Abstract:
Instrument having automatic fine-tuning of pitch with no moving parts. A flute is described as one embodiment. In flutes of the prior art the various notes are often slightly off pitch because of compromises in the locations of the holes, and musicians must compensate. In this invention a built-in electronic sensor picks up sound as a musician plays, and a circuit identifies the intended note. The played note is compared with, an accurate reference pitch and the frequency difference is used to generate a signal in phase quadrature with the played note. The quadrature signal is added to correct the pitch. The added vibration energizes a tuning actuator that is located in place of a conventional tuning plug of the flute. The tuning actuator pulls the pitch automatically to achieve a precisely correct pitch as each note is played. Also, a player can intentionally deflect the pitch.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     U.S. Pat. No. 5,808,218, issued Sep. 15, 1998, entitled “Expressive Musical Instrument With Which Accurate Pitch Can Be Played Easily,” of inventor Charles H. Grace, is incorporated in and made part of the instant patent application by reference. 
     U.S. Pat. No. 4,429,609, issued Feb. 7, 1984, entitled “Pitch Analyzer,” of inventor David J. Warrender is incorporated in and made part of the instant patent application by reference. 
     U.S. Pat. No. 5,668,340, issued Sep. 16, 1997, entitled “Wind Instrument with Electronic Tubing Length Control,” of inventors Hikaru Hashizume and Yutaka Washiyama is incorporated in and made part of the instant patent application by reference. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not Applicable. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to electronic fine tuning of the frequency of musical instruments in which there is a standing acoustic wave. 
     (2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     Acoustic standing waves in a musical instrument have both pressure vibrations and longitudinal air-velocity vibrations and are mostly confined within boundary walls of a resonant cavity. At certain points in the cavity, standing waves have velocity nodes of zero fluid velocity and maximum pressure. Between those velocity nodes are pressure nodes, which are points of zero pressure and maximum fluid velocity. 
     In the head joint of a conventional flute, an air-velocity node is always located at the face of the tuning plug, that is, the longitudinal air velocity is zero at the face of the tuning plug. Overall tuning is accomplished in a conventional flute by sliding the entire head joint relative to the body or by repositioning the tuning plug within the head joint, to change the length of the cavity. In a conventional instrument neither of those tuning procedures can be done note-by-note while playing music. 
     Many flutes are inaccurately intoned because some of the holes are used in differing combinations for producing more than one note, which requires design compromises in the positions and sizes of the holes. Overall tuning does not correct such compromises because the inaccuracies of intonation reside in the relative pitches of notes within the scale. Numerous design tricks attempt to alleviate the problem. 
     U.S. Pat. No. 5,808,218 discloses a wind instrument in which the resonant cavity is changed by physically moving the tuning plug with a servomechanism. 
     U.S. Pat. No. 4,429,609 discloses identification of a played musical note by means of a computer. 
     U.S. Pat. No. 5,668,340 discloses changing of the pitch of a wind instrument by electronically changing the length of a resonant cavity by injecting an audio tone into the resonant cavity. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention tunes the frequencies of standing acoustic waves in a resonant cavity by a control system having no moving parts. The system tunes correctly even when the musician changes the notes rapidly. 
     Using a flute as a exemplary embodiment, with this invention the pitch is controlled by replacing a tuning plug, at one end of the flute cavity, with a tuning actuator such as an electromagnetic speaker. The tuning actuator is energized with an audio signal that is designed to correct the pitch of the played note by pulling it. The energizing signal is in quadrature phase relationship to the main tone in the flute. Ir stays in quadrature as the pitch is corrected, so it does not cause undesired whistling tones. 
     Quadrature vibrations of the tuning actuator change the phase of the standing wave, so as to create a “virtual wall” spaced slightly behind or ahead of the tuning actuator itself. That has the effect of changing the effective length of the resonant cavity in the flute and hence the frequency. 
     The maximum amount of fine-tuning required in a flute is relatively small because the frequency ratio of two contiguous half-tones is only six percent of their center frequency and the tuning error is less than half of a half-tone. 
     The pitch is corrected whether the errors are caused by inherent defects of the flute, by inadvertent off-pitch playing by the musician, or both. There is a “bending” provision described below for intentionally playing off-pitch for artistic expression. U.S. Pat. No. 5,668,340 demonstrates that injection of additional vibrations into a resonant cavity can affect the pitch, but that uses a different approach requiring much more apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         FIG. 1  shows a flute as one embodiment of the invention. 
         FIG. 2  is a cross-sectional view of a head joint of the flute. 
         FIG. 3  is an electronic block diagram of the invention. 
         FIG. 4  is a flow chart of a computer program for the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Functions 
     In a flute embodiment of the present invention the tuning plug is replaced by a tuning actuator that is electronically energized, and whose vibrations change the effective cavity length. The tuning actuator&#39;s vibrations are substantially at quadrature (right angle) to the phase of the primary tone. That changes the pitch, not by introducing a second tone, but by changing the length of the cavity to amend the primary tone. 
     Using an internal pitch reference and a closed feedback loop, the instrument automatically brings a played note onto correct pitch. The volume and other aspects of the tone are controlled by the player, and the player can also deviate from the pitch for artistic effect with a “bending” lever, as explained later. 
     The instrument improves a player&#39;s “ear” because the player can always hear the correct pitch. Even expert musicians benefit from the invention because they need not subconsciously compensate for off-pitch intonation caused by the design limitations of the flute. 
     The instrument automatically energizes a tuning actuator  26  with a corrective quadrature audio-frequency signal, in the following way. 
     An electronic sensor  31  picks up a specimen of the flute tone to serve as a feedback signal in a feedback loop. The instrument measures the difference between (a) the frequency of the flute tone currently being played and (b) an internal reference pitch The difference is a frequency-error signal. Based in part on the frequency-error signal, the feedback signal drives the tuning actuator  26 . The feedback signal is substantially 90° away from the phase of the main signal. 
     The tuning actuator  26  produces a corrective acoustic vibration that brings the tone onto pitch by changing the effective length of the flute. 
     The error signal of the played tone is averaged over a short time to permit vibrato as in a conventional flute. That simple feature is not detailed herein. 
     Phasors can be used to describe the waves because the injected correction signal always tracks the frequency of the main resonant frequency, except for a minuscule error signal. 
     Equipment 
     1.  FIG. 1  shows the major sections of the flute  2 . They are a head joint  4 , a body  6 , and a foot  8 . An embouchure-hole pad  14  is affixed in the head joint  4  to serve as a primary source of tone in the flute. The body and foot sections have conventional finger keys  13  and an unusual pitch-bending lever  15 . A packet of external electronic circuits  20  is also shown, connected to the flute by a cable  18 .
 
