused Surces ) Figure 4 shws the wave frnts f a pint surce behind the array (a) and in frnt f the array (b) in a simulatin. The cncave wave frnts f Figure 4a achieve the synthesis f the signal f a virtual surce behind the array. WFS, hwever, is als capable f synthesizing a virtual surce in frnt f the array. Therefre the WFS array emits cnvex wave frnts which fcus in a pint that will be lcalized as the psitin f the fcused surce (Figure 4b). Naturally, the lcalizatin will nt be crrect fr listening psitins between the fcus pint and the array because the sund emissin f the virtual surce ccurs here reversely. Fr the practical applicatin, it is an enrmus prgress that virtual sund surces in the field between the listener and the ludspeakers can be created. As will be discussed in Sectin 4, sund engineers can be ffered cmpletely new tls fr the spatial sund design. 3. WFS PRACTICAL CONSTRAINTS Nt surprisingly, in practice it is nt pssible t match all theretical requirements fr a perfect result. The rendered WFS sund field differs frm the desired sund field t sme degree fr a number f reasns (fr details see [11]) Discreteness f the array (spatial aliasing) This effect prduces spatial and spectral errrs f the synthesized sund field due t the discretisatin f a cntinuus secndary surce distributin. Abve the spatial aliasing frequency f alias the time difference between tw successive ludspeaker signals interferes at the listener s psitin, depending n the spatial sampling interval, i.e. the ludspeaker / micrphne inter-spacing Reflectins f the reprductin rm (spatial interference) A WFS array can nt render the desired sund filed perfectly if reflectins f the reprductin rm prduce interference in spatial perceptin. In particular, perceptin f distance, depth and spatial impressin are affected, because fragile distance cues f synthesised surces can be dminated by the strnger distance cues generated by the array speakers. They interfere with the desired reflectin pattern f the synthesised surce. Special rm cmpensatin algrithms being under investigatin [12], [13] will perhaps be able t minimize this effect Restrictin t the hrizntal plane Thery des nt restrict WFS t the hrizntal plane. Hwever, the reductin f the array dimensin t the hrizntal plane is the practical apprach, having a number f cnsequences. First, virtual surces can be synthesized nly within the hrizntal plane. This includes virtual reflectins affecting the cmpleteness f a natural reflectin pattern and thus pssibly resulting in impairments f perceptin f distance, depth, spatial impressin and envelpment. Anther aspect is related t the measuring techniques used fr capturing the rm respnse. In practice there is sme mismatch with respect t elevated reflectins, because the measured rm respnse includes elevated reflectins althugh they are reprduced nly in the hrizntal plane. The effects f these types f DAFX DAFx'04 Prceedings 127

4 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Italy, Octber 5-8, 5-8, inaccurateness n spatial perceptin parameters are nt wellknwn yet. Furthermre, hrizntal arrays d nt generate real spherical waves, but cylindrical waves. In the case f imaging a plane wave fr example there results an errr with respect t the level rll-ff (3dB/dubling f distance), in cmparisn with the ideal plane wave (n rll-ff) [11], [14] Limitatin f array dimensins (diffractin) In practical applicatins the ludspeaker array will have a finite length. Due t a finite array s-called diffractin waves riginate frm the edges f the ludspeaker array [11], [14]. These cntributins appear as after-eches (and pre-eches respectively fr fcussed surces), and depending n their level and time-ffset at the receiver s lcatin may give rise t cluratin. Hwever, methds t reduce these truncatin effects are knwn, e.g. by applying a tapering windw t the array signals. This means that a decreasing weight is given t the ludspeakers near the edges f the array. In this way the amunt f diffractin effects can substantially be reduced at the cst f a limitatin f the listening area [14] Effects n perceptin Althugh a number f authrs have suggested methds t deal with the practical limits f rendering