Abstract:
An apparatus for effecting sound stage expansion in an audio system presenting two sound channels includes: (a) A first signal source coupled for providing at least one first signal representing a first sound channel to at least one first input locus of a first amplifying unit. The first amplifying unit participates in presenting the first sound channel. (b) A second signal source coupled for providing at least one second signal representing a second sound channel to at least one second input locus of a second amplifying unit. The second amplifying unit participates in presenting the second sound channel. (c) At least one first filter unit coupling the first signal source with at least one of the at least one second input locus. (d) At least one second filter unit coupling the second signal source with at least one of the at least one first input locus.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of prior filed copending Provisional Patent Application Ser. No. 60/676,167, filed Apr. 28, 2005. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to audio systems, and especially to audio systems presenting two sound channels. 
     BACKGROUND 
     The stereo sound stage of a stereo audio system may be regarded as the apparent physical separation between two speakers in the stereo audio system. The sound stage of a stereo audio system is generally reflective of the physical size of the embodiment of the system. By way of example and not by way of limitation small, compact stereo recording playback systems, mobile telephone systems, portable sound playback systems and other similar systems all suffer from a generally small perceived sound stage. A manifestation of such a limitation is a perception by a user of a reduced separation of audio playback channels (e.g., right channel and left channel) during presentation of a stereo audio output. 
     It is known that subtracting some of a right channel signal from a left channel signal while subtracting some of a left channel signal from a right channel signal can expand the perceived sound stage outside the actual physical separation of the audio output units (e.g., loudspeakers) of a stereo audio system. This system handling approach may be referred to as “cross differencing”. Low-frequency acoustic signals behave substantially like general pressure changes in a typical room or space and are generally non-directional. It is common practice among audio system designers to provide for the cross signals to be filtered so that they are significantly reduced at lower frequencies (e.g., below 400 Hertz; Hz) so as to prevent cancellation of bass sound reproduction in the audio system. 
     Sound stage expansion techniques may be used in connection with video imaging, but voices may be perceived as being displaced from their sources. Such a result may prove to be confusing to viewers, so care must be exercised in employing a sound stage expansion system in connection with video systems. 
     Sound stage expanding techniques are intended to make an existing stereo sound stage seem wider than the actual physical span of the speakers producing the stereo sound presentation. However, sound stage expansion techniques are generally not themselves a creator of a stereo audio effect. Prior art employments of sound stage expansion have produced a significant variation of center audio images in comparison with left side and right side signals. This variation of center audio images is commonly manifested in voices and instruments being decreased in volume generally at center-stage as compared with left and right audio signals. A result is that listeners have difficulty in overlooking the center audio image variance and the effect of the sound stage expansion is not fully perceived as listeners are distracted by relatively louder left and right output signals compared to output signals appearing at the center. Prior art sound expansion apparatuses have not provided a means for adjusting center stage audio image or volume. Some prior art employments have also produced substantial increases in high frequency components of left and right output signals, or treble boost. As a result, listeners may be distracted by the treble boost and so do not fully perceive the effect of a sound stage expansion. Prior art sound stage expansion apparatuses have not provided a means of controlling treble boost. 
     Turning to  FIG. 1 , a schematic diagram of an example of a conventional system can be seen. System  10  includes a left amplifier unit  12  and a right amplifier unit  14 . Left amplifier unit  12  has a non-inverting input terminal  20 , an inverting input terminal  22  and an output terminal  24 . A left signal providing unit  16  has a positive terminal  17  and a negative terminal  19 . Positive terminal  17  is coupled with non-inverting input terminal  20 . Negative terminal  19  is coupled with a ground terminal  25 . A feedback network  30  including resistors R 1 , R 2  couples output terminal  24  with inverting terminal  22 , negative terminal  19  and ground terminal  25 . Right amplifier unit  14  has a non-inverting input terminal  40 , an inverting input terminal  42  and an output terminal  44 . A right signal providing unit  36  has a positive terminal  37  and a negative terminal  39 . Positive terminal  37  is coupled with non-inverting input terminal  40 . Negative terminal  39  is coupled with a ground terminal  25 . A feedback network  50  including resistors R 3 , R 4  couples output terminal  44  with inverting terminal  42 , negative terminal  39  and ground terminal  25 . A cross differencing network  52  including cross differencing capacitor Cc and cross differencing resistor Rc couples a node or terminal  46  between resistors R 1 , R 2  with a node or terminal  48  between resistors R 3 , R 4 . 
