Tuneable UJT oscillator circuit
United States Patent 3879684
A relaxation oscillator with an offset control. A timing circuit including a charge storage device in the relaxation oscillator is energized from a normally constant voltage source and one or more variable voltage sources. The first source also establishes a reference signal in a discharge circuit in the relaxation oscillator. Changes in the voltage from the variable voltage source do not affect the reference signal, but do affect a timing signal from the charge storage device. As the discharge circuit discharges the charge storage device each time the timing signal exceeds the reference voltage, changing the voltage from the variable voltage source does alter oscillator frequency.
Application Number:
05/399921
Publication Date:
04/22/1975
Filing Date:
09/24/1973
View Patent Images:
Export Citation:
Assignee:
Inventronics, Inc. (Carlisle, MA)
Primary Class:
Other Classes:
331/177R, 84/DIG.018, 84/444
International Classes:
G10G7/02; H03K3/351; G10G7/00; H03K3/00; H03K3/28
Field of Search:
331/111,177R,177V
Other References:
Popular Electronics, Nov. 1968, p. 60..
Primary Examiner:
Grimm, Siegfried H.
Attorney, Agent or Firm:
Cesari, And Mckenna
Claims:
What I claim as new and desire to secure by Letters Patent of the United States is
1. A variable frequency oscillator circuit comprising:
2. A variable frequency oscillator circuit as recited in claim 1 wherein said variable voltage source comprises
3. A variable frequency oscillator circuit as recited in claim 2 wherein:
4. A variable frequency oscillator circuit as recited in claim 3 wherein said connecting means between said normally constant voltage source and said discharge circuit comprises a resistor.
5. A variable frequency oscillator circuit as recited in claim 1 additionally comprising another variable voltage source connected to said timing circuit input terminal for transmitting another variable voltage thereto, each of said variable voltage sources comprising:
6. An oscillator circuit comprising:
7. An oscillator as recited in claim 6 additionally comprising a second variable voltage source including a second potentiometer connected to said normally constant voltage source to provide a variable voltage at a second adjustable tap and a second resistor for coupling said tap and said timing circuit input terminal.
1. A variable frequency oscillator circuit comprising:
2. A variable frequency oscillator circuit as recited in claim 1 wherein said variable voltage source comprises
3. A variable frequency oscillator circuit as recited in claim 2 wherein:
4. A variable frequency oscillator circuit as recited in claim 3 wherein said connecting means between said normally constant voltage source and said discharge circuit comprises a resistor.
5. A variable frequency oscillator circuit as recited in claim 1 additionally comprising another variable voltage source connected to said timing circuit input terminal for transmitting another variable voltage thereto, each of said variable voltage sources comprising:
6. An oscillator circuit comprising:
7. An oscillator as recited in claim 6 additionally comprising a second variable voltage source including a second potentiometer connected to said normally constant voltage source to provide a variable voltage at a second adjustable tap and a second resistor for coupling said tap and said timing circuit input terminal.
Description:
BACKGROUND OF THE INVENTION
This invention generally relates to tuning musical instruments and more specifically to a tunable oscillator for use in such instruments.
Conventionally, a person listens to a reference note and adjusts a musical instrument until its note seems consonant with the reference note. Consciously, or not, the person tunes a note for a zero beat with the reference note, usually at some harmonic of either one or both the notes.
This type of tuning is possible because a diatonic scale is based upon mathematical relationships. In practice, however, pianos and other stringed instruments do not follow mathematical rules. Harmonics generated by a given note are more than integral multiples of the fundamental. This deviation, termed "stretch," may be defined as the difference between the measured and theoretical second harmonic frequencies of a note. Stretch is significant. In a piano, for instance, the second harmonic note from a string averages 2.002 to 2.006 times the fundamental frequency. Thus, if the fundamental notes are tuned mathematically, stretch causes a piano to sound out of tune.
Therefore, pianos and similar instruments must be tuned differently. The general approach is a complex, iterative process in which a tuner tries to reduce errors to a minimum step-by-step. Basically, a piano tuner starts tuning a piano in a "temperament octave" by adjusting a first note to a reference frequency. He adjusts the remaining notes in the temperament octave by listening to harmonics of third, fourth and fifth intervals. For example, in striking an interval of a third with a previously tuned lower note, the tuner adjusts the upper note while listening to the beat between the fifth harmonic of the lower note and the fourth harmonic of the upper note. He assumes the proper relationship exists when he obtains a predetermined beat frequency.
Listening to these harmonics reduces errors at the fundamental frequency because the harmonics multiply any error in terms of actual frequency differences. That is, a 4 Hz error at the fourth harmonic represents only a 1 Hz error at the fundamental. Also, the use of harmonics inherently tends to compensate for piano stretch. However, the process is not perfect and the tuner usually checks the temperament octave by retuning it using different intervals to minimize the tuning errors.