accurateness r t minimize their effects, there is still a lack f knwledge (sme details can be fund e.g. in [5], [6], [11], [12], [15]). Several effects f the cnstraints n specific perceptual attributes are nt knwn yet in detail. Hwever, this knwledge is imprtant fr further develpments f WFS systems in view f future applicatins. Current psychacustic studies are cncentrating n the subjective evaluatin f principle characteristics f WFS systems in cmparisn with sterephnic r binaural systems. They are necessary t evaluate the resulting impacts n attributes f spatial perceptin nt nly with respect t the develpment f WFS systems fr different applicatins but als in view f scientific knwledge. Particular attentin shuld be turned t the perceptin f directin, distance, spatial depth, spatial perspective, spatial impressin, reverberance, and envelpment, as well as sund clur. 4. WFS APPLICATIONS 4.1. The Eurpean CARROUSO Prject 1 The Eurpean CARROUSO Prject ( Creating, Assessing and Rendering in Real Time f High Quality Audi-Visual Envirnments in MPEG-4 Cntext ) has intended t break several limitatins f these current cmmercial systems by merging the new WFS rendering technique with the flexible new cding technlgy MPEG-4 standard, allwing bject-riented and interactive sund manipulatin. By means f the MPEG-4 frmat the signal f the surce ( Gestalt ) and its spatial prperties are transmitted separately. Fr reprductin, the dry surce signal is cnvlved with the measured r mdelled set f impulse respnses (cntaining the spatial infrmatin), and emitted by a ludspeaker array. In cntrast t sterephny WFS is able t prduce virtual stable surces lcalized at the same psitin thrughut the entire listening area, prduce virtual surces in frnt f the ludspeaker array ( fcused surces ) prduce plane waves that are lcalized in the same directin thrughut the entire listening area, enhance the sense f depth, spatial impressin and envelpment thrugh a realistic reprductin f the riginal rm respnse The key bjective f the prject CARROUSO was t prvide a new technlgy that enables t transfer a sund field, generated at a certain real r virtual space, t anther usually remte lcated space, in a bit efficient way at highest perceived quality. The principle blck diagram is illustrated in Figure 5, it shws three functinal cmpnents: Capturing Fr recrding f the sund field a micrphne array technlgy is applied. Signal prcessing calculates the psitin f the sund surces which culd be fixed r mving. The micrphne array as well as the vide cameras is used t gather relevant infrmatin n the acustical cnditins in the recrding rm. Acustic mdels can be btained t parameterise the acustic data set, thus making it suitable fr transmissin. Transmissin Encding f audi bjects is perated by the MPEG-4 standard and encapsulated int specific data streams. Fr bradcasting applicatins the transmissin adpts digital vide bradcasting (DVB) streams. The cding uses a subset f MPEG-4 cmpnents (n predefined prfile fr the given applicatin in the standard). Rendering In the rendering prcess the transmitted data are demultiplexed, decded and prcessed by a cmpsitr. It enables the reprductin f a recrded r simulated sund field via WFS ludspeaker arrays, ensuring immersive sund perceptin in a wide listening area. The riginal acustics f the reprductin rm, which may negatively influence the btained result, is ptimally crrected fr. All cmpnents were cmbined t a demnstratr, as a basis fr validatin. This was dne using perceptual experiments and field tests. The results f the prject are a first step twards a new quality f high quality spatial audi imaging. CARROUSO has shwn the pssibility t capture, transmit and render sund surces and their related acustic envirnment with mre realism, cmpared t existing sterephnic methds. This nvel spatial audi technlgy was develped fr applicatins in cnjunctin with mving pictures, using the recently intrduced MPEG-4 standard. It is cnsidered as a majr milestne fr immersive audi representatin at public places and in private husehlds. Tw applicatins have been targeted within this prject. The first ne cncerns high quality spatial audi with assciated vide fr bradcasting. The secnd applicatin is related t cperative and interactive wrk n immersive audi bjects. CARROUSO is expected t cntribute t infrmatin, cmmunicatin and media technlgy. 