HK1008878A - Stereo enhancement system - Google Patents

Description

Stereo enhancement system

Technical Field

The present invention relates generally to audio enhancement systems, and more particularly to systems and methods designed to improve the realism of stereo sound reproduction. More particularly, the present invention relates to an apparatus for widening a sound image generated by amplifying a stereo signal passing through a pair of speakers without introducing unnatural phase shift or time delay in the stereo signal.

Background

The audio or audio-video industry is actively working continuously to overcome the drawbacks of reproduced sound. Recently, with the breakthrough of interactive multimedia computer systems and the development of other audio and video, the interest in sound quality has increased. Therefore, there is a continuing effort to develop techniques for sound recording and playback in the audio industry.

Defects in the reproduced sound may be caused by such factors. Such as a microphone that inefficiently records sound, and a speaker that inefficiently reproduces the recorded sound. Efforts in the related industry to enhance sound images have resulted in methods of recording and encoding location information of sound sources together with information of the sound itself. These methods include multi-channel surround systems that use specifically encoded audio information, and that operate with specific decoding systems that account for this information.

Sound enhancement systems that do not require specific recording of sound are generally simpler and less costly. Such systems include introducing an unnatural time delay or phase shift between the left and right signal sources. Many of these systems address the drawback of the microphone failing to mimic the frequency response of the human ear. Due to the position of the loudspeaker, these systems also aim to compensate for the fact that the perceived direction of the sound emanating from the loudspeaker may not coincide with the original position of the sound. While the above systems are directed to reproducing sound in a more realistic and realistic manner, the use of these methods in the field of competitive audio enhancement has led to a wide variety of results.

Other sound enhancement techniques use signals called sum and difference signals. The sum and difference signals represent the sum between the left and right stereo signals and the difference between the left and right stereo signals, respectively.

It is well known that raising the level of the difference signal of a pair of stereo left and right signals can widen the perceptually prominent sound image emanating from a pair of loudspeakers, or other electroacoustic transducers, placed in front of the listener. A widened audio image resulting from the presence of background or reverberant sound amplified by the difference signal. This background is easily perceived at the appropriate level at live sound locations. However, in the recorded work, this background is covered by direct sound, and the background sound cannot be felt with the same level as in the live performance.

Much effort has been made to improve the background acoustic information from recorded programs. For example, indiscriminately adding a difference signal over a broad spectrum. However, indiscriminate addition of the difference signal can affect human perception of sound, which is undesirable. For example, a difference signal that increases the mid-range of the audio can result in a sound perception that is overly sensitive to the listener's head position.

U.S. patent nos.4,748,669 and 4,866,774 disclose a highly complimentary sound enhancement technique for processing sum and difference signals, made by Arnold Klayman, the same person as the inventor of the invention disclosed in this application.

As disclosed in the '669 and' 774 patents, sound enhancement systems are provided that provide dynamic or fixed equalization of the difference signal within a selected frequency band. In such systems, equalization of the difference signal is provided to boost the lower strength difference signal component without over boosting the stronger difference signal component. The stronger difference signal component typically occurs in a mid-range frequency of approximately 1 to 4 KHz. Such mid-range frequencies correspond to frequencies where the human ear has improved sensitivity. Various embodiments of the system disclosed by the '669 and' 774 patents also equalize the amplitude associated with the sum signal within a particular frequency band to prevent the sum signal from being covered by the difference signal. Furthermore, the increased level of the difference signal provided by the '669 and' 774 enhancement systems is a function of the sum signal itself.

U.S. patent No.4,748,669 and U.S. patent No.4,886,774 fully disclose the human auditory mild response characteristics, selectively enhancing the specific advantages of sum and difference signals.

Even with the above audio enhancement techniques, there is a general need for audio enhancement systems that provide high quality stereo image enhancement and that can meet all of the requirements of the evolving computer multimedia market, as well as the requirements of the audio and audio video market. The stereo enhancement system disclosed herein satisfies this need.

Summary of The Invention

The apparatus and method for producing a wider sound image disclosed herein is an improvement over the stereo enhancement systems disclosed in U.S. patent nos.4,738,669 and 4,866,744, which are incorporated herein by reference in their entirety. Improved systems have gained widespread acceptance. For example, in Multimedia World published at 1994.11, an author describes the invention as "it appears to be the next major event of a Multimedia PC and there is good reason to believe it can". Furthermore, the PC Gamer magazine published at 1994.9 writes "none of the various advances in audio technology over the last few years" in terms of the same stereo enhancement system.

The sound generated by a multimedia computer system is typically stored as digital information on a CD-ROM, or other digital storage medium. Unlike analog audio storage media, digital audio information, particularly stereo information, is more accurately stored over a wider frequency spectrum. The presence of this information has a great influence on the method of stereo enhancement. Furthermore, amplification or enhancement of such digitally stored sounds may overload a computer audio amplifier or a computer speaker. They may be relatively "low power" devices. This is particularly relevant for lower, i.e. bass, frequencies, where excessive amplification can "clip" the amplifier and may severely damage the low power speakers of a computer system or television set.