Once the tuner completes the temperament octave, he tunes other notes by comparing harmonics while playing octave intervals. He may, for example, listen to the beat between the fourth harmonic of a lower, tuned note and the second harmonic of the upper note while adjusting string tension for the upper note. Lower notes are tuned similarly, although not necessarily with octave intervals.
Each piano note has two or three strings. During the foregoing procedure, the tuner damps out strings so only one string actually sounds when a hammer strikes all the strings associated with that note. After the tuner completes the procedure, he must tune the other strings for each note by comparing either the fundamental frequencies of two strings associated with a given note or the fundamental of one note and a harmonic of another note an octave away.
As may be apparent, however, the entire procedure requires that a note sustain long enough to enable the tuner to determine the beat frequency. Obviously, the longer the interval the note sustains, the more accurately the tuner can determine the beat frequency. In tuning, each note struck sounds until it dies out naturally or the key is released. By "dying out" I mean that the note can no longer be heard. In the bass region, this poses no real problem. However, as the frequency increases, the time the tuner hears the note decreases. Hence, he cannot determine the beat notes in the treble range with as much accuracy.
In the bass region, the beat frequency becomes very low. For example, the full tone frequency difference at the low end of a piano is less than 3 Hz. The desired beat frequency is a small percentage of the difference in a semi-tone, so it is less than 1 Hz. This is a very difficult beat frequency to detect.
Although there are several tuning aids, no one aid has wide acceptance. In one, a high frequency oscillator produces an output clock signal at a selected frequency. A series of frequency dividers and an octave selector switch provide a means for generating a reference signal at a selected subharmonic frequency. The tuning aid combines this reference signal and an audio signal representing the note being tuned either to generate an audible beat note or to deflect a pointer on an indicating meter. Unfortunately, these aids lose accuracy as the tuned note comes into frequency with the reference. when the beat rate decreases below 20 Hz, the audible beat note becomes inaudible. Similarly, an indicating meter uses a frequency-to-current converter so the current level goes to zero at a zero beat. As the current approaches zero, the visual indication becomes less accurate. Both types of display, therefore, lose accuracy at the very time it is most necessary.
In another unit, the tuner attaches a piezoelectric transducer to a particular string or a sounding board to produce a corresponding electrical signal that is applied to the vertical deflection plates of a cathode ray tube. A selector switch, crystal controlled oscillator and a series of frequency dividers generate a selected reference signal which energizes the horizontal deflection plates of the tube. In using this circuit, one apparently assumes, erroneously, that a piano generates a constant, repetitive wave form. In fact, a piano string generates an extremely complex wave form with a fundamental and "harmonic" components, often of the same magnitude, but slightly out of tune with each other. Furthermore, many of the component frequencies are not necessarily constant in magnitude because a string vibrates in many modes, each with its own damping constant. These factors cause the waveform to change continuously, so the display is difficult to interpret.
Another problem relates to dynamic response. Initially, the amplitude of the signal is sufficient to drive the display off the screen. As the tone dies out, the input to the vertical deflection plates falls below the minimum level necessary for generating a usable display. An obvious solution is installing a variable gain amplifier to maintain the output at a constant value. However, a circuit which provides satisfactory results over the wide range of conditions and waveforms which the piano generates is difficult to attain in practice. If the variable gain circuit actually tracks the decay, it may follow the waveform and provide a dc output signal. Therefore, this solution is not practicable especially in view of the non-linear parameters or conditions and the short interval for a readable display. This effective dynamic range further complicates tuning because adjusting a string while monitoring the display is very difficult.
Still another tuning aid receives the audio signal from a piano and generates a corresponding electrical signal to energize the blanking or Z axis circuitry of a cathode ray tube. A circular generator energizes X and Y axis deflection plates with a reference frequency so the electron beam describes a circle on the screen. If a note is in tune with the reference, the audio signal blanks and unblanks the electron beam during the same part of each revolution to thereby display one arcuate segment. A second harmonic input signal produces two such arcuate segments; a third harmonic input signal, three segments; and so forth. If a given note is not exactly harmonically related to the reference, the segments rotate. The direction of rotation indicates whether the note is sharp or flat while the speed of rotation indicates the difference in frequencies. As notes in the upper piano produce a display with a number of segments, the spaces between adjacent sectors diminish; and the absolute frequency deviation which produces a persistent display tends to decrease. Furthermore, alternately blanking and unblanking the beam produces an indefinite segment termination on the screen. When the frequency deviation is small, the indefininte termination makes it difficult to determine whether the edges of the beams are moving. When notes in the lower range of the piano are tuned, the tuner must try to adjust while the tuning aid responds to harmonics, since subharmonics of the reference frequency generate complete circles on screen.
