Abstract:
The present invention is a method and apparatus for simply and inexpensively enhancing an electronic audio signal in such a way that the quality of audible sound produced from the audio signal more closely approaches that of the original sound heard live in an acoustically designed environment. The present invention restores the perception of harmonics that are normally missing in an electronic audio signal. The apparatus includes a circuit that causes an input audio signal to be distorted such that an enhanced audio signal is produced that exhibits an improved harmonic quality compared to that of the original input audio signal. This distortion is a non-linear amplification of enhancing harmonics or frequencies in the input audio signal. That is, a band of desirable harmonics or frequencies in the input audio signal are non-uniformly amplified across the band. This band of frequencies from the input audio signal can be a band of high frequencies and/or a band of low frequencies. The present circuit accomplishes this enhancement without having to transmit the electronic audio signal through a magnetic coil audio energy transfer system (i.e., a field inducing coil and an electromagnetic field receptor that are weakly coupled electromagnetically).

Description:
This appl. claims the benefit of Provisional No. 60/017,540 filed May 10, 1996. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the enhancement of electronic audio signals to improve the quality of sound produced from those signals, and more particularly to an apparatus and method for harmonically enhancing an electronic audio signal without the use of magnetic coupling to obtain the enhancement. 
     BACKGROUND OF THE INVENTION 
     It is usually considered more pleasurable to hear music, singing or other such complex sounds live, rather than hearing the same sound after it has been converted into an electronic audio signal and re-converted back into audible sound. 
     Many of the sounds we hear, especially musical notes, are often a composite. For example, a musical note having a basic pitch or fundamental frequency, usually contains components of the fundamental frequency called harmonics. These harmonics create the tonal quality or timbre of the sound, such as a musical note, that is often unique to the musical instrument or other sound producing source. In other words, these harmonics enrich the sound we hear. Numerous sound reproduction systems have been developed in an attempt to add harmonic enhancement to audio signals. However, these systems are often very sophisticated and expensive and the sound quality produced with such systems still falls short of the perceived quality of the original sound heard live. 
     A relatively unsophisticated and inexpensive system has been developed which produces an enhanced electronic audio signal which, when converted into audible sound, exhibits an improved harmonic quality compared to that of the original input audio signal and has been perceived as more closely duplicating the experience of hearing the original live sound in an acoustically designed environment. This system is disclosed in U.S. Pat. No. 5,361,306, which is assigned to the assignee of the present application. The exemplary circuits disclosed in the 5,361,306 patent include an input stage having a field inducing coil and an output stage having an electromagnetic field receptor (e.g., another coil) and an output. The field inducing coil and the electromagnetic field receptor are weekly coupled to form a magnetic coil audio energy transfer system. Input audio signals are transmitted through the inducing coil to set-up an electromagnetic field. The field inducing coil and the electromagnetic field receptor are weakly coupled such that when an input audio signal is transmitted through the field inducing coil, an enhanced audio signal is available at the output. 
     The present invention is an improvement to the invention disclosed in the U.S. Pat. No. 5,361,306. The present invention has a less complicated structure and is less expensive. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a number of methods and apparatus are provided for simply and inexpensively enhancing an electronic audio signal in such a way that the quality of audible sound produced from the audio signal is perceived to more closely approaches that of the original sound heard live in an acoustically designed environment. Sound produced from an audio signal enhanced in accordance with the present invention appears to resist degradation at high volumes and tends to eliminate, or at least significantly reduce, the formation of sweet spots. It is desirable for a circuit according to the present invention to be geared toward restoring the perception of harmonics that are normally lost due to the limitations of audio conversion equipment, the recording process, shortcomings of the recording media and/or the like. 
