US3881389A

US3881389A - Electronic guitar tuner

Info

Publication number: US3881389A
Application number: US361859A
Authority: US
Inventors: Frederick G Allen
Original assignee: F G ALLEN ASSOCIATES Inc
Current assignee: F G ALLEN ASSOCIATES Inc
Priority date: 1973-05-21
Filing date: 1973-05-21
Publication date: 1975-05-06
Anticipated expiration: 1992-05-06

Abstract

An electronic device for tuning the strings of the guitar or other musical instrument comprising of means for detecting the audible note and converting it into an electrical signal, means for filtering, amplifying, limiting and shaping such signal so that it drives a one-shot multivibrator whose average DC output is proportional to the frequency of the note, means for comparing this output with a DC reference voltage, and means for displaying the difference on a visible indicator showing whether the note sounded is flat, sharp or in tune.

Description

United States Patent 1 Allen 1 ELECTRONIC GUITAR TUNER [75] Inventor: Frederick G. Allen, Los Angeles,

Calif.

[73] Assignee: F. G. Allen Associates, Inc., Los Angeles. Calif.

[22] Filed: May 21, 1973 [21] Appl. No.: 361,859

[ May 6,1975

3/1973 Westhaver 84/454 1/1974 Zeiser ..84/454 [57] ABSTRACT An electronic device for tuning the strings of the guitar or other musical instrument comprising of means for detecting the audible note and converting it into an electrical signal, means for filtering. amplifying, limiting and Shaping such signal so that it drives a Fieid "g 5 455 shot multivibrator whose average DC output is proea c portional to the frequency of the note, means for comparing this output with a DC reference voltage, and [56] References Cited means for displaying the difference on a visible indica- UNITED STATES PATENTS tor showing whether the note sounded is flat, sharp or 2,901,699 8/1959 Motz et a1 84/454 X in tune. 3,180,199 4/1965 Anderson 84/454 3,433,166 9/1966 Althof 84/454 3,472,116 10/1969 Schott 84/454 3,631,756 l/l962 Mackworth 84/454 4 Claims, 2 Drawing Figures T a b c d T j u u T I T T zr'u I I I I I 1 iiilil lzl l l A II B D E F G IIIILTI- IIIBIIII- AIIPIIFY FILTER MIPLIFY SWITCHING VIBRATUR SWITCHING PIIIIIIE lIIIJ TRIIIISIS- UII'TIIIE TRANSIS- SIIAPE TllIlS T 10118 5SECOIIIIS I V aifi 3 H AR HER- llllllllllh, AGING all 1 1 [II I l I II l FILTER mm" A ENBE T 11 AM- 3 g "221: 1 I BATTERY 2 n astcoims SUPPLY PATENTEUMAY ems SHEET 1. OF 2 :21: h n EH5 2235 2 En 7 :2 5:552 55: L 5 wa -5 z :5 5 a -555 n 5:: 4 T g 1 M m mg 22 5 22 as; 22:: E25 225: a: =2: 25:5 =25; 25:5 :55 5:: :55 22: 52 a m u Q m 1 ELECTRONIC GUITAR TUNER DESCRIPTION The present invention is an electronic device that enables anyone to tune the notes of a stringed instrument such as a guitar exactly and simply to frequencies of standard pitch or to the pitch of a second instrument such as a piano. The device works on either a plucked or on a continuous note. Tuning is accomplished by selecting the desired frequency by a switch, plucking the note and adjusting the string tension till a visible indicator, such as a meter, reads zero. No audible tone discrimination by the operator is involved. The unit is portable, battery-operated, and small enough to be carried about in a guitar case with the guitar.

The device works by converting the audible note into a DC electrical signal. This is accomplished by driving a one-shot multivibrator with the note so that it delivers square pulses at the frequency of the note and ofa predetermined width or on-time. The average DC value of this signal, developed across a smoothing filter, is then proportional to the note frequency. This average value is then compared with a DC reference signal generated in the device, and the difference is indicated on an easy-to-read meter, showing whether the note is sharp, flat, or on tune.

This device differs from prior inventions principally in the fact that no oscillating elements, mechanical or electrical, are used as frequency references. This feature makes the present device one of great simplicity and reliability.

If desired, all reference frequencies are readily adjusted by the same ratio to match those of another instrument by making a single internal voltage adjustment till the meter reads zero when the device picks up a desired note of the other instrument.

The frequency selector switch may consist of six push buttons, each button being clearly labelled with the note of the corresponding guitar string, E, B, G, D, A and E. The buttons are arranged from low to high in the same order as the strings to make it simple for the operator to associate the buttons with the correct strings. The push button switch also permits instant change from one string to any other without going through all intermediate positions.

OPERATION OF CIRCUIT The operation of the circuit can be understood by reference to the Block Diagram, FIG. 1, and the Circuit Diagram, FIG. 2.

