Abstract:
The CPS (center point stereo) system of the present invention is directed to stereo sound reinforcement of live music, particularly with electronically-implemented instruments such as guitars and keyboards that provide a musical source signal at line level. A special CPS processor, converting regular left and right stereo signals to sum and difference signals, enables a center stage acoustic image to be created directly from a forward-directed loudspeaker unit driven from the sum signal and enables left and right spatialized stereo images to be created by recombination with a difference field received indirectly from a sideways-directed special dipole loudspeaker unit conveniently co-located with the forward-directed loudspeaker unit and driven from the difference signal. The CPS processor receives L and R input from the instrument or from an interposed FX (musical effects) or DSP (digital sound process) unit receiving mono or stereo input from the instrument. CPS market potential ranges from full CPS systems to unique add-on CPS processors and dipole loudspeaker units as system building blocks.

Description:
PRIORITY 
     Benefit is claimed under 35 U.S.C, § 119(e) of pending U.S. provisional application # 60/023,719 filed Aug. 8, 1996 by the present inventors. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of electronic musical instruments, and more particularly it relates to equipment for producing stereo sound effects from a centrally located speaker system, with particular benefit to live performances of electronic keyboard and guitar players. 
     BACKGROUND OF THE INVENTION 
     Although stereo sound has become universally accepted for sound reproduction, the exploitation of stereo effects in the field of amplified live music performance by solo performers and small groups has fallen short of its potential, even though many such performers are using instruments that provide capability of generating stereo output signals that could potentially enhance the overall effect of the musical performance. 
     The conventional practice for indoor stereo musical performance involves typically locating two spatially separated loudspeaker units at or near opposite ends of the stage or corners of the room, and driving each speaker with a dedicated amplifier, i.e. right and left channels. The physical separation between the two loudspeaker units is critical, depending on environmental factors such as the size and shape of the listening room, audience location, etc. 
     Excessive physical separation destroys the imaging performance, i.e. the “focus” of the point of origin of a sound image as perceived by a listener, e.g. an announcer, soloist or vocalist located at center stage where it most difficult for the two offset loudspeaker units to “synthesize” a focussed central image. 
     On the other hand, insufficient physical separation, while benefitting center imaging, degrades spatialization, i.e. the ability to create a full-width panoramic “sound stage” as perceived by the listener. 
     In practice, the limited physical separation selected as “optimal” is necessarily a tradeoff in which both imaging and spatialization are sacrificed. This “optimal” separation can perform acceptably for listeners located centrally, relative to the loudspeaker units. However, in most room or auditorium situations, some listeners will find themselves located too close to one of the loudspeaker units such that one channel will predominate: not only can the stereo effect be lost but in many cases the overall musical perception could even be judged as inferior to a monophonic performance of the same music. In the case where the musician is seeking to produce a highly spatialized stereo effect, a discriminating listener at an unbalanced listening location could perceive the performance as distracting or even totally unacceptable. The unbalance is also distracting to the musician if he is located too close to one of the stereo loudspeaker units. 
     If the musician or the sound technician attempts to mitigate this problem by placing the two speaker units closer together, or by utilizing a commercially available “combo-amp” unit having built-in forward-directed stereo speakers side by side built into a common case with the amplifier and thus having very little physical separation, this will tend to sacrifice the desired impact of the stereo effect, which could even become completely lost to all of the audience if the speakers are located so close together as to become practically monophonic. 
     While many keyboard and guitar players may develop special stereo effects in rehearsal and take great pains with these in recording, when it comes to live performance, in order to avoid ambivalence or controversy regarding stereo loudspeaker location, players may be reluctant to pursue excellent stereo performance and settle for an essentially monophonic loudspeaker setup such as a compact cluster or stack of speaker box units or the minimally separated “stereo” speakers of the “combo-amp”. While this may result in a perception of more uniform coverage throughout the audience, there is a great sacrifice of the potential enhancement that effective stereo effects could add to their live performance. 
     In basic stereo recording modes, an orchestra can be recorded with a pair of identical microphones located at strategic symmetrical locations, e.g. at opposite sides of the orchestra stage facing inwardly to center or located centrally facing outwardly to opposite sides, and in the basic stereophonic system the two resulting independent signals L and R are customarily handled by a pair of independent channels throughout, including re-recording and playback in various media, broadcast, and all forms of reproduction equipment including power amplifiers and speakers. This basic L and R stereo system is in widespread common use as the standard system in home entertainment systems. Pure L and R signals may be subjected to some degree of cross-coupling alteration in master-recording and also in spatial-enhancement compensation circuity in small radios and stereo players having closely spaced built-in stereo speakers. 