     In one aspect of the present invention, an apparatus is provided for enhancing the quality of an input audio signal made up of a plurality of signal components, each signal component having an amplitude and a frequency, and the frequencies of the input audio signal being within a band of frequencies having a low end and a high end. An apparatus, according to the principles of the present invention, includes a circuit that causes an input audio signal to be distorted such that an enhanced audio signal is produced that exhibits an improved harmonic quality compared to that of the original input audio signal. This distortion is a non-linear amplification of signal components enhancing harmonics or frequencies within a band of frequencies. signal components having frequencies within a band of desirable harmonics or frequencies are non-uniformly amplified across the band. This band of frequencies can be a band of high frequencies and/or a band of low frequencies. It is desirable for the signal components to be selected and amplified in such a way that the human ear is able to better perceive or pick up and register the harmonic character of the audio signal. The present circuit accomplishes this enhancement without having to transmit the electronic audio signal through a magnetic coil audio energy transfer system (i.e., a field inducing coil and an electromagnetic field receptor that are weakly coupled electromagnetically). 
     It is desirable for the present apparatus to include a circuit that is operatively adapted so that when an input audio signal as described above is transmitted therethrough, an enhanced audio signal is produced by amplifying signal components, having frequencies between a reference frequency and the high end, so as to increase in amplitude as per increasing frequencies from the reference frequency toward the high end and/or by amplifying signal components, having frequencies increase in amplitude as per decreasing frequencies from the reference frequency toward the low end, each over at least a portion of the band of frequencies. As used herein, the term reference frequency refers to the frequency or frequency range from which the response of all the other frequencies can be compared. In other words, the reference frequency can be seen as the frequency or frequency range after which the distortion (i.e., the enhancement) of the high frequencies and/or of the low frequencies can begin to be seen. It may be desirable for the circuit to be adapted such that the signal component having the reference frequency of the enhanced audio signal produced by the circuit has an amplitude that is substantially similar to (i.e., equal to or slightly above or below) the amplitude of the original input signal. 
     The present apparatus may include a circuit that uses a high pass filter-like network to non-linearly amplify a band of high frequencies of the input audio signal. Alternatively or in addition, the present apparatus may include a circuit that uses a low pass filter-like network to non-linearly amplify a band of low frequencies of the input audio signal. 
     In another aspect of the present invention, a method is provided for enhancing the quality of electronic audio signals. The method comprises the steps of: providing an input audio signal made up of a plurality of signal components, each signal component having an amplitude and a frequency, and the frequencies of the input audio signal being a band of frequencies having a low end and a high end; and distorting the input audio signal into an enhanced audio signal by non-linearly amplifying signal components having frequencies within the band of frequencies without the use of a magnetic coil audio energy transfer system and such that the enhanced audio signal exhibits an improved harmonic quality compared to that of the input audio signal. It is desirable for the audio signal being provided to have a bandwidth of frequencies with high frequencies and low frequencies, and for the step of distorting the signal to include non-linearly amplifying signal components having frequencies within a high frequency portion of the bandwidth, a low frequency portion of the bandwidth or both. 
     The present method can include the step of processing one or more of the enhanced audio signals into sound. The scope of the present invention is intended to include the sound that is so produced. The present enhanced audio signal and the sound produced therefrom includes audio signals and sounds having frequency bands which overlap or otherwise fall within the range of normal human hearing (i.e., approximately 20 Hz to 20 KHz). 
     The present method can also include the step of transmitting one or more audio signals enhanced according to the present invention from one location to another. The present method can further include the step of recording one or more of the present enhanced audio signals onto a recording medium. The scope of the present invention is also intended to include the recording medium having one or more of the present enhanced audio signals recorded thereon. The recording medium can be a magnetic recording medium (e.g., reel-to-reel tape, cassette tape, magnetic disk, etc.) or an optical recording medium (e.g, compact disk, video disk, etc.). The present invention is not intended to be limited to any particular type of recording medium or method of recording thereon. 
