Signal distortion circuit and method of use
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of patent application Ser. No. 667,115, filed Mar. 15, 1976 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to electronically processed sound. More particularly, this invention relates to a method and apparatus for enhancing the quality, clarity, presence, intelligibility and other characteristics of electronically processed sound. The invention is especially useful with, but not exclusively limited to, sound recording equipment, sound reproduction equipment, sound reinforcement equipment, sound analysis equipment and sound transmission equipment. 
     In U.S. Pat. No. 2,866,849 there is disclosed an apparatus for improving sounds of music and speech in which an audio signal is split into two signal paths and the signal in one path is passed successively through an amplifier, a 3KC high pass filter, a harmonic producer, a 6KC high pass filter and an amplifier and is then combined, either electrically or acoustically, with the signal in the other path. 
     In U.S. Pat. No. 2,852,604, there is disclosed a sound reproduction apparatus in which a monaural electrical signal is divided into two paths and the signal in one path is passed through a time delay mechanism. The two signals are then converted to sound through separate speakers. 
     In U.S. Pat. No. 3,219,757, there is disclosed a sound reproduction apparatus in which a monaural electrical signal is divided into two paths and the signal in one path is passed through a non-linear frequency dependent phase delay circuit. The two signals are then mixed in two different ways. One of the mixed signals is converted to sound through one speaker and the other mixed signal is converted to sound through another speaker. 
     In U.S. Pat. No. 3,560,656 there is disclosed a sound reproduction apparatus in which a monaural electrical signal is divided into two signal paths and the signal in one path is passed through a phase changing circuit. The two signals are then converted to sound through separate spaced apart speakers, with the sound emerging from both speakers having the same amplitude. 
     Other examples of electronic sound processing apparatus pertienent in one way or another to this invention may be found in U.S. Pat. Nos. 2,866,849; 2,953,644; 3,127,476; 3,591,699; 3,723,633; 3,745,254; 3,819,861; 3,828,133; and 3,890,466. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a method and apparatus for improving the quality and other characteristics of electronically processed sound. 
     It is another object of this invention to provide an improved method and apparatus for processing sound electronically. 
     It is still another object of this invention to provide a new and improved electronic circuit which can be used in a most beneficial manner with electronic signal processing equipment. 
     It is yet still another object of this invention to provide an electronic signal processing apparatus and method that can be used in conjunction with sound recording equipment, sound analysis equipment and sound transmission equipment. 
     It is another object of this invention to provide electronic signal processing apparatus that is relatively inexpensive to fabricate and relatively easy to use. 
     it is still another object of the invention to provide a new and improved circuit for generating harmonics of an electrical equivalent of an audio signal. 
     In its broadest aspects, this invention involves generating a low amplitude signal containing low order, odd and even, phase shifted, frequency dependent and amplitude dependent harmonics of the signal being processed and then adding the signal so generated to the signal being processed. 
     According to this invention, a monaural electrical signals is divided into two separate paths. The signal in one path is passed through an exciter circuit and an attenuator, and is then combined with the signal travelling along the other path. In the exciter circuit the signal is passed first through a high pass filter and then through a harmonic creator. The harmonic creator is designed to create low order harmonics of frequency present that are above a preselected threshold. The combining of the signals travelling along the two paths is achieved either electronically or acoustically. The amount by which the excited signal is attenuated is dependent on the type of the audio information being processed and the ultimate use of the electronically processed information. In most cases where the sound being processed is music or speech, the excited signal is attenuated to about between 20% and 70% of the amplitude of the unaltered signal. It has been found that sound that is electronically processed according to this invention has improved quality, clarity, intelligibility, presence, depth and reality. 
     One of the principal features of the invention is that the excited signal and the unaltered signal can be combined electrically and then fed into a single output transducer or can be combined acoustically by being fed into separate output transducers. 