Apparently, another reason professional piano tuners are reluctant to use prior aids is that each piano is tuned uniquely, so a generalized tuning aid that responds to the fundamental frequency of the note being tuned does not really help the tuner. The unique quality of each piano stems from its construction, string length, wear on hammers, and myriad factors. As a result, piano tuners continue to work conventionally and do not place any significant reliance on mechanical aids.
Therefore, it is an object of this invention to provide an oscillator circuit for use in a tuning aid which is readily adapted for tuning a wide variety of instruments.
SUMMARY
In accordance with my invention, a tuner selects a specific note in an octave and a specific octave on the tuning aid. This selection determines a nominal frequency for a clocking signal from a master clock oscillator including a timing circuit with a charge storage device. The timing circuit establishes a nominal frequency in response to the note selection. Comparison means in a discharge circuit discharge the timing circuit whenever a timing signal exceeds a reference signal. A normally constant voltage source establishes the reference signal in the discharge circut and other means connect the constant voltage source to a timing circuit input terminal. One or more variable voltage sources also connect to the timing circuit input terminal. When the voltage from the variable voltage source changes, it alters the charging current through the timing circuit only and thereby alters the clocking signal frequency.
This invention is pointed out with particularity in the appended claims. A more thorough understanding of the above and further objects and advantages of this invention may be attained by referring to the following description taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a tuning aid constructed in accordance with my invention; and
FIG. 2 is a detailed schematic of an embodiment of a master clock oscillator circuit shown in FIG. 1.
DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
1. General Discussion
As shown in FIG. 1, my tuning aid 10 comprises an input circuit 12, a reference circuit 14 and a detection circuit 16. The input circuit 12 comprises a microphone 18 which picks up signals generated as a musical instrument is tuned. For example, on a piano, it detects the sound emanating from a struck note. A conventional preamplifier 20 and an active filter 22 isolate the signal being tuned from other signals which the microphone 18 senses (i.e., an active bandpass filter). The filter 22 preferably is a tunable filter which has a quality factor greater than ten. Such bandpass filters are known in the art. The filter 22 it produces an audio output signal on a conductor 24 which connects to the detection circuit 16.
The reference circuit 14 produces a second input signal to the detection circuit 16. A variable frequency master clock oscillator 26 constructed in accordance with this invention covers the twelve notes two octaves above the highest octave to be tuned, for purposes which will become apparent later. A particular oscillator frequency is selected by a note selector 28 which simultaneously tunes the active filter 22. An octave selector 30 also controls the active filter 22 and further controls a frequency divider 32 which, in response to the signals from the master clock oscillator 26, provides a square wave output signal which is twice the frequency determined by the note selector 28 and octave selector 30. That is, if the selectors 28 and 30 are set to select a musical A at 440 Hz [hereinafter A(440)], the filter 22 has a center frequency of 440 Hz while the master clock oscillator 26 generates a 28.16 kHz output and an 880 Hz signal appears on the conductor 34 leading from the divider 32.
The detection circuit 16 has a detector 36 which receives both the audio signal on the conductor 24 and the reference signal on the conductor 34. It generates four output signals on output conductors 38-1, 38-2, 38-3 and 38-4. Each output is a constant-amplitude, pulse-width-modulated signal with pulse width varying as a function of the phase difference between a note signal on the conductor 24 derived from the instrument being tuned and a reference signal on the conductor 34, which is the output from the clock divider 32. The pulse repetition rate is equal to the selected reference frequency and the rate at which the pulse width changes on each conductor depends on the frequency difference between the note frequency and one-half the referency frequency, the pulses on each conductor having unvarying width if the struck note is in tune with the reference. Low-pass filters 40 couple the pulse signals from the detector 36 to a display 42. At any given time, a filtered ed dc output is proportional to the width of an input pulse. If there is a frequency deviation, each low-pass filter output varies from 0 to 200 percent of its normal value at a rate which is proportional to the frequency difference.
The display unit 42 preferably contains one pair of lamps (e.g., light-emitting diodes) energized by each low-pass filter output. Mechanically, each lamp in a pair may be diametrically opposed in a circle, with adjacent lamp pairs separated by 45°. As becomes apparent later, the signals which energize lamps in space quadrature are 180° out of phase electrically. If a first lamp pair is at full brilliance, a second lamp pair, displaced 90° from the first, is off. The lamp pairs that are displaced ±45 percent from the first are also off, for reasons I discuss later.
When an incoming note is in tune, one pair of lamps may be at or nearly at full brilliance or two pairs may be partially lit. However, the relative brilliance of the lamps does not change. As a result, the display appears stationary. If there is a frequency deviation, the individual lamp pairs reach full brilliance in one of two sequences. If the note is "shape" (i.e., at a higher frequency than the reference), then the lamps reach full brilliance in a clockwise sequence; so the display appears to rotate clockwise. When a note is flat, the sequence is reversed and the display appears to rotate counterclockwise. As the repetition rate at which a given set of lamps reaches full brilliance depends upon the frequency difference, the rate at which the display appears to rotate indicates the magnitude of the deviation.
