Stereo enhancement system

Abstract: A stereo enhancement system is described which includes a perspective correction system (200, 210) which is responsive to the left and right stereo signals. The perspective correction system includes circuitry (211, 213) for generating sum and difference signals from the left and right signals, equalization circuitry (215, 217, 221, 223) for providing fixed equalization for the sum and difference signals to compensate for the variation with direction of the frequency response of the human ear, and mixing circuitry (225) for combining the equalized sum and difference signals to produce left and right output signals. Stereo image enhancement circuitry is also disclosed.

Description: The disclosed invention generally relates to an enhancement system for stereo sound reproduction systems, and is particularly directed to a stereo enhancement system which broadens the stereo sound image, provides for an increased stereo listening area, and provides for perspective correction for the use of speakers or headphones. As used herein, the phrase "selectively altering the relative amplitudes of frequency components of said difference signal such that louder frequency components of said difference signal are attenuated relative to quieter frequency components of said difference signal" and similar phrases (such as "... modified such that louder frequency components of said input difference signal are attenuated relative to quieter frequency components of said input difference signal") includes both attenuation of the louder frequency components and boosting of the quieter frequency components. Equally, the phrase "selectively altering the relative amplitudes of frequency components of said sum signal such that selected sum signal components are boosted relative to other sum signal components" and similar phrases (such as "... modified such that selected input sum signal components are boosted relative to other input sum signal components") includes both boosting of the selected sum signal components and attenuation of the other components. Additionally, as used herein, the terms "boost", "enhance" and "emphasise" and the like are generally used interchangeably, as are the terms "attenuate" and "deemphasise" and the like. Also, as used herein, the term "fixed medium" includes phonograph records, digital discs, magnetic tape and the like. As is well known, a stereo sound reproduction system attempts to produce a sound image wherein the reproduced sounds are perceived as emanating from different locations, thereby simulating the experience of a live performance. The aural illusion of a stereo sound image is generally perceived as being between the speakers, and the width of the stereo image depends to a large extent on the similarity or dissimilarity between the information respectively provided to the left and right speakers. If the information provided to each speaker is the same, then the sound image will be centered between the speakers at "center stage." In contrast, if the information provided to each speaker is different, then the extent of the sound image will spread between the two speakers. While the general concept of stereo sound imaging is not complex, its use and implementation is more difficult. The width of the stereo sound image depends not only on the information provided to the speakers, but also on listener position. Ideally, the listener is equidistant from the speakers. With many speaker systems, as the listener gets closer to one speaker, the sound from the more distant speaker contributes less to the stereo image, and the sound is quickly perceived as emanating only from the closer speaker. This is particularly so when the information in each speaker is not very different. However, even with the listener equidistant from the speakers, the perceived sound image is generally between the physical locations of the speakers and does not extend beyond the region between the speakers. Some known speaker systems have been designed to reduce the limitation that a listener should ideally be located equidistant between speakers. However, such speaker systems are generally complex and the resulting stereo image is still limited to the region between the physical locations of the speakers. Another consideration in stereo sound reproduction is the fact that the sound transducers (typically speakers or headphones) are located at predetermined locations, and therefore provide sound emanating from such predetermined locations. However, in a live performance, the perceived sound may emanate from many directions as a result of the acoustics of the structure where the performance takes place. The human ears and brain cooperate to determine direction on the basis of different phenomena, including relative phase shift for low frequency sounds, relative intensity for sounds in the voice range, and relative time arrival for sounds having fast rise times and high frequency components. As a result of the predetermined locations of speakers or headphones, a listener receives erroneous cues as to the directions from which the reproduced sounds are emanating. For example, for speakers located in front of the listener, sounds that should be heard from the side are heard from the front and therefore are not readily perceived as being sounds emanating from the sides. For headphones or side mounted speakers, sounds that should emanate from the front emanate from the sides. Thus, as a result of the placement of speakers or headphones, the sound perspective of a recorded performance is incorrect. US-4349698 discloses a system which takes into consideration the situation in which sound from one speaker reaches the first ear of a listener at a different time to the time it reaches the second ear. US-4393270 discloses a system for affecting the perceived direction of sound by introducing a spatial distortion. There have been numerous attempts to spread and widen the stereo image with mixed results. For example, it is known that the left and right stereo signals may be mixed to provide a difference signal (such as left minus right) and a sum signal (left plus right) which can be selectively processed and then mixed to provide processed left and right signals. Particularly, it is well known that increasing or boosting the difference signal produces a wider stereo image. However, indiscriminately increasing the difference signal creates problems since the stronger frequency components of the difference signal tend to be concentrated in the mid-range. One problem is that the reproduced sound is very harsh and annoying since the ear has greater sensitivity to the range of about 1 KHz to about 4 KHz within the mid-range (herein called the "difference signal components of greater sensitivity"). Another problem is that the listener is limited to a position that is equidistant between speakers since the mid-range includes frequencies having wavelengths comparable to the distance between a listener's ears (which have frequencies in the range of between about 1 KHz and 2 KHz). As to such frequencies (herein called the "difference signal frequency components of increased phase sensitivity"), a slight shift in the position of the listener's head provides an annoying shift in the stereo image. Moreover, the perceived widening of the stereo image resulting from indiscriminate boosting of the difference signal is small, and is clearly not worth the attendant problems. Some known stereo imaging systems require additional amplifiers and speakers. However, with such systems, the stereo image is limited by the placement of the speakers. Moreover, placing speakers at different locations does not necessarily provide the correct sound perspective. With other systems, fixed or variable delays are provided. However, such delays interfere with the accuracy of the reproduced sound since whatever delays existed in the performance that was recorded are already present in the recording. Moreover, delays introduce further complexity and limit the listener's position. There have also been attempts to correct or compensate the improper sound perspective resulting from the use of headphones. However, considerations with known headphone enhancement systems include complexity and lack of effectiveness. It would be an advantage to provide a stereo enhancement system which extends the width of the stereo sound image beyond the region between the speakers. It would also be an advantage to provide a stereo enhancement system which does not place constraint on listening position. Another advantage would be to provide a stereo enhancement system which provides for a stereo sound image that may be perceived over a large listening area. Still another advantage would be to provide a stereo correction system which provides for sound perspective correction for use with speakers or headphones. The present invention seeks to achieve the foregoing and other features and advantages, and includes a stereo image enhancement system for providing a wider stereo image and listening area, and a perspective correction system which provides for sound perspective correction for use with speakers or headphones. The stereo image enhancement system and the perspective correction system may be utilized in combination or individually. A wider stereo sound image and listening area are achieved by generating sum and difference signals based on left and right stereo signals, selectively altering the relative amplitudes of the difference signal frequencies and the relative amplitudes of the sum signal frequencies and combining the processed sum and difference signals with the original left and right signals to produce enhanced left and right stereo signals. Particularly, selected frequency components of the difference signal are boosted (emphasised) relative to other difference signal frequency components, and selected frequency components of the sum signal are boosted relative to other sum signal frequency components. This selective alteration of the difference signal provides for a wider stereo image and a wider listening area, and the selective alteration of the sum signal can prevent the sum signal from being overwhelmed by the difference signal. In one preferred embodiment of the invention, a spectrum analyzer that is responsive to the difference signal controls the relative amplitudes of the difference signal frequency components so that the quieter difference signal frequency components are boosted relative to louder difference signal frequency components. The difference signal is also equalized by a fixed equalizer so that the difference signal frequencies having wavelengths comparable to the distance between a listener's ears are deemphasized. The spectrum analyzer further controls the relative amplitudes of the sum signal frequency components so that sum signal frequency components are boosted in proportion to the levels of corresponding difference signal frequency components. In another preferred embodiment of the invention, the difference signal is equalized with a fixed difference signal equalizer so that difference signal frequency components that statistically include quieter difference components are boosted relative to difference signal frequency components that statistically include louder difference signal frequencies. The sum signal is equalized with a fixed sum signal equalizer so that the sum signal in the frequency range that statistically includes difference signal frequency components are boosted. As a result of the selective alteration of the difference signal components, a wider stereo image is provided, and the harshness and image shifting problems