2.  FIG. 2  includes an electronic sensor  31 , whose inside surface is affixed smoothly on the inside surface of the flute an inch away from the embouchure hole toward the tuning plug. A package of electronic circuits  23  inside the head joint is also shown.
 
3,  FIG. 3  shows the electronic sensor  31 , connected to a preamplifier  27 .
 
4. Preamplifier  27  has a gain of 10 db. Its output is input to a digital computer  99 , within which it goes to a low-pass filter  33 .
 
5. The he low-pass filter  33  prepares the signal for the digital computer. The output of the low-pass filter  33  goes several places including to the signal input terminal of a frequency comparator  56  and a note-identification circuit  34 , which are parts of the computer.
 
6. The note-identification circuit  34  selects the on-pitch note that is closest to the fundamental frequency of the note played. Circuit  34  has contiguous ranges of frequencies whose widths are 6% of the center frequency of each range as in an equal-temperament scale. Each range has a unique identifying number corresponding to a note. The identifying number from circuit  34  is input to a frequency reference generator  36 .
 
7. The frequency reference generator  36  can generate all of the scale notes in the range of a flute. As in the prior art, the generator  36  comprises a crystal-controlled clock oscillator feeding a frequency divider. The divisor of the frequency divider is stored in a memory portion of generator  36  and is addressed by the note-identification circuit  34 . The output of the frequency reference generator  36  is a note of correct pitch, which is sent to the frequency comparator  56 .
 
8. Frequency comparator  56  is also conventional, and is also a portion of the computer  99 . It compares the frequency from the frequency reference generator  36  with the fundamental frequency of the played note. Frequency comparator  56  sends out an error signal that is a measure of the frequency difference, to an adder  41 .
 
9. A manual lever  15 , shown in  FIG. 1 , is located near the finger keys of the flute and enables the musician to “bend” the pitch of a note sharp or flat. The lever  15  is a center-off spring-return type. It causes the note to go sharp or flat depending upon the direction the lever is deflected from center, in an amount proportional to the distance. Its output signal goes to the adder  41 .
 
