Abstract:
The invention relates to a method of processing an input signal into a phantomized signal, including adding at least one attenuated delayed signal to the input signal, the phantomized signal having at least one delayed attenuated signal and the input signal. A phantomized signal according to the invention offers a new and impressive integration of a center channel signal of a multi channel audio system into the complete audio image provided by the system.

Description:
FIELD OF THE INVENTION 
   The invention relates to a method of processing an input signal into a phantomized signal, a phantomizer for processing a signal into a phantomized signal, and an inverse phantomizer according to claim  8 . 
   BACKGROUND OF THE INVENTION 
   For the past several decades, high quality loudspeakers for studio monitoring as well as domestic use have been designed and optimized for use in a traditional two-channel stereophonic setup. When optimizing the timbre of such loudspeakers, the primary objective is to improve the perceived reproduction quality of voices and musical instruments at the center of the stereo image, right between the loudspeakers, where the solo singer or instrument is usually located. This perceived location improvement is obtained simply by feeding the source signal to both stereo loudspeakers simultaneously at an identical level and phase. We will call this reproduction channel the phantom center channel. 
   When switching from stereophonic to multi-channel audio, the phantom center channel is usually replaced by a physical center channel: A loudspeaker located straight in front of the listener. This was originally maintained when extending cinema systems from monophonic to stereophonic format for the purpose of ensuring correct localization of the movie dialog for all seats in the theatre. Via home cinema systems, such a loudspeaker arrangement has now been standardized for multi-channel audio as well (according to ITU-R Recommendation BS.775-1, “Multi-channel Stereophonic Sound Systems with and without Accompanying Picture”, International Telecommunication Union, Geneva, Switzerland, 1992-1994). Also, the use of a physical center channel in audio-only systems will make the stereo image less sensitive to variations in the listener&#39;s position. 
   However, as the music industry gained experience with this new reproduction format, one problem became apparent: many highly esteemed music producers decided not to use the physical center channel at all, thus discarding its beneficial stabilizing effect on the stereo image. 
   It is the object of this invention to provide a solution which will make the use of the physical center channel attractive. 
   SUMMARY OF THE INVENTION 
   The invention relates to a method of processing a center input signal into a phantomized signal comprising the step of adding at least one attenuated delayed signal to the said input signal, phantomized signal comprising at least one delayed attenuated signal and said input signal. 
   A phantomized signal according to the invention offers a new and impressive illusion of a non-panned signal (center image) of a traditional stereo-system while maintaining a true reproduction of a non-panned signal into only one channel. 
   The obtained phantomized signal may e.g. be fed directly to the sound reproduction system or distribution medium or manipulated by further signal processing means such as reverberation or delay units. 
   Preferably, the complete signal contents of the original input signal and the delayed and attenuated signal should be fed to only one center channel of a multi-channel signal system. Evidently, insignificant signal components of the input signal or the phantomized signal may be fed to the other channels of the rendering system within the scope of the invention. 
   Evidently, both the original input signal and the added delayed signal may be colored by means of additional filtering means. 
   According to the invention, the term “phantomized signal” refers to a signal located in the center image of a traditional multi-channel reproduction system having no center channel. Such a signal may typically refer to a traditional two-channel stereo reproduction system and may typically be located in the center sound image by means of simple panning. 
   A phantomized center channel signal is a signal which has been manipulated in such a way that the timbre of the signal, when reproduced in the center channel, may be perceived by a listener to be a phantomized signal which has been established in the above-mentioned traditional multi-channel system having no center channel. 