1 EU-Prject IST (Jan June 2003): [4]. 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5 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Italy, Octber 5-8, 5-8, "Rm" Parameter Mdelling Live Sund Recrding RECORDING MPEG-4 Audi Encder BIFS & SAOL encder Natural audi encders MPEG-4 Audi Stream DVB frmat Multiplex MPEG-2/ MPEG-4 vide encder Multiplex - Synchrnizer NETW ORK MPEG-2/ MPEG-4 Vide Stream Server TRANSMISSION Cded vide Demultiplexer MPEG4 Stream Vide decder MPEG-4 Decder BIFS Decding SAOL prcessing Audi decders Cmpsitin Auralizatin WFS Rendering Display RENDERING User Interactin Figure 5: Principle blck diagram f the CARROUSO demnstratr Synthesis f WFS and sterephny This paper cncentrates n the synthesis f WFS and sterephny. Figure 6 illustrates the basic cncept by means f music recrding. Step ne is always the rm respnse measurement in the music hall, dne e.g. with a stepwise rtating micrphne. This measured spatial infrmatin is stred in the WFS prcessr. Fr recrding f rchestra and slist clsely spaced spt micrphnes are used. The sterephnic rchestra mix shuld be cmpsed in a way that it cntains as little rm infrmatin (reverb, reflectins, etc.) as pssible; but it shuld cntain the adequate spatial distributin f elements. This three channel sterephnic mix signals and the slist signal are being cnvlved with the apprpriate spatial impulse respnses. As a result, the rendered WFS sund field represents stable virtual surces lcated in the cncert hall. Listeners within the listening area perceive a three-channel sterephnic image f the rchestra and a pint surce image f the slist, whereby the reprduced characteristics f the cncert hall give a new sense f realism. On this basis apparent advantages f established cnventinal sterephnic recrding techniques n the ne hand and f WFS technlgies n the ther can, in principle, can be utilized thrugh a purpseful cmbinatin Virtual Panning Spts (VPS) The key tl is use f s-called Virtual Panning Spts (VPS) [16], virtual pint surces t be applied fr panning acrss any sterephnic imaging plane in the virtual WFS imaging area. VPS can be understd as virtual ludspeakers which reprduce the sterephnic sund image f a spacius sund surce (e.g. a chir) in the recrding rm (see als [16], [17], [18]). The suitable rm impulse respnses have t be measured in the riginal rm r t be created artificially in a suitable way. In the example in terms f Orchestra Recrding Sterephnic mix Slist Wave field synthesiser (Cnvlutin) Rm respnse measurement Strage Figure 6: WFS: Separate handling f surces and spatial infrmatin. Figure 7, the rchestra is imaged with the assistance f six VPS, which are reprduced via WFS and are relatively freely cnfigurable with regard t lcalizatin, expansin and distance. The sund design advantage f this cncept is self-explanatry: The sterephnic recrding f the rchestra accrding t Figure 6 prduces a spacius sund image f the surces as there is an image between the VPS in accrdance with the principles f phantm surces lcalizatin. The ludspeakers are virtual surces, generated thrugh WFS and prvided with the rm characteristics f the recrding rm. The lcatins f the VPS behave direc- DAFX DAFx'04 Prceedings 129

6 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Octber 5-8, 5-8, Wave field synthesiser (Cnvlutin) Virtual imaging area Orchestra rear imaging plane Orchestra frnt imaging plane VPS Listening area Listening area Listening area Figure 8: Example fr the cnfiguratin f the sterephnic imaging areas. Figure 7: Use f Virtual Panning Spts (VPS) tinally stable in the listening area. The knwn disadvantages f phantm surce lcalizatin, especially the lw directinal stability can be easily avided by emplying a sufficient number f VPS. The number f sterephnic imaging areas is in principle arbitrary. Frm an artistic pint f view, ne shuld rientate neself twards the number f spacius instruments r instrument grups (in large ensembles, e.g. string grups, brass player grups, chir). The number and spatial distributin f the VPS depends