     The present invention provides an apparatus and method for enhancing the harmonic quality of an electronic audio signal, in particular an audio signal having a complex wave form (i.e., multiple frequencies) such as, for example, music, singing, speech, animal or nature sounds, equipment noises, and the like. An audio signal enhanced according to the present invention exhibits an improved harmonic quality compared to that of the input electronic audio signal. Such a harmonic enhancement cat be obtained using a circuit like one of those disclosed in U.S. Pat. No. 5,361,306 and U.S. Pat. application Ser. No. 08/472,876, having a filing date of Jun. 7, 1995 and entitled APPARATUS AND METHOD OF ENHANCING ELECTRONIC AUDIO SIGNALS. 
     The present invention is predicated, at least in part, upon the discovery that the harmonic enhancement obtained using the invention described in the U.S. Pat. No. 5,361,306 and in the U.S. Pat. application Ser. No. 08/472,876 can be obtained without using a magnetic coil audio energy transfer system (i.e., a field inducing coil weakly coupled electromagnetically to an electromagnetic field receptor). The present teachings and disclosure reveal that there are a variety of other ways of obtaining the same or a similar harmonic enhancement in an electronic audio signal. Having been provided with the teachings and the exemplary circuits disclosed herein, it will be a matter of simple trial and error experimentation, if any, for one of ordinary skill in the art to design additional ways to produce the same or a similar enhancing effect. Accordingly, the general and specific circuits disclosed herein are examples only and the present invention is not intended to be so limited. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of one embodiment of the present invention; 
     FIG. 2 is a circuit diagram of another embodiment of the present invention; 
     FIGS. 3A,  3 B, and  3 C are circuit diagrams of the left channel circuit, right channel circuit and power supply circuit of a stereo embodiment of the present invention respectively; and 
     FIG. 4 is a graphical representation of the frequency response curve for the circuit of FIG. 3A and 3B, according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the present invention is herein described in terms of specific embodiments, it will be readily apparent to those skilled in this art that various modifications, re-arrangements, and substitutions can be made without departing from the spirit of the invention. The scope of the present invention is thus only limited by the claims appended hereto. 
     Each of the particular exemplary embodiments disclosed in the present application produce generally the same type of enhancement in an electronic audio signal. An apparatus, according to the principles of the present invention, comprises a circuit  8  capable of distorting an input audio signal transmitted therethrough by non-linearly (i.e., non-uniformly) amplifying enhancing harmonics or frequencies in the input audio signal. The present circuit  8  is operatively adapted to accomplish this enhancement without having to transmit the electronic audio signal through a magnetic coil audio energy transfer system (i.e., a field inducing coil and an electromagnetic field receptor that are weakly coupled electromagnetically), such as that disclosed in U.S. Pat. No. 5,361,306 and U.S. Pat. application Ser. No. 08/472,876, having a filing date of Jun. 7, 1995 and entitled APPARATUS AND METHOD OF ENHANCING ELECTRONIC AUDIO SIGNALS, both of which are incorporated herein by reference in their entirety. By increasing the amplitude of enhancing harmonics in this manner, the resulting enhanced audio signal exhibits an improved harmonic quality compared to that of the input audio signal. 
     EXEMPLARY EMBODIMENT NO. 1 
     Referring to FIG. 1, an exemplary embodiment  10  of the present circuit  8  can be separated into an input stage, a primary shaping stage, and an output stage. The input stage includes a coupling capacitor  12  that connects the balance of the circuit  10  to a source of electronic audio signals (not shown) through an input  28 . The capacitor  12  provides decoupling to remove any DC component that may be present in an input audio signal received from the signal source. A resistor  13  provides a bias path for the input of an operational amplifier  30 . A capacitor  14  is used for power supply filtering. The capacitor  14  is not connected to the input of the amplifier  30 . Two resistors  11  and  22  form a divider network to generate a reference voltage at one half of a supply voltage V S . For example, the divider network can divide a supply voltage V S  of 12 volts into a reference voltage of 6 volts. The resistor  13  biases the reference voltage before it reaches the input of the amplifier  30 . A capacitor  24  provides AC filtering for the supply voltage. The input amplifier  30  is a buffer amplifier exhibiting a voltage gain of one. The input buffer amplifier  30  isolates the primary shaping stage from the signal source. 