2. Specific Discussion
The heart of this invention is in the manner in which the detector 36 and low-pass filters 40 condition input signals and display the results. Still referring to FIG. 1, the signal the master clock oscillator 26 and the divider 32 place on conductor 34 has twice the frequency of the selected note. Division by at least two in the divider 32 means that the outputs from the master clock oscillator 26 must be four times the highest frequencies to be measured. In one specific embodiment using a "C" as a lower octave limit and a "B" as an upper limit, the master clock oscillator 26 generates nominal signals in the range between 16,744 and 31,609 Hz. Depending on the setting of the octave selector 30, the clock divider 32 divides the oscillator output by a faactor of 2 n where 1<n<8. When the octave selector 30 is set for the highest octave, the divider 32 divides the oscillator frequency by 2, while a division by 256 occurs when the octave selector 30 is set for the lowest octave. As a specific example, setting the note selector 28 to "A" causes the oscillator 26 to generate a 28,160 Hz signal. The frequency of the signal on the conductor 34 and the frequency which the tuning aid will sense are then as follows:
Signal On Frequency of Signal Octave Number Conductor 34 Being Measured ______________________________________ 8 14,080 7,040 7 7,040 3,520 6 3,520 1,760 5 1,760 880 4 880 440 3 440 220 2 220 110 1 110 55 ______________________________________
For the tuning aid to be effective, there should be some provision to vary the frequency of the master clock oscillator 26 shown in FIG. 1. The oscillator 26 generates signals in accordance with the known mathematical relationships of the equally tempered scale. Coarse and fine pitch variation controls 44 and 46 (FIG. 1) enable a tuner to vary the frequency of all the notes up to one-half a semi-tone in either direction (i.e., an offset of ±3 percent of the nominal oscillator frequency), while preserving the correct relationship among the notes.
As shown in FIG. 2, the master clock oscillator 26 comprises a unijunction transistor 150 in a relaxation oscillator circuit. A temperature-compensating resistor 152 connects "base 2" to a conductor 154 from a power supply which provides a substantially constant voltage. An output resistor 155 is between "base 1" and ground. Two elements in a timing circuit generally control the nominal oscillator frequency--a variable capacitor 156 and a variable resistor 158. Thus, the voltage between base 1 and base 2 (E bb ) is substantially constant.
To set the oscillator initially, the capacitor 156 is adjusted so that the oscillator 26 generates its highest required frequency. This is done with the resistor 158 at a minimum value. Usually the resistor 158 comprises a switched resistance ladder network which enables the frequency for each setting of the note selector 28 to be adjusted independently. During calibration, the frequencies are adjusted for the correct mathematical relationship. A buffer amplifier 160 couples the signal from the output resistor 155.
The capacitor 156 and resistor 158 constitute two distinct means varying the frequency of the oscillator 26. In accordance with this invention, the oscillator 26 includes a third means for independently varying frequency over the offset range. As known, the unijunction transistor 150 discharges when the voltage across the capacitor 156 (i.e., the emitter voltage) reaches a threshold which is a substantially constant percentage (η) of (E bb ) which establishes a reference voltage (ηE bb ). The time it takes the capacitor voltage to reach that threshold is a function of the resistor and capacitor values and the voltage applied to the timing circuit.
In the oscillator 26 in FIG. 2, this voltage appears across a capacitor 166 and is equal to the voltage on the conductor 154 minus the voltage across a resistor 162. The voltage across the resistor 162 depends on the current through it and the current has two components. A first component is constant for a given setting of the note selector 28 and depends upon the voltage from the constant voltage power supply on the conductor 154 and the series impedance of the resistors 162 and 158.
The second component is variable in response to the setting of the pitch controls which constitute two independent variable voltage sources. A conductor 164 carries this second component. As the pitch controls increase this current component, the voltage drop across resistor 162 increases so the voltage across capacitor 166 decreases. As a result, the oscillator frequency decreases.
The remaining circuitry shown in FIG. 2 provides this variable second current component. A first resistor network constitutes a first variable voltage source and comprises a resistor 172 for coupling the conductor 164 to the wiper of a potentiometer 174, the potentiometer 174 being energized from the conductor 154. Variations in the position of the coarse pitch control 44 offset the wiper arm, which constitutes an adjustable tap, from a normal position. Positioning the fine pitch control 46 similarly alters the wiper arm on a potentiometer 176 also energized from the conductor 154. A resistor 178 couples this wiper arm to the conductor 164 and, together with the potentiometer 176, constitutes another variable voltage source.