associated with indiscriminate increase of the difference signal are substantially reduced by the equalization provide by the fixed equalizer utilized by the invention. The selective alteration of the difference signal components further enhances the stereo image for the following reasons. Ambient reflections and reverberant fields at a live performance are readily perceived and are not masked by the direct sounds. In a recorded performance, however, the ambient sounds are masked by the direct sounds, and are not perceived at the same level as at a live performance. The ambient sounds generally tend to be in the quieter frequencies of the difference signal, and boosting the quieter frequencies of the difference signal unmasks the ambient sounds, thereby simulating the perception of ambient sounds at a live performance. The selective alteration of the difference signal also provides for a wider listening area for the following reasons. The louder frequency components of the difference signal tend to be in the mid-range which includes frequencies having wavelengths comparable to the ear-to-ear distance around the head of a listener (the previously mentioned "difference signal frequency components of increased phase sensitivity"). As a result of the selective alteration provided by the invention, the difference signal frequency components of increased phase sensitivity are not inappropriately boosted. Therefore, the stereo image shifting problem resulting from indiscriminate increase of the difference signal (discussed above in the background) is substantially reduced, and the listener is not limited to being equidistant from the speakers. In providing the selective alteration of the difference signal, the amount of enhancement, which is determined by the level of the selectively boosted difference signal that is mixed, may be automatically adjusted so that the amount of stereo provided is relatively consistent. Without such automatic adjustment, the amount of enhancement provided would have to be manually adjusted for the differing amounts of stereo in different recordings. The process of selectively altering the difference signal also boosts any artificial reverberation introduced in the recording process since artificial reverberation is predominantly in the difference signal. In accordance with another aspect of the invention, in order to avoid the inappropriate boosting of artificial reverberation, the enhancement system of the invention monitors the sum and difference signals for characteristics that indicate the possible presence of artificial reverberation. If the possibility of artificial reverberation is detected, the amount of boost provided for the difference signal is selectively reduced and the amount of boost for the sum signal is selectively increased. In accordance with the present invention there is provided a sound perspective correction system as defined in the claims which provides perspective correction for recorded performances reproduced with speakers located at different positions or with headphones. The perspective correction system selectively modifies sum and difference signals derived from the left and right stereo signals so that the reproduced sounds are perceived as emanating from the directions a listener would expect at a live performance. Thus, with speakers located in front of the listener, sounds that should be heard as emanating from the sides are perceived as emanating from the sides. With headphones, sounds that should be heard as emanating from the front are perceived as emanating from the front. The sound perspective correction system achieves perspective correction by generating sum and difference signals from left and right stereo signals, providing fixed equalization for the sum and difference signals to compensate for the variation with direction of the frequency response of the human ear, and combining the equalized sum and difference signals to produce left and right signals. For speakers located in front of the listener, the difference signal is selectively boosted so that side sounds are restored to the appropriate levels that would have been perceived had they been reproduced to emanate from the sides. For speakers located to the side or for headphones, the sum signal is selectively attenuated to restore front sounds to the appropriate levels that would have been perceived had they been reproduced to emanate from the front. As indicated previously, the sound perspective correction system of the invention may be utilized in conjunction with the above-summarized stereo image enhancement system of the invention or may be utilized alone with other audio components. Principles of the present invention are applicable both for playback of conventional stereo phonograph records, magnetic tapes and digital discs through a conventional sound reproducing system including a pair of loud speakers and for making unique recordings on phonograph records, digital discs or magnetic tape which recordings can be played on a conventional sound reproducing system to produce left and right stereo output signals providing the advantageous effects described above. Brief description of the drawings Preferred features of the invention will now be described, by way of example, with reference to the accompanying drawings wherein: FIG. 1 is a block diagram of the stereo enhancement system of the invention. FIG. 2 is a block diagram of a dynamic stereo image enhancement system in accordance with the invention which provides for dynamic equalization. FIG. 4 is a block diagram of a non-dynamic or fixed stereo image enhancement system in accordance with the invention which provides for fixed equalization. FIGS. 5A and 5B are plots of the equalization provided by the fixed stereo image enhancement system of FIG. 4 FIG. 6 is a block diagram of a sound perspective correction system in accordance with the invention. FIGS. 7A and 7B are frequency responses of the human ear which are helpful in understanding stereo image enhancement systems of FIGS. 2 and 4 and the sound perspective correction system of FIG. 5. FIG. 7C is the frequency response of FIG. 7A relative to FIG. 7B. FIG. 7D is the frequency response of FIG. 7B relative to FIG. 7A. FIGS. 8 and 9 illustrate sound reproducing and sound recording systems respectively, each of which employs either or both of the stereo image enhancement and perspective correction arrangements embodying principles of the present invention. DETAILED DESCRIPTION OF THE DISCLOSURE In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. In order to facilitate the understanding of the invention, the following discussion is provided in different sections, with each subsequent section being more detailed than the previous section. Thus, an overview is first presented wherein the overall functions provided are discussed. Then, the invention is discussed in more detail with more emphasis on operating parameters. I. OVERVIEW Referring now to FIG. 1, shown therein is a block diagram of the stereo enhancement system 300 of the invention, which includes a stereo image enhancement system 100 and a perspective correction system 200. The stereo image enhancement system 100 receives left and right stereo signals L and R and processes such signals to provide image enhanced left and right stereo signals L' and R' to the perspective correction system 200. The perspective correction system 200 processes the image enhanced stereo signals to provide image enhanced stereo signals which have been corrected to provide for proper sound perspective when amplified and played through speakers or headphones. For use with standard commercially available audio components, the stereo enhancement system 300 of the invention may be utilized in the tape monitor loop or, if available, in an external processor loop of a preamplifier. Such loops are not affected by the preamplifier controls such as tone controls, balance control, and volume control. Alternatively, the stereo enhancement system 300 may be interposed between the preamplifier and power amplifier of a standard stereo sound reproduction system. However, with such installation, the balance and tone controls are preferably disabled or nulled. The disclosed stereo enhancement system 300 may be readily incorporated for production into audio preamplifiers that are manufactured and sold as separate units, as well as into audio preamplifiers that are included in integrated amplifiers and receivers. As so incorporated, the stereo enhancement system 100 is preferably upstream of the tone and balance controls and preferably is capable of being bypassed. It should be noted that the enhancement provided by the disclosed stereo enhancement system 300 can be advantageously utilized to enhance recordings. Such recordings can be played back on an audio system which does not include the stereo enhancement system 300, or an audio system which does include the stereo enhancement system 300 and which has been bypassed. Thus, for example, a recording which includes image enhancement and perspective correction can be made for playback in an automobile with side mounted speakers. It should be noted that perspective correction may not be desired in making recordings unless the playback conditions are known, e.g., that playback will be only through side mounted automobile speakers. It should also be noted that the stereo image enhancement system 100 and/or the perspective correction 200 may be utilized independently in an audio system. Thus, for example, the perspective correction system 200 alone may be incorporated into an automobile audio system for correcting the improper sound perspective caused by side mounted speakers. Also, for cost considerations, the stereo image enhancement system 100 alone may be incorporated in an audio system for home use. Referring to FIG. 2, shown therein is a block diagram of a stereo image enhancement system 10 which may be utilized as the stereo image enhancement system 100 in the stereo enhancement system 300 of FIG. 1, and which provides for dynamic equalization of the sum and difference of left and right stereo signals to achieve a wider stereo image and a wider listening area. Particularly, subsonically filtered left and right stereo signals L and R at the outputs of subsonic filters 12, 14 are provided to a difference circuit 11 and a summing circuit 13 which respectively provide a difference signal (L-R) and a sum signal (L+R). A dynamic difference signal equalizer 19, a fixed difference signal equalizer 18, and a gain controlled amplifier 22 cooperate to selectively alter or modify the relative amplitudes of the difference signal frequency components (also referred to herein as "components" or "frequencies") to provide a processed difference signal (L-R) p . A dynamic sum signal equalizer 21 selectively alters or modifies the relative amplitudes of the sum signal frequency components (also referred to herein as "components" or "frequencies" to provide a processed sum signal (L+R) p .) A spectrum analyzer 17, which is responsive to the difference signal provided by the difference circuit 11, controls the dynamic difference signal equalizer 19 so that the quieter components of the difference signal are boosted relative to the louder components. More specifically, the dynamic difference signal equalizer 19 is controlled to attenuate the louder difference signal components more than the quieter difference signal components. The subsequent amplification of the equalized difference signal provides for a processed difference signal wherein the quieter components have been boosted relative to the louder difference signal components. The fixed