Accordingly, a stereo enhancement system is disclosed that produces a realistic stereo image that is transmitted over a large listening area. The stereo enhancement produced is particularly effective when using a pair of loudspeakers placed in front of the listener. However, current ambient acoustic type systems may also use the enhancement system disclosed herein to help widen the overall acoustic image and eliminate identifiable point sources.

The production of a widely pleasing stereo image surrounding a listener is achieved by a surprisingly simple circuit configuration. In a preferred embodiment, the stereo enhancement system includes circuitry for isolating background signal information, i.e., difference signals, from the left and right input source signals and monophonic signal information, i.e., sum information. The amplitude levels of the sum and difference signals may be fixed to predetermined levels or they may be manually adjusted by an operator of the stereo enhancement system. Furthermore, the left and right input source signals may generate a stereo signal, either truly or synthetically.

The background signal information is spectrally shaped, or equalized, to enhance the frequency components that are statistically of low intensity. Equalization of low intensity background signal components is performed without unduly boosting the corresponding mid-range frequency components. In sound systems where excessive background signal gain cannot be adjusted in the middle of the low frequencies, the high pass filter limits the amplification of these frequency components.

Shaping of the background signal information enhances the reverberant sound effect that may occur in the background signal information but is covered by the stronger direct field sound. The equalized background signal information is recombined with the monophonic signal and the left and right input signals, respectively, to produce enhanced left and right output signals.

The enhancement system disclosed herein can be readily implemented by digital signal processors having discrete circuit elements, or as a hybrid circuit structure. Enhancement systems are particularly popular in inexpensive audio systems that have relatively low power output signals and limit the space in which the enhancement system can be inserted due to its unique circuit configuration and inclusion of low power audio devices.

The above and other aspects, features and advantages of the present invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

Brief Description of Drawings

Fig. 1 is a schematic block diagram of a stereo enhancement system for producing a widened stereo image from a pair of input stereo signals.

Fig. 2 is a display of the frequency response of a projected enhancement curve for a stereo component of a difference signal.

Fig. 3 is a schematic diagram of a preferred embodiment of a stereo enhancement system for producing a widened stereo image from a pair of input stereo signals.

Fig. 4 is a schematic diagram of another embodiment of a stereo enhancement system for producing a widened stereo image from a pair of input stereo signals.

Description of The Preferred Embodiment

Referring initially to FIG. 1, a functional block diagram depicting a preferred embodiment of the present invention is shown. In fig. 1, a stereo enhancement system 10 inputs a left stereo signal 12 and a right stereo signal 14. The left and right stereo signals 12 and 14 are fed along paths 18 and 20, respectively, to a first summing device 16, e.g., an electrical summer. A sum signal representing the sum of the left and right stereo signals 12 and 14 is generated at its output 22 by the summing means 16.

The left stereo signal 12 is connected along path 24 to an audio filter 28, and the right stereo signal 14 is connected along path 26 to an audio filter 30. The outputs of the filters 28 and 30 are fed back to a second summing device 32. The summing device 32 produces a difference signal at a common output 34 that represents the difference of the filtered left and right input signals. Filters 28 and 30 are pre-tuned high pass filters designed to reduce the bass components present in the difference signal. The preferred embodiment performs the reduction of the difference signal bass component for the reasons described below.

Summing means 16 and summing means 32 form a summing network with output signals fed back to separate level-adjusting devices 36 and 38, respectively. The devices 36 and 38 are ideally potentiometers or similar variable-impedance devices. The adjustment of the devices 36 and 38 is typically done manually by the user to control the reference levels that appear in the sum and difference signals of the output signals. This allows the user to adjust the level and orientation of the stereo enhancement to his personal preferences and according to the type of sound played back. The increase in the level of the sum signal emphasizes the audio signal appearing in the central range between the pair of speakers. Conversely, an increase in the level of the difference signal emphasizes the background information that produces a wider perception of the sound image. In some known music genre parameters and system configurations, or audio devices that cannot be manually adjusted, the adjustment devices 36 and 38 may be eliminated and the sum and difference signal levels fixed at predetermined values.

The output of the means 38 is fed to an input 42 of an equalizer 40. Equalizer 40 spectrally shapes the difference signal appearing at input 42 by applying a low pass audio filter 44, a high pass audio filter 48, and an attenuation circuit 46, respectively, to the difference signal as shown. The output signals from filters 44, 48 and circuit 46 exit equalizer 40 along paths 50, 54 and 52, respectively.

The modified difference signal transmitted along paths 50, 52 and 54 constitutes a component of the processed difference signal (L-R) P. These components are fed back to a summing network comprising summing means 56 and summing means 58. The summing device 58 also receives the summed signal output from the device 36, in particular the original left stereo signal 12. All five signals are summed in summing device 58 to produce an enhanced left output signal 60.