The qualitative effect of varying either wiper arm position is the same. The component values are chosen so that a given physical displacement of the coarse pitch control 44 produces a large offset than the same displacement of the fine pitch control 46. Therefore, the following discussion relates only to the operation of the coarse pitch control 44.
Two relationships exist in this circuit. First, as apparent, the voltage on the conductor 154 is greater than the voltage on the conductor 164. Secondly, resistor 172 is at least an order of magnitude larger than resistor 162.
At a zero voltage offset position, there is a zero voltage drop across the resistor 172 so only the first current component flows through the resistor 162. If the coarse pitch control 44 is moved, the second current component from the conductor 164 changes the voltage across the resistor 162 and the capacitor 166.
Both pitch controls vary the frequency as a percentage of the base frequency, so these controls can be calibrated in "cents" difference to raise or lower the resulting frequency, assuming that the oscillator is calibrated with the ptentiometers 174 and 176 at their mid-points.
The tuning aid shown in FIG. 1 is sensitive and accurate. Tests shown that the display has visible motion when the phase shift is less than 10°, with the accuracy being dependent upon the time the tuning aid senses the tone and the stability of both the tone and note. This means that the tuning aid senses a frequency difference which produces less than a 10° phase shift over the interval the note signal exists. When operated from a battery power supply, the tuning aid is very stable. Tests against a tuning fork show no displacement after 10 seconds of tone. This increased sensitivity and stability have enabled me to analyze how pianos are tuned conventionally.
Therefore, it is apparent that there are many modifications and alterations which can be made to my tuning aid, the specifically described circuits and my method for tuning a piano. It is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.
This invention generally relates to tuning musical instruments and more specifically to a tunable oscillator for use in such instruments.
Conventionally, a person listens to a reference note and adjusts a musical instrument until its note seems consonant with the reference note. Consciously, or not, the person tunes a note for a zero beat with the reference note, usually at some harmonic of either one or both the notes.
This type of tuning is possible because a diatonic scale is based upon mathematical relationships. In practice, however, pianos and other stringed instruments do not follow mathematical rules. Harmonics generated by a given note are more than integral multiples of the fundamental. This deviation, termed "stretch," may be defined as the difference between the measured and theoretical second harmonic frequencies of a note. Stretch is significant. In a piano, for instance, the second harmonic note from a string averages 2.002 to 2.006 times the fundamental frequency. Thus, if the fundamental notes are tuned mathematically, stretch causes a piano to sound out of tune.
Therefore, pianos and similar instruments must be tuned differently. The general approach is a complex, iterative process in which a tuner tries to reduce errors to a minimum step-by-step. Basically, a piano tuner starts tuning a piano in a "temperament octave" by adjusting a first note to a reference frequency. He adjusts the remaining notes in the temperament octave by listening to harmonics of third, fourth and fifth intervals. For example, in striking an interval of a third with a previously tuned lower note, the tuner adjusts the upper note while listening to the beat between the fifth harmonic of the lower note and the fourth harmonic of the upper note. He assumes the proper relationship exists when he obtains a predetermined beat frequency.
Listening to these harmonics reduces errors at the fundamental frequency because the harmonics multiply any error in terms of actual frequency differences. That is, a 4 Hz error at the fourth harmonic represents only a 1 Hz error at the fundamental. Also, the use of harmonics inherently tends to compensate for piano stretch. However, the process is not perfect and the tuner usually checks the temperament octave by retuning it using different intervals to minimize the tuning errors.
Once the tuner completes the temperament octave, he tunes other notes by comparing harmonics while playing octave intervals. He may, for example, listen to the beat between the fourth harmonic of a lower, tuned note and the second harmonic of the upper note while adjusting string tension for the upper note. Lower notes are tuned similarly, although not necessarily with octave intervals.
Each piano note has two or three strings. During the foregoing procedure, the tuner damps out strings so only one string actually sounds when a hammer strikes all the strings associated with that note. After the tuner completes the procedure, he must tune the other strings for each note by comparing either the fundamental frequencies of two strings associated with a given note or the fundamental of one note and a harmonic of another note an octave away.
As may be apparent, however, the entire procedure requires that a note sustain long enough to enable the tuner to determine the beat frequency. Obviously, the longer the interval the note sustains, the more accurately the tuner can determine the beat frequency. In tuning, each note struck sounds until it dies out naturally or the key is released. By "dying out" I mean that the note can no longer be heard. In the bass region, this poses no real problem. However, as the frequency increases, the time the tuner hears the note decreases. Hence, he cannot determine the beat notes in the treble range with as much accuracy.
In the bass region, the beat frequency becomes very low. For example, the full tone frequency difference at the low end of a piano is less than 3 Hz. The desired beat frequency is a small percentage of the difference in a semi-tone, so it is less than 1 Hz. This is a very difficult beat frequency to detect.
