Guitar controller pickup and method for generating trigger signals for a guitar controlled synthesizer
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to my copending application Ser. No. 669,666 filed Nov. 8, 1984 now U.S. Pat. No. 4,630,520. 
     FIELD OF THE INVENTION 
     My present invention relates to a guitar controller pickup for an electronic music synthesizer and more particularly a device which can detect gate, velocity and trigger signals for control of the synthesizer. The invention also relates to a method for generating the aforementioned signals for a synthesizer guitar controller. 
     BACKGROUND OF THE INVENTION 
     When a keyboard is used to control a synthesizer, it provides three major control signals: NOTE, GATE and VELOCITY. The NOTE signal correspoonds to the key depressed and determines the pitch of the final sound. The GATE decides when the sound is initiated and stopped, which corresponds to the instant of key depression (for initiating the sound) and the instant key release (for stopping the sound). The VELOCITY is a parameter which is proportional to the force with which the key was struck. This may be interpreted by the synthesizer as the volume of the sound or can be used to control other timbral characteristics of the sound so that the dynamics are a direct function of the force of strike. 
     In a guitar synthesizer controller, preferably such as that which I have described in my copending patent application Ser. No. 669,666 filed Nov. 8, 1984 now U.S. Pat. No. 4,630,520, NOTE, GATE and VELOCITY signals must also be generated and must be derived from the normal guitar plating technique and made available for proper synthesizer operation. 
     In the case of a guitar controller NOTE information is determined by which string is depressed and the particular fret at which depression occurs. The method by which this information may be derived on the guitar controller previously disclosed is clearly outlined in that application. 
     The standard method of deriving GATE information on a guitar is to process the vibrating string through an envelope detector. However, I have discovered through experimentation, use of guitar controllers on the market and by reading published literature, that this method is inadequate for several reasons. 
     All of these earlier systems are fraught with various problems and drawbacks obviated by the present invention and some of which will be detailed below. 
     First, the speed with which an envelope detector responds to the onset of a plucked vibrating string depends on the fundamental frequency of that string, for example as described in Meno, U.S. Pat. No. 4,430,918. For instance, the Low E string on a guitar has a period of 12 milliseconds. This signal can theoretically be detected within one half cycle by an ideal envelope detector using full wave rectification. Thus, the fastest response possible for detection of the first vibratory peak would be 6 milliseconds (ms) on the low E string. This delay is detectable by the guitar player as a lag in response from the pick to the sound generated by the synthesizer. Admittedly, this minimum response time is less for higher pitched strings, however the problem becomes even more involved upon further examination. In addition, the lower frequencies of bass guitar strings makes this method totally unacceptable for bass guitar purposes. 
     The vibration characteristics of a guitar string are very complex. For instance, because of beating of nonharmonic overtones, the string vibration does not always reach its full peak of oscillation until well into its third or fourth cycle. Thus, any attempt at deriving a VELOCITY signal from a peak detector that follows and senses the peak of the envelope contour can cause a delay of 20 to 40 ms, which is totally unacceptable. 
     A further problem arises because GATE and VELOCITY information must be transmitted in immediate succession to conform with normal synthesizer protocols. Since the GATE is typically derived from the immediate rise of the envelope while the VELOCITY peak may be delayed by many milliseconds, it becomes obvious that either the GATE must be delayed to conform with the VELOCITY or the VELOCITY information must be forfeited. Thus, the standard method of deriving VELOCITY from an envelope is actually impossible. 
     Lastly, the complex shape of a typical guitar string vibration causes false peaks and valleys within one cycle. Thus, a fast envelope detector can actually be &#34;fooled&#34; into thinking that it has reached a peak of vibration when it has actually only captured a contour of the string vibration. 
     OBJECTS OF THE INVENTION 
     It is, therefore, the principal object of the present invention to provide a guitar controller pickup to derive GATE, VELOCITY, and triggering in signals for providing substantially instantaneous control signals to the synthesizer to obviate the disadvantages of the earlier systems described. 
     Another object is to provide an improved method of generating GATE, VELOCITY and trigger signals for a synthesizer guitar controller. 
     SUMMARY OF THE INVENTION 
     These objects and others will become apparent hereinafter are attained, in accordance with the present invention by measuring not the envelope of a vibration as described earlier but by measurement of the bend or deviation of a plucked strummed string. 
     According to the invention the GATE and VELOCITY sensor uses the measurement of the off-axial deviation of the guitar string, in a synthesizer guitar controller as described in my aforementioned U.S. patent application, from its at-rest, non-vibrating position at the bridge. A pickup with a DC response is necessary, so that standard magnetic pickup which rely upon the oscillation of the string in order to generate a signal are unsatisfactory. 