Similarly, the modified difference signal from equalizer 40, the sum signal, and the original right stereo signal 14 are combined in summing device 56 to produce an enhanced right output signal 62. The components of the difference signals produced along paths 50, 52 and 54 are inverted by summing device 56 to produce a signal for the right speaker (R-L)_PThe phase of the right speaker differs from the phase of the left speaker by 180 degrees.

Because the summing devices 56 and 58 combine the filtered and attenuated components of the difference signal to produce the left and right output signals 60 and 62, this results in full spectral shaping, i.e., normalization, of the difference signal. Thus, the enhanced left and right output signals 60 and 62 produce a much improved audio effect, since the background is selectively emphasized to completely encompass the listener in the reproduced sound range. The left and right output signals 60 and 62 are represented by the following mathematical formula:

Lout＝Lin+K₁(L+R)+K₂(L-R)_P (1)

Rout＝Rin+K₁(L+R)-K₂(L-R)_P (2)

it should be noted that the input signals Lin and Rin in the above formulas are typical stereo source signals, but may also be generated by single source synthesis at the same time. One such stereo synthesis method that can be used in the present invention is disclosed in U.S. patent No.4,841,572, also made by ArnoldKlayman and incorporated herein by reference. In addition, as discussed in U.S. patent No.4,789,669, the enhanced left and right output signals represented above may be stored magnetically or electronically in a variety of recording media, such as vinyl recording media, compact discs, digital or analog audio tapes, or computer data storage media. The same level of stereo image enhancement can be achieved by a conventional stereo playback system that can playback the enhanced left and right output signals that have been stored.

Signal in the above equation (L-R)_PRepresenting the difference signal that has been spectrally shaped according to the present invention. The improvement in the difference signal is represented by the frequency response depicted in fig. 2, which shows an enhanced projection, or normalized curve 70, according to a preferred embodiment.

The projected curve 70 represents the gain in dB as a function of audio frequency in logarithmic format. According to a preferred embodiment, the projection curve 70 has a maximum peak gain of about 10dB at point A, which is located at about 125 Hz. The gain of the projected curve 70 decreases at a rate of about 6dB per octave over a range of greater than or less than 125 Hz. The projected curve 70 uses a minimum gain of-2 dB for the difference signal at point B of about 2.1 KHz. At greater than 2.1KHz the gain increases at a rate of 6dB per octave to point C of about 7KHz and continues to increase to about 20KHz, the highest frequency audible to the human ear. Although the use of high-pass and low-pass filters results in equalization of the entire projection curve 70, it is also possible to use a band-stop filter with minimal gain at point B along with the high-pass filter to obtain a similar projection curve.

In a preferred embodiment, the gain difference between points A and B of the projected curve 70 is ideally designed to be 12dB, and the gain difference between points B and C should be about 6 dB. These numbers are design constraints and the actual numbers may vary from one circuit to another depending on the actual values of the components used. If the signal level devices 36 and 38 are fixed, the projection curve 70 will remain constant. However, the adjustment device 38 will slightly change the gain difference between points A and B, and points B and C. If the maximum gain magnitude difference is slightly less than 12dB, the resulting increase in mid-range amplification produces an unpleasant listening impression. Conversely, gain magnitude differences much greater than 12dB tend to reduce the listener's perception of mid-range sharpness.

The projection curve implemented by the digital signal processor reflects the design constraints discussed above more accurately in most cases. For the case of analog implementation, if corresponding to point A₁、B₁And C₁Is 20%, it is acceptable. This deviation from ideal conditions can produce the desired stereo enhancement effect, although less than optimal results.

As shown in fig. 2, a difference signal frequency of less than 125Hz, if possible, is increased by a reduced amount by applying the projection curve 70. This reduction avoids excessive amplification of very low frequencies, i.e. bass frequencies. For many audio playback systems, amplifying the audio difference signal in this low frequency range can produce an unpleasant and unrealistic sound image with too much bass response. These audio playback systems include near-field or low-power audio systems, such as multimedia computer systems, and home stereo systems.

The stereo enhancement provided by the present invention can fully exploit the advantages of high quality stereo recording. In particular, unlike previous analog tape or vinyl records, today's digital stored sound recordings contain difference signals, i.e., stereo, information throughout a wide spectrum including bass frequencies. There is no need to over-amplify the difference signal within these frequencies to get a proper bass response.

As a result, the number of interactive multimedia computer systems owned by common consumers and similar systems in commerce is rapidly increasing. These systems often contain integrated audio processors or peripheral sound devices, such as sound cards, to enhance their audio-video effects. The quality of sound produced by multimedia computers, and near-field audio systems such as portable stereo systems, is relatively low due to the power limitations of these systems themselves, limitations in speaker placement. And limits of listening positions. Although these limitations make near-field system life candidates for image enhancement, they must also overcome the strong unique problem imposed by any stereo enhancement system.