Although there are several tuning aids, no one aid has wide acceptance. In one, a high frequency oscillator produces an output clock signal at a selected frequency. A series of frequency dividers and an octave selector switch provide a means for generating a reference signal at a selected subharmonic frequency. The tuning aid combines this reference signal and an audio signal representing the note being tuned either to generate an audible beat note or to deflect a pointer on an indicating meter. Unfortunately, these aids lose accuracy as the tuned note comes into frequency with the reference. when the beat rate decreases below 20 Hz, the audible beat note becomes inaudible. Similarly, an indicating meter uses a frequency-to-current converter so the current level goes to zero at a zero beat. As the current approaches zero, the visual indication becomes less accurate. Both types of display, therefore, lose accuracy at the very time it is most necessary.
In another unit, the tuner attaches a piezoelectric transducer to a particular string or a sounding board to produce a corresponding electrical signal that is applied to the vertical deflection plates of a cathode ray tube. A selector switch, crystal controlled oscillator and a series of frequency dividers generate a selected reference signal which energizes the horizontal deflection plates of the tube. In using this circuit, one apparently assumes, erroneously, that a piano generates a constant, repetitive wave form. In fact, a piano string generates an extremely complex wave form with a fundamental and "harmonic" components, often of the same magnitude, but slightly out of tune with each other. Furthermore, many of the component frequencies are not necessarily constant in magnitude because a string vibrates in many modes, each with its own damping constant. These factors cause the waveform to change continuously, so the display is difficult to interpret.
Another problem relates to dynamic response. Initially, the amplitude of the signal is sufficient to drive the display off the screen. As the tone dies out, the input to the vertical deflection plates falls below the minimum level necessary for generating a usable display. An obvious solution is installing a variable gain amplifier to maintain the output at a constant value. However, a circuit which provides satisfactory results over the wide range of conditions and waveforms which the piano generates is difficult to attain in practice. If the variable gain circuit actually tracks the decay, it may follow the waveform and provide a dc output signal. Therefore, this solution is not practicable especially in view of the non-linear parameters or conditions and the short interval for a readable display. This effective dynamic range further complicates tuning because adjusting a string while monitoring the display is very difficult.
Still another tuning aid receives the audio signal from a piano and generates a corresponding electrical signal to energize the blanking or Z axis circuitry of a cathode ray tube. A circular generator energizes X and Y axis deflection plates with a reference frequency so the electron beam describes a circle on the screen. If a note is in tune with the reference, the audio signal blanks and unblanks the electron beam during the same part of each revolution to thereby display one arcuate segment. A second harmonic input signal produces two such arcuate segments; a third harmonic input signal, three segments; and so forth. If a given note is not exactly harmonically related to the reference, the segments rotate. The direction of rotation indicates whether the note is sharp or flat while the speed of rotation indicates the difference in frequencies. As notes in the upper piano produce a display with a number of segments, the spaces between adjacent sectors diminish; and the absolute frequency deviation which produces a persistent display tends to decrease. Furthermore, alternately blanking and unblanking the beam produces an indefinite segment termination on the screen. When the frequency deviation is small, the indefininte termination makes it difficult to determine whether the edges of the beams are moving. When notes in the lower range of the piano are tuned, the tuner must try to adjust while the tuning aid responds to harmonics, since subharmonics of the reference frequency generate complete circles on screen.
Apparently, another reason professional piano tuners are reluctant to use prior aids is that each piano is tuned uniquely, so a generalized tuning aid that responds to the fundamental frequency of the note being tuned does not really help the tuner. The unique quality of each piano stems from its construction, string length, wear on hammers, and myriad factors. As a result, piano tuners continue to work conventionally and do not place any significant reliance on mechanical aids.
Therefore, it is an object of this invention to provide an oscillator circuit for use in a tuning aid which is readily adapted for tuning a wide variety of instruments.
SUMMARY
In accordance with my invention, a tuner selects a specific note in an octave and a specific octave on the tuning aid. This selection determines a nominal frequency for a clocking signal from a master clock oscillator including a timing circuit with a charge storage device. The timing circuit establishes a nominal frequency in response to the note selection. Comparison means in a discharge circuit discharge the timing circuit whenever a timing signal exceeds a reference signal. A normally constant voltage source establishes the reference signal in the discharge circut and other means connect the constant voltage source to a timing circuit input terminal. One or more variable voltage sources also connect to the timing circuit input terminal. When the voltage from the variable voltage source changes, it alters the charging current through the timing circuit only and thereby alters the clocking signal frequency.
This invention is pointed out with particularity in the appended claims. A more thorough understanding of the above and further objects and advantages of this invention may be attained by referring to the following description taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a tuning aid constructed in accordance with my invention; and
FIG. 2 is a detailed schematic of an embodiment of a master clock oscillator circuit shown in FIG. 1.
DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
1. General Discussion
As shown in FIG. 1, my tuning aid 10 comprises an input circuit 12, a reference circuit 14 and a detection circuit 16. The input circuit 12 comprises a microphone 18 which picks up signals generated as a musical instrument is tuned. For example, on a piano, it detects the sound emanating from a struck note. A conventional preamplifier 20 and an active filter 22 isolate the signal being tuned from other signals which the microphone 18 senses (i.e., an active bandpass filter). The filter 22 preferably is a tunable filter which has a quality factor greater than ten. Such bandpass filters are known in the art. The filter 22 it produces an audio output signal on a conductor 24 which connects to the detection circuit 16.
The reference circuit 14 produces a second input signal to the detection circuit 16. A variable frequency master clock oscillator 26 constructed in accordance with this invention covers the twelve notes two octaves above the highest octave to be tuned, for purposes which will become apparent later. A particular oscillator frequency is selected by a note selector 28 which simultaneously tunes the active filter 22. An octave selector 30 also controls the active filter 22 and further controls a frequency divider 32 which, in response to the signals from the master clock oscillator 26, provides a square wave output signal which is twice the frequency determined by the note selector 28 and octave selector 30. That is, if the selectors 28 and 30 are set to select a musical A at 440 Hz [hereinafter A(440)], the filter 22 has a center frequency of 440 Hz while the master clock oscillator 26 generates a 28.16 kHz output and an 880 Hz signal appears on the conductor 34 leading from the divider 32.
The detection circuit 16 has a detector 36 which receives both the audio signal on the conductor 24 and the reference signal on the conductor 34. It generates four output signals on output conductors 38-1, 38-2, 38-3 and 38-4. Each output is a constant-amplitude, pulse-width-modulated signal with pulse width varying as a function of the phase difference between a note signal on the conductor 24 derived from the instrument being tuned and a reference signal on the conductor 34, which is the output from the clock divider 32. The pulse repetition rate is equal to the selected reference frequency and the rate at which the pulse width changes on each conductor depends on the frequency difference between the note frequency and one-half the referency frequency, the pulses on each conductor having unvarying width if the struck note is in tune with the reference. Low-pass filters 40 couple the pulse signals from the detector 36 to a display 42. At any given time, a filtered ed dc output is proportional to the width of an input pulse. If there is a frequency deviation, each low-pass filter output varies from 0 to 200 percent of its normal value at a rate which is proportional to the frequency difference.
The display unit 42 preferably contains one pair of lamps (e.g., light-emitting diodes) energized by each low-pass filter output. Mechanically, each lamp in a pair may be diametrically opposed in a circle, with adjacent lamp pairs separated by 45°. As becomes apparent later, the signals which energize lamps in space quadrature are 180° out of phase electrically. If a first lamp pair is at full brilliance, a second lamp pair, displaced 90° from the first, is off. The lamp pairs that are displaced ±45 percent from the first are also off, for reasons I discuss later.
When an incoming note is in tune, one pair of lamps may be at or nearly at full brilliance or two pairs may be partially lit. However, the relative brilliance of the lamps does not change. As a result, the display appears stationary. If there is a frequency deviation, the individual lamp pairs reach full brilliance in one of two sequences. If the note is "shape" (i.e., at a higher frequency than the reference), then the lamps reach full brilliance in a clockwise sequence; so the display appears to rotate clockwise. When a note is flat, the sequence is reversed and the display appears to rotate counterclockwise. As the repetition rate at which a given set of lamps reaches full brilliance depends upon the frequency difference, the rate at which the display appears to rotate indicates the magnitude of the deviation.
2. Specific Discussion
The heart of this invention is in the manner in which the detector 36 and low-pass filters 40 condition input signals and display the results. Still referring to FIG. 1, the signal the master clock oscillator 26 and the divider 32 place on conductor 34 has twice the frequency of the selected note. Division by at least two in the divider 32 means that the outputs from the master clock oscillator 26 must be four times the highest frequencies to be measured. In one specific embodiment using a "C" as a lower octave limit and a "B" as an upper limit, the master clock oscillator 26 generates nominal signals in the range between 16,744 and 31,609 Hz. Depending on the setting of the octave selector 30, the clock divider 32 divides the oscillator output by a faactor of 2 n where 1<n<8. When the octave selector 30 is set for the highest octave, the divider 32 divides the oscillator frequency by 2, while a division by 256 occurs when the octave selector 30 is set for the lowest octave. As a specific example, setting the note selector 28 to "A" causes the oscillator 26 to generate a 28,160 Hz signal. The frequency of the signal on the conductor 34 and the frequency which the tuning aid will sense are then as follows:
Signal On Frequency of Signal Octave Number Conductor 34 Being Measured ______________________________________ 8 14,080 7,040 7 7,040 3,520 6 3,520 1,760 5 1,760 880 4 880 440 3 440 220 2 220 110 1 110 55 ______________________________________
For the tuning aid to be effective, there should be some provision to vary the frequency of the master clock oscillator 26 shown in FIG. 1. The oscillator 26 generates signals in accordance with the known mathematical relationships of the equally tempered scale. Coarse and fine pitch variation controls 44 and 46 (FIG. 1) enable a tuner to vary the frequency of all the notes up to one-half a semi-tone in either direction (i.e., an offset of ±3 percent of the nominal oscillator frequency), while preserving the correct relationship among the notes.
