CA2252595A1 - Audio enhancement system for use in a surround sound environment - Google Patents

Abstract

An audio enhancement system and method for use in a surround sound environment creates a more diffuse and continuous sound field from a multi-channel, multi-speaker reproduction environment. Multiple audio source signals generated from an audio recording, which are intended for speakers placed in front of and behind a listener, are isolated into pairs and processed to create corresponding pairs of component audio signals. Each pair of component audio signals is generated, at least in part, from the information present in both corresponding audio source signals. The individual component audio signals are then selectively combined to form enhanced output signals so that each enhanced output signal is modified as function of a plurality of audio source signals.

Description

CA 022~2~9~ 1998-10-23 ~1 AUDIO ENHANCEMENT SYSTEM
FOR USE IN A SURROUND SOUND ENVIRONMENT

Back~round of the Invention 5This invention relat,es generally to audio enhancement systems and methods for improving the realism and dramatic effects ot: -" from stereo sound ll r .h F More particularly, this invention relates to apparatus and methods for ' -- g sound g - albd in a surround sound environment having separate front and rear audio channels.
The advent of stereo surround sound audio systems, i.e., audio systems having separate audio channels for 10 front and rear speakers, has brought a more realistic and u'~ 9 audio experience to listeners. Such systems, such as Dolby l: bcr. ~ibs Pro-Logic system, may use a matrixing scheme to store four or more separate audio channels on just two audio recording tracks. Upon dematrixing, the Pro Logic audio system delivers distinct audio signals to a left-front speaker, a right front speaker, a center speaker, and to surround speakers placed behind a listener.
15More recently, surround sound systems have emerged which can deliver completely separate forward and rear audio channels. One such system is Dolby Laboratories five-channel digital system dubbed "AC-3." An audio component which has Oolby AC 3 capability can deliver five discrete channels to speakers placed around a listening environment lleft-front, center, right-front, left-surround, and right-surround). Unlike previous surround sound systems, all five of the distinct channels of the Oolby AC 3 system have full bandwidth capability. This allows for more 20 dynamic and volume range of the rear, or "surround", channels.
The discrete full bandwidth channels of the Dolby AC 3 system have been touted as increasing localization of stereo sound effects within a sound field. This localization results from the distinct audio channels which feed a separate speaker within the surround sound environment. As a result, sound information can be r'l 'l d to any speaker within the system. Moreover, because the AC 3 audio channels are not limited in audio bandwidth, all of 25 the channels can be used for both ambient and direct sound effects.
Although localization of sounds to some extent is beneficial and may greatly increase realism upon audio playback, the capabilities of systems such as Dolby AC-3 and Pro Logic are limited. For example, a sound field which surrounds a listener can be created by directing sounds to five separate speakers placed around the listener.
However, the surround-sound field may be perceived by the listener as containing five discrete point sources from 30 which sounds emanate. In certain surround sound audio systems, sounds which are intended to move from one rear speaker to another rear speaker may seem, from a listener's ~ t;.~, to leap across the rear sound stage.
Similarly, sounds which are intended to move from a forward left speaker to a rear left speaker may likewise appear to leap across the left sound stage.
Despite the advances in audio reproduction systems, and pa, lib.d~rl~ those having surround sound capability, 35 there is a need for an audio enhancement system which can improve upon the realism of these audio reproduction systems. The audio enhancement system disclosed herein fulfills this need.

CA 022~2~9~ 1998-10-23 SummarY of the Invention An audio - ' - se I system and method is disclosed which is particularly designed for surround sound audio systems such as Dolby's AC 3 five channel audio system, Dolby's Pro Logic system, or similar multi channel audio surround systems. In a typical multi-channel audio enhancement system, four separate audio signals intended 5 for the front and rear speakers are s~ grouped in pairs. Each pair of audio signals is used to generate a pair of component audio signals modified relative to the original pair of audiosignals.
The level and type of modification made to the component audio signals may vary to emphasize certain ac~ tjr:~l features of the original audio signals. Individual component audio signals g dl~d from different pairs of original audio signals are then se'~ combined to create a cr~zns;te audio output signal. The composite 10 audio output signal is then fed directly to a speaker for acoustic ", ~d : ~m The remaining audio output signals are 9~ ~ dl~d in a similar fashion by combining selected component audio signals. This creates a group of four audio output signals which are enhanced as a function of at least some of the original audio signals.

Brief Descri~tion of the Drawin~s The above and other aspects, features, and advantages of the present invention will be more apparent from the following particular description thereof, ese.,led in conjunction with the following drawings, wherein:
Figure 1 is a schematic block diagram of an audio enhancement system for use in a surround-sound ."..;.~, 1.
Figure 2 is a schematic block diagram of an alternative embodiment of an audio enhancement system for use in a surround-sound environmednt.
Figure 3 is a high level block diagram of a preferred audio enh I system.
Figure 4A is a schl m~tic diagram of a summing circuit for use with the invention disclosed in Figure 1.
Figure 4B is a schematic diagram of a summing circuit for use with the invention disclosed in Figure 2.
Figure 5 is a schematic block diagram depicting one type of audio enhancement system which may be used as shown in Figures 1 and 2 in order to generate a b ~dl ~d stereo image.
Figure 6 is a graphical display of the ~,., ~ y response of an equalization curve, derived from the audio enhancement system of Figure 4, which is applied to the ambient stereo signal information.
Figure 7 is a schematic diagram of a first embodiment of the audio enhancement system shown in Figure 4.
Figure 8 is a schematic diagram of a second embodiment of the audio enhancement system shown in Figure 4.