     When the string is plucked, the degree by which it is moved off axis is directly proportional to the amount of energy imparted to its pick. When the string is released after a particular deviation, the resulting amplitude of vibration will directly correlate to the degree of off-axis movement prior to the pluck. 
     By using a DC sensor, such as an optical or Hall effect sensor, to detect when and by how much the string is moved off axis and released, it is possible to measure how far the string deviates from its rest position. This value is used as the VELOCITY signal. The flyback from the peak deflection initiates the GATE signal to turn on the sound. When the string stops vibrating, the GATE, and thus the sound, is turned off. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of the gate and velocity sensor; 
     FIG. 2 is a circuit diagram of the audio summing amps; 
     FIG. 3 is a diagrammatic section illustrating the biphase optical pickup; 
     FIGS. 4a-4e are diagrams illustrating the principles of the biphase optical pickup; 
     FIGS. 5A-5H are timing diagrams illustrating the various signals of the gate and velocity sensor; 
     FIG. 6 is a flow chart illustrating the logic for deriving gate qualifying retriggering, velocity and string bending; 
     FIG. 7 is a diagram illustrating the application of the invention; 
     FIGS. 8a-8d are diagrammatic section illustrating the principles of the monophonic optical pickup; 
     FIG. 9 is a block diagram of an alternate embodiment of gate sensor; 
     FIGS. 10a-10e are timing diagrams illustrating the various signals of an alternate embodiment of the gate sensor; 
     FIG. 11 is a block diagram of an alternate embodiment of the velocity sensor; and 
     FIGS. 12a-12d are timing diagrams illustrating the various signals of an alternate embodiment of the velocity sensor. 
    
    
     SPECIFIC DESCRIPTION 
     The GATE and VELOCITY signals generated by the technique described are directly applicable to the guitar controller described in the aforementioned application which is hereby incorporated in its entirety by reference. The controller will use the synthesizer of that application and the note selection means of that guitar controller. 
     FIG. 1 illustrates a preferred embodiment of the string vibration sensor. DC detaching means sense the vibration of the string 2701. Preferably two staggered reflective optical sensors 2702, 2703, (FIG. 3) from the DC detecting means and are placed near the bridge 2903 such that the string 2701 rests on the edge of the sensitive fields of each detector 2702 and 2703. This configuration allows the maximum sensitivity for sensing both AC-generating vibration and DC-signal generating movement off axis. Under the string is an infrared emitter 2704. 
     FIGS. 4a-4e illustrate the principles of the optical pickup. The string 2701 has a diameter d and is located a distance D from the optical emitter 2704. The string will cast a shadow of angle φ where: 
     
         tanφ=d/D 
    
     Typically, guitar strings range from 0.009 inches in diameter to 0.056 inches in diameter. The strings will typically be located a distance of 0.1 inches from the sensor so that angle φ ranges from: 
     
         minφ=arctan 0.009/0.1=5° 
    
     
         maxφ=arctan 0.056/0.1=29°. 
    
     The placement of the sensors 2702, 2703 makes them sensitive to both vertical and horizontal string movement. As the string is moved in one direction (FIG. 4b) the output of optical sensor 2702 is at its maximum and the output of optical sensor 2703 is at its maximum. 
     When the string is in the at-rest position both sensors 2702 and 2703, have equal outputs between their minimum and maximum outputs. Conversely, as the string is deflected in the other direction the output of optical sensor 2703 is at its minimum and the output of optical sensor 2702 is at its maximum. 
     FIG. 4e illustrates the magnitude of sensor 2702 and sensor 2703 a function of deflected distance, referred to as sensors 1 and 2, respectively. 
     As the string vibrates on the horizontal axis, the sensors 2702, 2703 generate out of phase signals, each peaking as the string approaches the optical axis on either side. In addition to sensing this horizontal movement, the DC output of the sensors will also vary in accordance with string movement in the vertical direction. Horizontal movement corresponds to string plucking, bending and vibration, while vertical movement corresponds to the string being fretted and moving closer to the fret board as it is pressed. 
     Since the vertical movement only occurs when the string is fretted, the magnitude of this signal depends upon which fret is pressed. The magnitude increases for frets nearer the bridge. Because it is a DC signal, FIG. 5B, this offset may inadvertently be interpreted as a pluck if it exceeds the processing threshold for normal plucking and so it must be minimized. 
     Fortunately, the outputs of the optical sensors are out of phase, so that summing their outputs in a summing means, preferably a differential amplifier 2705, cancels any common mode signals (vertical motion) while amplying differential signals (horizontal motion). Thus by summing the outputs of the optical sensors, the unwanted vertical movement is eliminated while the useful horizontal movement is enhanced. 
     It is necessary to turn on and turn off a gate signal in response to the string pluck. A gate is turned on when the string begins vibrating (FIG. 5A) after it flies back from being picked and turned off when the amplitude of vibration decreases to the point at which it is no longer detectable. 