n the fllwing criteria: Size and shape f the hmgeneus ensemble Circumstances f the prductin Artistic and sund balance-related intentin f the sund engineer Available transmissin capacity Virtual Panning Spts, VPS, are selected pints ( virtual ludspeakers ), which prduce a sterephnic representatin area. These can in principle be selected at chice in accrdance with the recrding situatin and the desired sund image. As an example, Figure 7 shws tw rear imaging planes and tw frnt imaging planes, ffering easy imaging f depth. The imaging area can be spread ut by an arbitrary number f VPS in a randm spatial expansin in accrdance with the situatin and intentin. The artistic arrangement f the ensemble upn the WFS transmissin cmmences with the chice, dedicatin and psitining f the VPS. Three parameters shuld be mentined, which lends the sund engineer t new pssibilities f spatial sund design (refer t the example in Figure 8): 1. In the case that ludspeaker arrays are installed lateral t the listening area, there are in principal, n prblems as far as directinal stability is cncerned, as a lateral sterephnic representatin sectr can be built up frm a sufficient number f stable VPS. The same applies t the sectr behind the listener. 2. A sterephnic imaging area des nt nly allw itself t be mved in all directins, stretched ut r cmpressed, but can als be presented in an extensive range with different distances. The representatin f depth is thus easily recgnizable. 3. With certain cnstraints (see Sectin and Figure 4), the VPS can be placed in a distance between the listener and the ludspeaker array and als with the sterephnic imaging field. Thus, the virtual imaging area theretically reaches in dense clseness t the listener and allws fr an expressive representatin f depth. The practical example shwn in Figure 8 cntains diverse new pssibilities f the spatial sund design. A ttal f 13 VPS were emplyed, whereby thse mre remte have been arranged in terms f the 3/4 stere ITU standard and act in the knwn fashin as sterephnic imaging areas (als see [3], Sectin 3). In the frnt (stage) area there are sterephnic representatin areas in three different distances and als the lateral sectrs have been prepared fr sterephnic imaging. In additin a mnphnic virtual surce has been prvided in the frnt between the listening area and the ludspeaker array, e.g. fr a slist. The individual sterephnic presentatins shuld cntain the (dry) direct sund f spacius sund surces r ensembles, with as little rm respnse as pssible. The cnventinal tls fr the representatin f spatial depth (generatin f suitable lateral reflectins are superfluus within the cncept f WFS and actually damaging, due t the fact that here by virtual surces (and VPS), the relevant distance infrmatin is available - in the frm f rm impulse respnses. In Figure 8, the parallel lines represent the plane waves fr the reverberatin synthesis. The reverberatin is nt prduced with the assistance f VPS. Rather is the effective surce t reprduce a plane wave infinitely far away [19] Creatin f VPS The lcatins and number f VPSs shuld be fund befre the WFS recrding starts. Suitable ludspeakers are psitined in the VPS areas f the recrding rm. The recrding rm is stimulated, the respnse is measured and the impulse respnse is calculated frm this (see Figure 6). Of curse, instead f a real measurement f the rm impulse respnse, mdel based prcedures DAFX DAFx'04 Prceedings 130

7 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Italy, Octber 5-8, 5-8, can be emplyed t create artificial rm impulse respnses. In practice, fr certain halls, such determinatin f the rm impulse respnses des nt have t be dne at each recrding. Rather, this nce determined data is permanently available in a data base. Als impulse respnses frm ther halls r pseudrealistic artificial (and smetimes better) characteristics can be desirable (cmparable with the input f mdern reverberatrs) Reductin f transmissin channels Thrugh the utilizatin f the VPS cncept, the number f transmissin channels as well as the cmputing capacity can be significantly reduced. The larger an ensemble, and als the number f individual participating instruments, the