In particular, boosting power in these systems during larger boosting periods may cause the amplifier to "clip," or it may damage components of the audio circuitry including the speaker. Limiting the bass response of the difference signal in most near-field audio enhancement applications also helps to avoid these problems.

Since the bass frequencies of the difference signal are not increased much according to the preferred embodiment, audio information in very low frequencies is also provided by the sum signal L + R, which is of course a mono tone. This is not relevant in near field systems, since bass information using a pair of loudspeakers as a sum signal will accurately create a sound image between the two loudspeakers where the listener is expected to be. However, the left and right signals determine bass information and provide bass directed to cue the near field by their respective amplitude levels.

The projected curve shown in fig. 2 provides sufficient low frequency image enhancement even if an audio system is not a near field system, i.e., it has speakers and a large listening area separated by a large distance. In particular, the bass frequencies have extremely large wavelengths that require a large listening area to effectively perceive a widened bass sound image. For example, a frequency of 30Hz has a wavelength of about 39 feet. A listener attempting to perceive directions at such low audio frequencies will require an equally large listening area. Thus, the stereo enhancement achieved by the projection curve of fig. 2 is also applicable to home stereo and other far-field applications.

Stereo enhancement can also be achieved without the occurrence of sum signal equalization, in accordance with the acoustic principles discussed herein, and with a minimum number of elements given by appropriate circuit design. The invention can therefore be easily and economically implemented in a variety of situations including those with such constraints that limit the space available to accommodate the stereo enhancement circuitry.

Fig. 3 depicts a circuit for generating a widened stereo image according to a preferred embodiment of the present invention. The stereo enhancement circuit 80 corresponds to the system 10 shown in fig. 1. In fig. 3, the left input signal 12 is fed to a resistor 82, a resistor 84 and a capacitor 86. The right input signal 14 is fed to a capacitor 88 and resistors 90 and 92.

The resistor 82 is in turn connected to the inverting terminal 94 of an amplifier 96. The same inverting terminal 94 is also connected to resistor 92 and resistor 98. Amplifier 96 is configured as a summing amplifier having a positive terminal 100 connected to ground via a resistor 102. An output 104 of the amplifier 96 is connected to the positive input 100 via a feedback resistor 106. A sum signal (L + R) representing the sum of the left and right input signals is generated at output 104 and fed to one end of a variable resistor 110 at the opposite end to ground. To obtain the proper sum of the left and right input signals through amplifier 96, resistors 82, 92, 98 and 106 in the preferred embodiment are 33.2 kilo-ohms, and resistor 98 is preferably 16.5 kilo-ohms.

The second amplifier 112 is configured as a "differential" amplifier. The amplifier 112 has an inverting terminal 114 connected to a resistor 116 in turn connected in series to the capacitor 86. Similarly, the positive terminal 118 of the amplifier 112 receives the right input signal through a resistor 120 and a capacitor 88 in series. Terminal 118 is also connected to ground via resistor 128. An output terminal 122 of the amplifier 112 is connected to the inverting terminal through a feedback resistor 124. The output 122 is also connected to a variable resistor 126 which in turn is connected to ground. Although the amplifier 112 is configured as a "differential" amplifier, its function may be embodied as the sum of the right input signal and the negative left input signal. Thus, amplifiers 96 and 112 form a summation network that produces a sum signal and a difference signal, respectively.

The two sets of connected RC networks, comprising elements 86/116 and 88/118 respectively, act as very low, or bass, frequency high pass filters that attenuate the left and right input signals. To obtain a suitable frequency response for the projected curve 70 of FIG. 2, the cut-off frequency, Wc' or the-3 dB frequency, of the high-pass filter should be about 100 Hz. Thus, in a preferred embodiment, the capacitance of capacitors 86 and 88 is 0.1 microfarads and the impedance of resistors 116, 120 is approximately 33.2 kiloohms. The values of the feedback resistor 124 and the attenuation resistor 128 are then selected by:output 122 will represent the amplified twice right difference signal (R-L). As a result of the high pass filtered input, the difference signal of output 122 will attenuate low frequency components below 125Hz at a 6dB per octave reduction ratio. It is possible to filter the low frequency components of the difference signal within equalizer 40 rather than using filters 28 and 30 (shown in fig. 1) to filter the left and right input signals, respectively. However, since the filter capacitor for low frequencies must be large, it is preferable to perform filtering in the input stage to avoid loading the previous circuits.

It should be noted that the difference signal relates to an audio signal containing information that is present in one input channel, i.e. either left or right, but not in the other channel. The particular phase of the difference signal is relevant for determining the final composition of the output signal. Thus, in general, the difference signals are represented as L-R and R-L, which are only 180 degrees out of phase. Thus, as will be appreciated by those skilled in the art, amplifier 112 may be constructed so that the difference signal of the left output (L-R) instead of (R-L) appears at output 122, so long as the difference signals of the left and right outputs are out of phase with each other.