     An alternative stereo recording technique known as “point source stereo” utilizes two different microphones centrally located close together, typically at front and center of the orchestra: one microphone is omnidirectional and the other is bidirectional (a.k.a. “figure 8” or “dipole”) in directivity. The signals from the two microphones, being sum (L+R) and difference ±(L−R) respectively, can be handled in two audio channels, but they would normally be processed in a matrixing circuit to produce L and R stereo signals for conventional handling through L and R channels. 
     Though not currently in high volume usage, sum-difference loudspeaker systems have been proposed for home stereo entertainment centers, utilizing a front-firing speaker and a dual-side-firing dipole speaker, driven respectively by sum and difference signals derived by matrixing from standard L and R stereo signals. 
     DISCUSSION OF RELATED KNOWN ART 
     U.S. Pat. No. 3,588,355 to Helm discloses a stereophonic system with loudspeakers oriented at right angles to each other and receiving matrixed sum and difference stereo signals. 
     U.S. Pat. No. 4,819,269 to Klayman discloses an extended imaging split mode loudspeaker system receiving sum and difference signals. 
     U.S. Pat. No. 5,109,416 to Croft discloses a side-firing dipole speaker, receiving a difference signal, to be used in conjunction with conventional direct-path speakers, for producing ambiance sound in multichannel sound reproduction. 
     OBJECTS OF THE INVENTION 
     It is a primary object of the present invention to provide, particularly for live musical performance on electronically-amplified instruments such keyboards and electric guitars, an electro-acoustic CPS (center point stereo) system that will process the musical source signal from the instrument&#39;s line level output and transduce it acoustically in a manner to create stereophonic sound images emanating from two loudspeaker units co-located at the center point of the CPS system, for embellishing live music performances with stereo effects. 
     It is a further object to disclose detailed circuitry of a CPS processor which includes an optional subwoofer line level output, for practicing the invention. 
     It is a further object to disclose several practical implementations of a dipole loudspeaker unit for practicing the invention. 
     It is a further object of the invention to provide the above described features of the primary object in a minimal electro-acoustic accessory package that will enable practice of the invention with maximal utilization of commercial electronic equipment that may be already available to a musician. 
     It is a still further object of the invention to disclose a total electro-acoustic embodiment including a sound processor, amplifiers and two-unit loudspeaker system that will operate from an instrument&#39;s line level output to create stereo sound images for embellishing live musical performances. 
     It is a still further object of the invention to disclose the further incorporation of additional stereo effects capabilities, e.g. FX and DSP, for which hardware may be available commercially. 
     It is a still further object to disclose a rack-mounted custom package unit containing CPS processing, special effects such as FX (musical effects, typically analog) or DSP (digital signal processing), amplification and loudspeaker units, providing the total CPS system with its sound radiators in a single stack. 
     It is a still further object to disclose several alternative practical CPS systems for practicing the invention. 
     SUMMARY OF THE INVENTION 
     In a basic package, the electro-acoustic element of the system according to the invention comprises, in addition to a forward-facing loudspeaker unit which may be an existing conventional unit having one or more speakers connected to vibrate in unison, a dipole loudspeaker unit having one or more speakers mounted vertically, connected to vibrate in unison and oriented to radiate transversely relative to the forward-facing loudspeaker unit, so as to radiate an acoustic field having a figure 8 dipole pattern: a pair of sideways lobes directed to opposite sides in opposite polarity. 
     The package also includes a CPS (center point stereo) processor of the present invention having L and R stereo audio line inputs for connection to stereo line outputs of the player&#39;s instrument and having a pair of line outputs providing (1) a “sum” signal input to a first power amplifier driving the forward-directed loudspeaker unit to radiate a “sum” acoustic field, and (2) a “difference” signal input to a second power amplifier driving the dipole loudspeaker unit, which radiates the “difference” acoustic field with the above-described dipole pattern. 
     The package embodiment may be expanded to include, ahead of the CPS processor, a commercial stereo FX or DSP unit that generates stereo line outputs: typically these are designed to operate from either mono or stereo line input from the musical instrument. 