     The primary shaping stage includes a capacitor  16  and a resistor  15  which, it is believed, form a high frequency enhancement network that causes a non-linear accentuation of a band of high frequencies in the input signal. A resistor  19  and a capacitor  18  are believed to function as a low pass filter which rounds off the high frequency accentuation. In other words, the network comprising the resistor  19  and the capacitor  18  is an attenuating circuit for limiting the highest amplitude of the accentuated high frequencies. The resistor  19  is an attenuating resistor and tends to reduce the output signal by reducing the current that flows through the capacitor  16 . A resistor  17  sums the output of the buffer amplifier  30  with the output of the loop containing the capacitor  16 . That is, at the node  26  to the right of the resistor  17 , the original input signal coming from the amplifier  30  is summed with the high frequency enhanced signal supplied through the loop containing the capacitor  16 . The resistor  17  can be used to control the intensity of the high frequency enhancement (i.e., how pronounced it is) in the signal. The high frequency enhancement becomes more pronounced as the resistance of the resistor  17  increases. 
     The output stage includes an output  31  and an output amplifier  32  which increases the voltage gain and buffers the summed signal from node  26 . It is desirable for the gain to be increased so that the final output signal has perceptibly about the same volume level as that exhibited by the original input signal. The amplifiers  30  and  32  can be separate components, but it is desirable for the amplifiers  30  and  32  to be in the form of one dual operational amplifier, as shown. Both amplifiers  30  and  32  are connected to the same supply voltage V S  through the same pin or terminal  33  and both are connected to the same ground through the same pin or terminal  35 . 
     The output stage also includes a low frequency enhancement network which includes a resistor  25  in series with a resistor  27  in parallel with a capacitor  20 . This low frequency enhancement network is believed to cause a non-linear accentuation of a band of the low frequencies in the signal. Resistors  25  and  27  and capacitor  20  boost the level (i.e., increase the amplitude) of the low frequencies relative to the high frequencies. In particular, resistors  25  and  27  and capacitor  20  function as a low pass filter-like network that allows low end frequencies of the signal to be selectively reamplified through amplifier  32  to a greater amplification. In other words, the output stage is designed to non-uniformly amplify a low frequency band or spectrum within the overall bandwidth of the signal. Thus, while the primary shaping stage is believed to selectively enhance (i.e., non-uniformly increase the amplitude of) a band of high frequencies in the signal, the output stage is believed to function, in part, as a secondary shaping stage to selectively enhance (i.e., non-uniformly increase the amplitude of) a band of low frequencies in the signal. 
     It is desirable for the volume of the input signal and the enhanced signal to be perceived as being generally the same. In order to obtain this perceived uniformity in volume, the output stage can be adapted so as to sufficiently amplify the harmonically enhanced signal. The overall gain provided by the output buffer amplifier  32  is determined, and can be controlled, by the values of a variable resistor or potentiometer  21  and a fixed resistor  23 , as well as the fixed resistors  25  and  27 . The variable resistor  21  and the fixed resistor  23  are used to uniformly adjust the amplitude of all the frequencies in the signal. Pot  21  is connected as a rheostat and together with resistors  23 ,  25  and  27  determine the DC and low frequency gain of amplifier  32 . The adjustability of resistor  21  can be used to compensate for component variations which affect the overall gain of the signal through the circuit  10 . In this way, the perceived amplitude of the output signal can be maintained generally the same as that of the input signal. Once a desired resistance for the potentiometer  21  is determined, the pot  21  and the fixed resistor  23  can be replaced with a single fixed value resistor. Thus, it is believed that the output stage uniformly amplifies all the frequencies of the high frequency enhanced signal at the same time it is non-uniformly favoring or amplifying the lower frequencies of the signal. 
     EXEMPLARY EMBODIMENT NO. 2 