The variable resistors 110 and 126, which may be simply voltage-divided, can be adjusted by providing the sliding contacts 130 and 132, respectively. The level of the difference signal present at the enhanced output signal may be controlled by manual, remote, or automatic adjustment of the sliding contact 132. Similarly, the level of the sum signal appearing as the enhanced output signal can be determined in part by the position of the sliding contact 130.

The summed signal present at the sliding contact 130 is fed through a series resistor 138 to the inverting input 134 of a third amplifier 136. The same summed signal of the sliding contacts 130 is also fed to the inverting input 140 of the fourth amplifier 142 through a separate series connected resistor 144. The amplifier 136 is configured as a differential amplifier having an inverting terminal 134 connected to ground via a resistor 146. The output 148 of the amplifier 136 is also connected to the inverting terminal 134 via a feedback resistor 150.

The positive terminal 152 of the amplifier 136 provides a common node connected to a set of summing resistors 156 and also to ground via resistor 154. The level adjusted difference signal from the sliding contact 132 is transmitted to a set of summing resistors 156 via paths 160, 162, and 164. This produces three separately defined difference signals that occur at points A, B and C, respectively. These defined difference signals are connected to the positive terminal 152 via resistors 166, 168 and 170 as shown.

At point a along path 160, the level adjusted difference signal from sliding contact 132 is passed to resistor 166 without any frequency response improvement. Therefore, the signal at point a is only attenuated by the divided voltage between resistor 166 and resistor 154. Ideally, the attenuation level of node A would be-12 dB with respect to the 0dB reference present at node B. This attenuation level is achieved by resistor 166 having an impedance of 100 kilo-ohms and resistor 154 having an impedance of 27.4 kilo-ohms. The signal at node B represents a filtered model of the level adjusted difference signal that appears through capacitor 172 connected to ground. The RC network of capacitor 172 and resistor 178 operates as a low pass filter with a cutoff frequency determined by the time constant of the network. According to a preferred embodiment, the cut-off frequency, or-3 dB frequency, of the low-pass filter is about 200 Hz. Thus, resistor 178 is preferably 1.5 kilo-ohms while capacitor 172 is 0.47 microfarads, and drive resistor 168 is 20 kilo-ohms.

At node C, the high pass filtered difference signal is fed back to the inverting terminal 152 of the amplifier 136 through the drive resistor 170. The cut-off frequency of the high-pass filter is designed to be about 7KHz and the gain with respect to node B is-6 dB. In particular, the capacitance value of capacitor 174 connected between node C and sliding contact 132 is 4700 picofarads, while the impedance of resistor 180 connected between node C and ground is 3.74 kiloohms.

The modified difference signal present at circuit positions a, B and C is also fed to the inverting terminal 140 of amplifier 142 through resistors 182,184 and 186, respectively. The three modified difference, sum and right input signal inputs are in turn connected to a set of summing resistors 188 of the amplifier 142. Amplifier 142 is configured as an inverting amplifier having a positive terminal 190 connected to ground and a feedback resistor 192 connected between terminal 140 and an output 194. To obtain proper summation of the signals by inverting amplifier 142, the impedance of resistor 182 is 100 kilo-ohms, the impedance of resistor 184 is 20 kilo-ohms, and the impedance of resistor 186 is 44.2 kilo-ohms. The exact values of the resistors and capacitors in the stereo enhancement system can be changed as long as the proper ratio is maintained to obtain the correct enhancement level. Other factors that may affect the passive component values are the power required by the enhancement system 80 and the characteristics of the amplifiers 104, 122, 136 and 142.

In operation, the modified difference signals are recombined to produce an output signal comprising the processed difference signal. In particular, the difference signal components appearing at points A, B and C recombine at terminal 152 of differential amplifier 136 and terminal 140 of amplifier 142 to form a processed difference signal (L-R)_P. Signal (L-P)_PRepresenting the difference signal that has been equalized by applying the projection curve of fig. 2. Ideally, the projected curve is characterized by a gain of 4dB at 7KHz, a gain of 10dB at 125Hz, and a gain of-2 dB at 2100 Hz.

Amplifiers 136 and 142 operate as mixer amplifiers that combine the processed difference signal with the sum signal and the left or right input signal. The signal on the output 148 of the amplifier 136 is fed to a drive resistor 196 to produce the enhanced left output signal 60. Similarly, the signal on output 194 of amplifier 142 produces the enhanced right output signal 62 by driving resistor 198. The drive resistor typically has a resistance on the order of 200 ohms. The enhanced left and right output signals can be represented by the mathematical equations (1) and (2) listed above. The value K in equations (1) and (2)₁The value K is controlled by the position of the sliding contact 130₂Controlled by the position of the sliding contact 132.

All of the individual circuit elements depicted in fig. 3 may be implemented digitally by software running on a microprocessor, or by a digital signal processor. Thus, separate amplifiers, equalizers, etc. may be implemented by corresponding portions of software or hardware.