     The foregoing and other objects and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawings which form a part thereof, and in which is shown by way of illustration, specific embodiments for practicing the invention. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be more fully understood, it will now be described by way of examples with reference to the drawings wherein like reference numerals represent like parts and wherein: 
     FIG. 1 is a block diagram of one embodiment of the invention; 
     FIG. 2 is a block diagram of another embodiment of the invention; 
     FIG. 3 is a circuit diagram of a vacuum tube version of the exciter circuit shown in FIGS. 1 and 2; 
     FIG. 4 is a graph illustrating the frequency dependent phase shifting characteristics of the exciter circuit in FIG. 3. 
     FIG. 5 is a block diagram of a two channel apparatus incorporating the invention; 
     FIG. 6 is a circuit diagram of a solid state version of the exciter circuit shown in FIGS. 1 and 2; 
     FIG. 7 is a plot of the transfer function of the harmonic creator in FIGS. 6; and 
     FIG. 8 is a plot of the gain and phase of the signal emerging from the high pass filter in FIG. 6. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, there is shown a source 11 of monaural electrical signals. Source 11 may be a microphone, a radio tuner and amplifier combination, an electrical pickup from a record player or a magnetic sound head associated with a magnetic tape or any other device whose output is a monaural electrical signal. The output from source 11 is passed through an amplifier 13 and is then divided into two discrete signal paths 15 and 17. The signal travelling along path 17 is passed in succession through an exciter circuit 19, an amplitude attenuator 21 and is then combined in a mixer 20 with the signal travelling along path 15. The combined signal is passed through an amplifier 23 and then fed into an output transducer 25, such as a loudspeaker or a magnetic tape or an engraved disc or a paper print-out. The exciter circuit 19 includes high-pass filter means and harmonic creator means. The harmonic creator means is designed to create low order harmonics above a preselected threshold of frequencies passed by the high pass filter means. 
     In the FIG. 2 embodiment, the signal from the amplitude attenuator 21 is passed through an amplifier 27 to an output transducer 29 instead of being passed through the amplifier 23 and is then combined with the signal travelling along path 15 as in the FIG. 1 embodiment. 
     In FIG. 3 there is shown a schematic diagram of a vacuum tube version of the exciter circuit 19. The circuit 19 is made up basically of six triodes, a filter circuit, a signal splitting network, two bridge circuits and various auxiliary components. 
     A monaural electrical signal is applied to the exciter circuit 19 through input terminal 51. The signal emerging from the input terminal 51 is passed through a coupling capacitor 53 to a signal splitting network 55 which is made up of a balance potentiometer 57 and two fixed resistors 59 and 61. 
     The portion of the signal that is passed through fixed resistor 61 is split up into two separate paths 63 and 65. The signal travelling along path 63 is passed through a high pass filter 67 to the grid of a triode 73. The filter 67 is made up of a diode 69, a resistor 71 and two capacitors 73 and 75. The purpose of the filter 67 is to cut off certain base frequencies from the signal conducted to the grid of triode 72. The plate of triode 72 is connected to the grid of triode 73 through a coupling capacitor 75 and the plate of triode 73 is connected to the output terminal 76 of the phase shifting circuit 19 through a capacitor 77. Each one of the triodes 72 and 73 includes the standard associated components. Thus, triode 72 includes a grid leak resistor 79, a cathode resistor 81 and a ground 83. Similarly, triode 73 includes a grid leak resistor 85, a cathode resistor 87 and a ground 89. Thus, the signal travelling along path 63 is passed through filter 67 to the grid of triode 73 and the output of triode 73 is applied to the output terminal 75. 
     The portion of the signal travelling along path 65 is passed through a filter circuit 91 to the grid of a triode 93. The plate of triode 93 is connected to the grid of a triode 95. 
     The portion of the signal passed through fixed resistor 59 is applied to the grid of a triode 97. The output of triode 97 is applied through a coupling capacitor 99 to the input grid of triode 72. Because the signal applied to the grid of triode 72 has passed through triode 97 it has gone through a 180° phase reversal. Thus, the two signals applied to the grid of tube 72 are completely out of phase with each other. 