     Referring to FIG. 2, an alternative embodiment  40  of the present circuit  8  includes many components which are substantially identical or at least similar to those found in the circuit  10  of FIG. 1, Therefore, such components are indicated by the same reference numerals and are not explained repeatedly herein. The circuit  40  is a modified version of the circuit  10  in that an RL high pass filter-like network made up of a resistor  15   a  and an inductor or coil  34  is substituted for the RC high pass filter-like network of the circuit  10  made up of the capacitor  16  and the resistor  15 . In the circuit  40 , the resistor  15   a  is located at the former site of the capacitor  16 , and the inductor  34  is located at the site previously occupied by the resistor  15 . The remaining components forming the circuit  40  are the same type as those used in the circuit  10  and are, therefore, indicated with the same reference numerals. 
     It is believed that the resistor  15   a  can have the same or a different value than that of the resistor  15 , depending on the value (i.e., the configuration) of the inductor  34  and the enhancement desired. When the resistors  15  and  15   a  have about the same value, it is believed that the circuits  10  and  40  can be made to produce the same enhanced output signals by modifying the inductor  34  (e.g., changing the number of turns, size wire, etc.). It may be necessary to vary the values of one or more of the various resistors and capacitors in the circuit  40  in order to obtain the same enhanced output signal as that obtained using the circuit  10 . The RC network of circuit  10  is believed to be more desirable due to the typically lower cost and size of the capacitor  16  versus the inductor  34 . 
     EXEMPLARY EMBODIMENT NO. 3 
     Referring to FIGS. 3A,  3 B, and  3 C, respectively, a further embodiment  48  of the present circuit  8  includes a left channel circuit  42 , a right channel circuit  44  and a power supply circuit  46 . Satisfactory results have been obtained when this dual-channel circuit  48  is made using the particular components and values listed in the Table below, according to their corresponding reference numerals. The circuit  48  of FIGS. 3A-C is basically a dual channel version of the circuit  10  of FIG.  1 . Therefore, in analyzing circuit  48 , reference should be made to the above discussion regarding FIG.  1 . Differences between the channel circuits  42  and  44  and the circuit  10  of FIG. 1 are described below. 
     The supply voltage V S  of the circuit  10  of FIG. 1 is halved by the divider network of resistors  11  and  22 , and the circuit  10  is connected to ground through the resistor  15  and the capacitor  18 . With the circuit  48 , in contrast, each of the channel circuits  42  and  44  is not connected to D.C. ground through its corresponding resistor  70  and capacitor  66 . In addition, the power supply circuit  46  (see FIG. 3C) supplies the amplifiers  50  and  52  of each channel circuit  42  and  44  with a reference voltage V R  of +6 volts and a supply voltage V S  of +12 VDC. This 12 volt supply voltage V S  is regulated by the voltage regulator  62  from a power supply of 14 to 24 volts connected to input  55 . The power supply circuit  46  has left channel network, comprising the resistors  83  and  84  and capacitor  81 , and a right channel network, comprising the resistors  85  and  86  and capacitor  82 , which provide the reference voltage V R  to the left channel circuit  42  and the right channel circuit  44 , respectively. These left and right channel reference voltage networks are identical. The power supply circuit  46  also includes an LED light which turns on and off with the actuation of the switch  54 , to indicate whether or not the input signal is being transmitted through the channel circuits  42  and  44 . 
     As with the coupling capacitor  12  of the circuit  10 , a coupling capacitor  63  connects the balance of each channel circuit  42  and  44  to a source of electronic audio signals (not shown) and provides decoupling to remove any DC component that may be present in an input audio signal received from the signal source. Unlike the circuit  10 , each channel circuit  42  and  44  also includes a coupling capacitor  68  and resistor  77  connected in line before a triple pole, double throw Bypass/Effect switch  54 . The capacitor  68  removes the DC component present at the output of the amplifier  52  and the resistor  77  provides a discharge path for any D.C. component coming from the output  58 . In this way, the capacitor  68  and the resistor  77  can eliminate an undesirable clicking noise that may be heard each time the corresponding switch  54  is actuated. Each pair of the amplifiers  50  and  52 , used in the particular circuit  48  disclosed in the Table, is in the form of a single dual operational amplifier exhibiting linear dynamic characteristics. 
     As similarly discussed above with regard to the circuit  10 , the variable 1 Kohm resistor  73  and the fixed 1%, 1.80 Kohm resistor  74  of the circuit  48  can be replaced with a 1%, 2.74 Kohm resistor. Furthermore, for automobile applications, it may be desirable for the voltage regulator  62  to be eliminated and the power input  55  connected directly to a 12 volt battery of the automobile. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Components for the Circuit of FIGS. 3A-C 
               