   It should be noted, that further coloring and more or less manipulated delay signals may be added to the input signal within the scope of the invention. 
   It should moreover be emphasized that the terms phantomizing/phantomizer/phantomized basically refers to an integration of a center channel signal of a multi channel rendering system into the complete sound image, thereby avoiding that the center channel dominates the sound image. 
   According to the invention, the method of processing, i.e. phantomizing the center channel, may be performed during the mixing of the multi channel audio signal, or the phantomizing may be performed as on-off operation in the rendering system in line with e.g. the traditional Dolby compressing and de-compressing systems. 
   Hence, the multichannel audio signal may comprise a distinct center channel, which may subsequently be broadened if the user of the rendering system actually prefers the phantomizing of the center channel. 
   A further way of implementing the processing according to the invention is that an engineer, when mixing the audio signal, phantomizes the center-channel according to the invention, thereby facilitating a rendering of a phantomized center channel. 
   The rendering system may then, if so desired, comprise a de-phantomizer (inverse phantomizer), which may be activated if the user of the rendering system prefers a distinct rendering of the center channel. 
   The invention facilitates a motivation to the sound engineers to utilize the center channel in multi-channels sound reproducing system having a center channel. 
   Moreover, it should be noted that the term delay basically refers to a mutual relationship between the direct signal and the delayed part of the direct signal. 
   Moreover, it should be noted that the delay may be established in numerous ways within the scope of the invention as long as the delayed signal (which may evidently comprise further delayed signals and processed signal components) represents an approximation to HRTF based transfer functions between the listener(s) and the available loudspeakers. 
   A phantomized signal according to the invention offers a new and impressive integration of a center channel signal of a multi channel audio system into the complete audio image provided by the system. 
   In one embodiment the phantomized signal is fed to the center channel of a multi-channel audio reproduction system or medium. 
   When adding an attenuated delayed center channel signal to the center channel signal, a phantomized center channel signal has been obtained by means of simple signal processing without the requirement of redesign of the loudspeaker used for center channel reproduction. 
   According to the invention, an advantageous processing method of center channel sound signal has been obtained. 
   According to the invention, a center channel signal may be manipulated by means of simple signal processing into a center channel signal having artificial panning corresponding to panning of traditional sound reproduction systems like stereo. It should be noted that the desired two-channel effect has been obtained by manipulation and reproduction of only one channel. 
   Hence, a sound engineer may now use the center channel in a music production. It should nevertheless be noted that a method according to the invention may be applied to both speech and music signals. 
   The method comprises comb-filtering of the input signal. 
   Comb-filtering of the center-channel represents a very simple and cost effective implementation, and the filter may be applied by a sound engineer by means of a simple optional on-/off switch. 
   Evidently, the comb-filtering may be applied to the center channel by means of e.g. an equalizer being carefully adjusted to fit to the desired comb-filtering characteristics. 
   The at least one delayed attenuated signal is delayed 0.2 ms to 0.4 ms with respect to the signal added to the said at least one delayed attenuated signal, preferably 0.3 ms+/−0.05 ms. 
   The at least one delayed attenuated signal is attenuated 5 dB to 20 dB with respect to the signal added to the said at least one delayed attenuated signal, preferably 9 to 12 dB. 
   The phantomized signal ( 58 ; PMCCS) comprises said at least one delayed attenuated signal and said input signal ( 59 ; MCCS). 
   Moreover, the invention relates to a phantomizer for processing of a signal into a phantomized signal for reproduction in the center channel of a multi-channel reproduction system, said phantomizer comprising at least one signal input connected to the comb-filtering means, said comb-filtering means feeding at least one signal output. 