     Other package embodiments can include one or both power amplifiers, woofer speaker units and associated power amplifiers, preamplifiers and accessories. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further objects, features and advantages of the present invention will be more fully understood from the following description taken with the accompanying drawings in which: 
     FIG. 1 is a pictorial diagram of an illustrative basic embodiment for practice of a CPS (center point stereo) system of the present invention in a typical musical performance environment. 
     FIG. 2 is a functional block diagram of the subject matter of FIG.  1 . 
     FIGS. 3-7 are functional block diagrams of alternative embodiments in which CPS of the present invention may be practiced, derived from the basic-embodiment of FIGS. 1-2. 
     FIG. 8 is a schematic diagram of the CPS processor unit of FIGS. 1,  2 ,  4 - 7 . 
     FIG. 8A is a schematic diagram of a power supply for the CPS processor of FIG.  8 . 
     FIGS. 9,  9 A and  9 B are three views of a first implementation of a dipole loudspeaker unit with which the present invention can be practiced, utilizing a single speaker mounted on a central baffle board. 
     FIG. 10 is a front elevational view of a second implementation of a dipole loudspeaker unit with which the present invention can be practiced, utilizing a pair of speakers mounted face-to-face on a central baffle board. 
     FIGS. 11,  11 A and  11 B are three views of a third implementation of a dipole loudspeaker unit with which the present invention can be practiced, utilizing a pair of speakers mounted side-by-side on a central baffle board. 
     FIGS. 12 and 12A are two views of a fourth implementation of a dipole loudspeaker unit with which the present invention can be practiced, utilizing a pair of speakers mounted on opposite side panels of an enclosure. 
     FIG. 13 is an elevational pictorial diagram of a CPS system related to FIG. 5, utilizing a forward-directed commercial mono guitar amplifier-speaker combination in the sum channel. 
     FIG. 14 is an elevational pictorial block diagram of a CPS system related to FIG. 5, showing an elevational view of a triple-unit loudspeaker stack having a modular sub-woofer loudspeaker unit added beneath the dipole loudspeaker unit and the forward-directed loudspeaker unit. 
     FIG. 15 is an elevational pictorial diagram of a rack-mounted CPS system operating from a stereo source as in FIG. 4 but otherwise containing essentially all of the components of FIG. 14 in a single stack. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates pictorially a preferred standard embodiment of the present invention. A musical signal source  10 , providing stereo L and R signals as indicated at audio line outputs, represents practically any conventional musical instrument such as an “electric” guitar having one or more string pickups, a “miked” acoustic guitar or piano, a guitar- or keyboard-controlled synthesizer or tone generator, typically already owned and operated by the musician/user. Such instruments commonly provide stereo output signals L and R, which are normally directed in two independent channels leading to left and right loudspeakers through stereo amplifiers. 
     Designations L and R refer primarily to the perception of physical relationship in the final sound reproduction. With regard to musical source signals, while L and R may sometimes correspond to locations of different instruments and/or performers, in most instances, including the present text, L and R do not necessarily relate to any left/right relationship at the source since the two channels are often created synthetically and/or designated arbitrarily. 
     In FIG. 1, the L and R signals are delivered to the input of CPS (center point stereo) processor  16 , wherein the L and R stereo signals are matrixed in a manner to provide a first line output sum signal s=L+R, i.e. the sum of the two input signals, and to provide a second line output difference signal d=L−R, i.e. the difference between the two input signals. 
     The sum signal s is amplified by power amplifier  14 A which generates an amplified sum signal S that drives a conventional forward-directed loudspeaker unit  16 , from which an L+R acoustic field emanates in a generally forward direction as indicated by the outlined arrow. 
     Similarly the difference signal d is amplified by power amplifier  14 B and the amplified difference signal D is applied to the dipole speaker unit  18 , which radiates acoustic signals from both sides as indicated: R−L on one side and L−R on the other side, thus emanating in mutually opposite phase relation to each other. 
     The difference acoustic fields L−R and R−L from the dipole loudspeaker unit  18  reach the listeners mainly via room or other environmental reflections, while the sum acoustic field R+L from the forward-directed loudspeaker  16  reaches the listeners mainly in a direct path: these two fields add algebraically at the listener&#39;s ear, thus creating a perception of the intended L and R relationship. 
     The gain of sum amplifier  14 A can be set to provide a desired basic level of loudness, then the gain of difference amplifier  14 B can be adjusted to provide a desired degree of stereo effect, ranging from monophonic through a condition of normal stereo separation as found at source  10 , to a condition of exaggerated stereo separation. Such adjustment is typically performed in a manner to seek optimal compensation for the influence of room or auditorium acoustics. 