difference signal equalizer 18 selectively attenuates the equalized difference signal provided by the dynamic difference signal equalizer 19 to provide deemphasis in a predetermined manner. The spectrum analyzer 17 also controls the sum signal equalizer so that components of the sum signal are boosted as a direct function of the levels of corresponding difference signal components. More specifically, the sum signal equalizer 21 boosts the sum signal to provide a processed sum signal wherein the sum signal components have been boosted in proportion to the amplitudes of correspondening difference signal amplitudes. A feedback and reverberation control circuit 30 controls the gain of the gain controlled amplifier 22 so that the amount of stereo provided is relatively consistent from recording to recording. The control circuit 30 also controls the difference signal equalizer 19 and the sum signal equalizer 21 so that difference signal components that may include artificial reverberation are not inappropriately boosted when the possibility of artificial reverberation is detected. When the possibility of artificial reverberation is detected by the control circuit 30, the reverberation control signal RCTRL controls the dynamic difference signal equalizer 19 to provide further attenuation in selected frequency bands where artificial reverberation statistically occurs, and the dynamic sum signal equalizer 21 to provide further boost in such selected frequency bands. In this manner, any artificial reverberation which may be present in the difference signal is not inappropriately boosted in the subsequent amplification of the difference signal. The further boost of the sum signal ensures that the sum signal frequencies in the selected frequency bands are of sufficient level to compensate any artificial reverberation which may not have been sufficiently attenuated by the dynamic difference signal equalizer 19 pursuant to the reverberation control signal RCTRL. The control circuit 30 is responsive to the sum and difference signals provided by the summing circuit 13 and the difference circuit 11, and also to the processed difference signal provided by the gain controlled amplifier 22. Referring now to FIG. 4, shown therein is a block diagram of a further embodiment of a stereo image enhancement system 110 which may be utilized as the stereo image enhancement system 100 in the stereo enhancement system of FIG. 1, and which provides for respective fixed equalization of the sum and difference of left and right stereo signals to achieve a wider stereo image and a wider listening area. Particularly, subsonically filtered left and right stereo signals L and R from subsonic filters 112, 114 are provided to a difference circuit 111 and a sum circuit 113 which generate respective difference and sum signals (L-R) and (L+R). A fixed difference signal equalizer 115, a gain controlled amplifier 125, and a reverberation filter 129 cooperate to selectively boost certain difference signal components relative to other difference signal components. A fixed sum signal equalizer 117 and a gain controlled amplifier 127 cooperate to selectively boost certain sum signal components relative to other sum signal components. Effectively, the sum and difference signals are respectively spectrally shaped or equalized in a fixed predetermined manner. Particularly, the difference signal is equalized so that the frequencies where the quieter difference signal components statistically occur more frequently are boosted relative to the frequencies where the louder difference signal components statistically occur more frequently. The sum signal is equalized so that frequencies where the difference signal components statistically occur are boosted relative to other frequencies. The stereo image enhancement system 110 further includes a feedback and reverberation control circuit 40 which is substantially similar to the control circuit 30 of FIG. 2 and provides substantially similar functions. Particularly, the control circuit 40 cooperates with the gain controlled amplifier 125 so that substantially consistent stereo is provided for differing amounts of stereo within a given recording and between different recordings. The control circuit 40 further cooperates with the gain controlled amplifier 127 and the reverberation filter 129 to compensate the effects of artificial reverberation. When the possibility of artificial reverberation is detected, the gain controlled amplifier 127 boosts the sum signal, and the reverberation filter 129 attenuates the difference signal components that statistically include artificial reverberation relative to other difference signal components. In this manner, the difference signal components that may include artificial reverberation are not inappropriately boosted. The further boost to the sum signal is to compensate for any artificial reverberation which may not have been sufficiently attenuated by the reverberation filter 129. Referring now to FIG. 6, shown therein is a block diagram of a sound perspective correction system 210 which may be utilized as the sound perspective correction system 200 in the stereo enhancement system of FIG. 1. The perspective correction system 210 is responsive to left and right signals provided by the outputs of a stereo image enhancement system in accordance with the invention as discussed above relative to FIGS. 2 and 4. Alternatively, as discussed with reference to the stereo enhancement system 300 of FIG. 1, the left and right signals may be provided by an appropriate audio preamplifier. The sound perspective correction system 210 includes a summing circuit 211 and a difference circuit 213 for respectively providing sum and difference signals (L+R) and (L-R). The sum and difference signals are respectively equalized by a fixed sum signal equalizer 215 and a fixed difference signal equalizer 221, which provide different equalization characteristics. Particularly, the fixed sum signal equalizer 215 provides for one equalization output, and the fixed difference signal equalizer 221 provides for one equalization output. A pair of two position switches 217, 223 control whether equalized or non-equalized sum and difference signals are provided to a mixer 225. The selection of the signals provided to the mixer 225 is determined by the type of sound transducers (e.g., speakers or headphones) and/or the location of the sound transducers (e.g., front or side) used for sound reproduction. The mixer 225 mixes the sum and difference signals to provide processed left and right output signals which are the outputs of the sound perspective correction system 210. As discussed above relative to the stereo enhancement system 300 of FIG. 1, the outputs of the sound perspective system 210 may be provided to the preamplifier tape monitor loop input or to a standard power amplifier. II. DETAILED BLOCK DIAGRAM DISCUSSION A. The Dynamic Stereo Image Enhancement System Referring again to FIG. 2, the stereo image enhancement system 10 of the invention includes a left input signal subsonic filter 12 and a right input signal subsonic filter 14 which are responsive to left and right stereo signals L and R provided by a stereo sound reproduction system (not shown). For example, the left and right stereo signals L and R may be provided by a preamplifier tape monitor loop output. The subsonic filters 12, 14 provide subsonically filtered input signals L in and R in to a difference circuit 11 and a summing circuit 13. Each of the subsonic filters 12, 14 is a high pass filter having a -3 dB frequency of 30 Hz and a roll-off of 24 dB per octave. The sharp roll-off provides some protection against damage to speakers in the event a phono cartridge is accidentally dropped. Vertical displacement of a stylus due to dropping a phono cartridge is manifested as low frequency difference signal components with large amplitudes, which could be potentially damaging to speakers. The sharp subsonic filter roll-off cuts off such low frequency components to reduce the possibility of damage. The difference circuit 11 subtracts the right subsonically filtered signal R in from the left subsonically filtered signal L in to provide a difference signal (L-R), while the summing circuit 13 adds the left and right subsonically filtered input signals L in and R in to provide a sum signal (L+R). The difference signal (L-R) is provided to a multi-band spectrum analyzer 17. The difference signal (L-R) is further provided to a multi-band dynamic difference signal equalizer 19 which is controlled by control signals provided by the spectrum analyzer 17. The sum signal (L+R) is provided to a multi-band dynamic sum signal equalizer 21 which is also controlled by the control signals provided by the spectrum analyzer 17. The multi-band spectrum analyzer 17 is responsive to predetermined frequency bands and provides respective control signals associated with each of the predetermined frequency bands. Particularly, such control signals are proportional to respective average amplitudes of the difference signal (L-R) within the respective predetermined frequency bands. By way of example, the multi-band spectrum analyzer 17 includes a plurality of one octave wide bandpass filters respectively centered in the predetermined frequency bands and respectively having roll-offs of 6 dB per octave. The respective outputs of the bandpass filters are rectified and appropriately buffered to provide the control signals. The dynamic difference signal equalizer 19 is also responsive to the predetermined frequency bands and selectively cuts (attenuates) the difference signal frequencies in such predetermined frequency bands in response to the control signals provided by the spectrum analyzer 17. Specifically, the difference signal equalizer 19 attenuates the difference signal components within the respective predetermined frequency bands as a direct function of the respective control signals provided by the spectrum analyzer 17. That is, for a given frequency band, attenuation increases as the average amplitude of the difference signal (L-R) within such frequency band increases. The output of the dynamic difference signal equalizer 19 is provided to a fixed difference signal equalizer 18 which attenuates selected frequencies of the dynamically equalized difference signal in a predetermined manner. An appropriate equalization characteristic for the fixed difference signal equalizer 18 is shown in FIG. 5A. By way of example, the fixed difference signal equalizer 18 may include a plurality of parallel filter stages including a low pass filter and a high pass filter having the following characteristics. The low pass filter has a -3 dB frequency of about 200 Hz, a roll-off of 6 dB per octave, and a gain of unity. The high pass filter has a -3 dB frequency of about 7 KHz, a roll-off of 6 dB per octave, and a gain of one-half. The fixed equalization of the fixed difference equalizer 18 is provided (a) so that frequencies to which the ear has greater sensitivity (about 1 KHz to about 4 KHz) are not inappropriately boosted, and (b) so that difference signal components having wavelengths comparable to the distance between the ears of a listener (the previously discussed "difference signal components of increased phase sensitivity") are not inappropriately boosted. Alternatively, such fixed equalization may be provided prior to dynamic equalization. The difference signal provided by the fixed difference signal equalizer 18 is amplified by a gain controlled amplifier 22 to provide a processed difference signal (L-R) p . The dynamic sum signal equalizer 21 is also responsive to the predetermined frequency bands