   According to a further preferred embodiment of the invention, the phantomizer comprises one signal input connected to comb-filtering means which again feeds exactly one signal output. 
   A device according to the invention implies a very simple and cost effective solution, and the device may be applied in any multi-channel system. 
   The input of the phantomizer is feeding a summing node, said input also feeding the said summing node via a feed forward line, said feed-forward line comprising a delay line serially connected to an attenuator. 
   According to the above-mentioned embodiment, a very simple and cost-effective implementation of the invention has been obtained. 
   The delay line has a delay of approximately 0.2 to 0.4 ms, preferably 0.3 ms+/−0.05 ms. 
   The attenuator attenuates 5 to 20 dB, preferably 9 to 12 dB. 
   Moreover, the invention relates to an inverse phantomizer for processing of at least two-panned signals into de-phantomized signals for reproduction in a multi-channel rendering system, said phantomizer comprising two signal inputs each connected to inverse comb-filtering means, each inverse comb-filtering means feeding one signal output. 
   This embodiment makes it possible to use loudspeakers optimized for physical center channel speakers in a traditional stereophonic setup by canceling the sound coloring effect of acoustic comb-filtering inherent in traditional stereos. 
   Finally it should be noted that a very simple phantomizer according to the invention may be built into consumer multi-channel reproduction systems having a center channel, providing a simple sound-coloring option without adding significant costs to the system. Such a feature may e.g. comprise of both a phantomizer and an inverse phantomizer offering the user simple sound-editing features, i.e. adding phantomized signal components to the center channel, or optionally removing phantomized signal components from phantomized program material during reproduction. 
   Moreover, the invention relates to a method of phantomizing a center channel signal of a multi-channel reproduction system, said signal being manipulated in such a way that the timbre of the signal, when reproduced in the center channel loudspeaker of a multi channel reproduction system, may be perceived by the listener as a phantomized signal which has been established in a multi-channel reproduction system having no center channel. 
   A multi-channel reproduction system having no center channel may e.g. be a two-channel stereo loudspeaker system. 
   A loudspeaker may be understood as one or several loudspeaker transducers arranged in one or several cabinets. 
   Moreover, the invention relates to a method of phantomizing a center channel signal of a multi-channel reproduction system, said signal being manipulated in such a way that the timbre of the signal, when reproduced in the center channel loudspeaker of a multi channel reproduction system, may be perceived by a listener as an audio-signal reproduced by at least two neighboring channels when established in a multi-channel reproduction system. 
   Basically, the invention deals with integration of a center channel into a multi channel audio signal including a center channel, in such a way that the listener may perceive the sound image within being disturbed by the center channel. 
   A center channel signal of a multi-channel reproduction system is phantomized by processing of the center channel approximated to a HRTF based transfer function. 
   The approximation comprises at least one comb-filtering of the center channel. The approximation comprises a HRTF based head related transfer functions (HRTF; H 17 L, H 18 R, H 17 R H 18 L, H 36 L, H 36 R)) and where the approximation comprises a HL=(H 17 L+H 18 R)/H 36 L and a HR=(H 17 R+H 18 L)/H 36 R filtering of the center channel ( 59 ). 
   For reasons of symmetry, the above signals HL and HR are equal. Hence, they may be established by one filter. 
   The HRTF based transfer functions may e.g. be established by KEMAR measurements performed by MIT Media Lab. 
   Basically, the H 36 R component may typically be approximated to one, or the complete function may be established on the basis of a experience based tuning of filters. 
   Hence one of several applicable filters within the scope of the invention being approximated to the above stated function may be a comb filter. 
   Moreover, the invention relates to a phantomizer for processing of sound input signals. 
   A center channel signal may be understood broadly as a signal intended for reproduction in the center channel loudspeaker of a multi channel reproduction system such as broadcast, webcast, the loudspeaker signal itself or a corresponding signal stored on a medium such as DVD. 