     The dipole loudspeaker  18  and CPS processor  12 , being the non-conventional components shown in FIG. 1 in solid lines, can be marketed as a basic add-on package for musicians who already possess the conventional components shown in dashed lines: source  10 , amplifiers  14 A and  14 B, and forward-directed loudspeaker system  16 . 
     Speaker  18  is shown supported by a rectangular box  22  which could be merely a speaker stand to raise the loudspeaker units to a suitable listening height, or it could be a woofer loudspeaker unit for handling bass musical content in essentially a monophonic manner apart from the stereo midrange content. 
     Whereas a conventional stereo system for musical instruments requires physically separated L and R speakers, the present dipole speaker system, utilizing mainly reflected paths for the side-directed difference signals, produces the perception of stereo sound to listeners in the room or auditorium, with all of the loudspeakers in the player&#39;s system located close together, typically in a central stack and even combined in a common enclosure, providing wide coverage over most or all parts of the room or auditorium, which plays an acoustically interactive role in creating the perception of the stereo musical effect. 
     For a keyboard or guitar player, the capability of stereo performance as enabled by the CPS system with all the loudspeaker units stacked at one location is clearly an advantage in dealing with sound stage setup scenarios for live performances. With the further potential enhancement of a more uniform audience coverage from CPS, the player is motivated to more fully exploit the live performance potential of the stereo capabilities already existing in his/her electronic instrument or easily added thereto. 
     FIG. 2 is a functional block diagram of the preferred embodiment illustrated pictorially in FIG.  1 . 
     FIGS. 3-7 are functional block diagrams of CPS systems of the present invention in different embodiments having, in common with FIGS. 1 and 2, a pair of amplifiers  14 A and  14 B receiving as input respectively a sum signal s and a difference signal d which are delivered as amplified replicas S and D respectively to a forward-directed sum loudspeaker unit  16  and a sideways-directed dipole loudspeaker unit  18 . 
     FIG. 3 is a version that is functionally equivalent to FIG. 2 but with the summing and subtracting portions of the CPS processing performed separately rather than both in a CPS processor module. When a power amplifier is available equipped with dual gain-controlled inputs, these can be utilized to perform L+R addition as indicated in amplifier  14 C. A minimal CPS processor  12 B is required only to perform the subtractive portion of the CPS processing; thus the cost of adding a CPS processor function to an existing system can be reduced. 
     FIG. 3A shows an alternative passive summing circuit that does not require dual amplifier inputs and thus enables the use of two similar single-input amplifiers  14 B: a pair of equal resistors R 3  and R 4  connected as shown with the common terminal connected to the single input of amplifier  14 B. A 6 dB insertion loss in the sum channel must be taken into account in setting the gains of amplifiers  14 B for overall system S/D balance. 
     FIG. 4 is an upgrade modification that is applicable to the subject matter of FIGS. 1-3, with the addition of a stereo FX (effects) unit  20  interposed between source  10  and CPS processor  12 . Such FX units are commercially available, providing special sound processing such as reverb, flanging, delay, echo and other musical effects. There is a trend to implement such FX units digitally using DSP (digital sound processing) to greatly expand their variety, capabilities and flexibility. 
     FIG. 5 is a mono-sourced version applicable to the subject matter of FIGS. 2-4 wherein a monaural musical source  10 A provides only a monophonic signal M. Many FX units are designed to operate from either a mono or a stereo source, thus FX unit  20  accepts a monophonic signal M as input and converts it to a pair of (synthesized) stereo signals delivered as L and R outputs driving the CPS processor. 
     FIG. 6 is a version of the subject matter of FIG. 4 wherein the FX and CPS processing functions are combined in a custom FX+CPS processing unit  12 A which accepts a mono source signal and delivers  5  and d signals for driving the power amplifiers  14 B. 
     In FIG. 7, the source  10 C incorporates a built-in CPS processor, equivalent to CPS processor  12  in FIGS. 1,  2 ,  4 , and  5 , contained within the body of a guitar or keyboard unit, so that CPS according to this invention can be practiced with only the addition of the special dipole loudspeaker unit  18  along with conventional components: amplifiers  14 A, and forward-directed loudspeaker unit  16 . 