and selectively boosts the sum signal frequencies in such predetermined frequency bands in response to the control signals provided by the spectrum analyzer 17. Specifically, the dynamic sum signal equalizer 21 boosts the sum signal components within the respective predetermined frequency bands as a direct function of the respective control signals provided by the spectrum analyzer 17. That is, for a given frequency band, boost increases as the average amplitude of the difference signal (L-R) within such frequency band increases. The output of the dynamic sum signal equalizer 21 is a processed sum signal (L+R) p . The predetermined frequency bands for the spectrum analyzer 17, the dynamic difference signal equalizer 19, and the dynamic sum signal equalizer 21 include seven (7) bands of one octave width each which are respectively centered at 125 Hz, 250 Hz, 500 Hz, 1 KHz, 2 KHz, 4 KHz, and 8 KHz. A larger or smaller number of predetermined frequency bands may be readily utilized. The dynamic difference signal equalizer 19 provides for each of the frequency bands a maximum attenuation of 12 dB for the maximum level of the corresponding control signals provided by the spectrum analyzer 17. No attenuation would be provided for a control signal having a zero level. Similarly, the dynamic sum signal equalizer 21 provides for each of the frequency bands a maximum boost of 6 dB for the maximum level of the corresponding control signals provided by the spectrum analyzer 17. No boost would be provided for a control signal having a zero level. The control signals provided by the spectrum analyzer 17 have a range between 0 volts and 8 volts. The corresponding range of attenuation provided by the dynamic difference signal equalizer 19 would be between 0 dB and -12 dB, while the corresponding range of boost provided by the sum signal equalizer 21 would be between 0 dB and 6 dB. It should be readily apparent that for a given control signal for a particular frequency band, the value of the boost provided by the dynamic sum signal equalizer 21 is one-half of the value of the attenuation provided by the dynamic difference signal equalizer 19. Other ratios may be utilized, but it is important that the level of boost provided by the dynamic sum signal equalizer 21 be less than the corresponding level of attenuation provided by the dynamic difference signal equalizer 19. Such reduced boost has been found to be appropriate since most recordings include more sum signal than difference signal. A maximum boost level approaching the maximum attenuation level would result in inappropriately high levels of the processed sum signal (L+R) p . As discussed further herein, selected frequency bands of the dynamic difference signal equalizer 19 and the dynamic sum signal equalizer 21 are further responsive to other control signals. The foregoing discussion of the responses of such equalizers to the control signals provided by the spectrum analyzer were based on such other control signals having zero levels. To the extent that other control signals have non-zero levels, the total attenuation or boost is the superposition of the individual attenuation or boost due to the individual control signals. In other words, the respective control signals are added. It should be noted that preferably the dynamic difference signal equalizer 19 is configured to provide for each of the frequency bands a maximum attenuation, such as 12 dB, in order to avoid inappropriate levels of attenuation. Similarly, the dynamic sum signal equalizer 21 is preferably configured to provide for each of the frequency bands a maximum boost, such as 6 dB, in order to avoid inappropriately high levels of boost. The stereo image enhancement system 10 further includes a feedback and reverberation control circuit 30 which cooperates with other elements in the system to provide for automatic adjustment of the stereo image enhancement provided and for reverberation compensation. The characteristics of recordings that make automatic enhancement adjustment and reverberation compensation desirable are discussed further below. The control circuit 30 (described in more detail below relative to FIG. 3) is responsive to the difference signal (L-R) provided by the difference circuit 11 and the sum signal (L+R) provided by the sum circuit 13. The control circuit 30 provides a gain control signal CTRL for controlling the gain controlled amplifier 22 which varies the gain applied to the difference signal provided by the fixed difference signal equalizer 18. The control circuit 30 is further responsive to the processed difference signal (L-R) p provided by the gain controlled amplifier 22, thereby providing a closed loop system for controlling the processed difference signal (L-R) p . The control circuit 30 controls the gain of the gain controlled amplifier 22 to maintain a constant ratio between (1) the sum signal (L+R) provided by the summing circuit 13 and (2) the processed difference signal (L-R) p output of the gain controlled amplifier 22. By way of example, the gain controlled amplifier 22 may be an appropriate voltage controlled amplifier. The control circuit 30 further provides a reverberation control signal RCTRL to the difference signal equalizer 19 and the sum signal equalizer 21 for controlling the amount of equalization provided in the frequency bands centered at 500 Hz, 1 KHz, and 2 KHz (herein the "reverberation bands"). The presence of artificial reverberation, which is almost always in difference signal frequencies in the reverberation bands, is indicated by a larger than expected ratio between the sum signal and the difference signal, since a large ratio indicates the presence of a center stage soloist (vocalist or instrumentalist), which in turn indicates the possibility of artificial reverberation. The control circuit 30, therefore, monitors the ratio between the sum signal (L+R) and the difference signal (L-R). When the possible presence of artificial reverberation is detected (for example, when the sum signal to difference signal ratio is greater than a predetermined value), the reverberation control signal RCTRL provides further control of the reverberation bands in the difference signal equalizer 19 and the sum signal equalizer 21. As to the difference signal equalizer 19, the reverberation control signal RCTRL causes further attenuation in the above specified reverberation bands in addition to the attenuation resulting from the control signals provided by the spectrum analyzer 17. As to the sum signal equalizer 21, the reverberation control signal RCTRL causes further boost in the above specified reverberation bands in addition to the boost resulting from the control signals provided by the spectrum analyzer 17. The further attenuation of the difference signal components within the reverberation bands is to prevent any artificial reverberation which may be accompanying a soloist from being inappropriately boosted when the processed difference signal is subsequently amplified. The further boost of the sum signal components within the reverberation bands insures that the sum signal components in the reverberation bands are of sufficient level to compensate any artificial reverberation that is not sufficiently attenuated by the dynamic difference signal equalizer 19. The dynamic difference signal equalizer 19 provides for each of the above specified reverberation bands a maximum attenuation of 12 dB for the maximum level of the reverberation control signal RCTRL, with no corresponding control signal from the spectrum analyzer 17 present. The total attenuation provided in response to both the reverberation control signal RCTRL and a corresponding control signal from the spectrum analyzer 17 would be the superposition of the respective attenuations in response to the individual control signals. However, as noted previously, the dynamic difference signal equalizer 19 is preferably configured to provide a predetermined maximum attenuation, such as 12 dB, regardless of the levels of the control signals. The dynamic sum signal equalizer 21 provides for each of the above specified reverberation bands a maximum boost of 6 dB for the maximum level of the reverberation control signal RCTRL, with no corresponding control signal from the spectrum analyzer 17 present. The total boost provided in response to both the reverberation control signal RCTRL and a corresponding control signal from the spectrum analyzer 17 would be the superposition of the respective boosts in response to the individual control signals. However, as noted previously, the dynamic sum signal equalizer 21 is preferably configured to provide a predetermined maximum boost, such as 6 dB, regardless of the levels of the control signals. Alternatively, reverberation compensation for the processed sum signal may be achieved by utilizing a gain controlled amplifier (not shown) to vary the gain applied to the equalized sum signal provided by the dynamic sum signal equalizer 21. Such gain controlled amplifier would amplify the processed sum signal as a function of the reverberation control signal RCTRL. If a gain controlled amplifier to amplify the processed sum signal is utilized to compensate the effects of artificial reverberation, the reverberation control signal RCTRL would not be provided to the dynamic sum signal equalizer 21. The output of the gain controlled amplifier 22 is coupled to one fixed terminal of a potentiometer 23 which has its other fixed terminal coupled to ground. The wiper contact of the potentiometer 23 is coupled to a mixer 25 which therefore receives the processed difference signal (L-R) p having a level controlled by the gain controlled amplifier 22 and the potentiometer 23. As mentioned previously, the control circuit 30 and the gain controlled amplifier 22 control the ratio between the sum signal (L+R) provided by the summing circuit 13 and the processed difference signal (L-R) p provided by the gain controlled amplifier 22. As discussed further herein, that ratio is controlled by circuitry within the control circuit 30. The potentiometer 23 provides further control over the amount of stereo enhancement provided. The output of the dynamic sum signal equalizer 21 is coupled to one fixed terminal of a potentiometer 27 which has its other fixed terminal coupled to ground. The wiper contact of the potentiometer 27 is coupled to the mixer 25 which therefore receives the processed sum signal (L+R) p having a level controlled by the potentiometer 27. The potentiometer 27 controls the level of the sound image at center stage. The left and right subsonically filtered input signals L in and R in are provided as further inputs to the mixer 25. The mixer 25 combines the processed sum signal (L+R) p and the processed difference signal (L-R) p with the left and right input signals L in and R in to provide left and right output signals L out and R out . Particularly, the left and right output signals L out and R out are provided by the mixer 25 in accordance with the following: L out = L in + K 1 (L+R) p + K 2 (L-R) p R out = R in + K 1 (L+R) p - K 2 (L-R) p The value of K 1 is controlled by the potentiometer 27; and the value of K 2 is controlled by the potentiometer 23. The overall effect of processing the difference signal (L-R) is that the quieter difference signal components are boosted relative to the louder difference signal components. That is, the selective attenuation of the difference signal followed by amplification provides a processed difference signal wherein the louder components may be comparable in level to their original levels while the quieter difference signal components have levels greater than