Detailed D~ n of the Preferred Embodiment Figure 1 depicts a block diagram of a multi-channel audio e ~ ~oment system 10 for use in a surround sound environment. The audio enhancement system 10 operates in cc- - 1U with a stereo signal decoder 12 having multi-channel audio source signals. The decoder 12 of Figure 1 is a six channel audio decoder which provides CA 022~2~9~ 1998-10-23 audio signals that ultimately drive a group of six speakers. Each of the six audio channels is intended for a different pne of the six speakers. In particular, an audio source signal 14, representing the center information (e.g., dialogue), is ultimately directed to a center speaker 16. An audio source signal 18 containing low ~nrl ~ sounds is ultimately directed to a subwoofer 20.
The remaining four audio source signals 20, 22, 24, and 26 of the stereo decoder 12 represent the signals ordinarily intended for c- -~1 )r lafter amplification) to a left rear speaker 28, a left front speaker 30, a right-front speaker 32, and a right-rear speaker 34" , 5.,1~. However, as shown in Figure 1, the audio source signals 20, 22, 24, and 26 are instead ..,I~.,t;. 'y routed to a group of audio enhancement devices 40, 42, 44, and 46. In this manner, all of the source signals are isolated in pairs such that no two pairs are identical but two separate pairs may contain the same source signal.
Specifically, a first audio enhancement device 40 receives the left-front source signal 22 ILf), and the right front source signal 24 IRf). The audio enhancement device 40 outputs a first enhanced component signal 50 ILf1) and a second enhanced component signal 52 IRf1). In a similar manner but with different inputs, a second audio Dnt device 42 receives the left rear source signal 20 llrl and the source signal 22 (Lf). In turn, the device 42 outputs first and second component signals 54 (Lf2), and 56 ILr11.
Likewise, a third audio enhancement device 44 receives the source signal 24 IRf) and the right-rear source signal 26 (Rr)~ The device 44 outputs first and second component signals 58 (Rf2) and 60 (Rr1). Finally, a fourth audio enhancement device 46 receives the source signal Lr and the source signal 26 IRr). The device 46 outputs first and second component signals 62 ILr2) and 64 (Rr2). For ease of explanation and clarity, the '- - mPnt system 10 is shown having four separate audio enhancement devices 40, 42, 44, and 46. It can be ~ c;dlcd by one of ordinary skill in the art that the resultant component signals may be 6r dlLd by a single audio ~ ' -r device receiving all four source signals and modifying them appropriately.
Selected pairs of the component signals Iderived from different pairs of source signals) are combined at one of four summing junctions 70, 74, 78, or 82. Specifically, the component signals Lf1 and Lf2 are combined at the summing junction 70 to create a composite enhanced output signal 72 ll ~; . ,j~ for driving the left front speaker 30. At the summinp, junction 74, the component signals 52 IRf1) and 58 (Rf2) combine to create a composite enhanced output signal 76 IR~t Il) for driving the right-front speaker 32. A composite enhanced output signal 80 (I ,; , ) drives the left-rear speaker 28. The signal l ,; ~; is ~ dlLd at the summing junction 78 from component signals Lr1 and Lr2. Lastly, the component signals 60 (Rr1) and 64 IR~2) are combined at the summing junction 82 to create a composite enhanced output signal 84 IR,; . ~j~ To summarize, 1~; . ,; - K1lLf1 + L 2); ~fl .. - K2(Rf1 + Rf2); 1; ~ - K3(Lr1 + Lr2); and ~; r, - K41Rr1 + Rr2)~ where each of the component signals is generated as a function of two audio source signals. The independent variables K1-K4 are determined by the gain, if any, of the summing junctions 70, 74, 78, and 82.In operation, the audio enhancement system 10 creates a set of four enhanced audio output signals 72, 76, 80, and 84. Each of these four enhanced audio signals is modified as a function of a plurality of the original source signals 20, 22, 24, and 26. The ~ rm I system 10 operates on the decoded pre-amplified audio source CA 022~2~9~ 1998-10-23 WO 97/41711 PCT/US97/0699~

signals which are designated for separate speakers placed within a listening environment. Accordingly, the resultant enhanced output signals 72, 76, 80, and 84 must be amplified before reproduction by the speakers 28, 30, 32, and 34. Audio signal amplifiers are not sep~ld~ shown in Figure 1 but may possibly be included in the speakers 28, 30, 32, and 34.
The enhanced output signal I f; , is g a ~ as a composite of signals Lfl and Lf2. The signal Lfl is g dl~d by the audio enhancement device 40 as a function of the two audio source signals 4 and Rf. Yarious audio enhancement apparatus and methods may be used for the device 40. In a preferred embodiment, however, the device 40 creates a signal Lf1 which, in - : with the signal Rf1, broadens a perceived spatial image when these signals are played through the speakers 30 and 32, r~ . This creates a more diffuse soundfield 10 between the speakers 30 and 32 and eliminates excessive localization of sound which can detract from realism.
In addition to the component signal Lf1, a second component signal Lf2, is a al~,d by the audio enhancement device 42. The signal Lf2 is 9 al~d as a function of the audio source signals 20, L" and 22, 4.
The signal Lf2 I.LJe~.i,,ts one of a pair of audio signals Ithe other being Lrl) which, in ~ P- -e with a preferred embodiment, generate an enhanced spatial image when amplified and played through the speakers 28 and 30.
Accordingly, the composite enhanced left output signal, I F~ ; comprises a portion of the signal Lfl and the signal Lf2. Thus, the acoustics generated through the speaker 30 will be -', d( nt upon both of the audio source signals Lr and Rf, which without the enhancement system 10, would be directly ~ to the speakers 28 and 32, I. pc ~ ly. The signal I F~ will thus create an improved spatial image which is dependent on the front audio source signals, Lf and Rf, and the left side audio source signals, Lr and 4.
In a similar manner, the composite enhanced output signals R~ L ' and R,~ . are 9~ d from c- ~ pc--nt signals outputted from the: ' I devices 40, 42, 44, and 46. In particular, the signal R~i ~; is a function of the front source signals, 4 and Rf, and the right side source signals, Rf and Rr;
the signal l ,; . ~; is a function of the left side source signals, Lf and Lr~ and the rear source signals, Lr and R,;
and the signal R,; . ,~ is a function of the right side source signals, Rf and Rr~ and the rear source signals, Lr 25 and Rr~
In ae- dance with the embodiment shown in Figure 1, each of the audio output signals supplied lafter amplificationl to a respective one of the speakers 28, 30, 32, and 34 is a function of at least three of the audio source signals 20, 22, 24, and 26. Thus, a given audio output signal played through a speaker becomes dependent upon original source signals intended (before enhancement) for other nearby or adjacent speakers. By blending the 30 output signals in this manner an improved sound eA~.e,i ~e can be achieved. Depending on the level and type of audio e ' - e I devices used, the F 1~ - of speaker point sources can be ~ d, and instead, a perceived array of loudspeakers is created. Thus, a sound reproduction environment originally intended as a "surround"
environment can be made into an environment which envelops or immerses the listener in sound.
In addition to the enhancement of the source signals 20, 22, 24, and 26, the signals 14 and 16 may 35 require level adjustment to balance these signal levels with those of the enhanced source signals 20, 22, 24, and 26. Such level adjustment may be preset and fixed or may be manually adjustable by a user of the system 10.