   
     FIGURES 
     The invention will be described below with reference to the drawings where 
       FIG. 1  shows the traditional stereophonic speaker-setup 
       FIG. 2  shows how the traditional speaker-setup emulates a phantom center channel speaker 
       FIG. 3  shows a traditional multi-channel speaker-setup 
       FIG. 4  shows the cause of the comb-filtering effect 
       FIG. 5   a  shows an embodiment of the invention, the Audio Center Channel Phantomizer 
       FIG. 5   b  shows a block diagram of a simple comb-filter 
       FIG. 5   c  shows the frequency response of a simple comb-filter 
       FIG. 6  shows another embodiment of the invention, the Inverse Phantomizer 
       FIG. 7  shows the Inverse Phantomizer applied in a stereo-setup, and where 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a traditional two-channel stereophonic setup  10  comprising a left channel loudspeaker  11  and a right channel loudspeaker  12 . Further, it comprises a listener, a human being  13 , having a left ear  14  and a right ear  15 . 
   The listener  13  is located in front of the two loudspeakers  11 ,  12 , facing their center. 
   When producing music, the different instruments or voices are located in the sound image by feeding the left channel speaker  11  and the right channel speaker  12  with different voltage amplitudes or phases. E.g. if the left speaker  11  plays louder than the right speaker  12 , it will seem to the listener  13  that the instruments playing are to the left of his center. 
     FIG. 2  shows how the traditional stereo system  10  localizes a voice or musical instrument in the center of the sound image. The hardware of the system corresponds to that described in  FIG. 1 , except for the addition of a phantom center channel loudspeaker  16 . 
   Placing voices or musical instruments right at the center of the stereo image is obtained by simply feeding the source signal to both loudspeakers  11 ,  12  simultaneously with an identical level and phase. Traditionally, a listener  13  will perceive the place of sound origin to be somewhere between the speakers  11 ,  12  which is from the phantom center speaker  16 . 
     FIG. 3  shows a multi-channel setup  30  with a left channel loudspeaker  31 , a right channel loudspeaker  32  and a physical center channel loudspeaker  36 . Additionally, it comprises a listener  33  with a left ear  34  and a right ear  35 . 
   Evidently, according to the invention, the system may comprise further loudspeakers, such as a preferred five channel system comprising one center loudspeaker, two front speakers and two rear speakers. 
   The listener  33  is placed in front of the center channel speaker  36 , facing it. 
   The multi channel rendering system having a center channel facilitates a relatively simple and accurate localization of a source, when an audio signal has to be located in the center of the sound image established by the system. 
   Audio signals emitted from the center loudspeaker  36  are basically received as two “separate” signals  36 L and  36 R by the left and right ear  34 ,  35  respectively. 
   Obviously, a sound engineer could simply feed a voice or musical instrument to the center channel loudspeaker  36  in order to locate the signal in the center of the sound image. Such mixing would definitely place the sound source at the center of the sound image and it would also make the obtainable localization less sensitive to variations in the listener&#39;s position  33 . 
   However, as the music industry has gained experience with this new reproduction format  30 , one problem has become apparent: Many highly esteemed music producers have decided not to use the physical center channel speaker  36 , thus discarding its beneficial stabilizing effect on the stereo image. The reason is that even when using 3 identical loudspeakers  31 ,  32 ,  36  at the front in compliance with the ITU Recommendation BS.775-1 “Multi-channel Stereophonic Sound Systems with and without Accompanying Picture”, the perceived timbre of the physical center channel  36  deviates too much from that of the familiar phantom center channel  16  for which the loudspeakers were optimized in the first place. 