their original levels. The processing of the sum signal (L+R) is to raise the level of the sum signal so that it is not overwhelmed by the selective boosting of difference signal components. The potentiometers 23, 27 are user controlled elements to allow the user to control the respective levels of the processed sum signal (L+R) p and the processed difference signal (L-R) p that are mixed by the mixer 25. For example, the potentiometers 23, 27 may be adjusted to minimize the processed difference signal and to maximize the processed sum signal. With such adjustment, the listener would hear primarily any center stage soloist present in the recording being played. The left and right output signals L out and R out are provided to the sound perspective correction system 200 of the stereo enhancement system 300 of FIG. 1. Alternatively, as discussed relative to the stereo enhancement system 300 of FIG. 1 and to the extent that the sound perspective correction system 200 is not utilized, the left and right output signals L out and R out are appropriately provided, for example, to the tape monitor loop input of the preamplifier tape monitor loop that provided the left and right stereo signals L and R. In the foregoing stereo image enhancement system 10, the sum signal equalizer 21 and the difference signal equalizer 19 are dynamically controlled by the spectrum analyzer 17, and in that sense the system is referred to as the dynamic stereo image enhancement system 10. Alternatively, a simplified non-dynamic equalization or fixed equalization stereo image enhancement system may be provided which does not include the spectrum analyzer 17 and which provides for fixed equalizatior of the sum and difference signals. C. The Fixed Stereo Image Enhancement System Referring particularly to FIG. 4, shown therein is a block diagram of a statistical or fixed stereo image enhancement system 110 which includes a left input signal subsonic filter 112 and a right input signal subsonic filter 114 which are responsive to left and right stereo signals L and R provided by a stereo sound reproduction system (not shown). For example, as discussed above relative to the stereo enhancement system 300 of FIG. 1, the left and right stereo signals L and R may be provided by a preamplifier tape monitor loop output. The subsonic filters 112, 114 provide subsonically filtered input signals L in and R in to a summing circuit 111 and a difference circuit 113. As discussed above relative to the dynamic stereo image enhancement system of FIG. 2, the subsonic filters 112, 114 afford protection against damage due to dropping a phono cartridge. The difference circuit 111 subtracts the right signal R in from the left signal L in to provide a difference signal (L-R), and the summing circuit 113 adds the subsonically filtered left and right input signals L in and R in to provide a sum signal (L+R). The difference signal (L-R) provided by the difference circuit 111 is provided to a fixed difference signal equalizer 115 which selectively attenuates the difference signal as a function of frequency. The fixed difference signal equalizer 115 is substantially similar to the fixed difference signal equalizer 18 of the dynamic stereo image enhancement system 10 of FIG. 2, and an appropriate equalization characteristic is shown in FIG. 5A. By way of example, the fixed difference signal equalizer 115 may include a plurality of parallel filter stages including a low pass filter and a high pass filter having the following characteristics. The low pass filter has a -3 dB frequency of about 200 Hz, a roll-off of 6 dB per octave, and a gain of unity. The high pass filter has a -3 dB frequency of about 7 KHz, a roll-off of 6 dB per octave, and a gain of one-half. As discussed further herein, further amplification for the equalized difference signal output of the fixed difference equalizer 115 is provided by a gain controlled amplifier 125. Such amplification may also be provided, at least in part, by the fixed difference signal equalizer 115. The output of the gain controlled amplifier 125 is coupled to a reverberation filter 129 which provides a processed difference signal (L-R) p as its output. Referring again to FIG. 5A, it should be noted that the difference signal is particularly attenuated in the range of about 1 KHz to about 4 KHz since the human ear has greater sensitivity to such frequencies and since such frequency range includes difference signal components having wavelengths that are comparable to the distance between a listener's ears (the "frequencies of increased phase sensitivity"). As discussed previously and relative to the prior art, loud difference signals within such frequencies result in annoying harshness and limit a listener to being located equidistant between the speakers. By attenuating such frequencies, the harshness and the limitation on location are substantially reduced. The sum signal (L+R) provided by the summing circuit 113 is coupled to a fixed sum signal equalizer 117. An appropriate equalization characteristic for the fixed sum signal equalizer 117 is shown in FIG. 5B. By way of example, the fixed sum signal equalizer 117 includes a bandpass filter which has -3 dB frequencies at 200 Hz and 7 KHz and rolls off at 6 dB per octave. The 200 Hz to 7 KHz bandwidth of the bandpass filter approximates the operating range of the dynamic sum signal equalizer 21 of the dynamic stereo image enhancement system 10 of FIG. 2. It should be noted that the equalization characteristic of the fixed sum signal equalizer 117 rolls off below 200 Hz at 6 dB per octave to avoid overly emphasized bass. Moreover, there is very little difference signal in that range, so that the processed sum signal in that range does not have to be boosted very much. As discussed further herein, further amplification of the processed sum signal (L+R) p is provided by a gain controlled amplifier 127 which also provides for artificial reverberation compensation. Such amplification may also be provided, at least in part, by the fixed sum signal equalizer 117. The equalization characteristics of the fixed equalizers 115, 117 and the gains associated with the processed sum and difference signals are intended to approximate the average behavior of the dynamic enhancement system 10 of FIG. 1 for a large variety of recordings. Thus, the difference signal components in the frequency ranges that statistically include predominantly quiet components are boosted relative to the difference signal components in the frequency ranges that statistically include predominantly louder components. The louder components of the difference signal are typically in the mid-range, and the quieter components are on either side of the mid-range. Particularly, the difference signal components in the mid-range are attenuated to a greater degree than the difference signal components on either side of the mid-range. The equalized signal is then boosted so that the difference signal components on either side of the mid-range are boosted relative to the difference signal components in the mid-range. The enhancement system 110 further includes a feedback and reverberation control circuit 40. The control circuit 40 cooperates with other elements in the system to provide for automatic adjustment of stereo enhancement and for reverberation compensation. The control circuit 40 is responsive to the difference signal (L-R) provided by the difference circuit 111 and the sum signal (L+R) provided by the summing circuit 113. The control circuit 40 provides a gain control signal CTRL for controlling the gain controlled amplifier 125 which varies the gain applied to the equalized difference signal provided by the fixed difference signal equalizer 115. The control circuit 40 is further responsive to the amplified difference signal provided by the gain controlled amplifier 125. Particularly, the output of the gain controlled amplifier 125 would be provided to the bandpass filter 36 of the control circuit 30 of FIG. 3. The control circuit 40 controls the gain controlled amplifier 125 so as to maintain a constant ratio between the sum signal (L+R) provided by the summing circuit 111 and the difference signal provided by the gain controlled amplifier 125. The control circuit 40 further provides a reverberation control signal RCTRL to the gain controlled amplifier 127 which provides for reverberation compensation. By way of example, the gain controlled amplifier 127 may be an appropriate voltage controlled amplifier. The reverberation filter 129 is a variable rejection filter that includes two one octave wide filters respectively centered at 500 Hz and 1.5 KHz, and each having a low Q to provide sufficient bandwidth. Each of the filters of the reverberation filter 129 may be similar to one of the equalizer bands in the dynamic difference signal equalizer 19 of the dynamic stereo image enhancement system 10 of FIG. 2, and provides a maximum attenuation of 12 dB for the maximum level of the reverberation control signal RCTRL. By way of example, the gain controlled amplifier 125 may be an appropriate voltage controlled amplifier. The processed sum signal (L+R) p output of the gain controlled amplifier 127 is provided to a fixed terminal of a potentiometer 123 which has its other fixed terminal coupled to ground. The wiper contact of the potentiometer 123 is coupled to a mixer 121 which therefore receives the processed sum signal (L+R) p having a level controlled by the potentiometer 123. The processed difference signal (L-R) p output of the reverberation filter 129 is coupled to a fixed terminal of a potentiometer 119 which has its other fixed terminal coupled to ground. The wiper contact of the potentiometer meter 119 is coupled to the which therefore receives the processed difference signal (L-R) p having a level controlled by the potentiometer 119. The gain controlled amplifier 127 and the reverberation filter 129 are preferably controlled by the reverberation control signal RCTRL so that the resulting increase in the processed sum signal (L+R) provided by the gain controlled amplifier 127 is less than the decrease in the processed difference signal (L-R) p provided by the reverberation filter 129. Increasing the level of the processed sum signal (L+R) p provided by the gain controlled amplifier 127 is to provide for a sufficient level of the processed sum signal (L+R) p to compensate for any artificial reverberation not sufficiently attenuated by the reverberation filter 129. The left and right subsonically filtered input signals L in and R in are provided as further inputs to the mixer 121. The mixer 121 combines the processed difference signal (L-R) p and the processed sum signal (L+R) p with the left and right input signals L in and R in to provide left and right output signals L out and R out . The mixer 121 may be similar to the mixer 25 of the dynamic stereo enhancement system 10 of FIG. 1, and would provide the left and right output signals L out , R out in accordance with the following: L out = L in + K 1 (L+R) p + K 2 (L-R) p R out = R in + K 1 (L+R) p - K 2 (L-R) p The value of K 1 is controlled by the potentiometer 123; and the value of K 2 is controlled by the potentiometer 119. The potentiometers 119, 123 are user controlled elements to allow the user to control the levels of the processed difference signal (L-R) p and the processed sum signal (L+R) p that are mixed by the mixer 121. For example, the potentiometers 119, 123 may be adjusted to minimize the processed difference signal and to maximize the processed sum signal. With such adjustment, the listener would hear primarily any center stage soloist present in the recording