CA 022~2~9~ 1998-10-23 WO 97/41711 PCT/U~97/06995 Level control devices are common to one of ordinary skill in the art and would be placed between the decoder 12 and the si~nal amplifier (not shown) used to power the- 1" ~F iale speaker.
In some surround sound svstems, such as the Dolby Pro Logic system, there is a single audio signal used to simulate surround effects. This single audio signal is transmitted to both of the rear speakers. In such systems, the signals Lr and Rr of Figure 1 would be identical and there would be no need for the rear audio ' -- ament unit 46.
Figure 2 depicts a multi-channel audio enhancement system 100 which employs the techniques just described in c ~ )r with Figure 1. In addition, the enhancement system 100 has two additional audio enhancement devices 102 and 104. Like the other devices 40, 42, 44, and 46, the enhancement devices 102 and 104 provide component 10 signals which c~r'.ibule to the final audio output signals 72, 76, 80, and 84. The component signals are determined as a function of their ,. pe~i;e source signals.
Unlike the other four enhancement devices 40, 42, 44, and 46, the devices 102 and 104 provide crossover audio enhancement. Crossover audio enhancement modifies sounds as a function of those source signals intended for playback by speakers placed diagonally from each other. In particular, the enhancement device 102 inputs the 15 source signals Lr and Rp The resultant component signals Rf3 and Lr3 are ~ dl~,d by the device 102. The signal Rf3 is combined at a summing junction 110 with two other component signals, RF1 and Rf2. This creates a composite output signal 112 (R~; ~ll I which is modified as a function of all four source signals 20, 22, 24, and 26. Similarly, the signal Lr3 is combined at the junction 114 to generate the composite signal 116 (I,; . )) which powers lafter amplification) the left-rear speaker 28.
The operation of the second crossover enhancement device 104 is similar to that of the device 102.
Specifically, the device 104 receives source signals 4 and Rr intended for diagonally positioned speakers 30 and 34.
The device 104 O dles a first component signal 120 (Rr3) which is combined at a summing junction 122 with Rr1 and Rr2 to produce the final output signal 124 (R,; . 1 Likewise, a second component signal 126 is combined at a summing junction 128 with Lf1 and Lf2 to produce the final output signal 130 ~
Figure 3 depicts the multi-channel audio s ' - ; system 10 c lod to a host system 132 and a storage media device 134. In the preferred embodiment, the host system 132 is an audio receiver which is compatible with surround systems such as the Dolby Laboratories five channel digital system dubbed "AC 3." In other embodiments, the host system 132 is an audio receiver which is compatible with Dolby I r~ ~rat iSS' Pro-Logic system. Furthermore, while a multi channel surround system such as AC 3 is preferred, the present invention is not 30 limited to surround sound systems and can be used to enhance a wide variety of multi-channel sound systems. In other embodiments, for instance the host system 132 may also comprise a laser disk system, a video tape system, a stereo receiver, a television receiver, a computer based sound system, a digital signal processing system, a Lucasfilm THX entertainment system or the like.
While the stora~e media device 134 in the preferred embodiment provides an AC 3 compatible bitstream, 35 other embodiments can use a wide range of storage mediums and storage formats. The format of the AC-3 bitstream is defined by Dolby Laboratories and is well known to those of ordinary skill in the art. Thus, one of CA 022~2~9~ 1998-10-23 ordinary skill in the art will recognize that the storage media device 134 may include a wide variety of optical .storage mediums, magnetic storage mediums, computer a~ce "l storage systems or the like. For example, the storage media device 134 may comprise laser disc players, digital video devices, compact discs, video tapes, audio tapes, magnetic recording tracks, floppy disks, hard disks, etc. Furthermore, other embodiments of the storage media device 134 support a wide variety of data formats such as analog frequency modulation, pulse code modulation and the like. In addition, the storage media device 134 may be part of a cable l~ system, an interactive video device, a computer network, the Internet, a television ~' ~7d~ system, a high definition television br.? 'l -st system or the like.
In the preferred embodiment, the multi-channel audio signal decoder 12 receives sound data from the host 10 system 132 or the storage media device 134 via a communications bus 136. For example, a composite radio 1,., s~ signal containing an AC 3 bitstream is sent from the storage media device to the multi-channel audio signal decoder 12 via the communications bus 136. However, one of ordinary skill in the art will recognize that the communications bus 136 can be configured to carry a wide variety of audio signal formats.
In other embodiments, the host system 132, the storage media device 134, and the communications bus 15 136 may be integrated into a single device. For example, a digital video device may integrate the host system 132, the storage media device 134 and the communications bus 136. In addition, as discussed in more detail below, other embodiments may integrate the host system 132, the storage media 134 and the systems 10 or 100 with discrete analog components, a semiconductor substrate, through software, within a digital signal processing (DSP) chip, i.e., firmware, or in some other digital format. For example, an audio receiver may contain a digital signal r ~ ~
20 which accesses the storage media 134 via communications bus 136, performs host system 134 functions and performs the functions of systems 10 or 100 to produce enhanced signals, Figures 4A and 4B depict the summing junctions disclosed in Figures 1 and 2. The two signal summing junction 70 of Figure 1 is ~ s~ Ld by the circuit shown in Figure 4A. The remaining junctions 74, 78, and 82 are identical to the junction 70 except for the particular input signals received. The summing junction 70 is 25 configured as a standard inverting amplifier having an r, al ~' amplifier 142. The amplifier 142 receives the signals Lf1 and Lf2. Lf~ and Lf2 are then combined, or added together, at an inverting terminal 144 of the amplifier 142. The relative ~qain of the circuit 70 is determined by the resistors 146, 14B and 150. In a preferred embodiment, the gain for each of the signals Lf1 and Lf2 will be unity. However, slight adjustments in gain may be required depending on the particular audio environment and the personal ,,.~,.l of a listener.
-Figure 4B depicts the summing junction 128 of Figure 2. The junction 12B and the junction 70 are similarly configured as summing, inverting amplifier circuits. The junction 128, however, has an ~F di' ' amplifier 152 which combines three inputs, Lf1, Lf2, and Lf3, instead of just two inputs.
The audio enhancement techniques disclosed in Figures 1 and 2 improve the immersive effect of a surround sound audio system. The systems 10 and 100 of Figures 1 and 2 depict a typical home audio ~p,.d 35 environment having four primary speakers placed along the front and rear areas of a sound stage. However, the concepts of the present invention are 1" 1i( '' to sound; .:.. ~ having additional speakers which may be CA 022~2~9~ 1998-10-23 WO 97/41711 PCTtUS97/06995 ~7 placed at any location within a sound stage. For example, speakers may be placed along side walls or even at .different Lk..~ --' levels from one another or with respect to a listener. In addition, the concepts of the present invention can be applied to any pair of audio source signals that may be selected for enhancement. The resultant crn~pr-- l si~qnals are then combined with other component signals created from a second pair of audio source signals. This same process may be continued for each possible pair of audio source signals v ~ dt~d by a stereo signal decoder or the like.
The systems 10 and 100 may be implemented in an analog discrete form, in a semiconductor substrate, through software, within a digital signal processing tDSP) chip, i.e., firmware, or in some other digital format.
The multi channel audio enhancement system 10 of Figure 1, or the enhancement system 100 of Figure 2, may employ a variety of audio enhancement devices for generating the component audio signals. For example, the devices 40, 42, 44, 46, 102, and 104 may use time-delay techniques, phase-shift techniques, signal equalization, or a combination of all of these techniques to achieve a desired audio effect. Moreover, the audio enhancement t ' , s applied by the individual r ' - - I devices 40, 42, 44, 46, 102, and 104 need not be identical.
In aoc d e with a preferred embodiment of the present invention, the enhancement devices 40, 42, 44, and 46 of Figure 1 equalize an ambience signal component found in a pair of stereo signals. As a result, many sounds emanating from a given speaker will not be localized to that speaker. In addition, sounds intended to move across the sound stage from one speaker to another, will do so gradually as if additional speakers were present.
The ambience signal component represents the differences between a pair of audio signals. An ambient signal component derived from a pair of audio signals is therefore often referred to as the "difference" signal component.
An example of one audio enhancement device (and methods for implementing same) which is suitable for use with the present invention is discussed in co- -~t with Figures 5 8. Such a device broadens and blends a perceived sound stage ~ at~d from a pair of stereo audio signals by enhancing the ambient sound information.
The audio ~ ' mPnt device and method disclosed in Figures 5-8 is similar to that disclosed in pending application serial number 081430751 filed on April 27, 1995, which is h.CGI~O dt d herein by reference as though fully set forth.
Related audio enhancement devices are disclosed in U.S. Patent Nos. 4,738,669 and 4,866,744, issued to Arnold 1. Klayman, both of which are also incr ~ ai d by reference as though fully set forth herein.
Referring initially to Figure 5, a functional block diagram is shown depicting an audio enhancement device 160. In a preferred embodiment of the present invention, the device 160 represents each of the devices 40, 42, 44, 46, 102, and 104. The enhancement system 160 receives first and second stereo source signals (S1 and S2) at inputs-162 and 164"~ pr li.~l~. These stereo source signals are fed to a first summing device 166, e.g., an LIh~IIL adder. A sum signal, representing the sum of the stereo source signals received at the inputs 162 and 164, is v alLd by the summing device 166 at its output 168.
~ The signal S1 is also c -etcd to an audio filter 170, while the signal S2 is connected to a separate audio filter 172. The outputs of the filters 170 and 172 are fed to a second summing device 174. The summing device 174 v c al~;~ a di~L.I c e signal at an output 176. The difference signal represents the ambient information present ,, . ~