             
          
           
               
                 Component 
                 Description 
                 Value/ Part No. 
               
               
                   
               
               
                 50/52 
                 Motorola Dual Op Amp 
                 LM 1458 N 
               
               
                 54 
                 3PDT Bypass/Effect 
                 — 
               
               
                   
                 Switch 
                   
               
               
                 55 
                 Power Supply Input 
                 — 
               
               
                 56 
                 RCA Phono Input Jack 
                 — 
               
               
                 58 
                 RCA Phono Output Jack 
                 — 
               
               
                 60 
                 On/Off LED Light 
                 — 
               
               
                 62 
                 National Semiconductor 
                 LM 7812 or 
               
               
                   
                 Voltage Regulator 
                 LM 340T-12 
               
               
                 63 
                 5% MYLAR, CAPN 
                 .47 uF, 50 V 
               
               
                 64 
                 5% MYLAR, CAPN 
                 .01 uF, 50 V 
               
               
                 65 
                 5% MYLAR, CAPN 
                 .047 uF, 50 V 
               
               
                 66 
                 5% MYLAR, CAPN 
                 .033 uF, 50 V 
               
               
                 67 
                 5% MYLAR, CAPN 
                 .015 uF, 50 V 
               
               
                 68 
                 POLCAP, Radial Alum. 
                 1 uF, 50 V 
               
               
                 69 
                 1% Resistor 
                 100 KOHM 
               
               
                 70 
                 1% Resistor 
                 249 OHM 
               
               
                 71 
                 1% Resistor 
                 20 KOHM 
               
               
                 72 
                 1% Resistor 
                 1.1 KOHM 
               
               
                 73 
                 Variable Resistor 
                 1 KOHM 
               
               
                 74 
                 1% Resistor 
                 1.8 KOHM 
               
               
                 75 
                 1% Resistor 
                 24.9 KOHM 
               
               
                 76 
                 1% Resistor 
                 49.9 KOHM 
               
               
                 77 
                 1% Resistor 
                 100 KOHM 
               
               
                 79, 81, 82 
                 POLCAP, Radial Alum. 
                 100 uF, 50 V 
               
               
                 80 
                 POLCAP, Radial Alum. 
                 1 uF, 50 V 
               
               
                 83-86 
                 1% Resistor 
                 1 KOHM 
               
               
                 87 
                 1% Resistor 
                 2 KOHM 
               
               
                   
               
             
          
         
       
     