The plucked note from the guitar or other stringed instrument dies off exponentially over several seconds, (typically seconds) as shown at@, FIG. 1. The audi ble note is picked up by a Microphone A, converted thereby to an electrical signal and fed to an initial stage of amplification at B, which may conveniently be an operational amplifier with feedback, as shown in FIG. 2. The signal from an electrical guitar can be fed directly into the phone jack at A, which then disconnects the microphone.

Because the notes of stringed instruments are extremely rich in harmonics, extensive filtering is required to eliminate all but the fundamental tone. This is achieved in the Filter C which may be an RC active filter using several operational amplifiers, C C and C The bandpass must be at least one half-note wide (6 percent fractional bandwidth) for ease in tuning but must reject the second and higher harmonics by a factor of 10 to 20. The filter is tuned to any one of the six frequencies used on the guitar by the push-button switch S which changes resistors R in the RC network of the filter comprising R and the two capacitors labelled C in FIG. 2. The filter also provides some amplification of the signal.

The shaping stage D clips the sine waves from the filter by overdriving an operational amplifier, D. Thus the exponentially decaying train of sine waves entering this stage leaves it as a train of equal amplitude nearly square waves as shown at@ of FIG. 1. The train is several seconds long, and decays only when the amplitude of the signal entering D no longer saturates the operational amplifier.

The nearly square waves leaving the shaping stage are further shaped into sharp, exact square waves of amplitude equal to the battery voltage V,, by the switching transistors E to ensure a precisely-timed trigger pulse to the multivibrator, F.

The multivibrator F is triggered into the on condition by the positive-going leading edge of the square wave from E. It remains on for a time T which is determined by the resistance R and the capacitance C in an RC discharge circuit, and then returns to its initial state, ready for the next input pulse. The correct on time T for each of the six notes of the guitar is determined by the value of R selected in the multivibrator by the same push button switch S that selects the correct frequency in filter C.

The output signal of the multivibrator is squared up and restored to full battery voltage amplitude, V,,, by the switching transistors G. This squared output is then fed to an averaging filter, l-I, consisting of a lowpass R-C combination R and C whose time constant is long compared to the period of the lowest guitar note (Hz) but short enough to allow the meter to respond quickly when a note is struck. (An R-C time constant of 0.09 seconds is satisfactory). The output of the averaging filter H is then.a DC component equal to V with a small ripple superposed as shown at@, FIG. 1. It rises from zero to its final value in afraction of a second after the note is struck, remains constant for several seconds, then decays again, as shown at@, lower left of FIG. 1.

The average DC signal, V will be just the ontime, T divided by the total time of one cycle, T, times the supply voltage, V,,, or

When the note is correctly tuned, its period T will be equal to T and the correct average output voltage for that note will then be 0 (TM/T0) 11 By choosing T as a constant fraction r, of the cor- -rect period of the note T,,, or T /T r, for all six notes will be'in error by the inverse ratio of the frequency, or thus f fn o/ Then since T T,,+AT we have tfo+ f)/f To/(To-i-AT) Then the average voltage delivered will now be Thus the average or DC output voltage of G, V will be linearly proportional to any frequency error in the guitar note; it will be larger or smaller than the correct reference voltage, V depending on whether Af is positive (note is sharp), or negative (note is flat).

Note also that the voltage output of G is directly proportional to the battery voltage, V,,.

A reference voltage V 'is supplied by a voltage divider taking a fraction r of the battery voltage V,,, so that V r V,,. Only one value of V is needed.

The fraction r is chosen so that the reference voltage will match the correct note voltage for all notes, so that Thus, we must choose r r For our present case we use a value of r and r slightly greater than V2. This keeps the multivibrator locked in the on position so that it cannot respond to a second rising edge after one half a period as caused by the undesired second harmonic frequency, most likely to get through.

The outputs of the note, V and the reference voltage V are now compared by applying them to the two inputs of a difference amplifier, 1, whose output drives the zero-center null meter J, visible on the front panel. When the note is exactly in tune we will have so that there is no difference to be amplified and the meter reads zero. When the note is sharp, V is too high by the amount (Af/f,,)V and the meter deflects upward; when the note is flat the meter deflects downward by the same amount. The sensitivity of the difference amplifier and null meter is such that a frequency error of 1 percent can be readily detected. This corresponds to 1/6 of one half-note on the musical scale. For the lowest note of the guitar (f 80Hz) this corresponds to about lHz, and for the highest note (f--32OHz) this corresponds to about 3H2.

Because all six guitar notes, when properly tuned, produce the same output signal, V a given per cent change in the frequency of any note will produce the same null meter deflection. Since a musical half-tone is always the same percent change in frequency from a given note (6 percent), our null meter has the same sensitivity in tuning all guitar notes.

To tune instruments satisfactorily, the device must indicate frequencies reproducibly and accurately to about 1% for any battery voltage and temperature to be encountered.