larger saving in transmissin channels is. In the case f a small ensemble r individual instruments, hwever, the VPS cncept des nt save transmissin channels r nly saves a few (see [16]) Handling aspects A majr gain in the utilizatin f the VPS cncept is the imprved handling f the recrding techniques and the imprvement in the spatial quality f the WFS reprductin. The handling f the VPS cncept is based n the applicatin f sterephnic techniques, which all sund engineers are familiar with. There are nt any cmpletely new micrphne techniques; the mde f peratin remains the same as during cnventinal prductins (except fr the representatin f distance, which is autmatically given within the cnfiguratin f VPS). Als the mix t the VPS psitins requires n new mdus perandi - cnventinal panning and mixing techniques are being used. The number f micrphnes remains apprximately the same as in the case f an apprpriate stere recrding. The recrding in accrdance with the VPS cncept thus has the significant advantage that the sund engineer can directly emply familiar prcesses f wrk. The imprvement n spatial quality by way f WFS ges hand in hand with the incrpratin f sterephnic techniques. The sund engineer has clearly mre influence ver imprtant quality characteristics f a recrding, he has mre creative rm fr manipulating the spatial parameters and can mre strngly influence the sund clr Virtual ludspeaker reprductin An imprtant applicatin f the VPS technique is a special preset f the VPS setup n the reprductin side, which enables the reprductin f cnventinal multichannel recrdings in a virtual listening rm [16]. Fr this purpse, tw mdificatins are suggested fr the WFS decder, which can be activated in the event f need fr applicatin (see Figure 9): 1. The cnfiguratin f the VPS with regard t rm impulse respnses and spatial arrangement is dne in accrdance with the preset setup f virtual ludspeakers in a virtual listening rm. Arbitrary arrangements f the virtual ludspeakers can be preset and be activated dependent n the sterephnic frmat t be reprduced. 2. The virtual surce signal is nt received via the transmissin channel, but frm the multichannel decder n the reprductin side (e.g. that f a DVD player). Surrund sund surce (e.g. DVD) Subw. Wave field renderer (Cnvlutin) LS L VPS cnvlutin data: Respnses frm the reprductin rm C Listening area R RS Cnvlutin data: Respnses frm the recrding rm Figure 9: VPS cnfiguratin fr rendering virtual multichannel ludspeakers. The WPS reprductin unit perates cmpletely detached frm WFS transmissin, and can principally ffer three attractive advantages: 1. Diverse sterephnic multichannel frmats can be easily reprduced ptimal thrugh the selectin f a VPS preset, withut having t apprpriately adjust the ludspeaker arrangement within the living-rm. 2. The virtual ludspeakers can als be placed utside the living-rm, i.e. als in a cnfined area situatin, the listening area fr multi-channel sterephny is sufficiently large. 3. A future high quality WFS reprductin unit will allw fr an electrnic cmpensatin f diverse defects in the reprductin rm [12], especially the reductin f the effect f the early reflectins and the balancing f asymmetrical arrangements f the speaker array. Frm the technical and practical pint f view the applicatin f WFS fr multichannel stere reprductin culd be the first step twards acceptance in the market place. In this regard, the develpment f the s-called MAP technlgy (see e.g. [20], [21]) is imprtant. The flat panels, e.g. fed with glass fiber cables, can ften be better integrated int the living-rm and are mre attractive than cnventinal ludspeakers. Thereby nt nly the applicatin f virtual ludspeakers within the hme is envisaged, but als the emplyment in cinemas, theaters as well as fr high quality sund reinfrcement. 5. ACKNOWLEDGMENT I wish t thank my clleague Helmut Wittek fr his basic cntributins t this paper. DAFX DAFx'04 Prceedings 131