     There are various different ways in which a CPS processor  12  of FIGS. 1,  2 ,  4 , and  5 , and the dipole loudspeaker unit  18 , as the two special building blocks, taught by this invention, can be combined with conventional components, i.e. musical sound source  10 , amplifiers  14 A and  14 B, and a forward-facing loudspeaker unit  18  to form functional systems for practicing this invention. The realization of two special components of the invention, CPS processor  12  and dipole loudspeaker unit  18 , will now be described in greater detail in connection with FIGS. 8-12A 
     FIG. 8 is a schematic diagram of CPS processor  12  of FIGS. 1,  2 ,  4  and  5 . Four op-amps,  0 A 1 - 0 A 4 , perform addition and subtraction of the L and R audio signals applied to the input at jacks J 1  and J 2 , and thus provide the difference signal d at J 3  and the sum signal S at J 4  as line level outputs. These four op-amps  0 A 1 - 0 A 4  may be implemented by two dual IC&#39;s type 4560. 
     The sum signal  5  is also directed through a low pass filter LPF and level-adjustment potentiometer R 23  to output jack J 5  which thus provides a line output that is equalized for driving a sub-woofer amplifier/speaker system. 
     The sub-woofer line output is an optional deluxe feature that is not essential to basic practice of the invention. 
     FIG. 8A is schematic of a dual regulated power supply for providing D.C. power to CPS processor  12  of FIG.  8 . 
     FIG. 9 is a front elevational view of a dipole loudspeaker assembly  18  having a substantially open housing  24 . Speaker  26  is mounted on a baffle board  24 A which is located within housing  24  at or near the center thereof. The front panel of housing  24  is configured with a large opening, defined by edge  24 B, that can be dimensioned to influence sound dispersion as a matter of design choice. Loudspeaker  26  is shown directed (arbitrarily) to the left thus its forward acoustic wave emanates to the left and its back wave emanates to the right; since these waves are opposite in polarity, the directivity pattern is inherently a “figure 8”, typical of the dipole configuration, e.g. a dipole antenna. Due to open basket structure of speaker  26 , the back wave directed to the right is practically equal in sound pressure to the front wave directed to the left, so that the “figure 8” dipole directivity pattern is practically symmetrical, especially in the mid-frequency range that plays the major role in stereo imaging and spatialization. 
     It is an advantage of the CPS system that high and/or low frequency reinforcement can be readily implemented by adding a tweeter and/or (sub) woofer to the forward-directed unit only, whereas in conventional RL stereo, these must be added to both channels or implemented via an additional central loudspeaker unit with special cross-over and matrixing. 
     FIG. 9A is the right hand end elevational view of the dipole loudspeaker assembly  18  of FIG.  9 : the rear view of speaker  26 , mounted on baffle board  24 A is seen through a large opening defined by edge  24 C configured in housing  24 . 
     FIG. 9B is a plan view of the dipole loudspeaker assembly  18  as seen looking down from above: a topside opening defined by edge  24 D reveals speaker  26  mounted on baffle  24 A. 
     The outline of enclosure  24  can be dimensioned to conform to other available loudspeakers or equipment enclosures, e.g. for stacking purposes. 
     FIG. 10 shows a front elevational view of a dual-speaker dipole loudspeaker assembly  18 A that is functionally similar to loudspeaker assembly  18  shown in FIGS.  9 - 9 B: an open housing  28  is configured with a central baffle board  28 A. However instead of a single speaker, there are two speakers  26 A, seen through the opening defined by edge  28 B, mounted face-to-face on a common opening in the central vertical baffle board  28 A which defines a central axis of the housing  28 . The two speakers  26 A are connected (in series or parallel) in opposite polarity so that their diaphragms are caused to move in unison as indicated by the two arrows, thus they act together to provide the equivalent effect of a single diaphragm; however their complementary diaphragm movement serves to balance inherent voice coil travel non-linearities of each speaker  14 B and thus provide superior linearity and fidelity compared to a single speaker. 
     FIG. 11 shows a front elevation of a dipole loudspeaker assembly  18 B that is functionally similar to the assemblies  18  and  18 A in FIGS. 9 and 10 in that housing  30  is open and is configured with a central baffle board  30 A, however speakers  26 B are surface-mounted on opposite sides of baffle board  30 A, facing in opposite directions, and, as seen in the right hand end elevational view, FIG. 11A, and in the plan view, FIG. 11B, as viewed from above, the two speakers  26 B are offset from each other in a side-by-side oppositely-facing disposition. 