     An electronic audio signal can have a bandwidth of frequencies, with a low end and a high end. It has been found that when an electronic audio signal in the form of a standard symmetrical square wave, comprising a plurality of frequencies with a high end and a low end, is transmitted through the exemplary embodiments of FIGS.  1  and  3 A-C, the resulting output audio signal is non-linear. This is evident from FIG. 4, which illustrates an exemplary frequency response curve C generated by each of channel circuit  42  and channel circuit  44 . The frequency response curve was obtained by connecting input jack  56  to a signal source S, a conventional signal generator, which generated a sine wave input signal which was swept through a band of frequencies from about 20 Hz to about 22 KHz and connecting the output jack  58  to a resistive load of 50 KOhms, which load represented an equivalent impedance that either channel circuit  42  or channel circuit  44  might see when connected to the input of a conventional audio amplifier. From that curve, it is clear that the resulting output signal includes frequency components which increase in amplitude as per increasing frequencies from a desired reference frequency toward the high end of the signal to form a high frequency peak. This resulting output signal also includes frequency components which increase in amplitude as per decreasing frequencies from the reference frequency toward its low end to form a low frequency peak. The amplitude of the frequency components can begin to decay after the high frequency peak and/or after the low frequency peak, depending on the width of the frequency band of the input signal. There are up to only two amplitude peaks, between the low end and the high end of the input signal bandwidth, see the frequency response curve shown in FIG.  4 . The increasing amplitude as per increasing frequencies and increasing amplitude as per decreasing frequencies each occur over at least a portion or band of the overall frequency bandwidth of the signal. The low end of the bandwidth can be at one end of the range of normal human hearing and the high end can be at the other end of the range of normal human hearing. It is believed that the exemplary embodiment of FIG. 2 can produce the same type of enhancing effect as that produced by the exemplary embodiment of FIG.  1 . 
     With regard to the output signal produced when the standard symmetrical square wave is transmitted through the exemplary embodiment of FIG. 1 or of FIGS. 3A-C, respectively, it has been found that changing the value of the resistor 15 or 70 can shift the high frequency peak from one frequency to another; changing the value of the capacitor  16  or  65  can change the sharpness (i.e., the slope) of the increase in amplitude of the frequencies toward the high end; changing the value of the resistor 19 or 72 can change the height (i.e., amplitude) of the high frequency peak; and changing the value of the capacitor 18 or 66 can change the downward slope of decay, toward the high end, after the high frequency peak. In addition, it has been found that changing the value of the resistor 27 or 76 can change the height of the low frequency peak; changing the value of the capacitor 20 or 67 can change the slope of the increase in amplitude of the frequencies, toward the low frequency peak; and changing the value of the resistor 25 or 75 can shift the low frequency peak from one frequency to another. 
     As used herein, the term reference frequency refers to the frequency or frequency range after which the non-linear amplification (i.e., the enhancement) of the high frequencies and/or of the low frequencies can begin to be seen. The reference frequency can be changed by changing one or more of the components or the component parameters of the particular circuit  8 . For example, with regard to the circuit  10  of FIG. 1, the reference frequency can be changed by varying the values of one or more of the components 15, 16, 18-20, 25 and 27. Changing the reference frequency may affect the perceived desirability of the resulting enhancement. For the particular circuit  48  disclosed in detail in FIGS. 3A-C and the Table, the selected reference frequency is a range from about 400 Hz to about 1 KHz. The amplitude of this reference frequency is substantially similar to that of the fundamental frequency of the square wave input signal (i.e., about 1 volt). It is believed desirable for the circuit  48  to have a reference frequency that is closer to about 1 KHz. It may even be more desirable for the reference frequency to be up to about 3.5 KHz (e.g., a range of about 3.2 to about 3.8 KHz). 
     It is understood that it may also be desirable for a circuit according to the principles of the present invention to produce an output signal, from a square wave input audio signal, that is non-linear with frequencies which either: (1) increase in amplitude as per increasing frequencies from a desired reference frequency toward the high end of the signal, or (2) increase in amplitude as per decreasing frequencies from the reference frequency toward its low end; as well as one which does both. In other words, it may be desirable for the present invention to be used to enhance only a band of high frequencies of an audio signal or only a band of low frequencies of an audio signal, rather than to enhance both a band of the highs and a band of the lows of the signal. 
     An enhanced audio signal, according to the present invention, exhibits an improved harmonic quality compared to that of the original input audio signal. Additional advantages and modifications will readily appear to those skilled in the art. For instance, it may be desirable for two or more of the above described circuits to be used in series. In addition, using well known techniques and the teachings of the present invention, it is understood that a circuit according to the present invention can be reproduced into an integrated circuit (I.C.) in the form of a hybrid I.C., a monolithic chip, etc. Such an I.C. would be particularly useful in applications where size is important such as, for example, in hearing aids, cellular and other portable telephones, etc. In light of the present teachings, it is also understood that the operation of a circuit according to the present invention (i.e., the present enhancing effect) can be readily reproduced using well known digital signal processing techniques. 
     Furthermore, when electronic audio signals from a compact disc of music and vocals was transmitted through a circuit of the present invention and the resulting enhanced electronic audio signal re-recorded onto a cassette tape using a consumer cassette player/recorder, the sound quality of the music and vocals produced from the recorded cassette tape was perceptibly better than the same music and vocals produced directly from the compact disc. This occurred even though the compact disc format is widely recognized as producing superior sound quality compared to the cassette tape format. 
     It is believed that the present invention can be used to enhance electronic audio signals from sound converting equipment, for example a hearing aid, a microphone or the like, before being either recorded onto a recording medium (for example, magnetic tape or optical disk) or converted directly into acoustic sound or other sound impulses. It is also believed that an audio signal enhanced according to the present invention can be transmitted through the air or some other medium, for example, for television, radio, sonar, computer or cellular telephone use; can be transmitted through transmission lines, for example, for telephone, cable TV or computer use; can be converted directly into audible sound, for example, for use at a concert, a play, in a restaurant, or a bar; and that it can be used in any other application which includes an audio signal such as, for example, in distinguishing sonar images, etc. 
     The present invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described herein. Departures may be made from such details without departing from the spirit or scope of the general inventive concept of the present invention. Therefore, the scope of the invention should be limited only by the following claims and equivalents thereof.