being played. The left and right output signals L out and R out are provided to the sound perspective correction system 200 of the stereo enhancement system 300 of FIG. 1 Alternatively, as discussed relative to FIG. 1, to the extent that the sound perspective correction system 200 is not utilized, the left and right output signals L out and R out are appropriately provided, for example, to the tape monitor loop input of the preamplifier that provided the left and right stereo signals L and R. D. The Perspective Correction System The sound perspective correction system 210 of FIG. 6 provides perspective correction for (a) speakers located in front of the listener ("front located speakers"); (b) headphones; and (c) speakers located to the side of the listener ("side located speakers"), such as those in automobile doors. As used herein, the term headphones shall refer to all headphones, including those sometimes characterized as airline headsets. Generally, headphones can be categorized as being (a) circumaural where the earcup surrounds the entire large outer ear known as the pinna, (b) supraaural where the earcup sits on the outer surface of the pinna, and (c) intraaural where the earcup fits within the entrance to the ear canal. Referring specifically to FIG. 6, the sound perspective correction system 210 includes a summing circuit 211 and difference circuit 213 which are both responsive to left and right input L in and R in signals provided by a stereo image enhancement system as described above or by a stereo sound reproduction system (not shown). For example, as discussed above relative to the stereo enhancement system 300 of FIG. 1, the left and right input signals L in and R in may be provided by the preamplifier tape monitor loop output of such a stereo system. The summing circuit 211 adds the left and right input signals L in and R in to provide a sum signal (L+R), and the difference circuit 213 subtracts the right signal R in from the left signal L in to provide a difference signal (L-R). The sum signal (L+R) is provided to the input of a fixed sum signal equalizer 215 which provides for one equalization output that is coupled to the switchable terminal 2 of a two-position switch 217. The switchable terminal 1 of the two-position switch 217 is coupled to the output of the summing circuit 211. The switched terminal of the switch 217 provides a switched sum signal (L+R) s . The difference signal (L-R) is provided to the input of a fixed difference signal equalizer 221 which provides for one equalization output that is coupled to the switchable terminal 1 of a two-position switch 223. The switch 223 is ganged together with the switch 217 so that each is in the same corresponding position. The switchable terminal 2 of the switch 223 is coupled to the output of the difference circuit 213. The switched terminal of the switch 223 provides a switched difference signal (L-R) s . The ganged two-position switches 217, 223 are controlled by the user, and are set as a function of whether (a) front located speakers are to be used, or (b) headphones or side located speakers are to be used. It should be readily apparent that in position 1, the fixed sum signal equalizer 215 is bypassed, and in position 2 the fixed difference signal equalizer 221 is bypassed. The switched terminal of the switch 217 is connected as an input to a mixer 225, and the switched terminal of the switch 223 is also connected as an input to the mixer 225. The mixer 225 combines the switched sum signal (L+R) s and the switched difference signal (L-R) s to provide left and right output signals L out and R out . Particularly, the left and right output signals L out and R out are provided by the mixer 225 in accordance with the following: L out = (L+R) s + (L-R) s R out =(L+R) s - (L-R) s Position 1 of the switches 217, 223 corresponds to sum and difference signals for use with front located speakers. Position 2 of the switches 217, 223 corresponds to sum and difference signals for use with headphones or side located speakers, such as in an automobile. From the foregoing it should be evident that only the difference signal is equalized when front located speakers are utilized, and that only the sum signal is equalized when headphones or side located speakers are utilized. Referring again to the fixed sum signal equalizer 215 and to the fixed difference signal equalizer 221, each includes a plurality of equalization bands which are about one-third octave wide. The following Tables I and II set forth the respective center frequencies of such equalizer bands and the amount of equalization provided. Table I sets forth the equalization provided by the fixed difference signal equalizer 221 for the output connected to the switchable terminal 1 of the switch 223. As discussed above, the fixed sum signal equalizer 215 is bypassed when the switches 217, 223 are in position 1 (front speakers). Center Freq Difference Signal Equalizer 500 Hz +5.0 dB 1 KHz +7.5 dB 8 KHz +15.0 dB Table II sets forth the equalization provided by the fixed sum signal equalizer 215 for the output connected to the switchable terminal 2 of the switch 217. As discussed above, the fixed difference signal equalizer 221 is bypassed when the switches 217, 223 are in position 2 (headphones or side speakers). Center Freq Sum Signal Equalizer 500 Hz -5.0 dB 1 KHz -7.5 dB 8 KHz -15.0 dB The values set forth in Table I are representative values only and may be modified on the basis of factors including speaker location and speaker characteristics. Similarly, the values set forth in Table II are representative values only, and with side located speakers may be modified on the basis of factors including speaker location and speaker characteristics. With headphones, the values of Table II may also be modified on the basis of factors including the type of headphone, as well as specific headphone characteristics. It should be noted that the equalization for headphones may differ from the equalization for side placed speakers. With side located speakers, the sound reaches the ear with little interference. However, with headphones, the combined structure of the headphones and the ear influences the spectrum of the sound reaching the eardrum. Moreover, the concha (the section leading into the ear canal) and part of the ear canal may be occluded by the headphone structure, which would further influence the spectrum of sound reaching the eardrum. A discussion of the effects of airline entertainment headsets on sound reproduction is set forth in "Some Factors Affecting the Performance of Airline Entertainment Headsets," S. Gilman, J. Audio Eng. Soc., Vol. 31, No. 12, December 1983, pp. 914-920. The equalization provided by the sound perspective control system 210 can be further understood by reference to FIGS. 7A through 7D. FIG. 7A represents a statistical average frequency response of the human ear to sound emanating from zero degrees azimuth or straight ahead (herein the "front response"). FIG. 7B represents a statistical average frequency response of the human ear for sound emanating from 90 degrees azimuth as measured relative to straight ahead (herein the "side response"). FIG. 7C is the front response relative to the side response, i.e., the response of FIG. 7A (front) minus the response of FIG. 7B (side). Equalization is required for sounds which should be emanating from the front but with side located speakers or headphones are emanating from the sides. The response of FIG. 7C is indicative of the equalization that would restore front sounds to their appropriate levels when such sounds are reproduced by side located speakers or headphones. FIG. 7D is the side response relative to the front response, i.e., the response of FIG. 7B (side) minus the response of FIG. 7A (front) provides the response of FIG. 7D. Equalization is required for sounds which should be emanating from the sides but are emanating from the front. The response of FIG. 7D is indicative of the equalization that would restore side sounds to their appropriate levels when such sounds are reproduced by forward placed speakers. The equalization characteristics of the equalizers 215, 221 are based on the response of FIGS. 7C and 7D, but do not provide the entire equalization indicated by such responses. It has been determined that equalization bands of one-third octave widths respectively centered at 500 Hz, 1 KHz, and 8 KHz are sufficient. The characteristics of each equalization band have been discussed previously. The foregoing has been a disclosure of a stereo sound perspective correction system which provides for a stereo image having a corrected stereo sound perspective. It is readily utilized with or without the disclosed stereo image enhancement systems. Its use with a stereo image enhancement system would provide for a stereo image which is wider, a greater listening area when used with speakers, and proper sound perspective. The disclosed implementation of the sound perspective correction system of the invention is not complex and effectively utilizes only a few narrow equalization bands. As discussed above, the relative responses of the front and side responses to one another tend to indicate that wider ranges of equalization should be utilized, but the few narrow equalization bands have been found to be a reasonable approximation over the entire audio bandwidth. As previously mentioned, principles of the present invention are applicable either for playback of conventional stereo sound recordings or for the manufacture of unique stereo sound recordings which will provide advantages described above when played back through conventional sound responsive systems. Thus, as illustrated in Fig. 8, for playback of a conventional sound recording, an exemplary system having the enhancement described herein includes a conventional playback apparatus 300 which may respond to a digital record, such as a laser disc, a phonograph record, a magnetic tape, or the sound channel on video tape or motion picture film. The playback apparatus provides left and right channel stereo signals L,R to a preamplifier 302 from which the left and right signals are fed to the stereo image enhancement system 100 described above to provide processed output signals L out and R out fed either directly to a pair of conventional loudspeakers 304, 306 or fed to the speakers via the perspective correction system 200 previously described. A similar arrangement is used in making a recording that will itself bear data in the form of physical grooves of a phonograph record, magnetic domains of a magnetic tape or like medium, or digital information that may be read by optical means. Such data defines left and right stereo signals formed of signal components that, when played back on a conventional sound reproducing system, produce all of the advantages described above. Thus, as illustrated schematically in Fig. 9, a recording system for making a sound recording embodying principles of the present invention may receive left and right stereo input signals from either a pair of microphones 310 or a conventional stereo playback system 312 which is adapted to provide left and right stereo input signals L,R. The playback system 312, like the system 300 of Fig. 8, may provide its output signals from any conventional record medium including digital records such as a laser disc, phonograph records, magnetic tape, or video or film sound track media. Ganged switches 314, 316 schematically indicate in FIG. 9 that the system may use either left and right signals from a playback device or the left and right signals from a pair of microphones. These signals are fed to a preamplifier 318 and