As shown in FIG. 2, the master clock oscillator 26 comprises a unijunction transistor 150 in a relaxation oscillator circuit. A temperature-compensating resistor 152 connects "base 2" to a conductor 154 from a power supply which provides a substantially constant voltage. An output resistor 155 is between "base 1" and ground. Two elements in a timing circuit generally control the nominal oscillator frequency--a variable capacitor 156 and a variable resistor 158. Thus, the voltage between base 1 and base 2 (E bb ) is substantially constant.
To set the oscillator initially, the capacitor 156 is adjusted so that the oscillator 26 generates its highest required frequency. This is done with the resistor 158 at a minimum value. Usually the resistor 158 comprises a switched resistance ladder network which enables the frequency for each setting of the note selector 28 to be adjusted independently. During calibration, the frequencies are adjusted for the correct mathematical relationship. A buffer amplifier 160 couples the signal from the output resistor 155.
The capacitor 156 and resistor 158 constitute two distinct means varying the frequency of the oscillator 26. In accordance with this invention, the oscillator 26 includes a third means for independently varying frequency over the offset range. As known, the unijunction transistor 150 discharges when the voltage across the capacitor 156 (i.e., the emitter voltage) reaches a threshold which is a substantially constant percentage (η) of (E bb ) which establishes a reference voltage (ηE bb ). The time it takes the capacitor voltage to reach that threshold is a function of the resistor and capacitor values and the voltage applied to the timing circuit.
In the oscillator 26 in FIG. 2, this voltage appears across a capacitor 166 and is equal to the voltage on the conductor 154 minus the voltage across a resistor 162. The voltage across the resistor 162 depends on the current through it and the current has two components. A first component is constant for a given setting of the note selector 28 and depends upon the voltage from the constant voltage power supply on the conductor 154 and the series impedance of the resistors 162 and 158.
The second component is variable in response to the setting of the pitch controls which constitute two independent variable voltage sources. A conductor 164 carries this second component. As the pitch controls increase this current component, the voltage drop across resistor 162 increases so the voltage across capacitor 166 decreases. As a result, the oscillator frequency decreases.
The remaining circuitry shown in FIG. 2 provides this variable second current component. A first resistor network constitutes a first variable voltage source and comprises a resistor 172 for coupling the conductor 164 to the wiper of a potentiometer 174, the potentiometer 174 being energized from the conductor 154. Variations in the position of the coarse pitch control 44 offset the wiper arm, which constitutes an adjustable tap, from a normal position. Positioning the fine pitch control 46 similarly alters the wiper arm on a potentiometer 176 also energized from the conductor 154. A resistor 178 couples this wiper arm to the conductor 164 and, together with the potentiometer 176, constitutes another variable voltage source.
The qualitative effect of varying either wiper arm position is the same. The component values are chosen so that a given physical displacement of the coarse pitch control 44 produces a large offset than the same displacement of the fine pitch control 46. Therefore, the following discussion relates only to the operation of the coarse pitch control 44.
Two relationships exist in this circuit. First, as apparent, the voltage on the conductor 154 is greater than the voltage on the conductor 164. Secondly, resistor 172 is at least an order of magnitude larger than resistor 162.
At a zero voltage offset position, there is a zero voltage drop across the resistor 172 so only the first current component flows through the resistor 162. If the coarse pitch control 44 is moved, the second current component from the conductor 164 changes the voltage across the resistor 162 and the capacitor 166.
Both pitch controls vary the frequency as a percentage of the base frequency, so these controls can be calibrated in "cents" difference to raise or lower the resulting frequency, assuming that the oscillator is calibrated with the ptentiometers 174 and 176 at their mid-points.
The tuning aid shown in FIG. 1 is sensitive and accurate. Tests shown that the display has visible motion when the phase shift is less than 10°, with the accuracy being dependent upon the time the tuning aid senses the tone and the stability of both the tone and note. This means that the tuning aid senses a frequency difference which produces less than a 10° phase shift over the interval the note signal exists. When operated from a battery power supply, the tuning aid is very stable. Tests against a tuning fork show no displacement after 10 seconds of tone. This increased sensitivity and stability have enabled me to analyze how pianos are tuned conventionally.
Therefore, it is apparent that there are many modifications and alterations which can be made to my tuning aid, the specifically described circuits and my method for tuning a piano. It is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.