     In examining the summed sensor outputs, the sub-audio DC component of the signal corresponds to the off-axis string movement by plucking, while the AC signal corresponds to the string vibration. In order to extract the DC signal, the outputs of the sensors 2702 and 2703, are summed in a summing means, forming a guitar signal, preferably a differential amplifier 2705, whose output is passed through a lowpass filtering means, preferably a sharp low pass filter (2706), whose cutoff is below the fundamental frequency of vibration for the string being sensed. The DC signal now corresponds to the position of the string in the horizontal plane, FIG. 5B. Because of the large phase shifts in the sharp low pass filters, this signal will be lagged by several milliseconds. Since the cutoff frequencies are on the order of 40 Hz, this lag may be on the order of 10 to 20 milliseconds. 
     To overcome this, the signal is further processed by a differentiating means, preferably a differentiator 2702, whose output corresponds to changes in the slope of the DC signal, FIG. 5C. Thus, when the DC signal is rising, the differentiator will generate a negative peak, while falling slopes will generate a positive peak. These positive and negative peaks are used to generate trigger signals, that are then used to qualify string movements as valid gates. 
     Using the differentiated DC signals, we can derive trigger pulses by passing the differentiated signal through a comparing means, preferably comparators 2708 and 2709, comparing the signal to either a trigger up reference voltage 2710 or a trigger down reference voltage 2711, FIG. 5D and 5E, that correspond to string movement in either the &#34;up&#34; or &#34;down&#34; direction, called TRIG UP and TRIG DOWN. Thus, it is now possible to use the information of &#34;which direction was the string picked&#34; to generate another control parameter for the synthesizer. 
     The availability of a trigger for either direction means that two synthesizer sounds can be generated for each string, depending upon which direction it is plucked. For instance, a down pluck may trigger a trumpet sound while an up pluck will trigger a violin sound. This is virtualy impossible with anything but a DC-based pick detection system and is a substantial enhancement to the many virtues of the guitar synthesizer controller. 
     These trigger pulses must be distinguished as having been caused by a pick rather the than string motion due to bending the string off of its resting position by the fretting hand. To do this, the vibration or AC information is used to qualify the trigger pulses. 
     Whenever a string is picked, a burst of vibration occurs because of the energy that is imparted by the pick. This eventually decays and forms the typical plucked guitar timbre. This initial burst of AC signal may be used to qualify the pick triggers as being valid. The signal called AC ON is used to gate the TRIG UP and TRIG DOWN pick triggers so that a GATE is initiated only if an UP or DOWN TRIG precedes a valid AC ON. 
     In the logic used for the system presently implemented, the AC ON signal is valid on its falling edge. So, a GATE will only go high (its valid state) when AC ON falls after a TRIG UP or TRIG DOWN, FIG. 5H. 
     A more sensitive AC detector determines when the AC signal on the string has decayed to an inaudible level. This is called AC OFF and is used to turn the GATE off. 
     The AC ON and AC OFF signals, FIGS. 5F and 5G respectively, are derived from the output of the differential amplifier 2705, by passing the summed signal through a high pass filtering means 2713. The output of the rectifying means 2713 is connected to a comparing means, preferably a set of comparators 2714 and 2717. By comparing the signal to either a reference AC ON voltage, 2716, or reference AC OFF voltage, 2717, an AC ON or AC OFF signal is formed. 
     The output of rectifying means is also connected to a low pass filtering means 2718, for deriving the guitar string envelope. 
     While the string is vibrating, it may be plucked very quickly so that the AC ON detector remains low. This is because it cannot turn off in such a short time. The DC UP and DOWN detectors, however, can be made sensitive enough to capture the short DC pulses that occur in even the fastest picking and thus can be used to monentarily set the GATE low so as to retrigger the synthesizer sound. 
     This multiple retriggering is impossible with an AC based systems because many times the AC signal cannot even be visually distinguished as having been picked when viewed on a storage oscilloscope. Thus, a system that relies solely on AC variations cannot derive the retriggers that actually exist. The DC method, however, accurately extracts this information and thus makes the multiple strum possible on a guitar synthesizer controller. 
     VELOCITY is derived by sampling the strings&#39; maximum DC deviation from its nominal DC value at rest. At that time, the TRIG UP or DOWN signal is used to sample the peak which is then digitized and held until the GATE ON is triggered. The synthesizer is then sent both GATE ON and VELOCITY information. The output of low pass filter 2706, forms the velocity signal. 
     Thus, unlike the peak detection method of deriving GATE and VELOCITY, the velocity is actually available BEFORE the gate turns on. 