10. The adder  41  adds the bending signal from lever  15  to the error signal from comparator  56 . The output of adder  41  goes to a gain-control amplifier  28 .
 
11 Pitch correction is accomplished by controlling the amplitude of the output of the gain-control amplifier  28 . Amplifier  28  sets the gain of the feedback channel.
 
     The amplifier  28  receives its tone input signal from the low-pass filter  33  at its tone input terminal  30 . In addition to a tone input signal terminal  30 , the amplifier  28  has a gain-control terminal  29 . In analog circuits this would be called automatic gain control (AGC) amplifier, in which the gain is controlled by a DC signal applied to a gain-control terminal. In this embodiment the AGC function is performed by the digital computer  99 . 
     12. One output of gain-control amplifier  28  is connected to a phase shifter  35 , which shifts the signal into substantially quadrature phase relationship with the principal tone in the flute. Optionally, the quadrature signal may actually be biased to be a few degrees greater than 90° to enhance stability of the loop. 
     A second output of amplifier  28  goes to a selector switch  37 , further described below, which selects a different phase delay if the principal tone is sharp than if the principal tone is flat. 
     Phase shifter  35  has two outputs. A delay of nominally 270° comes out at a terminal  40  and a tap at nominally 90° exits at a terminal  38 . The word nominally is employed because an optional bias of a few degrees increases the delay to a few degrees greater than 90°. The exact amount of bias depends upon phase shifts that occur depending upon design details of the feedback loop. 
     The nominal 270° delay is equivalent to a nominal 90° lead, so it provides a nominal 90° advancement of phase to the next-following cycle. 
     If the note is too flat the signal is delayed nominally 270° and if the note is too sharp the delay is nominally 90°. An nominally 270° phasor makes the pitch higher. 
     A delay of 90 degrees is different in seconds for different frequencies, of course, so a circuit  39  is provided to make the clock rate of the delay line proportional to the frequency of the tone being played. The delay line therefore accomplishes a given delay, say 90o, irrespective of the frequency. 
     13. The two outputs of phase shifter  35  go to a digital selector switch  37  that selects the correct phase shift depending upon whether the note is too flat or too sharp. 
     14. The output of switch  37  goes to a digital-to-analog converter (D/A)  57 . 
     15. An amplifier  61  receives the output of the D/A  57  and amplifies it with fixed gain. The output of amplifier  61  is always a tone of the same frequency as the tone in the flute except for an insignificant error signal. It is in quadrature phase relationship to the main tone in the flute. The, output of amplifier  61  is connected to the tuning actuator  26 .
 
16. The tuning actuator  26  is in effect a plug stop whose location is electrically controllable. Its “virtual” position (effective location) is controlled by the power with which it is energized. The reason the tuning actuator  26  controls the pitch is that the signal driving the tuning actuator  26  has a substantially quadrature relationship relative to the phase of the main tone.
 
     The amplitude of the signal that drives the tuning actuator  26 . is proportional to the loudness of the original in-flute tone (as well as to the amount of frequency error). In the preferred embodiment that amplitude is automatically proportional to the loudness because the signal at the amplifier  28  originates from the acoustic sensor  31 . 
     The tuning actuator  26  produces a quadrature acoustic wave to change the effective length of the flute&#39;s resonant cavity slightly. It changes the phase of the audio tone by adding a quadrature phosor to the principal tone.\. Audio vibration of the tuning actuator  26  brings the standing wave in the flute onto the desired pitch. There is only one fundamental frequency in the flute at any time, but of course there are harmonics 
     Operation 
     The following is a list of the events carried out by the feedback program in playing an on-pitch note. Most of the events involve the computer  99 . The numbered events below have the same computer step numbers as  FIG. 4 . 
     The flute is first tuned overall in a conventional manner by blowing a note such as a 440 Hz note and sliding the entire head joint to bring the pitch to 440 Hz. There is a Zero Switch for overall tuning; after overall tuning it should be placed in the Play position. 
     Blow the flute to play a note. The acoustic sensor  31  produces an electronic signal. 
     Preamplifier  27  in the computer receives the signal from sensor  31  and amplifies it with fixed gain. 
     1. The low-pass filter  33  in the computer smoothes the amplifier signal and conditions it for computer use. 
     2. One output from the low-pass filter  33  goes to the note-identification circuit  34 . There, the intended note is identified by comparing it with an array of frequency ranges to find the frequency range into which the played frequency fits. The ID circuit  34  sends out a note-identifying number.
 