   The invention deals with this problem. 
   Again,  FIG. 4  shows the traditional stereo system  10 , with two physical speakers  11 ,  12 , the phantom center speaker  16  and the listener  13 . Additionally,  FIG. 3  comprises four paths of sound  17 L,  17 R,  18 L,  18 R. 
     17 L illustrates the path of sound from the left loudspeaker  11  to the nearest ear of the listener  13 , i.e. the left ear  14 .  17 R illustrates the path of sound from the right loudspeaker  12  to the nearest ear of the listener  13 , i.e. the right ear  15 . 
     18 L illustrates the path of sound from the left loudspeaker  11  to the farthest ear of the listener  13 , i.e. the right ear  15 . 
     18 R illustrates the path of sound from the right loudspeaker  12  to the farthest ear of the listener  13 , i.e. the left ear  14 . 
   Sound paths  17 L and  17 R are the direct paths of sound from speaker to ear, and paths  18 L and  18 R are moving around the head to the ear farther away. 
   The problem with the sound engineers not using the physical center channel  36  can now be explained. 
   The signal reaching each ear from the physical center channel speaker  36  is that of one loudspeaker  36  placed in front of the listener. But the signal reaching each ear from the phantom center channel speaker  16  is the sum of two signals: One following the paths  17 L,  18 R from the loudspeakers  11  and  12  and one following the paths  18 L,  17 R from the loudspeakers  11  and  12 , respectively. 
   Thus, the center channel signal of the stereo system  10  fed to each ear is a summation of two different and mutually delayed signals, both of which differ—primarily due to different degrees of shadowing effect of the head  13 —from the signal reaching each ear from a physical center channel of the multi-channel system  30 . 
   In a first approximation (the physically correct analysis would include head-related transfer functions, which are fairly complicated, individual, orientation- and position-dependent, as well as the acoustics of the listening room) the difference between the signals reaching the ears from the two types of center channels  16  and  36  can be described as a comb-filtering effect: Adding a delayed and attenuated copy of the sound signal to itself. As a further refinement, a filter may be inserted into the model&#39;s delay-and-attenuation signal path in order to approximate the frequency dependence of the above-mentioned shadowing effect. 
   Thus, because loudspeakers are optimized for phantom center channel  16  reproduction, an undesirable coloration of the physical center channel  36  is perceived, primarily due to the absence of this comb-filter effect. 
   Using the above introduced terms of  FIG. 3  and  FIG. 4 ,
     when H 36L  (= 36 L) represents the transfer function between the center channel loudspeaker  36  and the left  34  ear of a listener located in a certain position relative to the loudspeaker,   when H 36R  (= 36 R) represents the transfer function between the center channel loudspeaker  36  and the right ear  35  of a listener located in a certain position relative to the loudspeaker,   when H 17L  (= 17 L) represents the transfer function between the illustrated left speaker  11  and the left ear  14  of a listener  13 ,   when H 18L  (= 18 L) represents the transfer function between the illustrated left speaker  11  and the right ear  15  of a listener  13 ,   when H 17R  (= 17 R) represents the transfer function between the illustrated right speaker  12  and the right ear  15  of a listener  13 , and   when H 18R  (= 18 R) represents the transfer function between the illustrated right speaker  12  and the left ear  14  of a listener  13 ,
 