     In operation, amplifier unit  12  will force left feedback voltage VL f  at inverting input terminal  22  to equal left input voltage VL i  from left signal providing unit  16 . This in effect applies an inverted left input voltage VL i  to output terminal  44  via cross differencing network  52  and inverting input terminal  42  of right amplifier unit  14 . The result is a reducing of right output voltage VR o  at output terminal  44  by an amount related with an inverted left input signal VL i . Similarly, amplifier unit  14  will force right feedback voltage VR f  at inverting input terminal  42  to equal right input voltage VR i  from right signal providing unit  36 . This in effect applies an inverted right input voltage VR i  to output terminal  24  via cross differencing network  52  and inverting input terminal  22  of left amplifier unit  12 . The result is a reducing of left output voltage VL o  at output terminal  24  by an amount related with an inverted right input signal VR i . This cross differencing signal effects sound stage expansion using apparatus  10 . However, cross differencing network  52  has a deleterious effect in that it increases gain for both of amplifier units  12 ,  14  above its characteristic frequency f c : 
                       f   c     =     1     2   ⁢   π   ⁢           ⁢     R   c     ⁢     C   c           ,           (   1   )               
where R c  is the value of resistor R c  in network  52  and C c  is the value of capacitor C c  in network  52 . Increasing gain for amplifier units  12 ,  14  at frequencies higher than characteristic frequency f c  is manifested as increased volume for higher frequency signals, such as treble audio output signals. As mentioned earlier herein, such variation of treble signals is distracting to listeners. The effect of the sound stage expansion is not fully perceived as listeners concentrate on louder treble signals to the left and right. Sound stage expansion apparatus  10  does not provide a means for adjusting high frequency response or center stage audio image or volume.
 
     There is a need for an apparatus and method for effecting sound stage expansion that permits mitigating of center audio image variation and controls treble boost in sound presentation. 
     SUMMARY 
     In accordance with a preferred embodiment of the present invention, an apparatus for expanding sound stage representation of an audio system presenting two sound channels is provided. The apparatus comprises a first amplifier unit having a first inverting input terminal, a first non-inverting input terminal and a first output terminal; a second amplifier unit having a second inverting input terminal, a second non-inverting input terminal and a second output terminal; a right channel signal source coupled for providing a first right channel signal and a second right channel signal to the first amplifier unit; one right channel signal of the first and second right channel signals being provided to the first inverting input terminal, an other right channel signal of the first and second right channel signals being provided to the first non-inverting input terminal; a left channel signal source coupled for providing a first left channel signal and a second left channel signal to the second amplifier unit; one of the first and second left channel signals being provided to the second inverting input terminal, an other one of the first and second left channel signals being provided to the second non-inverting input terminal; a first filter unit coupled for providing filtered the one right channel signal to the second non-inverting input terminal; a second filter unit coupled for providing filtered the other right channel signal to the second inverting input terminal; a third filter unit coupled for providing filtered the one left channel signal to the first non-inverting input terminal; and a fourth filter unit coupled for providing filtered the other left channel signal to the first inverting input terminal. 
     In accordance with a preferred embodiment of the present invention, the first and second right channels are provided as substantially fully differential signals, and wherein the first and second left channels are provided as substantially fully differential signals. 
     In accordance with a preferred embodiment of the present invention, at least one selected filter unit of the first filter unit, the second filter unit, the third filter unit and the fourth filter unit includes at least one capacitive unit coupled with at least one resistive unit. 