CA 022~2~9~ 1998-10-23 in the filtered signals S1 and S2. The filters 170 and 172 are pre-conditioning high pass filters which are designed to avoid over-amplification of the bass components present in the ambient component of a pair of stereo signals.
The summing device 168 and the summing device 174 form a summing network havin~q output signals individually fed to separate level-adjusting devices 1 BO and 182. The devices 180 and 182 are ideally potentiometers or similar l,a~ pp' r devices. Adjustment of the devices 180 and 182 is typically performed manually by a user to control the base levels of sum and dil~ signals present in the output signals. This allows a user to tailor the level and aspect of stereo enhancement according to the type of sound reproduced, and depending on the user's personal p~ . -s~ An increase in the level of the sum signal emphasizes the audio signals appearing at a center stage positioned between a pair of speakers. Cor..~ lt, an increase in the level of difference signal emphasizes the ambient sound information creating the F I, ~ of a wider sound image. In some audio arrangements where the parameters of music type and system configuration are known, or where manual adjustment is not practical, the adjustment devices 180 and 182 may be eliminated and the sum and difference signal levels fixed at a predetermined value.
The output of the device 182 is fed into an equalizer 184 at an input 186. The equalizer 184 spectrally shapes the difference signal appearing at the input 186. This is accomplished by 1~ atel~ applying a low-pass audio filter 188, a high pass audio filter 190, and an dli i a circuit 192 to the difference signal as shown.
Output signals from the filters 188, 190, and the circuit 192 exit the equalizer 184 along paths 194, 196, and 198, e ~
The modified ';'~ ru signals transferred along paths 194, 196, and 198 make up the components of a , ocesscd difference signal, (S1-S2)p. These components are fed into a summing network comprising summing devices 200 and 202. The summing device 200 also receives the sum signal output from the device 180, as well as the original stereo source signal S1. All five of these signals are added within the summing device 200 to produce an enhanced audio output signal 204.
Similarly, the modified d;f~ b - signals from the equalizer 184, the sum signal, and the signal S2 are combined within the summing device 202 to produce an enhanced audio output signal 206. The components of the difference signal originating along paths 194, 196, and 198 are inverted by the summing device 202 to produce a , ..cesscd difference signal for one speaker, IS2-Sl)p, which is 180 degrees out of-phase from that of the other speaker.
The overall spectral shaping, i.e., normalization, of the ambient signal information occurs as the summing devices 200 and 202 combine the filtered and a: l~d components of the difference signal to create the audio output signals 204 and 206. Accordingly, the audio output signals 204 and 206 produce a much improved audio effect because ambient sounds are 5~'- 'K~ emphasized to fully encompass a listener within a reproduced sound stage. The audio output signals 204 and 206 are ,., ~eal~d by the following mathematical formulas:
-AUDIO OUT~ S1 + K1IS1 + S2) + K21S1 S2)p 11) AUDIO OUTI2) - S2 + K11S1 + S2) - K21S1 S2)P 12) CA 022~2~9~ 1998-10-23 WO 97/41711 rcT/usg7/06995 It should be noted that input signals S, and S2 in the equations above are typically stereo source signals, .but may also be synthetically ~ dtOd from a monophonic source. One such method of stereo synthesis which may be used with the present invention is disclosed in U.S. Patent No. 4,841,572, also issued to Arnold Klayman and ~ -O ~ di I herein by ,-c~ e Moreover, as discussed in U.S. Patent No. 4,748,669, the enhanced output sipnals 5 I~"rb~"i ~ above may be O i -lly or elb~ rll~ stored on various recording media, such as vinyl records, compact discs, dipital or analo~g audio tape, or computer data storage media. Enhanced audio output si~gnals which have been stored may then be reproduced by a cc . .inn-' stereo reproduction system to achieve the same level of stereo image enhancement.
The signal (S1S2)p in the equations above Ib, c~ i the r ~re ~ d,fl~.~ace signal which has been 10 spectrally shaped according to the present invention. In a~ d- e with a preferred embodiment, modification of the dilld~. re signal is ,e, e~.,t~d by the 1~ response depicted in Figure 6, which is labeled the enhancement F pr li.~, or normalization, curve 210.
The pe,~pr ~ curve Z10 is displayed as a function of yain, measured in decibels, against audible ~I~, e r - displayed in log format. According to a preferred embodiment, the F~ ~ prcl;.~ curve 210 has a peak gain of approximately 7 dB at a point A located at approximately 125 Hz. The gain of the p.,. pr ~ curve 210 decreases above and below 125 Hz at a rate of approximately 6 dB per octave. The p ~,e.,li.~ curve 210 applies a minimum gain of 2 dB to a difference signal at a point B of approximately 2.1 Khz. The gain increases above 2.1 Khz at a rate of 6 dB per octave up to a point C at approximately 7 Khz, and then continues to increase up to approximately 20 Khz, i.e., approximately the highest treguency audible to the human ear. Although the overall equalization of the perspective curve 210 is accomplished usin~g high-pass and low pass filters, it is possible to also use a band-rejection filter, having a minimum gain at point B, in conjunction with a high-pass filter to obtain a similar perspective curve.
In a preferred embodiment, the ~gain s, dC - between points A and B of the perspective curve 210 is ideally desi~gned to be 9 dB, and the ~gain separation between points B and C should be approximately 6 dB. These figures are design constraints and the actual figures will likely vary from circuit to circuit depending on the actual value of components used. If the signal level devices 180 and 182 are fixed, then the p.,., I;.c curve 210 will remain constant. However, adjustment of the device 182 will sli~ghtly vary the gain separation between points A
and B, and points B and C. In a surround sound environment, a gain separation much larger than g dB may tend to reduce a listener's perception of mid-range definition.
-Implementation of the r . ~ curve by a digital signal processor will, in most cases, more accurately reflect the design constraints discussed above. For an analog implementation, it is acceptable if the frequencies r~ rdin~g to points A, B, and C, and the constraints on gain separation, vary by plus or minus 20 percent. Such a deviation from the ideal ~ will still produce the desired stereo enhancement effect, althou~gh with less than optimum results.
As can bs seen in Figure 6, difference signal ll.~ ~ e below 125 Hz receive a dr ~asEd amount of boost, if any, throu~gh the application of the perspective curve 210. This decrease is intended to avoid over CA 022~2~9~ 1998-10-23 ~10 amplification of very low, i.e., bass, 1~ s With many audio reproduction systems, and especially surround $ound audio systems, amplifying an audio difference signal in this low frequency range can create an unpleasurable and unrealistic sound image having too much bass response.
The stereo enhancement provided by the present invention is uniquely adapted to take advantage of high 5 quality stereo recordings. Spc '; 'I~, unlike previous analog tape or vinyl album recordings, today's digitally stored sound recordings contain difference signal, i.e. stereo, information throughout a broader frequency spectrum, including the bass 1,, ~-- .., amplification of the difference signal within these 1,., - ~ is therefore not required to obtain adequate bass response.
Figure 7 depicts a circuit 220 for creating a ' .-' ' stereo sound image. The audio enhancement circuit 220 co l~_r~n ~ to the device 160 shown in Figure 5. In Figure 7, the source signal S1 is fed to a resistor 222, a resistor 224, and a capacitor 226. The source si~qnal S2 is fed to a capacitor 228 and resistors 230 and 232.
The resistor 222 is r ~ct d to a n~a i .~ terminal 234 of an amplifier 236. The same non inverting terminal 234 is also c~ ~: d to the resistor 232 and a resistor 238. The amplifier 236 is configured as a summing amplifier having an inverting terminal 240 c-- e ~ to ground via a resistor 242. An output 244 of the amplifier 236 is sr t~,d to the inverting terminal 240 via a feedback resistor 246. A sum signal (Sl+S2), representing the sum of the first and second source signals, is 9: dl~d at the output 244 and fed to one end of a variable resistor 250 which is grounded at an opposite end. For proper summing of the source signals S1 and S2 by the amplifier 236, the values of resistors 222, 232, 238, and 246 in a preferred embodiment are 33.2 kohms while resistor 238 is preferably 16.5 kohms.
A second amplifier 252 is configured as a "difference" amplifier. The amplifier 252 has an inverting terminal 254 rr e ~ to a resistor 256 which is in turn - P - ' in series to the capacitor 226. Similarly, a positive terminal 258 of the amplifier 252 receives the signal S2 through the series connection of a resistor 260 and the capacitor 228. The terminal 258 is also connected to ground via a resistor 262. An output terminal 264 of the amplifier 252 is c1 ertPd to the inverting terminal through a feedback resistor 266. The output 264 is also connected to a variable resistor 268 which is in turn c-- et~d to ground. Although the amplifier 252 is configured as a "difference" amplifier, its function may be chdlal,t~,.iL~d as the summing of the right input si~qnal with the negative left input si~qnal. Accordingly, the amplifiers 236 and 252 form a summing network for ~qenerating a sum signal and a difference signal, rG, '~.
The two series c: ~t~d RC networks comprising elcments 2261256 and 2281260, respectively, operate as high pàss filters which attenuate the very low, or bass, frequencies of the left and right input signals. To obtain the proper frequency response for the pars~o~.li.~ curve 210 of Figure 6, the cutoff frequency, wt, or 3 dB
y, for the high pass filters should be approximately 100 Hz. Accordingly, in a preferred embodiment, the capaLitu,~ 226 and 228 will have a s~par --e of .1 micro farad and the resistors 256, 260 will have an impedance of 3\ 1. OXillld~ 33.2 kohms. Then, by choosing values for the feedback resistor 266 and the attenuating resistor 262 such that:

CA 022~2~9~ 1998-10-23 W O 97/41711 PCTrUS97/06995 R,20 R"6 ( ) Rl28 R~24 the output 264 will represent a difference signal, IS2 S,), amplified by a gain of two. As a result of the high-pass filtering of the inputs, the d;'~ e signal at the output 264 will have attenuated low ~ y components below approximately 125 Hz decreasing at a rate of 6 dB per octave. It is possible to filter the low ~ components of the d;f~ .nce signal within the equalizer 184 tshown in Fig. 5~, instead of using the filters 170 and 172 Ishown 5 in Fig. 5), to separately filter the input source signals. However, because the filtering c ~a - ~ for use at low ~nqu( - must be fairly large, it is preferable to perform this filtering at the input stage to avoid loading of the preceding circuit.
The variabie resistors 250 and Z68, which may be simple pQi: 1 ~, are adjusted by placement of wiper contacts 270 and 272"~ ECI;.G'Y. The level of the ambience signal - , c, t, i.e., ' '~.. ce signal, present in the enhanced output signals may be controlled by manual, remote, or automatic adjustment of the wiper contact 272. Similarly, the level of mono signal component, i.e., sum signal, present in the enhanced output signals is determined in part by the position of the wiper contact 270.
The sum signal present at the wiper contact 270 is fed to an inverting input 274 of a third amplifier 276 through a series connected resistor 278. The same sum signal at the wiper contact 270 is also fed to an inverting input 280 of a fourth amplifier 282 through a separate series c~ : ' resistor 284. The amplifier 276 is configured as a difference amplifier with the inverting terminal 274 CL- -: ' to ground through a resistor 286.
An output 288 of the amplifier 276 is also csr ct~,d to the inverting terminal 274 via a feedback resistor 290.
A positive terminal 292 of the amplifier 276 provides a common node which is cc --ct~,d to a group of summing resistors 294 and is also con ~ d to ground via a resistor 296. The level adjusted difference signal from the wiper contact 272 is 1, '~.lLd to the group of summing resistors 294 through paths 300, 302, and 304. This results in three separately conditioned difference signals appearing at points A, B, and C"~ ;pr ~;. 'y. These conditioned difference signals are then c -cl.,d to the positive terminal 292 via resistors 306, 308, and 310 as shown.
At point A along the path 300, the level adjusted difference signal from wiper contact 272 is transferred to the resistor 306 without any ~", ~ response modification. Accordingly, the signal at point A is merely dli led by the voltage division between the resistor 306 and the resistor 296. Ideally, the level of attenuation at node A will be 9 dB relative to a 0 dB reference appearing at node B. This level of dll 6UI~ is implemented ~ by the resistor 306 having an impedance of 100 kohms and the resistor 296 having an impedance of 21 kohms.
The signal at node B ", L~..ls a filtered version of the level adjusted difference signal appearing across a capacitor 312 which is c -cl~d to ground. The RC network of the capacitor 312 and a resistor 314 operate as a low pass filter with a cutoff ~ determined by the time constant of the network. In ~ e with a preferred embodiment, the cutoff 1,~, ~, or ~3 dB frequency, of this low-pass filter is approximately 200 Hz. Accordingly, . . ~