     Connected to the grid of triode 97 is an inductive circuit 101 whose purpose is to establish the phase slope. The inductive circuit 101 includes a pair of identical variable inductors 103 and 105, a resistor 107, a diode 109, a resistor 111, a capacitor 113 and a ground 115 and is connected to the grid of triode 117. 
     The supply voltage for the circuit is conducted through terminal 119 to a filter circuit 121 which filters out any ripple content present in the supply voltage and is then split up into three paths. One path 123 supplies the plate voltage to triode 97 and triode 117 through plate resistors 125 and 127. Another path 129 supplies voltage to bridges 131 and 133. Bridge 131 is made up of resistors 135, 137, 139, 141 and diode 143 and bridge 133 is made up of resistors 145, 147, 149, 151 and diode 153. The output of bridges 105 and 107 at junction 155 is applied to the grid of the tube 117. Junction of bridge 131 is connected to the plate of tube 72. Junction 161 of bridge 133 is connected to the plate of tube 73. 
     The exciter circuit 19 further includes cathode grounding networks 163 and 165. 
     Representative values and other information relating to the component parts of phase shifting circuit 19 as designed for application with sounds of speech and music are as follows: 
     
         ______________________________________                                    
All triodes             6K6112Wa                                          
Inductors 103, 105      KIP                                               
Resistor 57             20K                                               
All diodes              1N3484                                            
Capacitors (electrolytic)                                                 
201, 202, 203           50mF/20V                                          
77                      10mF/6V                                           
113                     1mF/3V                                            
Capacitors (non-polarized)                                                
25, 53, 211             0.1mF                                             
213                     0.05mF                                            
214                     0.02mF                                            
215, 217                0.01mF                                            
73, 75, 76, 99          0.001mF                                           
Resistors                                                                 
221, 222                15K                                               
127. 223, 224, 225, 228, 231, 233                                         
                        220 ohms                                          
135, 137, 139, 141, 145, 147, 149, 151                                    
                        100K                                              
150                     440K                                              
71, 171                 10K                                               
79, 179, 85, 185        1M                                                
87, 187, 81             1.5K                                              
191, 111                4.5K                                              
107, 108                51K                                               
241                     8.2K                                              
227, 125                220K                                              
61                      220K                                              
59                      4.7K                                              
______________________________________                                    
 
    
     An audio plot depicting the frequency dependent phase shift and gain of the exciter circuit 19 of FIG. 3 for the values noted above is shown in FIG. 4. As can be seen the circuit 19 produces a linear frequency dependent phase shift of about 360° over a frequency range from about 100 HZ to about 22 KHZ with a gain of between ±5DB over a frequency range of about 200 HZ to at least 22 KHZ. The point of zero phase shift is about 2KHZ. Although it cannot be said with absolute certainty which specific elements in exciter circuit 19 perform which function, a comparison actually performed of the input and output waveforms of a circuit 19 actually constructed and tested has shown that the circuit 19 passes frequencies above a certain level and generates low order, odd and even, phase shifted and amplitude dependent harmonics of the frequencies so passed. 