Fig. 4 depicts an alternative embodiment of the stereo enhancement circuit 80. The circuit of fig. 4 is similar to the circuit of fig. 3 and represents another approach for using the projection curve 70 (shown in fig. 2) for a pair of stereo audio signals. The stereo enhancement system 200 is constructed using an alternative summing network that produces sum and difference signals.

In alternative embodiment 200, left and right input signals 12 and 14 are still ultimately fed to the negative inputs of mixer amplifiers 204 and 226. However, to generate the sum and difference signals, the left and right signals 12 and 14 are first fed through resistors 208 and 210, respectively, to the negative terminal 212 of a first amplifier 214. The amplifier 214 is configured as an inverting amplifier having a grounded input 216 and a feedback resistor 218. The sum signal, or in this case the inverted sum signal- (L + R), is generated at output 220. The summed signal component is then fed to the holding circuit after being level adjusted by the variable resistor 222. Since in an alternative embodiment the summed signal is inverted, it is fed back to the non-inverting input 224 of the amplifier 226. Thus, the amplifier 226 now requires a current balancing resistor 228 disposed between the non-inverting input 224 and ground potential. Similarly, a current balancing resistor 230 is provided between the inverting input 232 and ground potential. These slight modifications to amplifier 226 in alternative embodiments are necessary to obtain the correct sum to produce right output signal 62.

To generate the difference signal, the inverting summing amplifier 236 receives the left input signal and the summed signal at inverting input 238. In particular, the left input signal 12 passes through a capacitor 240 and a resistor 242 before reaching the input 238. Similarly, the inverted sum signal on output 220 passes through capacitor 244 and resistor 246. The RC network created by element 240/242 and element 244/246 provides bass frequency filtering of the audio signal as described in the preferred embodiment.

Amplifier 236 has a non-inverting input 248 connected to ground and a feedback resistor 250. A difference signal R-L is generated at output 252 when resistors 208, 210, 218, and 242 have a resistance value of 100 kilo-ohms, resistors 246 and 250 have a resistance value of 200 kilo-ohms, capacitor 244 has a capacitance value of 0.15 microfarads, and capacitor 240 has a capacitance value of 0.33 microfarads. The difference signal is then conditioned by the variable resistor 254 and fed back to the hold circuitry. The holding circuitry of fig. 4 is identical to the holding circuitry of the preferred embodiment disclosed in fig. 3, except as described above.

The entire stereo enhancement system 80 of fig. 3 uses a minimum number of components to achieve acoustic requirements and produce complimentary stereo sound. System 80 may be comprised of only four active elements, corresponding to the generally operational amplifiers of amplifiers 104, 112, 136 and 142. These amplifiers are readily available as quad packages on separate semiconductor chips. The additional components necessary to implement the stereo enhancement system 80 include only 29 resistors and 4 capacitors. System 200 can also be implemented using one quad-wound amplifier, 4 capacitors and only 29 resistors including potentiometers and output resistors. Because of its unique design, the enhancement system 80 and 200 can be implemented at a minimum cost with a minimum component space and still provide an incredibly widening effect of the existing stereo image. In fact, the entire system 80 can be formed as a single semiconductor substrate, or integrated circuit.

In addition to the embodiments described in fig. 3 and 4, there are possible additional ways of interconnecting the same elements to obtain a perspective enhancement of the stereo signal. For example, a pair of amplifiers configured as differential amplifiers may receive the left and right signals, respectively, and may also each receive a sum signal. In this manner, the amplifier may generate a left differential signal L-R, and a right differential signal L-R, respectively.

Suitable modifications of the difference signal generated from the enhancement systems 80 and 200 have been carefully made and have achieved the best results in a variety of applications and for the input audio signal. The adjustments currently made by the user include only the application of the sum and difference signal levels of the limiting circuitry. However, it is possible to use potentiometers instead of resistors 178 and 180 to allow for proper equalization for the difference signal.

From the above description and the accompanying drawings it has been shown that the present invention has significant advantages over current stereo enhancement systems. Although the foregoing detailed description has shown an overview of the invention and pointed out fundamental novel features thereof, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims.

Claims (37)

1. A system for enhancing a pair of audio left and right stereo signals, comprising:

a first high pass filter having a first cut-off frequency, said first high pass filter receiving said left and right stereo signals for providing improved left and right stereo signals having reduced bass information;

first means for isolating background signal information representing a difference of the modified left and right stereo signals;

second means for isolating monophonic signal information representing a sum of the left and right stereo signals;

a second high pass filter having a second cutoff frequency greater than said first cutoff frequency, said second high pass filter processing said background signal information to provide a first modified signal;

a low pass filter having a third cutoff frequency greater than said first cutoff frequency and less than said second cutoff frequency, said low pass filter processing said background signal information to provide a second modified signal;

third means for combining said first modified signal, said second modified signal, said monophonic signal information and said left signal to provide an enhanced stereo left output signal; and

fourth means for combining said first modified signal, said second modified signal, said monophonic signal information and said right signal to provide an enhanced stereo right output signal.