a desired transfer function between a multi-channel rendering system having a center channel and the listener according to the invention may be established as
 
 HL =( H 17 L+H 18 R )/ H 36 L  and
 
 HR =( H 17 R+H 18 L )/ H 36 R  
   

   The transfer function is here described with respect to a three channel system for the purpose of explaining the basic features of the invention, and the invention may also be applied in other multi-channel rendering systems, such as five channel systems, etc. Hence, basically the term “left” speaker  31 ,  11  and “right” speaker  32  refers to all other speakers in the rendering system than the center speaker  36 . 
   According to a simple preferred embodiment of the invention, the center channel may be reproduced according to the properties of only the two illustrated speakers  11 ,  12  for reasons of simplicity. 
   It should be noted that the above stated transfer function are equal for reasons of symmetry due to the fact that the invention deals with the center channel. Therefore, both signals may be established by the same filter. 
     FIG. 5   a  shows a block diagram of a first embodiment of the invention. It comprises a multi-channel audio rendering system  51 , e.g. a mixing console, representing audio signals in a 2x+1 channel format, i.e. with x rights channels R 1 , R 2 , . . . Rx, x left channels L 1 , L 2 , . . . Lx and a center channel CC. According to the illustrated embodiment of the invention the center channel CC is fed to a phantomizer  52  as a center channel output  59 . The other channels L . . . , R . . . are fed directly to corresponding loudspeakers LLS and RLS. 
   The Audio Center Channel Phantomizer  52  is connected to the center channel  59  of the audio rendering system  51 , and a center channel loudspeaker  58  is connected to the output of a Audio Center Channel Phantomizer  52 . 
   The Audio Center Channel Phantomizer  52  comprises a direct connection  53  between the input from the center channel  59  of the audio rendering system  51  and the center channel loudspeaker  58  through an algebraic summing point  57 . 
   Further, the Audio Center Channel Phantomizer  52  comprises a delay-line DL  54 , a filter F  55  and an attenuator ATT  56 . 
   DL  54 , F  55  and ATT  56  form a feed-forward path which is fed from the center channel  59  and ends at the summing point  57 . 
   In order to make the physical center channel  36  useful in music (re-)production without having to re-design the center channel loudspeaker, it is only necessary to add the above-mentioned comb-filtering back into the signal path. Preferably, this should be done at the production stage, so the consumer will not have to change anything. Thus, according to the illustrated embodiment of the invention, the processing circuit comprises a delay line  54  and an attenuator  56 —which forms a comb-filter—applied to the center channel output  59  of a multi-channel mixing console or any other means for rendering audio onto any audio media or reproduction setup comprising a physical center channel. This may even include mono. Additional filtering  55  may be added to the delay path. The delay line  54 , filter  55  and attenuator  56  may be interchanged arbitrarily with no effect on the function of the invention. 
     FIG. 5   b  shows a block diagram of an ordinary, simple comb-filter  80 . It comprises one input  81  and one output  82 . The input  81  and the output  82  are connected by a direct connection  83  through a summing point  86 . Further, the comb-filter  80  comprises a feed forward path comprising a delay line DL  84  and an attenuator ATT  85 . The DL  84  and ATT  85  are fed from the input  81  and end at the summing point  86 . 
     FIG. 5   c  shows the frequency response of a simple comb-filter  80 . This is the effect that the Audio Center Channel Phantomizer  52  contributes to the original center channel signal in its simplest embodiment. 
   The delay is 0.3 ms and the attenuation has been set to 9 dB. 
   The comb-filter  80  has two parameters: The delay of the delay line DL  84  and the attenuation of the attenuator ATT  85 . 
   The delay required can be determined simply by considering the physical lengths from each loudspeaker to one ear. This involves only simple trigonometry, i.e. the cosine relations, and the speed of sound. By choosing a certain delay, the distance from the speakers to the listener is also determined. 
   Determining the attenuation required is a bit harder to do theoretically, and is done by ear in relation to this embodiment. 
   According to a preferred embodiment of the invention, parameter settings are approximately a delay=0.3 ms and an attenuation=9 dB. 
   It should be noted that the simple above illustrated preferred embodiment according to the invention is a simple approximation to the above stated transfer functions
 