     The category of central-baffle open-housing dipole loudspeaker assemblies as exemplified above in FIGS. 10-11B, can be implemented with any number of additional speakers mounted on the central baffle; typically an even total number of speakers is divided into two groups of oppositely-facing speakers arranged in a complementary pattern, all connected so as to vibrate in unison. 
     In this central-baffle category, the housing can be made to have open regions as indicated on any or all of its six panel areas; the size and location of the openings are allocated in design to control the sound dispersion. 
     FIG. 12 shows a front elevational view of a dual-speaker dipole loudspeaker assembly  18 C that differs from the open-housing category described above in that the housing  18 C is constructed in the manner of a conventional loudspeaker enclosure, having no central baffle or large openings or ports other than those associated directly with the two speakers  26 C shown in dashed outline mounted back-to-back at opposite sides of enclosure  18 C. 
     As in dipole loudspeaker units generally, speakers  26 C are connected with polarity such to cause their diaphragms to vibrate in unison as indicated by the two arrows. This arrangement results in much different operating conditions than are found in conventional speaker enclosures: the unison vibration condition tends to cancel sound pressure buildup inside the enclosure  18 C even if it is made practically air-tight. 
     As with the dual speaker central-baffle approach, each of the two side speakers  28 C could be replaced by two or more speakers, typically with smaller ones. 
     Generally in the dipole loudspeaker implementations, including all of those described above, the speakers in a dipole loudspeaker unit can be made identical; they are generally not required to handle low bass frequencies since such lower frequencies contribute little to stereo effect, so the lower frequencies are typically handled elsewhere in the overall loudspeaker system, e.g. in a woofer or sub-woofer unit that could be made as part of the forward-directed unit. 
     The foregoing examples of dipole loudspeaker units, enclosure openings and speaker openings are shown without covering for clarity of illustration; however in actual practice these will generally be covered with grill cloth or screen of known art for appearance and protection. 
     FIG. 13 depicts a two-unit loudspeaker stack having a dipole loudspeaker unit  18 , shown as a single-speaker unit, similar to unit  18  in FIGS. 9-9B. The housing of dipole loudspeaker unit  18  is dimensioned to a modular size and supports a combo-amp unit having forward-directed loudspeaker unit  16 , driven by a built-in amplifier unit  14 A. Monophonic input from the musical instrument source  10  is applied as input to amplifier  14 A and directed internally via a “FX send” line to FX unit  20  which provides L and R stereo line outputs, applied as input to the CPS processor  12 . 
     Output d (difference L−R) from CPS processor  12  is applied to dipole loudspeaker unit  18  driven from power amplifier  14 B shown located beneath dipole loudspeaker unit  18 . 
     The s (sum L+R) output of CPS processor  20  is applied to amplifier  14 A via an “FX return” line input. 
     This arrangement allows the player, with only a monophonic output from the instrument, to very conveniently switch back and forth, using a switch on amplifier  14 A, between (a) a regular monophonic system utilizing only forward-directed speaker unit  16 , and (b) a multi-function stereophonic system utilizing FX unit  20 , CPS processor  12  and dipole loudspeaker unit  18 . 
     FIG. 14 depicts a three-speaker stack including a modular sub-woofer unit  38  beneath the dipole loudspeaker unit  18 . Sub-woofer unit  38  is driven from power amplifier  40 , which receives input from the sub-woofer line output of CPS processor  12 . 
     An input preamp  44 , receiving a monophonic source signal from a musical instrument source  10 , applies the source signal to the monophonic line input of FX unit  20 , whose stereo L and R line outputs are applied to the line inputs of CPS processor  16 . The s and d line outputs of CPS processor  16  are applied to the two line inputs of a stereo amplifier  14 D, which drives forward-directed loudspeaker unit  16  and dipole loudspeaker unit  18  so as to generate center point stereo sound. 
     FIG. 15 is a front elevation representation of a rack-mounted system that provides in a single tower unit all of the components of FIG.  13 : starting from the bottom, sub-woofer unit  38 , dipole loudspeaker unit  18 , forward-directed loudspeaker unit  16 , sub-woofer power amplifier  40 , CPS processor  12 , stereo power amplifier  14 C, stereo FX unit  20  and input preamplifier  44 . This single tower requires only a monophonic musical source such as a guitar or keyboard/tone generator to provide a full operating system for live performance that will fill an auditorium with stereo sound effects emanating entirely from the rack as the center point source of stereo sound. 
     The invention may be embodied and practiced in other specific forms without departing from the spirit and essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description; and all variations, substitutions and changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.