2. The enhancement system of claim 1 wherein said first means includes an operational amplifier configured as a differential amplifier for combining said left and right stereo signals to isolate said background signal information.

3. The enhancement system of claim 1 wherein said first means includes an operational amplifier configured as an inverting amplifier for combining said left stereo signal and said sum signal to isolate said background signal information.

4. The enhancement system of claim 1 wherein said first cutoff frequency of said first high pass filter is in the range of 125 to 200Hz, said second cutoff frequency of said second high pass filter is in the range of 5.6 to 8.4KHz, and said third cutoff frequency of said low pass filter is in the range of 160 to 240 Hz.

5. The enhancement system according to claim 1 further comprising attenuating means for attenuating said background signal information substantially through all audio frequency levels, said attenuating means coupled to said third and fourth means for providing said left and right enhancement output signals containing said attenuated background signal information.

6. The enhancement system of claim 1 further comprising means for manually adjusting the background signal information level.

7. The enhancement system of claim 1 wherein said first, second, third and fourth means are operational amplifiers and said low pass filter and said first and second high pass filters are first order RC filters comprising passive circuit elements.

8. An enhancement system as claimed in claim 1, characterized in that said system is implemented in digital format in an audio signal processor formed as an integrated circuit.

9. The enhancement system of claim 1 wherein said stereo left and right signals are generated synthetically from a monophonic audio signal source.

10. An enhancement system as claimed in claim 1, characterized in that said stereo left and right signals are part of an audio-video composite signal.

11. An audio enhancement system for producing a widened stereo image from stereo left and right signals reproduced through a pair of loudspeakers, comprising:

a first amplifier for receiving left and right signals and providing a difference signal representing a difference between said left and right signals;

a second amplifier for receiving the left and right signals and providing a sum signal representing the sum of said left and right signals;

a low pass filter for receiving the difference signal from the first amplifier;

a high pass filter for receiving the difference signal from the first amplifier;

a third amplifier having a first input connected to an output of said low pass filter and an output of said high pass filter, a third amplifier having a second input connected to said left stereo signal and said sum signal, wherein said output of said low pass filter, said output of said high pass filter, said left signal, and said sum signal are combined by said third amplifier to produce a left composite output signal;

a fourth amplifier for receiving the output of the low pass filter, the output of the high pass filter, the right signal, and the sum signal, wherein the output of the low pass filter, the output of the high pass filter, the right signal, and the sum signal are combined by the fourth amplifier to produce a right composite output signal.

12. The audio enhancement system of claim 11 wherein said first, second, third and fourth amplifiers are operational amplifiers.

13. The audio enhancement system of claim 12 wherein said operational amplifier is formed on a semiconductor substrate.

14. The audio enhancement system of claim 11 wherein said audio enhancement system is implemented in digital format by a digital signal processor.

15. The audio enhancement system of claim 11 further comprising an attenuator that attenuates said difference signal that actually passes through the audio spectrum by a fixed amount, wherein said third and fourth amplifiers input said attenuated difference signal, and wherein said left and right composite output signals comprise said attenuated difference signal.

16. The audio enhancement system of claim 11 further comprising a potentiometer connected between said first amplifier and said outputs of said low pass and high pass filters for adjusting the level of the difference signal input to said low pass and high pass filters.

17. The audio enhancement system of claim 11 further comprising a first bass filter connected between said left signal and said first amplifier and a second bass filter connected between said right signal and said first amplifier, said first and second bass filters attenuating extremely low frequency components of said left and right signals.

18. The audio enhancement system of claim 17 wherein the cut-off frequency of said first and second bass filters is in the range of 125 to 200 Hz.

19. The audio enhancement system of claim 11 wherein the cutoff frequency of said low pass filter is in the range of 160Hz to 240Hz and the cutoff frequency of said high pass filter is in the range of 5.6KHz to 8.4 KHz.

20. The audio enhancement system of claim 11 wherein said system comprises no more than 4 active amplifiers, no more than 4 capacitors and no more than 30 resistors.

21. A stereo enhancement system for producing a wide stereo image from a pair of stereo signals represented as left and right input signals, said left and right input signals being modified by said enhancement system and converted to sound by an electro-acoustic transducer to produce the image, whereby the amount of stereo information present in the left and right input signals is represented by a difference signal equal to the difference between the left and right stereo signals and the amount of mid-range information present in the left and right input signals is represented by a sum signal equal to the sum of the left and right stereo signals, said stereo enhancement system comprising:

a circuit for improving a frequency response of the stereo information to produce processed stereo information characterized by a maximum gain and a minimum gain, the gain of the processed stereo information varying with respect to frequency components of the processed stereo information, the circuit comprising:

a first audio filter for attenuating bass audio components present in said stereo information in relation to said maximum gain;

a second audio filter for attenuating audio of a mid-range of said stereo information associated with said maximum gain to produce said processed stereo information, said mid-range of audio corresponding to frequencies for which the human ear has increased sensitivity;

a first amplifier for combining the processed stereo information and the sum signal with the left input stereo signal to produce an enhanced left output signal; and

a second amplifier for combining the processed stereo information and the sum signal with the right input stereo signal to produce an enhanced right output signal.