 HL =( H 17 L+H 18 R )/ H 36 L  and
 
 HR =( H 17 R+H 18 L )/ H 36 R  
 
   Several, more complicated approximations to the transfer function may be applied within the scope of the invention. 
   One of such approximations may be applied when applying the so-called KEMAR HRTF provided by MIT Media Lab as approximation to the HRTF and applying corresponding suitable filters. 
     FIG. 6  shows a second embodiment of the invention comprising a multi-channel audio rendering system  61 , e.g. a mixing console, with a center channel output  69 C, a left channel output  69 L and a right channel output  69 R. Further, it comprises an embodiment of the invention  62 , the Inverse Phantomizer with an output  68 . 
   It also comprises two summing points  70 L,  70 R, and two loudspeakers  71 L,  71 R. 
   Again, the Inverse Phantomizer  62  is connected to the center channel  69 C of the audio rendering system  61 , and the output  68  is connected to the two summing points  70 L,  70 R. The left channel output  69 L of the audio rendering system  61  is added to the output  68  from the Inverse Phantomizer  62  at the left channel summing point  70 L, and the left speaker  71 L is connected to the output from the left summing point  70 L. The right channel output  69 R of the audio rendering system  61  is added to the output  68  from the Inverse Phantomizer  62  at the right channel summing point  70 R, and the right speaker  71 R is connected to the output from the right summing point  70 R. 
   The Inverse Phantomizer  62  comprises a direct connection  63  between the input from the center channel  69 C of the audio rendering system  61  and the output  68  through an algebraic subtraction point  67  in this particular embodiment. 
   Further, the Inverse Phantomizer  62  comprises a delay line DL  64 , a filter F  65  and an attenuator ATT  66 . 
   DL  64 , F  65  and ATT  66  now form a feed-back-path which is fed from the center channel output  68  and ends at the subtraction point  67 . 
   Contrary to the first embodiment  52 , this second embodiment  62  of the invention solves the inverse problem: The use of audio signals adapted for reproduction in physical center channel speakers in a stereophonic setup. In this way, the two speakers  71 L,  71 R are fed with a modified center channel signal, i.e. inverse-phantomized (also called de-phantomized). Thus, the center channel signal fed to the loudspeakers  71 L and  71 R are established on the basis of a phantomized center channel signal  69 C including the added delay composants providing panning illusions. The signal  69 C are fed to the inverse phantomizer  62  in which the added delay composants (and coloring) are removed or decompensated. 
   Instead of adding an attenuated, delayed and possibly filtered signal to the output as was the case in the first embodiment  52 , it is now subtracted at the input  67 . As long as the gain through the feedback path  64 ,  65 ,  66  is less than unity (which is the case with the above-mentioned typical 9 dB of attenuation), the system is stable and the exact inverse of the first embodiment  52 . Hence, it can be used to cancel the effect of the first embodiment  52 , if desired. 
     FIG. 7  shows the second embodiment of the invention, the Inverse Phantomizer  62 , used in a stereo setup. It comprises a multi-channel audio rendering system  61 , e.g. a stereo mixer with a left channel output  59 L and a right channel output  59 R. Further, it comprises two Inverse Phantomizers  62 L,  62 R, a left channel loudspeaker  68 L and a right channel loudspeaker  68 R, both optimized for use as physical center channel speakers. 
   It should be noted that the illustrated embodiment may be applied for establishing a virtual center channel in e.g. a stereo system  71 L,  71 R. Hence, if a “normal” multi-channel signal (i.e. not-phantomized) is fed to the loudspeakers  71 L,  71 R via an inverse phantomizer  62 , the comb-filtering of the center channel may be established as a kind of decompensation of a normal signal, thereby obtaining that the sound is colored in such a way that it may be perceived as a signal which has been “monoficied”. 
   The first Inverse Phantomizer  62 L is connected to the left output-channel of the audio rendering system  69 L, and the second Inverse Phantomizer  62 R is connected to the right output-channel of the audio rendering system  69 R. The left channel loudspeaker  68 L is connected to the first Inverse Phantomizer  62 L, and the right channel loudspeaker  68 R is connected to the second Inverse Phantomizer  62 R. 
   This embodiment makes it possible to use audio signals adapted for reproduction in a physical center channel speaker in a traditional stereophonic setup by canceling the sound coloring effect of the acoustic comb-filtering effect, se  FIG. 4 , and feeding the de-phantomized center channel signals to the two stereo channels. 
   Overview of cases in which a Phantomizer and Inverse Phantomizer are used: 
   Two types of setups: 
   We will divide the reproduction setups into two cases, Phantom Center, where acoustic comb-filtering at the ears of the listener occurs, and Physical Center where no filtering occurs: 
   
     
       
         
           
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   Two loudspeaker types 
   We will divide all loudspeakers into two groups: Those designed for mono, which (presumably) have neutral timbre, and those designed for stereo (more common these days), which have been designed with something like an inverse comb-filter response to compensate for the acoustic comb-filtering of the center image in a stereo setup: 
   
     
       
         
           
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   Three Phantomizer usage types 
   There are three ways to use the Phantomizer: Non-use, use the Phantomizer or use the Inverse Phantomizer: 
   
     
       
         
           
             H 
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             H 
             
               Inverse 
               ⁢ 
               _ 
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               Phantomize 
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               r 
             
           
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   Combinations yielding neutral timbre 
   
     
       
             
             
             
             
           
         
             
                 
             
             
                 
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               Type 
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               Phantom 
               Stereo 
               None 
               comb · comb −1  · 1 = 1 
             
             
               Center 
               Mono 
               Inverse_Phantomizer 
               comb · 1 · comb −1  = 1 
             
             
               Physical 
               Stereo 
               Phantomizer 
               1 · comb −1  · comb = 1 
             
             
               Center 
               Mono 
               None 
               1 · 1 · 1 = 1