     In accordance with a preferred embodiment of the present invention, at least one selected filter unit of the first filter unit, the second filter unit, the third filter unit and the fourth filter unit includes at least one capacitive unit coupled with at least one resistive unit. 
     In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises a first amplifier having a inverting input terminal, a non-inverting input terminal, and an output terminal; a second amplifier having a inverting input terminal, a non-inverting input terminal, and an output terminal; a first resistor-capacitor (RC) network that receives a first input signal from a left channel signal source and that is coupled to inverting terminal of the first amplifier and the non-inverting terminal of the second amplifier; and a second RC network that receives a second input signal from a right channel signal source and that is coupled to the inverting terminal of the second amplifier and the non-inverting terminal of the first amplifier. 
     In accordance with a preferred embodiment of the present invention, the apparatus further comprises a resistor network that is coupled between the non-inverting terminals of the first and second amplifiers. 
     In accordance with a preferred embodiment of the present invention, the first RC network further comprises: a capacitor that receives the first input signal; a first resistor that receives the first input signal and that is coupled to the inverting terminal of the first amplifier; a second resistor that is coupled between the capacitor and the inverting terminal of the first amplifier; a third resistor that is coupled between the capacitor and the non-inverting terminal of the second amplifier; and a fourth resistor that is coupled between the output terminal and the inverting terminal of the first amplifier. 
     In accordance with a preferred embodiment of the present invention, the second RC network further comprises: a capacitor that receives the second input signal; a first resistor that receives the second input signal and that is coupled to the inverting terminal of the first amplifier; a second resistor that is coupled between the capacitor and the inverting terminal of the second amplifier; a third resistor that is coupled between the capacitor and the non-inverting terminal of the first amplifier; and a fourth resistor that is coupled between the output terminal and the inverting terminal of the second amplifier. 
     In accordance with a preferred embodiment of the present invention, the first and second input signals are differential, and wherein the first RC network receives a negative portion of the first input signal, and wherein the second RC network receives a negative portion of the second input signal. 
     In accordance with a preferred embodiment of the present invention, the apparatus further comprises: a third RC network that receives a positive portion of the first input signal and that is coupled to the non-inverting terminal of the first amplifier; and a fourth RC network that receives a positive portion of the second input signal and that is coupled to the non-inverting terminal of the second amplifier. 
     In accordance with a preferred embodiment of the present invention, the capacitor further comprises a first capacitor, and wherein the third RC network further comprises: a second capacitor that receives the positive portion of the first input signal; a fourth resistor that receives the first input signal and that is coupled to the non-inverting terminal of the first amplifier; a fifth resistor that is coupled between the second capacitor and the non-inverting terminal of the first amplifier; a sixth resistor that is coupled between the second capacitor and the inverting terminal of the second amplifier. 