22. A stereo enhancement system as claimed in claim 21, characterized in that the cut-off frequency of the first audio filter is in the range 125 to 200 hz.

23. A stereo enhancement system as defined in claim 21, wherein the circuitry is formed in a digital signal processor.

24. A stereo enhancement system as claimed in claim 21, characterized in that the second audio filter comprises a low-pass filter and a high-pass filter having a cut-off frequency which is larger than the cut-off frequency of the low-pass filter.

25. A stereo enhancement system as defined in claim 24, wherein said cut-off frequency of said low-pass filter is in the range of 160 to 240Hz and said cut-off frequency of said high-pass filter is in the range of 5.6 to 8.4 KHz.

26. A stereo enhancement system for generating a wide stereo image from a pair of stereo signals represented as left and right input signals, said left and right input signals being modified by said enhancement system and converted to sound by an electro-acoustic transducer to generate an image, whereby the amount of stereo information present in the left and right input signals is represented by a difference signal equal to the difference between the left and right stereo signals and the sum of said left and right signals is represented as a sum signal, said stereo enhancement system comprising:

a circuit for normalizing a frequency response of the difference signal to produce a processed difference signal characterized by a maximum gain and a minimum gain, the normalized level of the processed difference signal being used to vary with respect to frequency components of the processed difference signal, the circuit comprising:

a first audio filter for attenuating bass audio components present in said difference signal related to said maximum gain to produce a first improved difference signal;

second and third audio filters for attenuating mid-range audio frequencies of said first modified difference signal associated with said maximum gain to produce second and third modified difference signals, said mid-range audio frequencies corresponding to those frequencies to which the human ear has increased sensitivity, said processed difference signal comprising the sum of said first, second and third modified difference signals;

a first amplifier for combining the first, second and third modified difference signals with the sum signal and the left input stereo signal to produce an enhanced left output signal; and

a second amplifier for combining the first, second and third modified difference signals with the sum signal and the right input stereo signal to produce an enhanced right output signal.

27. The stereo enhancement system of claim 26 further comprising a third amplifier for producing the difference signal and wherein the first audio filter comprises a first high pass filter connected between the left input signal and the third amplifier for attenuating bass components of the left input signal, and wherein the first audio filter comprises a second high pass filter connected between the right input signal and the third amplifier for attenuating bass components of the right input signal.

28. A stereo enhancement system as claimed in claim 27, characterized in that the cut-off frequencies of the first and second high-pass filters are in the range of 125 to 200 Hz.

29. A stereo enhancement system as defined in claim 26, characterized in that the second audio filter is a low-pass filter with a cut-off frequency in the range of 160 to 240 Hz.

30. A stereo enhancement system as defined in claim 26, characterized in that the third audio filter is a high-pass filter having a cut-off frequency in the range of 5.6 to 8.4 KHz.

31. A method for generating enhanced left and right stereo output signals from left and right stereo input signals to produce a widened stereo image when the left and right output signals are played back through a pair of loudspeakers, said method equalizing background signal information that is actually present in the left and right signals by an audio spectrum to produce processed background signal information, said processed background signal information having a varying frequency response, said frequency response being characterized by a maximum gain in a frequency range of 100 to 150Hz and greater than 7khz and said frequency response being characterized by a minimum gain in a frequency range of 1680 to 2520Hz and less than 30 Hz.

32. A method of generating an enhanced left and right stereo output signal as defined in claim 31, wherein the spacing between said maximum gain and said minimum gain is adjustable at a level between 10dB and 14 dB.

33. A method of producing enhanced left and right stereo output signals according to claim 31 and wherein the separation between said maximum gain and said minimum gain is fixed at about 12 dB.

34. A stereo sound recording having stored thereon audio information that is replayable to produce an enhanced stereo effect, the stereo sound recording comprising:

a recording medium containing said audio information, wherein said audio information is accessible to an audio playback device to produce left and right stereo output signals having a difference signal component representing a difference between the left and right output signals, said difference signal having an improved frequency response characterized by a maximum gain in a first frequency range of 100 to 150Hz and a minimum gain in a second frequency range of 1680 to 2520Hz and a third frequency less than about 30 Hz; and

wherein said frequency response decreases at a rate of about 6dB per octave less than said first frequency range and increases to said second frequency range at greater than said first frequency range, said frequency response increases at a rate of about 6dB per octave greater than said second frequency range.

35. A stereo recording as claimed in claim 34, characterized in that the recording medium is an analog or digital optical storage medium.

36. A stereo recording as claimed in claim 34, characterized in that the recording medium is an analog or digital tape recording medium.

37. Stereo recording according to claim 34, characterized in that the separation between the maximum gain and the minimum gain is in the range of 10dB to 14 dB.