CA 022~2~9~ 1998-10-23 WO 97141711 PCTrUS97/0699S
~12 the resistor 314 is, c~o~ ") 1.5 kohms and the capacitor 312 is .47 m;~.ufai~ds, the drive resistor 308 is 33.2 kohms, and the feedback resistor 290 is 121 kohms.
In surround sound audio systems, there is often an abundance of bass or low~ y information resulting from the b~ o~ and the additional speakers. Therefore, it may be desirable to ~, dlLI) control the level of 5 low-lt' ~ cy d;f~L. ce signal appearing at node B. As should be apparent to one of ordinary skill in the art, this can be accomplished by connecting the output 264 of the amplifier 252 to a second variable gain resistor which, instead of the wiper contact 272, directly drives the resistor 314. In this manner, the time constant of the low pass filter is maintained and the lower l"~q - of the difference signal can be more precisely and directly c~rIIl 'l~ d At node C, a high pass filtered difference signal is fed through the drive resistor 310 to the non inverting terminal 292 of the amplifier 276. The high pass filter is designed with a cutoff ~ of approximately 7 Khz and a relative gain to node B of 6 dB. ~pe~ 'ly, a capacitor 316 CUmlcL~Ld between node C and the wiper contact 272 has a value of 4700 I ~alads, and a resistor 318 c~ Cl~d between node C and ground has a value of 3.74 kohms.
The modified difference signals present at circuit locations A, B, and C are also fed into the inverting terminal 280 of the amplifier 282 through resistors 320,322 and 324,1~ ~I cIi.Jy. The amplifier 282 is configured as an inverting amplifier having a positive terminal 332 C-- -ct ' to ground and a feedback resistor 334 c ectPd between the terminal 280 and an output 336. To achieve proper summing of the signals by the inverting amplifier 282, the resistor 320 has an impedance of 100 kohms, the resistor 322 has an impedance of 33.2 kohms, and the resistor 324 has an pc' e of 44.2 kohms. The exact values of the resistors and c p, ~ in the audio enhancement system 220 may be altered as long as the proper ratios are maintained to achieve the correct level of F.'- ce :. Other factors which may affect the desired value of the passive components are the power requirements of the enhancement system 220 and the chàl~l.,IL.i,Ii"s of the amplifiers 236, 252, 276, and 282.
In ., r. the modified ''l~c,~.~ce signals are recombined to generate output signals comprised of a , .cess~d difference signal. SFe~ f; -'I~, difference signal components found at points A, B, and C are recombined at the terminal 292 of the difference amplifier 276, and at the terminal 280 of the amplifier 282, to form a processed difference signal (S,-S2)p. The signal (S1 S2)p ,., c~..ls the difference signal which has been equalized through application of the perspective curve 210 of Figure 6. Ideally then, the r- pf~ , curve is chdldclLH~cd by a gain of 4 db at 7 Khz, a gain of 7 dB at 125 Hz, and a qain of 2 dB at 2100 Hz.
The amplifiers 276 and 282 operate as mixing amplifiers which combine the r ~ ~ d difference signal with the sum signal and either the left or right input signal. The signal at the output 288 of the amplifier 276 is ted through a drive resistor 340 to produce an enhanced audio output signal 342. Similarly, the signal at the output 336 of the amplifier 282 travels through a drive resistor 344 to produce an enhanced audio output signal 346. The drive resistors will typically have an impedance on the order of 200 ohms. The enhanced output signals 342 and 346 can be , c;,sed by the mathematical equations (1) and (2) recited above. The value of Kl in e~
and (2) is CUUllL'Ief' by the position of the wiper contact 270 and the value of K2 is s I~ 'ILd by the position of the wiper contact 272.

... .