     In accordance with a preferred embodiment of the present invention, the capacitor further comprises a first capacitor, and wherein the third RC network further comprises: a second capacitor that receives the positive portion of the second input signal; a fourth resistor that receives the first input signal and that is coupled to the non-inverting terminal of the second amplifier; a fifth resistor that is coupled between the second capacitor and the non-inverting terminal of the second amplifier; a sixth resistor that is coupled between the second capacitor and the inverting terminal of the first amplifier. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of an example of a conventional system; and 
         FIGS. 2 ,  3 , and  4  are circuit diagrams of examples of systems in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Turning to  FIG. 2 , an example of a system in accordance with a preferred embodiment of the present invention can be seen. System  60  includes a left amplifier unit  62  and a right amplifier unit  64 . Each of amplifier units  62 ,  64  has a differential input and a single-ended output. Left amplifier unit  62  has a non-inverting input terminal  70 , an inverting input terminal  72  and an output terminal  74 . Right amplifier unit  64  has a non-inverting input terminal  80 , an inverting input terminal  82  and an output terminal  84 . A left signal providing unit  66  has a positive terminal  67  and a negative terminal  69 . Positive terminal  67  is coupled with inverting input terminal  72  via a capacitor Cc 1  and a resistor RL 3 . Positive terminal  67  is also coupled with non-inverting terminal  80  via a cross differencing filter unit  90  including capacitor Cc 1  and a resistor RR 4 . Negative terminal  69  is coupled with a ground terminal  75 . A right signal providing unit  96  has a positive terminal  97  and a negative terminal  99 . Positive terminal  97  is coupled with inverting input terminal  82  via a capacitor Cc 2  and a resistor RR 3 . Positive terminal  97  is also coupled with non-inverting terminal  70  via a cross differencing filter unit  92  including capacitor Cc 2  and a resistor RL 4 . Negative terminal  99  is coupled with ground terminal  75 . A feedback network  100  including resistors RL 1 , RL 2  couples output terminal  74  with inverting terminal  72  and positive terminal  67 . A feedback network  102  including resistors RR 1 , RR 2  couples output terminal  84  with inverting terminal  82  and positive terminal  97 . A network  104  including resistors R bias1 , R bias2  couples non-inverting input terminals  70 ,  80  with ground terminal  75 . 
     System  60  avoids increasing perceived center treble signals as occurred in prior art apparatus  10  ( FIG. 1 ) because network  104  is not coupled to participate in either of feedback networks  100 ,  102  and therefore does not affect gain of either of amplifier units  62 ,  64 . System  60  effects cross differencing in order to realize sound stage expansion via cross differencing filter units  90 ,  92 . Cross differencing connection is effected so that a portion of right channel input signal VR i  from right signal providing unit  96  is applied to non-inverting input  70  to subtract that cross difference connected right signal portion from output signal VL o . Cross differencing connection is also effected so that a portion of left channel input signal VL i  from left signal providing unit  66  is applied to non-inverting input  80  to subtract that cross difference connected left signal portion from output signal VR o . 
     Cross differencing filter units  90 ,  92  permit adjustment of center audio image presented by system  60 . Changing values of capacitors Cc 1 , Cc 2  or resistors RL 4 , RR 4  can alter the center image presented by system  60  to a significant degree. Filter circuitry established by capacitor Cc 1  with resistor RL 3  and established by capacitor Cc 2  with resistor RR 3  may also be altered to adjust sound stage extension performance of system  60 . 
     Mathematical explanations describing sound stage expansion are available. An intuitive explanation suffices for purposes of describing the present invention: inverted cross signals of a particular frequency that emanate from one side of a system negate or cancel out a direct signal of the particular frequency from the opposite side of the system, thereby causing the brain of a listener to infer that the direct signal is further away than it actually is. 
     Turning to  FIG. 3 , another example of a system in accordance with a preferred embodiment of the present invention can be seen. System  160  includes a left amplifier unit  162  and a right amplifier unit  164 . Each of amplifier units  162 ,  164  has a differential input and a differential output. Left amplifier unit  162  has a non-inverting input terminal  170 , an inverting input terminal  172  and output terminals  174 ,  176 . System  160  is configured for fully differential signal operation so that left amplifier unit  162  presents a differential output signal so that output signal +VL o  is presented at output terminal  174  and output signal −VL o  is presented at output terminal  176 . Output signals +VL o , −VL o  are preferably fully differential output signals so that they are substantially equal in amplitude and opposite in phase with respect to each other. Right amplifier unit  164  has a non-inverting input terminal  180 , an inverting input terminal  182  and output terminals  184 ,  186 . System  160  is configured for fully differential signal operation so that right amplifier unit  164  presents a differential output signal so that output signal +VR o  is presented at output terminal  184  and output signal −VR o  is presented at output terminal  186 . Output signals +VR o , −VR o  are preferably fully differential output signals so that they are substantially equal in amplitude and opposite in phase with respect to each other. 