     A problem arises when attempting to measure VELOCITY peaks while the string is bent off axis. In this case, the true value of the string movement due to picking is masked by the offset caused by the string being pushed off axis by the fretting finger. Of course, since it is impossible to bend an unfretted string, this case is not a problem when picking open strings. 
     In a synthesizer guitar controller, as described in my aforementioned U.S. patent application, a string bending sensor may be placed at the nut on the guitar neck. The output of this bend sensor can be used to compensate the DC sensor at the bridge so that the DC offset caused by string bending is cancelled at the bridge. 
     By subtracting the bend sensor output from the DC pick detector output, any string bending offsets may be nulled out at the bridge pickup. The scale factor for the null will be dependent upon the fret at which the bending occurs because the nut and bridge sensors are inversely proportional with respect to bend sensing. Thus, a scaled compensation is necessary. This can easily be accomplished by the control computer that is used to gather and interpret the data in the guitar synthesizer controller. 
     FIG. 6 is a flow chart summarizing how the gate qualifying, retriggering velocity and string bending is derived by the pickup. 
     Yet another major advantage of the bi-phase pickup is that the sensors 2801 i  (FIG. 2) will have different amplitudes and tones depending upon which direction the string is plucked. Thus, the sensors on one side may be summed and brought out independent of the summed sensors 2801 i , 2801 i+1 , . . . 2801 n  on the other side. 
     If these summed outputs are brought out to independent amplifiers, 2802, 2803, the sound of a plucked string will come out of one amplifier when it is plucked &#34;up&#34; and the other amplifier when it is plucked &#34;down&#34;, thus producing a stereo effect on each string. Thus, a bi-phase audio signal for each string is available for independently processing the string vibration in two picking directions. This I have not found possible to achieve in any other way and provides a unique richness to the guitar audio that is independent of its synthesizer controlling qualities. 
     In FIG. 7, I have shown a guitar 100 having a nut 101, strings 102, a bridge 103, a neck 104 and conductive frets 105 along the neck. The note selection circuit of my prior application mentioned above is shown diagrammatically at 200 and, since it is identical in construction and operation to that of the aforementioned application it will not be described further herein except to note that the inputs to this circuit have only been shown representationally. A microprocessor unit 300, which can include a multiplexer, receives all necessary inputs from the note selection circuit 200 and the biphase circuit 400 (see FIG. 1) and outputs via a cable as described in the prior application to the synthesizer 500 which has also been described therein. 
     An alternate embodiment of the DC detecting means is illustrated in FIGS. 8a-8d. 
     FIGS. 8a-8d illustrate the principles of a monophonic optical sensor 801. An infrared emitter 803 is placed under string 802, sensor 801 is tilted so that its entire radiant sensitive area is affected by the infrared beam. 
     FIG. 8a illustrates the string 802 at an at-rest position. The output of sensor 801 has an intermediate output between its maximum output and its minimum output. When string 802 is deflected to its maximum &#34;up&#34; position, no shadow is cast on the sensor 801. Consequently, the sensor 801 is at its maximum output. 
     When string 802 is deflected to its maximum &#34;down&#34; position, a shadow totally eclipses the sensor. Consequently, the sensor 801 is at its minimum output. The vertical motion is cancelled out by virtue of the fact that the change in the shadow cast on the sensor 801 from the string moving further or closer to the emitter is very minimal as compared to the horizontal movement. 
     An alternate embodiment of the gate sensor is illustrated in FIG. 9. The output of the sensor FIG. 10a is passed through a low pass filter means, preferably a sharp low pass filter 901, generating a DC signal FIG. 10b. The DC signal, FIG. 10b normally rests at V1. When the string is plucked in the &#34;up&#34; direction, the voltage will become more positive, while if the string is plucked in the &#34;down&#34; direction the voltage will become more negative. By sampling the direction of the voltage deviation from the resting voltage VI whenever a gate transition is generated, it can be determined whether the picking was in the &#34;up&#34; or &#34;down&#34; direction. 
     The output of the low pass filter means is passed through a differentiating means 902, generating a differentiated signal FIG. 10c. The differentiated DC signal is further processed by a full wave rectifier 903, where output is illustrated in FIG. 10d. The output of the full wave rectifier is compared to a reference voltage to generate the trigger signal as illustrated in FIG. 10e. 
     An alternate embodiment of the velocity sensor is shown in FIG. 11. The output of the sensor is passed through a full wave rectifier 1101. The output of the full wave rectifier is further processed by a peak detector 1102. 
     FIGS. 12a-12d illustrate the output signals of the sensor, full wave rectifier and peak detector respectively. Since the output of the full wave rectifier contains the unprocessed AC and DC components of the string movement, peak detection of this will allow the control computer to sample whenever a gate transition occurs and obtain a valid velocity level, just as in the DC sample mode.