CA 022~2~9~ l998-l0-23 WO 97/41711 PCT~US97/06995 ~13 All of the individual circuit components depicted in Figure 7 may be implemented digitally through software run on a m;L,lr ~c ~r, or through a digital signal processor. Accordingly, an individual amplifier, an equaliler, or other components, may be realized by a cc ,l ~ eding portion of software or firmware.
An alternative embodiment of the audio enhancement device 220 is depicted in Figure 8. The device 350 5 of Figure 8 iS similar to that of Figure 7 and r~ ts another method for applying the p re ~i~ curve 210 (shown in Fig. 6) to a pair of stereo audio signals. The audio enhancement system 350 utilizes an alternative summing network configuration for generating a sum and difference signal.
In the alternative embodiment 350, the audio source signals S1 and S2 are ultimately fed into the ne~qative input of mixing amplifiers 352 and 354. To generate the sum and ' 'f~.. signals, however, the signals S1 and S2 are first fed through resistors 356 and 358, l~ pr ti~ ~y, and into an inverting terminal 360 of a first amplifier 362. The amplifier 362 iS configured as an inverting amplifier with a grounded input 364 and a feedback resistor 366. The sum signal, or in this case the inverted sum signal -IL+R), is g aled at an output 368. The sum signal corpr ( is then fed to the remaining circuitry after being level adjusted by the variable resistor 370. Because the sum signal in the alternative embodiment is now inverted, it is fed to a nvr ~ lin~q input 372 Of the amplifier 354.
Accordingly, the amplifier 354 requires a current balancing resistor 374 placed between the non-inverting input 372 and ground potential. Similarly, a current-balancing resistor 376 iS placed between an invertin~q input 378 and ground potential. These slight modifications to the amplifier 354 in the alternative embodiment are ce y to achieve correct summing to generate the enhanced audio output signal 380.
To ~qenerate a difference signal, an inverting summing amplifier 383 receives the signal Sl and the sum signal at an inverting input 384. More specifically, the source signal Sl is passed through a capacitor 386 and a resistor 388 before arriving at the input 384. Similarly, the inverted sum signal at the output 368 is passed through a capacitor 390 and a resistor 392. The RC networks created by components 3861388 and components 3901392 provide the bass ~ filtering of the audio signal as described in c Fi 'nLt r with a preferred embodiment.
The amplifier 382 has a grounded n~r, ..,.ling input 394 and a feedback resistor 396. A di~.l ce signal, 25 S2-Sl, is 9: at~d at an output 398 with impedance values of 100 kohm for the resistors 356, 358,366, and 388, impedance values of 200 kohms for the resistors 392 and 396, a r1p, - -e of .15 micro farads for the capacitor 390, and a capacitance of .33 micro farads for the capacitor 386. The difference signal is then adjusted by the variable resistor 400 and fed into the remaining circuitry. Except as described above, the remaining circuitry of Figure 8 is the same as that of a preferred embodiment disclosed in Figure 7.
The entire audio enhancement system 220 of Figure 7 uses a minimum of components. The system 220 may be CV1 IL..,tcd with only four active cor pr !~, typically operational amplifiers CG~ ,O ding to amplifiers 236, 252, 276, and 282. These amplifiers are readily available as a quad package on a single semiconductor chip.
Additional components needed to construct the audio enhancement system 220 include only 29 resistors and 4 c pa- ~ s. The system 350 of Figure 8 can also be manufactured with a quad amplifier, 4 capacitors, and only 35 29 resistors, including the put ~ and output resistors. Because of its unique design, the audio ' - -systems 220 and 350 can be produced at minimal cost utili~ing minimal component space and still provide r CA 022S2S9S 1998-10-23 / W O 97/41711 rCTl~'Sn7/0~995 u,lbel ~n,-ab'e broadening of an existing stereo image. In fact, the entire system 220 can be formed as a single semiconductor substrate, or ;,ltog,d~ed circuit.
Apart from the c",bod;".cnts depicted in Figures 7 and 8, there are cont~ivaLly additional ways to interconnect the same components and obtain perspective enhancement of stereo signals as described herein. For example, a pair of amplifiers configured as difference amplifie~s may receive a pair of source signals, respectively~
and may also each receive the sum signal. In this manner, the amplifiers would generate a first difference signal, L R, and a second difference signal, R l, respectively.
In addition, still other embodiments of audio enhancement devices may not separately ~o,enerate a difference signal at all. Of main importance is the fact that ambient information, information represented by a difference signal, is properly equalized. This can be accomplished in any number of ways without specifically generating a difference signal. For example, the isolation of the difference signal information and its subsequent equalization may be performed digitally, or performed simultaneously at the input stage of an amplifier circuit.
The perspectiie modification of the difference signal resulting from the enhancement systems 220 and ~50 has been carefully engineered to achieve optimum results for a wide variety of applications and inputted audio signals. Adju~l,,,Er,t~ by a user currently include only the level of sum and difference signals applied to the conditioning circuitry. However, it is conceivable that potentiometers could be used in place of resistors 314 and 318 to allow for adaptive E.~uali~ation of the difference signal.
Other audio enhancement apparatus and methods which may be used as the devices 40, 42, 44, 46, 102, and 104 include time delay techn~ ,es as disclosed in U.S. Patent No. 4,355,203 ~incorporated herein by reference as though fully set forth), and phase shifting techniques as disclosed in U.S. Patent No. 5,105,462 (incorporated herein by reference as though fully set forth).
Thraugh the foregaing d~ .tion and accompanying drawings, the present invention has been shown ta have important advantages over current stereo reproduction and enhancernellt systems. While the above detailed descri~.tion has shown, described, and pointed out the fundamental novel features of the invention, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those skilled in the art~ .out ~EFertjr~o from ~ of the inventJ!q~J Therefore, the invention should be limited in its scope only by the following claims.

AMENDE~ SHEET
. . . ~ , . .

Claims (35)

Claims

1. An audio enhancement device (10) which broadens the perceived spatial image of sound signals, said audio enhancement device (10) comprising:
a first audio enhancer (40) which is configured to receive at least a first pairof said audio source signals (22, 24), said first audio enhancer (40) configured to process said first pair of audio source signals (22, 24) in a manner which generates a first component signal (50);
a second audio enhancer (42) which is configured to receive at least a second pair of said audio source signals (20, 22), said second audio enhancer (42) configured to process said audio source signals (20, 22) in a manner which generates a second component signal (54);
a third audio enhancer (44) which is configured to receive at least a third pairof said audio source signals (24 26), said third audio enhancer (44) configured to process said audio source signals (22, 24) in a manner which generates a third component signal (58); and a plurality of combining junctions (70 78), each of said combining junctions in communication with at least two of said component signals (50, 54, 58), said combining junctions (70 78) configured to combine said component signals (50, 5458) to generate a plurality of audio output signals (72, 76) such that said plurality of audio output signals (72, 76) broadens a perceived spatial image when said audio output signals (72, 76) energize a plurality of speakers (30, 32).

2. The audio enhancement device (10) of Claim 1 further comprising a first crossover enhancer (104) which is configured to receive at least a fourth pair of audio source signals (22, 26), wherein said fourth pair of audio source signals (22, 26) differs from said first, second and third pairs of said audio source signals (22, 24), said first crossover enhancer (104) configured to process said fourth pair of said audio source signals (22, 26) in a manner which generates a first crossover component signal (126).

3. The audio enhancement device (10) of Claim 2 wherein one of said combining junctions (128) is configured to combine at least two of said component signals (50, 54) with said first crossover component signal (126) to generate one of said audio output signals (130).

4. The audio enhancement device (10) of Claim 1 wherein one of said audio enhancers (40, 42, 44) modifies ambient information existing in at least two of said audio source signals (20, 22, 24, 26) by inserting a time delay within said ambient information.

5. The audio enhancement device (10) of Claim 1 wherein one of said audio enhancers (40, 42, 44) modifies ambient information existing in at least two of said audio source signals (20, 22, 24, 26) by phase shifting said ambient information.

6. The audio enhancement device (10) of Claim 1 wherein one of said audio enhancers (40, 42, 44) modifies ambient information existing in at least two of said audio source signals (20, 22, 24, 26) by selectively emphasizing relative amplitudes in said ambient information.

7. The audio enhancement device (10) of Claim 1 wherein one of said combining junctions (70, 74) adds said component signals (50, 54) together to generate one of said audio output signals (72, 76).

8. The audio enhancement device (10) of Claim 1 wherein one of said combining junctions (70, 74) comprises an inverting amplifier.

9. The audio enhancement device (10) of Claim 1 wherein one of said first combining junctions (70, 74) comprises an operational amplifier.

10. The audio enhancement device (10) of Claim 1 wherein said plurality of audiosource signals (20, 22, 24, 26) comprise a left-rear audio signal (20), a left-front audio signal (22) and a right-front audio signal (24) and a right-rear audio signal (26).

11. The audio enhancement device (10) of Claim 10 wherein said first audio enhancer (40) is configured to generate of plurality of front component signals (50, 52), said second audio enhancer (42) is configured to generate a plurality of left component signals (54, 56) and said third audio enhancer (44) is configured to generate a plurality of right component signals (58, 60).