     A left signal providing unit  166  has a positive terminal  167  and a negative terminal  169 . Positive terminal  167  is coupled to provide an input signal +VL i  at an input terminal  400 . Negative terminal  169  is coupled to provide an input signal −VL i  at an input terminal  402 . Input signals +VL i , −VL i  are fully differential input signals so that input signal +VL i  may be regarded as a primary signal and input signal −VL i  may be regarded as an anti-primary signal so that input signals +VL i , −VL i  are substantially equal in amplitude and opposite in phase with respect to each other. 
     Input signal +VL i  is provided from input terminal  400  to non-inverting input terminal  170  via a first filter unit  290  including a capacitor Cc 1  and a resistor RL 3 . Input signal +VL i  is provided from input terminal  400  to inverting input terminal  182  via a second filter unit  292  including capacitor Cc 1  and a cross differencing resistor RRc 1 . Input signal −VL i  is provided from input terminal  402  to inverting input terminal  172  via a third filter unit  294  including a capacitor Cc 2  and a resistor RL 6 . Input signal −VL i  is provided from input terminal  402  to non-inverting input terminal  180  via a fourth filter unit  296  including capacitor Cc 2  and a cross differencing resistor RRc 2 . 
     A right signal providing unit  196  has a positive terminal  197  and a negative terminal  199 . Positive terminal  197  is coupled to provide an input signal +VR i  at an input terminal  404 . Negative terminal  199  is coupled to provide an input signal −VR i  at an input terminal  406 . Input signals +VR i , −VR i  are fully differential input signals so that input signal +VR i  may be regarded as a primary signal and input signal −VR i  may be regarded as an anti-primary signal so that input signals +VR i , −VR i  are substantially equal in amplitude and opposite in phase with respect to each other. 
     Input signal +VR i  is provided from input terminal  404  to non-inverting input terminal  180  via a fifth filter unit  298  including a capacitor Cc 3  and a resistor RR 6 . Input signal +VR i  is provided from input terminal  404  to inverting input terminal  172  via a sixth filter unit  300  including capacitor Cc 3  and a cross differencing resistor RLc 2 . Input signal −VL i  is provided from input terminal  406  to inverting input terminal  182  via a seventh filter unit  302  including a capacitor Cc 4  and a resistor RR 3 . Input signal −VL i  is provided from input terminal  406  to non-inverting input terminal  170  via an eighth filter unit  304  including capacitor Cc 4  and a cross differencing resistor RLc 1 . 
     A feedback network  200  including resistors RL 1 , RL 2  couples output terminal  176  with non-inverting terminal  170  and positive terminal  167 . A feedback network  202  including resistors RL 3 , RL 4  couples output terminal  174  with inverting terminal  172  and negative terminal  169 . A feedback network  204  including resistors RR 4 , RR 5  couples output terminal  186  with non-inverting terminal  180  and positive terminal  197 . A network  206  including resistors RR 1 , RR 2  couples output terminal  184  with inverting terminal  182  and negative terminal  199 . 
     Cross differencing filter units  292 ,  296 ,  300 ,  304  permit adjustment of center audio image presented by system  160 . Changing values of capacitors Cc 1 , Cc 2 , Cc 3 , Cc 4  or resistors RLc 1 , RLc 2 , RRLc 1 , RRc 2  can alter the center image presented by system  160  to a significant degree. Filter units  290 ,  294 ,  298 ,  302  established by capacitors may also be adjusted by changing values of capacitors Cc 1 , Cc 2 , Cc 3 , Cc 4  or resistors RL 3 , RL 6 , RR 3 , RR 6  to alter sound stage extension performance of system  160 . 
     Turning to  FIG. 4 , another example of a system in accordance with a preferred embodiment of the present invention can be seen. System  161  is coupled to receive input signal from an input device  660  at input terminals  400 ,  402 ,  404 ,  406 . System  161  is substantially the same as system  160  described in connection with  FIG. 3  except that system  161  receives input signals from an input device  660  rather than receiving input signals directly from signal providing units (e.g., signal providing units  166 ,  196 ;  FIG. 3 ). In order to avoid prolixity, a detailed description of system  161  will not be provided here. One may refer to the description of system  160  ( FIG. 3 ) for an understanding of the structure and operation of system  161 . 