3. The note-identifying number addresses the frequency reference generator  36 , which responds with an on-pitch reference frequency corresponding to the intended fundamental note. For example, if an A4 note is played, generator  36  produces a 440 Hz note.
 
The on-pitch reference frequency is sent from the reference generator  36  to the frequency comparator  56 .
 
4. Frequency comparator  56  compares the fundamental frequency of the tone in the flute with the reference frequency received from the frequency reference generator  36  and produces a DC error signal indicative of the difference in frequency. The error signal is positive if the note is sharp or negative if the note is flat. The error signal is conducted from comparator  56  to an adder  41 .
 
5. If there is a “bending” signal from the player for deflecting the pitch for artistic expression, a bending signal component is added to the error signal in the adder  41 . The bending signal can call for sharp or flat deflection.
 
6. The output of the adder  41  goes to the gain-control amplifier  28 . The gain-control amplifier  28  sets the gain of the feedback signal to whatever amount is required to correct the pitch. In addition to the error signal and the bending signals, at terminal  29 , the volume of the tone presently in the flute also controls to the amplitude of the feedback signal, via terminal  30 .
 
     The audio input signal at terminal  30  has a frequency the same as the sound in the flute and has amplitude that is approximately proportional to the amplitude of the sound in the flute. 
     The sign of the error from adder  41  indicates whether the played note is to be made sharper or flatter. In the case of a note that is too sharp, the feedback moves the “virtual position” of the tuning actuator  26  to make the resonant cavity longer. The cavity is made shorter if the note is too flat. 
     7. The phase shifter  35  is a delay line that shifts the phase of the signal it receives from AGC amplifier  28 . The outputs of the delay line are in quadrature with the main tone in the flute. 
     8. The desired phase shift is measured in degrees, not a certain number of seconds. The clock frequency of the phase shifter  35  is therefore controlled by the frequency of the audio signal in the flute. The clock rate is set by a clock frequency circuit  39 . Circuit  39  simply applies a fixed multiple of the flute frequency to the clock terminal of the phase shifter  38 . That makes the shifter&#39;s clock rate proportional to the frequency of the played tone. 
     Tpnes coming out of the phase shifter  35  at a terminal  40  are delayed 90° and tones coming out at a terminal  38  are delayed 270°. The 270° delay creates a 90° advance of the next-occurring cycle. 
     9. A signal from the AGC circuit  28 , at a terminal  42  of the switch  37  tells whether the note is too flat or too sharp. The digital switch  37  selects terminal  40  to provide a 90° delay to correct notes that are too sharp, and selects terminal  38  to provide the 270° delay for notes that are too flat.
 
10. The output of the digital switch  37  drives the digital-to-analog converter (D/A)  57 .
 
The D/A converter  57  drives the fixed gain amplifier  61 .
 
The amplifier  61  drives the tuning actuator  26 . Because of its quadrature phase, the signal sent to the tuning actuator  26  controls the effective length of the flute cavity and hence the pitch. It the preferred embodiment of a flute the change of pitch is very slight.
 
     When the pitch becomes correct, the frequency comparator  56  indicates an error very nearly zero. 
     The foregoing steps are repeated when the player plays the next note. 
       FIG. 4  outlines briefly the program of the computer  99 . All of the computer steps are conventional in the prior art. 
     When a musician blows a note, acoustic sensor  31  picks up the note and preamplifier  27  amplifies it. Steps are: 
     1. Filter  33  in the computer smoothes the signal and makes it suitable for use by the computer. Computer step No.  60 . 
     2. Note ID circuit  34  identifies the note. Computer step No.  62 . 
     3. Reference frequency generator  36  produces an on-pitch note. Computer step No.  64 . 
     4. Frequency comparator  56  compares the played tone with the reference frequency and produces the error signal Computer step No.  66 . 
     5. Adder  41  adds the error signal and the bending signal if any. Computer step No.  68 . 
     6. AGC circuit  28  receives the signal from filter  33  at a terminal  30  and amplifies it by an amount dictated by a signal received at a terminal  29  from the adder  41 . Computer step No.  70 . 
     7. The clock frequency of phase shifter  35  is controlled by the auxiliary clock oscillator  34 . Clock frequency circuit  34  multiplies a signal from the filter  33  to set the clock rate at which the phase shifter  35  will operate. Computer step No. 72. 
     8. Phase shifter  35  shifts the phase of the tone of the AGC output signal. Phase shifter  35  actually produces two phase shifter signals, one for a “too sharp” note at a terminal  40  and one for a “too flat” note at a terminal  42 . Both have close to a quadrature phase shift but they may have slightly different phase bias if optional bias is employed. Computer step No.  74 .
 