thence to the stereo image enhancement circuit 100 described above. From the stereo image enhancement circuit 100, the processed left and right output signals are fed either directly to a recording device 320 or indirectly to the recording device via the above described perspective correction circuit 200. The recording device conventionally records the left and right output signals L out and R out on a record medium 322 which may be any one of the record medium types commonly employed. It will be noted that the output signals L out and R out that are fed to recording device 320 are derived, in the case of the stereo image enhancement, from mixer 25 of Fig. 2 or mixer 21 of Fig. 4, or in the case of the perspective correction from the mixer 225 of Fig. 6. The output signal L out recorded on the medium 322 includes the several left channel output signal components described, namely the described combination of L in + K 1 (L+R) p + K 2 (L-R) p for the left channel output. Similarly, the output signal R out is recorded upon the record medium by the recording apparatus and includes the components described above as R in + K 1 (L+R) p - K 2 (L-R) p . The record medium 322, when recorded with the arrangement illustrated in Fig. 9, is simply played back on a conventional sound recording responsive device to provide the above-described advantages. These advantages are derived from the fact that the record medium so produced embodies signal-producing means that cooperates with the sound recording responsive device to produce left and right output signals that comprise a combination of signal components including a processed difference signal and a processed sum signal. The processed difference signal is a modification of an input difference signal formed in the stereo image enhancement circuit 100. This input difference signal represents the difference of the left and right input signals L and R, and as previously described, has relative amplitudes of certain components modified to boost those of its components that are within frequency bands wherein the input difference signal has lowest amplitude relative to those components of such input difference signal that are within frequency bands wherein the input difference signal components have highest amplitude. Similarly, the recording will produce a right stereo output signal component as a processed sum signal formed in the stereo image enhancement circuit 100. This processed sum signal component is a modification of the sum of the left and right channel input signals, and, as previously described, has relative amplitudes of certain components modified to boost those of its components in frequency bands where the input difference signal has higher amplitudes relative to those components of the input sum signal that are within frequency bands where the difference signal has lower amplitude. Thus, the record cooperates with the sound responsive system to cause the speakers to produce left and right stereo signals each having sum and difference components wherein amplitudes of such components are relatively attenuated or boosted, respectively, within those frequency bands wherein the difference signal has lower amplitudes. Moreover, the operation of the gain control amplifier 22 and control circuit 30 of Fig. 2, and the corresponding circuits of Fig. 4 cause the stereo output signals produced by playback of record 322 to have a substantially constant ratio of the sum signal to the modified or processed difference signal, all as previously described. When the system of Fig. 9 is employed to make a record having perspective correction in addition to or instead of image enhancement, such a record will cooperate with the conventional stereo player to produce left and right stereo output signals having components including a processed sum signal which are increasingly attenuated in frequency bands centered on 500 Hz, 1 KHz and 8 KHz, respectively, as described above, and a component comprising a difference signal. Such a record is made specifically to be played back through a system including side mounted speakers. Where a perspective corrected record made with the system of FIG. 9 is specifically made for playback in a system with front mounted speakers, the record when played back on a stereo player will provide left and right stereo output signals wherein one output signal has components comprising a sum signal and a component comprising a difference signal, where such difference signal has amplitudes thereof increasingly boosted in frequency bands centered respectively at 500 Hz, 1 KHz and 8 KHz, as described above. Stated otherwise, the recording having perspective correction for front speakers, when played in a stereo player, produces a left output signal which is formed of the sum of a first component comprising the sum signal and a second component comprising the processed difference signal as set forth in equation 5 above and will provide a right output stereo signal formed of the difference between the sum signal and the processed difference signal as set forth in equation 6 above. When such a recording is made for use with side mounted speakers, only the sum signals in equations 5 and 6 are equalized whereas when the recording is made for use with front mounted speakers only the difference signals of equations of 5 and 6 are equalized. It will be seen that a method of making unique stereo sound recordings having stereo image enhancement, or perspective correction, or both may be carried out by the apparatus illustrated in Fig. 9. The method generally comprises combining left and right input signals to generate sum and difference signals, and creating a processed sum signal by selectively altering relative amplitudes of components of the sum signal within respective predetermined frequency bands so as to boost those of the sum signal components which are within frequency bands of highest difference signal component amplitudes relative to those of the sum signal components which are within frequency bands of lowest difference signal component amplitudes. The method also includes the step of creating a processed difference signal by selectively alterating the relative amplitude of components of the difference signal within the predetermined frequency bands so as to attenuate those of the difference signal components which are within frequency bands where difference signal components are highest relative to those of the difference signal components which are within frequency bands wherein the difference signal components are lowest. The method also combines the left and right signals with the processed sum and difference signals to provide enhanced right and left output signals which are fed to a sound recording device to make a sound recording. Other features of the method include the described electronic analysis of the frequency spectrum of the difference signal and generation of control signals as a function of the amplitudes of the difference signal within respective predetermined frequency bands, and utilizing the control signals to determine the extent to which amplitudes of components of the sum and difference signals are altered within the respective frequency bands. According to an important aspect of the method described herein, right and left signals are added and subtracted to generate sum and difference signals, a dynamic control signal is generated representing the amount of stereo in the input signals, the sum and difference signals are processed for enhancement of the output signals and at least one of the processed signals is modified in accordance with the amount of stereo in the input signals. A specific feature of this aspect of the method involves modification of one of the processed signals, which is accomplished so as to maintain a constant ratio between one of the sum and difference signals and the processed signal. In use of the described method, for making a recording that is corrected for perspective with side mounted speakers, left and right signals are combined to provide sum and difference signals, the sum signal is equalized as previously described and combined with the unprocessed difference signal to provide a left output formed of the sum of the processed sum signal and the unprocessed difference signal and to form a right output signal comprising the difference between the processed sum signal and the unprocessed difference signal. These output signals are fed to the recording mechanism to provide a record medium having perspective correction for side mounted speakers. For front mounted speakers, a perspective corrected record medium is made by combining the right and left input signals to provide sum and difference signals, equalizing the difference signal as previously described, and combining the unprocessed sum signal with the equalized difference signal to provide a left output formed of the sum of the unprocessed sum signal and the processed or equalized difference signal and to provide a right output signal formed of the difference between the unprocessed sum signal and the equalized difference signal. These output signals are fed to a recording mechanism to produce a record medium having perspective correction for front speakers. A record made by the apparatus and method described herein is uniquely distinguished from other stereo records in that unique signal generating data is embodied in the record. Whether such data is in the form of variable magnetic elements, varying grooves of a phonograph record or digital information such as variations in optical reflectivity of a laser or digital disc, for example, the unique aspects of such a record medium are readily recognizable. Upon playback of such an unique record by conventional record playing medium, stereo sound will be produced having all of the above-described advantages and composed of the specified signal components. The amount of enhancement is continually and automatically adjusted by control circuit 30 and gain controlled amplifier 22 to compensate for variation in the amount of stereo information from one recording to another when using the described system for playback of conventional recordings. So too, such continuous and automatic adjustment is embodied in a recording made as indicated in Fig. 9. Thus, if the stereo information contained in a record employed in the playback system 312, or, if the stereo information reaching the microphone pair 310, should vary either from one recording to the next or should vary during any given performance or recording, the described control circuit 30 and gain control amplifier 22 will result in adjustment of the amount of enhancement in the information recorded on the record medium 322 and, therefore, result in such adjustment of output signals when record medium 322 is played back in a conventional system. As described above and illustrated in FIG. 4, where fixed sum and difference equalizers are employed, amplitude of the processed sum channel signal is boosted, and certain frequencies of the processed difference signal are attenuated under control of the reverberation control signal RCTRL. This arrangement provides automatic control of the amount of reverberation by automatically increasing the level of the processed sum channel signal and concomitantly decreasing the level of certain frequencies of the difference channel signal. These increases and decreases in signal levels are effected in the reverberation bands, as described above, to reduce boost of natural or artificial reverberation that may be present, which boosts is provided by the enhancement circuits described herein. A similar reverberation control is also described above