8 Prc. f the 77th Int. Cnference n n Digital Audi Effects (DAFx'04), (DAFX-04), Naples, Italy, Italy, Octber 5-8, 5-8, [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] 6. REFERENCES G. Theile, On the lcalisatin in the superimpsed sundfield, Ph.D. Thesis, Techn. University Berlin, Online at ISATION_english.pdf G. Theile, On the Naturalness f Tw-Channel Stere Sund, J. Audi Eng. Sc., vl. 39, pp , G. Theile, Multichannel natural music recrding based n psychacustic principles, Prc. 19th AES Int. Cnf., 2001, pp Fr an updated versin see hauptmikrfn/theile/multich_recrding_30.oc t.2001_.pdf S. Brix, et al., CARROUSO An Eurpean Apprach t 3D- Audi, 110th AES Cnv., 2001, Preprint A. J. Berkhut, D. de Vries, and P. Vgel, Acustic Cntrl by Wave Field Synthesis, J. Acust. Sc. Am., vl. 93, pp , M. M. Bne, E. N. G. Verheijen, and P. F. van Tl, Spatial Sund Field Reprductin by Wave Field Synthesis, J. Audi Eng. Sc., vl. 43, pp , H. Geiger, and K. Scheel, Handbuch der Physik, Band VIII. Verlag Julius Springer, Berlin, U. Hrbach, and M. Bne, Practical Implementatin f Databased Wave Field Reprductin System, 108th AES Cnv., 2000, Preprint M. Bne, and W. de Bruijn, Imprving Speech Intelligibility in Telecnferencing by using Wave Field Synthesis, 114th AES Cnv., 2003, Preprint M. Bne, Acustic Rendering with wave field synthesis, ACM SIGGRAPH and EUROGRAPHICS campfire, Snwbird, Utah, May 26-29, H. Wittek, Perceptin f Spatially synthesized sund fields Literature review abut WFS, available nline at S. Sprs, A. Kuntz, and R. Rabenstein, An apprach t listening rm cmpensatin with wave field synthesis, Prc. 24 th AES Int. Cnf., 2003, pp E. Crteel, and R. Nicl, Listening rm cmpensatin fr Wave Field Synthesis. What can be dne? Prc. AES 23rd Int. Cnf., Cpenhagen, J.-J. Snke, Variable Acustics by Wave Field Synthesis, Thela Thesis, Amsterdam, Netherlands, 2000, ISBN E. Verheijen, Sund Reprductin by Wave Field Synthesis, Delft University f Technlgy, G. Theile, H. Wittek, and M. Reisinger, Ptential Wavefield Synthesis Applicatins in the Multichannel Sterephnic Wrld, Prc. AES 24 th Int. Cnf., 2003, pp M. Bne, et. al., Virtual surrund speakers with wave field synthesis, 108th AES Cnv., 1999, Preprint C. Kuhn, R. Pellegrini, D. Leckschat, and E. Crteel, An Apprach t Miking and Mixing f Music Ensembles Using Wave Field Synthesis, 115th AES Cnv., M. Bne, W. de Bruijn, and T. Piccl, Sund Recrding Techniques fr Wavefield Synthesis and Other Multichannel Systems, 104th AES Cnv., 1998, Preprint [20] [21] M. Bne, and W. de Bruijn, On the applicability f DML panels fr wave field synthesis based sund reprductin, 108th AES Cnv., 1999, Preprint E. Crteel, et al., Multichannel Inverse Filtering f Multiexciter Distributed Mde Ludspeakers fr Wave Field Synthesis, 108th AES Cnv., 2002, Preprint Günther Theile has btained his Ph.D. frm the Techn. University f Berlin in In his thesis he intrduced a general thery n sund surce lcalisatin. Based n this, he prpsed the sphere stere micrphne and the diffuse field equalizatin, which has becme ITU-R BS. 708 fr studi mnitr headphnes. In 1984 he invented the masking threshld based subband cding technique and started the develpment f the MUSICAM system, which is nw ISO/IEC Standard (MPEG 1 Audi Layer II). He was engaged in investigatins n dynamic range cntrl and n multichannel sund, including multichannel surce cding (MPEG 2 Audi Layer II). Under his chairmanship ITU-R TG 10-1 has prduced Recmmendatin BS. 775, knwn as 5.1 surrund standard. His current activities are cncentrating n further develpments in the field f multichannel sund and research n spatial audi systems based n binaural techniques and wave field synthesis. In his fields f wrk Dr. Theile has published abut ne hundred papers, and he has been granted a number f Internatinal Patents. In 1992 he received the Lthar- Cremer-Medal fr his basic wrk n perceptual surce cding. He has received the AES Fellwship and AES Bard f Gvernrs Awards. He was Chairman f the 102nd AES Cnventin in Munich and f the AES 19th Cnference n Surrund Sund. He has chaired ITU-R Wrking Party 10C in the perid Currently he is serving as Chairman f the AES Suth German Sectin, as Vice-Chairman f the AES Technical Cmmittee n Multichannel and Binaural Audi Technlgies, as Chairman f the VDT specialist grup Research & Develpment and Surrund Sund Frum, as well as Chairman f the Prgramme Cmmittee f the German Tnmeistertagung. DAFX DAFx'04 Prceedings 132