     In FIG. 5, there is shown in block diagram form an apparatus 400 according to this invention for processing two channels of monaural electrical signals. The apparatus 400 consists essentially of two processing channels, identified as channel 401 and channel 402 and a monitor section 403. The two channels 401 and 402 are essentially identical. In channel 402, a monaural electrical signal from an external source (not shown) is applied to input 420 which is an active transformer. The signal emerging from the input 420 is split into three separate parts. One part is fed into a switching circuit 422 which is connected to a buffer amplifier 423 at the output section 424. When the signal travels along this direct path, it is in the by-pass mode. Output 424 further includes a gain control 425 and an output transformer 426. A second part of the signal is fed into an input switching circuit 427 in the monitor section 403. The third part of the signal is fed into a peak limiter circuit 441 which is made up of a peak limiting control 443, a threshold control 445 and an amplifier 447. The signal emerging from the amplifier 447 is fed into an indicator circuit 451 and also into an exciter circuit 461 (which corresponds to the exciter circuit 19 of FIGS. 1 and 2). The output from the exciter circuit 461 is fed into an amplifier 463 which restores the signal to a level to compensate for losses occurring in passing through the exciter circuit 461. The output from the amplifier 463 is fed into an attenuator 465 and then into the switching circuit 422 where it feeds into either of the other two modes. In one of these modes a pure excited signal is fed into amplifier 423. In the third mode a mixed signal, that is a pure signal mixed with an attenuated excited signal is fed into amplifier 423. Thus, the signal emerging from output 424 can be selected so as to be either the original pure signal, an excited shifted signal or a mixture of the original signal and an excited signal. Channel 402 further includes a meter switch 467 and a meter amplifier 469 and an indicator 471. 
     Monitor section 403 further includes output section 473 and level section 475. 
     In channel 401 elements 520, 522, 523, 524, 525, 526, 541, 543, 545, 547, 551, 561, 563, 565, 567, 569 and 571 correspond to elements 420, 422, 423, 424, 425, 426, 441, 443, 445, 447, 451, 461, 463, 465, 467, 469 and 471 in channel 402. 
     As can be appreciated, the apparatus 400 can be used in basically any application where either one or two channels of audio information are being processed electronically. The apparatus 400 is particularly useful with sound recording equipment, sound reinforcing equipment and sound reproduction equipment. 
     In FIG. 6 there is shown a solid state version of the exciter circuit 19. The circuit comprises a filter 621, a variable gain amplifier 623 whose output is coupled to the output of the filter 621 and a harmonic creator 625 whose output is coupled to the output of the variable gain amplifier 623. 
     The filter 621 is a 2-pole slow roll off Butterworth high pass filter and is made up of two capacitors 629 and 631, a pair of resistors 633 and 635 and an operational amplifier 637. The frequencies passed by the filter 621 are fed into the variable gain amplifier 623 which allows the level to the next element, namely the harmonic creator, to be varied. The variable gain amplifier 623 includes an adjustable potentiometer 639 and an operational ampifier 640 and a pair of resistors 641 and 642. 
     The harmonic creator 625 includes a diode 643, an adjustable potentiometer 645 and an operational amplifier 647 and a resistor 648. The harmonic creator 625 utilizes the voltage to current characteristics of the diode 643 to softly clip the peaks of the incoming signal at a threshold determined by the adjustable potentiometer 645. By clipping the peaks softly only low order harmonics are created. By selecting the proper threshold only transient portions of the signal become clipped. By clipping the signal on one side only both odd and even harmonics are created. 
     Representative values and other information relating to the component parts of the exciter circuit 19 of FIG. 6 as designed for application with sounds of speech and music are as follows: 
     
         ______________________________________                                    
Element                                                                   
______________________________________                                    
621                4KHZ, 2 pole,                                          
                   high pass                                              
629                0.01μf                                              
631                0.01μf                                              
633                56K                                                    
635                27K                                                    
637                #4131                                                  
639                10K                                                    
640                1/2#4558                                               
641                10K                                                    
642                1K                                                     
643                1N914                                                  
645                10K                                                    
647                1/2#4558                                               
648                47K                                                    
______________________________________                                    
 
    
     The transfer function of the harmonic creator 625 for the values above is shown in FIG. 7 and the gain and phase of the signal emerging from the filter 621 for the values above is shown in FIG. 8. It is believed that the phase change in the signal resulting from passing the signal through the filter produces a directional or &#34;third dimensional&#34; effect in the resulting sound. 
     It will be understood that various changes in the details, materials, arrangements of parts and operating conditions which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principles and scope of the invention.