in connection with the arrangement illustrated in FIG. 2, in which the reverberation control signal is employed to cause the dynamic difference signal equalizer 19 to provide further attenuation in the reverberation bands and to cause the dynamic sum signal equalizer 21 to provide further boost to the sum signal components. Overall, the foregoing has been a disclosure of systems for substantially improving the stereo image resulting from recorded performances, both in playback of conventional records and in the production of improved recordings. Such systems are readily utilized with standard audio equipment and are readily added to installed audio equipment. Further, the disclosed systems may be easily incorporated into preamplifiers and/or integrated amplifiers. Such incorporation may include provisions for bypassing the disclosed systems. The disclosed stereo enhancement system is readily implemented using analog techniques, digital techniques, or a combination of both. Further, the disclosed stereo enhancement system is readily implemented with integrated circuit techniques. Also, the disclosed systems may be utilized with or incorporated into a variety of audio systems including airline entertainment systems, theater sound systems, recording systems for producing recordings which include image enhancement and/or perspective correction, and electronic musical instruments such as organs and synthesizers. Further, the disclosed systems would be particularly useful in automotive sound systems, as well as sound systems for other vehicles such as boats. It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Claims: A sound perspective correction system (210) for use in a stereo system having respective left and right input signals whereby the left and right signals are played through the speakers placed in front of a listener, comprising: means for combining (213) the left and right input signals to create a difference signal representing the difference between the left and right input signals; characterised in that the correction system further comprises: means for equalizing (221) the difference signal by a level of equalization to create a processed difference signal, the level of equalization representing a statistically-averaged response of the human auditory system for sounds emanating from the sides of a listener diminished by a statistically-average response of the human auditory system for sounds emanating from the front of a listener; and means (225) for combining the left and right input signals with the processed difference signal to create left and right perspective corrected output signals. The sound perspective correction system (210) of claim 1, wherein said level of equalization is further characterised by boosting of components of the difference signal below approximately 2 kHz and above 4 kHz relative to components of the difference signal between approximately 2 kHz and 4 kHz. The sound perspective correction system (210) of claim 1, wherein said means for equalizing (221) comprises means for boosting components of the difference signal below approximately 2 kHz and above approximately 4 kHz relative to components of the difference signal between 2 kHz and 4 kHz. The sound perspective correction system (210) of claim 1, wherein said means for equalizing (221) comprises a low pass filter for filtering components to the difference signal below approximately 2 kHz and a high pass filter for filtering components of the difference signal above approximately 4 kHz, and means for boosting components of the difference signal below approximately 2 kHz and above 4 kHz. The sound perspective correction system (210) of claim 1, wherein said means for equalizing (221) is a digital signal processor. The sound perspective correction system (210) of claim 1, wherein said level of equalization is further characterised by first, second and third equalization bands of one-third octave width, the first equalization band centred at approximately 500 Hz and providing a boost of approximately 5 dB, the second equalization band centred at approximately 1 kHz and providing a boost of approximately 7.5 dB, and third equalization band centred at approximately 8 kHz and providing a boost of approximately 15 dB. A sound perspective correction system (210) for use in a stereo system having respective left and right input signals whereby the left and right signals are played through speakers placed at the sides of a listener, comprising: means for combining (211) the left and right input signals to create a sum signal representing the sum of the left and right input signals; characterised in that the correction system also comprises means for equalizing (215) the sum signal by a level of equalization to create a processed sum signal, the level of equalization representing a statistically-averaged response of the human auditory system for sounds emanating from the front of a listener diminished by a statistically-averaged response of the human auditory system for sounds emanating form the sides of a listener; means for combining (225) the left and right input signals with the processed sum signal to create left and right perspective corrected output signals. The sound perspective correction system (210) of claim 7, wherein said level of equalization is further characterised by the attenuation of components of the sum signal below approximately 2 kHz and above approximately 4 kHz relative to components of the sum signal between approximately 2 kHz and 4 kHz. The sound perspective correction system (210) of claim 7, wherein said means for equalizing (215) comprises means for attenuating components of the sum signal below approximately 2 kHz and above approximately 4 kHz relative to components of the sum signal between 2 kHz and 4 kHz. The sound perspective correction system (210) of claim 7, wherein said means for equalizing (215) comprises a low pass filter for filtering components of the signal below approximately 2 kHz and a high pass filter for filtering components of the sum signal above approximately 4 kHz, and means for attenuating components of the sum signal below approximately 2 kHz and above 4 kHz. The sound perspective correction system (210) of claim 7, wherein said means for equalizing (215) is a digital signal processor. The sound perspective correction system (210) of claim 7, wherein said level of equalization is further characterised by first, second and third equalization bands of one-third octave width, the first equalization band centred at approximately 500 Hz and providing a level of attenuation of approximately 5 dB, the second equalization band centred at approximately 1 kHz and providing a level of attenuation of approximately 7.5 dB, and the third equalization band centre at approximately 8 kHz and providing a level to attenuation of approximately 15 dB. A method of enhancing left and right stereo input signals to achieve perspectively corrected left and right output signals for use in a stereo system whereby the left and right signals are played through speakers placed in front of a listener, comprising the steps of: electronically combining (213) the left and right input signals to create a difference signal representing the difference between the left and right input signals; characterised in that the method includes equalizing (221) the difference signal by a level of equalization to create a processed difference signal, the level of equalization representing a statistically-averaged response of the human auditory system for sound emanating from the sides of a listener diminished by a statistically-averaged response of the human auditory system for sounds emanating from the front of a listener; and electronically combining (225) the left and right input signals with processed difference signal to create left and right perspective corrected output signals. The method of claim 13, wherein said level of equalization is further characterised by boosting of components of the difference signal below approximately 2 kHz and above approximately 4 kHz relative to components of the difference signal between approximately 2 kHz and 4 kHz. The method of claim 13, wherein said step of equalizing the difference signal comprises the steps of boosting components of the difference signal below approximately 2 kHz and above approximately 4 kHz relative to components of the difference signal between 2 kHz and 4 kHz. The method of claim 13, wherein said step of equalizing the difference signal comprises the steps of filtering components of the difference signal below approximately 2 kHz and filtering components of the difference signal above approximately 4 kHz, and the step of boosting components of the difference signal below approximately 2 kHz and above 4 kHz. The method of claim 13, wherein said step of equalizing the difference signal comprises the steps of equalizing three bands of the difference signal of one-third octave width, the first equalization band centred at approximately 500 Hz and boosted by approximately 5 dB, the second equalization band centred at approximately 1 kHz and boosted by approximately 7.5 dB, and the third equalization band centred at approximately 8 kHz and boosted by approximately 15 dB. A method of enhancing left and right stereo input signals to achieve perspectively corrected left and right output signals for use in a stereo system whereby the left and right signals are played through speakers placed at the sides of a listener, comprising the steps of: electronically combining (211) the left and right input signals to create a sum signal representing the sum between the left and right input signals; characterised in that the method includes equalizing (215) the sum signal by a level of equalization to create a processed sum signal, the level of equalization representing a statistically-averaged response of the human auditory system for sounds emanating from in front of a listener diminished by a statistically-averaged response of the human auditory system for sounds emanating from the sides of a listener; electronically combining (225) the left and right input signals with the processed sum signal to create left and right perspective corrected output signals. The method of claim 18, wherein said level of equalization is further characterised by attenuating components of the sum signal below approximately 2 kHz and above approximately 4 kHz relative to components of the sum signal between approximately 2 kHz and 4 kHz. The method of claim 18, wherein said step of equalizing the sum signal comprises the steps of attenuating components of the sum signal below approximately 2 kHz and above approximately 4 kHz relative to components of the sum between 2 kHz and 4 kHz. The method of claim 18, wherein said step of equalizing the sum signal comprises the steps of filtering components of the sum below approximately 2 kHz and filtering components of the sum signal above approximately 4 kHz, and the step of attenuating components of the sum signal below approximately 2 kHz and above 4 kHz. The method of claim 18, wherein said step of equalizing the sum signal comprises the steps of equalizing three bands of the sum signal of one-third octave width, the first equalization band centred at approximately 500 Hz and attenuated by approximately 5 dB, the second equalization band centred at approximately 1 kHz and attenuated by approximately 7.5 dB, and the third equalization band centered at approximately 8 kHz and attenuated by approximately 15 dB.

Inventors: KLAYMAN ARNOLD I, KLAYMAN, ARNOLD I.
Applicants: SRS LABS INC, SRS LABS, INC.
Application year: 1987
Main classification: H04S1
IPC codes: H04S1