     Input device  660  includes a left amplifier unit  662  and a right amplifier unit  664 . Left amplifier unit  662  has a non-inverting input terminal  670 , an inverting input terminal  672  and output terminals  674 ,  676 . Input device  660  is configured for fully differential signal operation so that left amplifier unit  662  presents a differential output signal so that output signal +VL 1  is presented at output terminal  674  and output signal −VL 1  is presented at output terminal  676 . Output signals +VL 1 , −VL 1  are preferably fully differential output signals so that they are substantially equal in amplitude and opposite in phase with respect to each other. Output signal +VL 1  is provided to input terminal  400  of system  161 . Output signal −VL 1  is provided to input terminal  402  of system  161 . Right amplifier unit  664  has a non-inverting input terminal  680 , an inverting input terminal  682  and output terminals  684 ,  686 . Input device  660  is configured for fully differential signal operation so that right amplifier unit  664  presents a differential output signal so that output signal +VR 1  is presented at output terminal  684  and output signal −VR 1  is presented at output terminal  686 . Output signals +VR 1 , −VR 1  are preferably fully differential output signals so that they are substantially equal in amplitude and opposite in phase with respect to each other. Output signal +VR 1  is provided to input terminal  404  of system  161 . Output signal −VR 1  is provided to input terminal  406  of system  161 . 
     A left signal providing unit  666  has a positive terminal  667  and a negative terminal  669 . Positive terminal  667  is coupled to provide an input signal +VL i  to non-inverting input terminal  670  via a resistor RL i1 . Negative terminal  669  is coupled to provide an input signal −VL i  to inverting input terminal  672  via a resistor RL i3 . Input signals +VL i , VR i  are illustrated in  FIG. 4  as being differential signals. Alternatively, input signals +VL i , VR i  may be presented as single-ended signals and, if so presented, input signals +VL i , VR i  may be converted to differential signals at input terminals  400 ,  402 ,  404 ,  406 , as may be understood by one skilled in the art of audio circuit design. 
     A feedback network  700  including resistors RL i1 , RL i2  couples output terminal  674  with non-inverting terminal  670  and positive terminal  667 . A feedback network  702  including resistors RL i3 , RL i4  couples output terminal  676  with inverting terminal  672  and negative terminal  669 . A feedback network  704  including resistors RR i1 , RR i2  couples output terminal  684  with non-inverting terminal  680  and positive terminal  697 . A network  706  including resistors RR i3 , RR i4  couples output terminal  686  with inverting terminal  682  and negative terminal  699 . 
     Input signals +VL 1 , −VL 1 , +VR 1 , −VR 1 , are provided from input terminals  400 ,  402 ,  404 ,  406  for use by system  161  substantially as described with respect to signals arriving at terminals  400 ,  402 ,  404 ,  406  in apparatus  160  ( FIG. 3 ). 
       FIG. 4  illustrates that the apparatus of the present invention may be employed with originating signal providing units (e.g., signal providing units  166 ,  196 ;  FIG. 3 ) or may be employed to effect sound stage expansion for a sound presenting unit such as a stereo amplifying unit (e.g., input device  660 ;  FIG. 4 ). Signals provided to the apparatus of the present invention, therefore, may be already amplified signals, or filtered signals or amplified and filtered signals. Signals provided to the apparatus of the present invention may have already been subjected to sound expansion treatment. Said another way, the apparatus of the present invention may be embodied in original equipment integrally included in an audio system. Alternatively, the apparatus of the present invention may be employed as an additional, add-on or after market module receiving output signals from an audio system and effecting sound stage expansion with regard to those output signals received from the audio system. 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.