9. A sign-error signal comes from the AGC circuit  28  to terminal  42  the selector switch  37 . Selector switch  37  selects a phase shift depending upon whether the sign of the error shows at that the tone is too sharp or too flat. Computer step No.  76 .
 
10. D/A  57  converts the signal and sends it out of computer  99 . Step No.  78 . The signal is substantially 90° displaced from the primary tone in the flute. That&#39;s the end of the program of computer  99 .
 
Amplifier  61  drives the actuator  26  with a quadrature signal.
 
     External Circuits 
     In the embodiment described above, external sub-circuits  20  are in a separate packet for placement in the player&#39;s clothing or elsewhere and connected to the flute by a thin electrical cable  18 . 
     The external sub-circuits  20  include a conventional line-operated power supply that provides low-voltage power required by various sub-circuits. DC power devices known as converters or line adaptors are available in retail stores, but one that is purpose-built for the required voltages and current capacities is preferable here. 
     Other Embodiments 
     1. Player-Generated Vibrato 
     Player-generated vibrato is preserved by including a ¼-second averaging circuit in the feedback loop, which is not described or claimed herein. 
     2. Equipment-Generated Vibrato 
     A player may choose to turn on an optional electronic vibrato circuit that is built into the flute. Numerous conventional vibrato circuits are suitable. 
     3 Harmonics 
     Harmonics above the fundamental tone can be processed in the same way as the fundamental tone and added as harmonics to the correction signal. However, harmonic correction has not been described herein, as it is ordinarily corrected automatically upon correcting the fundamental. 
     4. Alternative Tone Pickup 
     The location and type of the tone pickup  26  affect the phase delay of feedback, and the phase adjustment is readily tailored to the pickup&#39;s type and location during detailed design. 
     5 Alternative Reference-Frequency Generator 
     Although the instrument preferably employs a conventional reference-frequency generator comprising a clock oscillator and scalar, a non-volatile addressable storage memory in which a plurality of reference frequencies are stored in advance, could be employed instead. 
     6. Alternative Note-Identification Circuits 
     Although the preferred embodiment identifies the intended note by electronically comparing the frequency being played with an array of frequency ranges, various other conventional means of identifying the closest intended-note could be used instead. 
     For example, in one alternative embodiment, key-actuated switches detect the positions of the finger keys  13 . This alternative method for identifying a selected note is commonly used in the prior art by Yamaha and Casio companies. 
     Yet another alternative means for sensing the positions of keys is to use conventional capacitive proximity sensors. 
     7. Alternative Tuning Actuator 
     Instead of being an electromagnetic loudspeaker as in the preferred embodiment, the tuning actuator  26  can be stacked piezoelectric crystals or other acoustic transducers. 
     8. All Equipment Mounted on the Flute 
     If desired, all of the equipment can be sufficiently miniaturized to be located in the flute itself—including the batteries. 
     9. Instruments Other than Flutes 
     The invention also applies to reed instruments, brass instruments, acoustic organs and other musical instruments that involve standing waves whose pitch can be tuned by a mechanical element of the resonant cavity. The mechanical element can be partially or wholly replaced by an electronic tuning actuator of the present invention. 
     10. Alternative Designs 
     Wherever digital circuits are employed in the preferred embodiment, analog circuits could be used instead, and wherever analog circuits are employed, digital circuits could be used instead. 
     11. Independent Oscillator. 
     An audio oscillator could be used for providing a corrective signal to the tuning actuator  26 . The oscillator&#39;s frequency would be synchronized with the signal from the sensor, and be tuned to an approximately quadrature phase relative to the sensor&#39;s signal. The oscillator&#39;s amplitude would track the error signal. 
     12. Over-all Electronic Tuning. 
     Electronic overall tuning of the instrument to different reference pitches such as A4 of 435 Hz can be readily accomplished with this invention.