12. The audio enhancement device (10) of Claim 11 wherein one of said combining junctions (78) is configured to combine one of said left component signals (56) and one of said rear component signals (62) to generate a left-rear audio output signal (80).

13. The audio enhancement device (10) of Claim 12 further comprising:

a fourth audio enhancer (44) which is configured to receive said right-front audio signal (24) and said right-rear audio signal (26), said fourth audio enhancer (44) configured to process said right-front audio signal (24) and said right-rear audio signal (26) in a manner which generates a plurality of right component signals (58, 60), and wherein one of said combining junctions (82) is configured to combine one of said right component signals (60) and one of said rear component signals (64) to generate a right-rear audio output signal (84).

14. The audio enhancement device (10) of Claim 13 further comprising a fourth combining junction (74) configured to combine one of said front component signals (52) and one of said right component signals (58) to generate a right-front audio output signal (76).

15. The audio enhancement device (10) of Claim 14 further comprising a second crossover enhancer (102) which is in communication with said right-front audio signal (24) and said left-rear audio signal (20), said second crossover enhancer (102) configured to process said right-front audio signal (24) and said left-rear audio signal (20) in a manner which generates a second crossover component signal (108).

16. The audio enhancement device (10) of Claim 15 wherein said second combining junction (114) is further configured to combine said second crossover component signal (108), one of said left component signals (56) and one of said rear component signals (62) to generate said left-rear output signal (116).

17. A computer system which broadens the perceived spatial image of sound signals, said computer system comprising:
a computer processor (132) configured to access audio data stored on a computer accessible storage medium (134), said computer processor (132) further configured to transfer said audio data to a data bus (136);
an audio decoder (12) in communication with said data bus (136), said audio decoder (12) configured to generate at least four audio source signals (20, 22, 24, 26);
a first audio enhancer (40) which is configured to receive at least a first pairof audio source signals (22, 24), said first audio enhancer (40) configured to process said first pair of audio source signals (22, 24) in a manner which generates a first component signal (50);

a second audio enhancer (42) which is configured to receive at least a second pair of audio source signals (20, 22), said second audio enhancer (42) configured to process said second pair audio source signals (20, 22) in a manner which generates a second component signal (54); and a third audio enhancer (44) which is configured to receive at least a third pairof audio source signals (24 26), said third audio enhancer (44) configured to process said third pair audio of source signals (22, 24) in a manner which generates a third component signal (58); and a plurality of combining junctions (70 78), each of said combining junctions in communication with at least two of said component signals (50, 54, 58), said combining junctions (70 78) configured to combine said component signals (50, 5458) to generate a plurality of audio output signals (72, 76).

18. The computer system of Claim 17 wherein said audio decoder (12) is a digital signal processor.

19. The computer system of Claim 17 wherein said audio signals (20, 22, 24, 26) are AC-3 compatible audio signals.

20. The computer system of Claim 17 wherein each of said four audio signals (20, 22, 24 26) corresponds to a discrete, full bandwidth audio channel.

21. The computer system of Claim 17 wherein said computer accessible storage medium (134) is a hard disk.

22. The computer system of Claim 17 wherein said computer accessible storage medium (134) is a compact disk.

23. The computer system of Claim 17 wherein said computer accessible storage medium (134) is a laser disk.

24. The computer system of Claim 17 wherein said computer processor (132) transfers an audio bitstream to said data bus (136).

25. The computer system of Claim 17 wherein said computer processor (132) transfers an AC-3 compatible bitstream to said data bus (136).

26. The computer system of Claim 17 wherein said computer processor (132) also processes television signals.

27. A method of enhancing sound in a surround sound reproduction environment wherein the surround sound environment has at least four separate audio source signals designated for speakers situated within the reproduction environment and placed around a listener, the method of enhancing comprising the following steps:
providing four audio source signals (20, 22, 24, 26) generated from a stereo signal decoder (12) during playback of a recorded audio signal;
modifying different combinations of said audio signals (20, 22, 24, 26) to generate a plurality of component signals (50, 52, 54, 56, 58, 60, 62, 64); and processing combinations of said component signals (50, 52, 54, 56, 58, 60, 62, 64) to create at least four corresponding enhanced audio signals (72, 76, 80, 84) wherein each of said four enhanced audio signals (72, 76, 80, 84) is modified as a function of at least three of said audio source signals (20, 22, 24, 26), said enhanced audio signals (72, 76, 80, 84) creating an immersive sound experience for the listener when the enhanced audio signals (72, 76, 80, 84) are amplified and played through speakers of a reproduction environment.

28. The method of Claim 27 wherein said audio source signals (22, 24, 26, 28) comprise a left-front signal L f (22), a right-front signal R f (24), a left-rear signal L r (20) and a left-front signal L f (22).
29. The method of Claim 28 wherein one of said enhanced audio output signals (72) is an enhanced left-front output signal L f(enhanced) (72) for reproduction in a surround sound environment.
30. A method of Claim 29 wherein said enhanced left-front output signal L f(enhanced) (72) is enhanced according to the following equation L f(enhanced) = K1(M1(L f, R f) +
M2(L f, L r)), where M1-M8 are independent variables which dictate the level and type of modification to the audio source signals, and K1-K4 are independent variables which determine the gain of the enhanced audio signals.

29. The method of Claim 28 wherein one of said enhanced audio output signals (76) is an enhanced right-front output signal R f(enhanced) (76) for reproduction in a surround sound environment.

30. A method of Claim 29 wherein said enhanced right-front output signal R f(enhanced) (76) is enhanced according to the following equation R f(enhanced) = K2(M3(L f, R f) +
M4(R f, R r)), where M1-M8 are independent variables which dictate the level and type of modification to the audio source signals, and K1-K4 are independent variables which determine the gain of the enhanced audio signals.

31. The method of Claim 28 wherein one of said enhanced audio output signals (80) is an enhanced left-rear output signal L r(enhanced) (80) for reproduction in a surround sound environment.

32. A method of Claim 31 wherein said enhanced left-rear output signal L r(enhanced) (80) is enhanced according to the following equation L r(enhanced) = K3(M5(L f, L r) + M6(L r, R r)), where M1-M8 are independent variables which dictate the level and type ofmodification to the audio source signals, and K1-K4 are independent variables which determine the gain of the enhanced audio signals.

33. The method of Claim 28 wherein one of said enhanced audio output signals (84) is an enhanced right-rear output signal R r(enhanced) (84) for reproduction in a surround sound environment.

34. A method of Claim 33 wherein said enhanced right-rear output signal R r(enhanced) (84) is enhanced according to the following equation R r(enhanced) = K4(M7(R f, R r) +
M8(L r, R r)), where M1-M8 are independent variables which dictate the level and type of modification to the audio source signals, and K1-K4 are independent variables which determine the gain of the enhanced audio signals.

35. The method of Claim 34 wherein the independent variables M1-M8 represent equalization of ambient audio information present in said audio source signals (20, 22, 24, 26).