The circuit is automatically compensated for the principal variable, the battery voltage V since both the average signal output V and the reference voltage V are directly proportional to V,,. Their difference, V V is given by and will thus be zero when T /T r (note correctly tuned) regardless of the battery voltage V,,. Thus, changes in battery voltage do r change the accuracy of the null reading or the pitch of the note. Such changes will, of course, change the sensitivity of the meter, but these changes are relatively small.

The only other critical part of the circuit is the multivibrator that determines the on-time T Commercially available multivibrators hold variation of T with battery voltage to less than 1 percent over a 20 percent change in V R and C in the multivibrator are chosen stable to /2 percent over the operating temperatures to be encountered.

If it is desired to tune the guitar to another instrument such as a piano not in standard pitch, the present tuner can readily have all its reference notes shifted up or down by exactly the same ratio by adjusting the reference divider ratio r up or down until the proper piano notes read null on the meter. r then becomes a FrequencyAdjust setting. Thus, supposing the piano notes are all too high by the ratio f,,/f m. Then the periods of all pianonotes are low by the same ratio, so that If we now adjust the ratio r to r until one note of the piano reads null on the meter, then we will have M o 2') n 0 so that the corrected r is given by r T IT mT /T mr Thus, the new setting of the Frequency Adjust ratio potentiometer r will be m times its former value, and thus all reference notes of the tuner will now be shifted upward by the exact ratio in as desired, by a simple potentiometer adjustment of r The functions described in the Block Diagram, FIG. 1, are carried out by the circuit elements shown in the diagram of FIG. 2. All transistors, operational amplifiers and the multivibrator used are commercially available, are non-critical as to exact specifications, and 0perate from a low voltage (5 volts) supplied by several (4) nickel-cadmium cells, re-chargeable from any 1 10 volt 60l-lz power outlet by the circuit shown on FIG. 2.

The circuit works well for the following parameter values: open loop gain of operational amplifiers of 1000 or more; gain of stage B at 20; gain of Stage I of 15; R of about K ohms; R of about 50 K ohms; C, of 0.01 microfarads; C of 0.2 microfarads; R of 31 K ohms; C of 3 microfarads; transistors are silicon high beta, low current drain, (Fairchild 7163 and National S.C. PT-4-7l64); multivibrator is Texas Instrument SN74Ll2l.

I claim:

1. An electronic device for tuning a stringed instrument comprising: transducing means for converting into an electrical signal the acoustic vibrations of a struck, plucked or bowed string; filter means for removing unwanted noise and overtones from said signal; amplifier and limiting means for rendering all excursions of said signal of equal amplitude for a sufficient time after the striking of the string to permit frequency measurement thereof; one-shot multivibrator means responsive to said signal for delivering pulses having frequency of said signal and having predetermined width determined by the internal parameters of said multivibrator means, so that the average DC value of said pulses becomes proportional to the frequency of said signal; filter means to extract the average DC value of said pulses; a DC reference voltage; means for comparing said DC reference voltage and said average DC value, thereby to indicate whether the fundamental frequency of the struck string is above or below a predetermined frequency; a single DC supply voltage; transistor switching means following said multivibrator means for making said average DC value of pulses proportional to said DC supply voltage; voltage divider means for making said DC reference voltage also proportional to said DC supply voltage, so that both average DC value of pulses and reference voltage rise and fall together with changes in supply voltage, with the result that when the two voltages are matched by tuning to the correct frequency, their difference continues to be zero over a considerable range of supply voltage, making regulation unnecessary.

2. An electronic device for tuning a stringed instrument comprising: transducing means for converting into an electrical signal the acoustic vibrations of a struck, plucked or bowed string; filter means for removing unwanted noise and overtones from said signal; means for altering the center frequency of said filter means; amplifier and limiting means for rendering all excursions of said signal of equal amplitude for a sufficient time after striking the string to permit frequency measurement thereof; one-shot multivibrator means responsive to said signal for delivering pulses having the frequency of said signal and having predetermined width so that the average DC value of said pulses becomes proportional to the frequency of said signal; filter means to extract the average DC value of said pulses; at least one DC reference voltage; means for comparing said DC reference voltage and said average DC value, thereby to indicate whether the fundamental frequency of the struck string is above or below a predetermined frequency; and means for altering the width of said pulses, whereby a plurality of different notes of the stringed instrument may be frequency measured by the device.

3. Device in accordance with claim 2 wherein said means for altering the width of said pulses is inversely proportioned to the string frequency to be measured, such that the pulse width is the same fraction of the multivibrator periodicity for each string note to be tuned, thereby producing the same average DC value of multivibrator output for all notes to be measured, so that a single value for said DC reference voltage accommodates all notes to be measured.

4. Device in accordance with claim 3 comprising: means for adjusting said DC reference voltage; a single adjustment of said reference voltage serving to shift each of said predetermined frequencies by exactly the same percentage.