US 3722353 A
An electronic tuning device for tuning stringed instruments provides a selectable sinusoidal reference frequency which is converted to pulses of the same frequency. The fading picked-up signal from a string of the instrument is passed to a high-gain saturating amplifier and converted to a sustained square wave which lasts of the order of 5 to 10 seconds as the string signal becomes minimal. The frequency of the square wave is then compared with the pulse frequency by supplying the pulses and the square waves to a comparator gate whose output controls a glow discharge lamp attached to the device. When the string of the instrument is tuned to bring the frequency of the square wave close to that of the pulses, the frequency difference to be minimized is indicated by the observable blink frequency of the lamp. The instrument responds similarly to one octave above the tuned frequency. Provision is also made to deliver the selected reference frequency to the usual musical instrument amplifier to enable aural tuning by a group of musicians.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Westhaver Mar. 27, 1973  ELECTRONIC TUNING DEVICE FOR VISUAL TUNING OF STRINGED INSTRUMENTS [76 Inventor: Lawrence A. Westhaver, 13001 Old Stagecoach Road. Laurel. Md.
221 Filed: June11,1971 21 Appl.No.: 152,246
52 US. cl... ..s4/454, 324/79 R 51 Int. Cl. ..G10g 7/02 58 Field Of Search ..84/454; 324/791:
 References Cited UNITED STATES PATENTS 3,509,454 4/1970 Gossel ...s4/454ux 3,180,199 4/1965 Anderson"... ....s4/4s4 2,207,450 7/1940 Bergan et al. ....84/454 3,144,802 8/1964 Faber et al. ....84/454 3,631,756 1/1972 Mackworth-Young ..84/454 Primary Examiner-Richard B. Wilkinson Assistant Examiner-John F. Gonzales Attamey-James W. Westhaver  ABSTRACT An electronic tuning device for tuning stringed instruments provides a selectable sinusoidal reference frequency which is converted to pulses of the same frequency. The fading picked-up signal from a string of the instrument is passed to a high-gain saturating amplifier and converted to a sustained square wave which lasts of the order of 5 to 10 seconds as the string signal becomes minimal. The frequency of the square wave is then compared with the pulse frequency by supplying the pulses and the square waves to a Y 4 Claims, 6 Drawing Figures SELECTABLE STANDARD PULSE DRIVER a FREQUENCY 32:37: moiialoR OSCILLATOR L GUITAR SATURATING COMPARISON S'GNAL AMPLIFIER m GATE PATENTEUHAR2H975 3,722,353
sum 1 or 5 SELECTABLE PULSE DRIVER a STANDARD m S'GNAL AMPLIFIER m GATE LI Ll U L! U U H H H Fl H FIG. 3
INVENTOR LAWRENCE A. WESTHAVER PATEHTEDHmHm 3,122,353
sum 2 OF 5 INVENTOR LAWRENCE A. WESTHAVER PATENTEnnARznma SHEET 3 [IF 5 Q GE mm @N INVENTOR LAWRENCE A. WESTHAVER PATENTEUmznma SHEET '4 OF 5 INVENTOR LAWRENCE A. WESTHAVER mm m mm mm vm mm PATENTFDmzmu SHEET 5 [IF 5 INVENTOR LAWRENCE A. WESTHAVER ELECTRONIC TUNING DEVICE FORVISUAL TUNING OF STRINGED INSTRUMENTS PURPOSES OF THE INVENTION The present invention was developed to meet the need for a compact unitary device capable of visually accurate tuning of stringed instruments.
The primary object of the invention is to provide a relatively small portable electronic device capable of accurate tuning by visual observation of a lamp which indicates the disparity between string frequency and a standard frequency.
Another objec'tis to provide such a device with a selection of standard frequencies for the various strings of a guitar or other stringed instrument.
A further object is to provide such a device witha selection of standard frequencies for input to the'usual instrument amplifier-speaker to enable aural tuning of other instrument in a band while also providing visual tuning for the operator of the device.
THE PRIOR ART In' prior systems of the electronic tuning, agreement of the instrument pickup frequency has been sensed either visually or audially. Various visual sensing arrangements are described in U. 8. Pat. No. 3,144,802 and include use of an oscilloscope, a stroboscope, or a digital'read-out of the frequency of the instrument being tuned.
Audialsensing is illustrated in U. S. Pat. No.
3,501,992, this patent employing a resistor-capacitor type of generator for the reference frequency. It is also known to use digital circuitry to compare the instrument frequency with a reference signal. In U. S. Pat. No. 3,472,116, digital circuitry is used to compare the divided frequencies representing the twelve tones of an octave.
SUMMARY OF THE PRESENT INVENTION In the present invention, the device includes an electronic system compactly built into a casing having dimensions of about two by .eight by eight inches and having in visible position a glow lamp responsive to the frequency disparity. The device is readily portable and may be mounted on a music stand or in the usual guitar instrument amplifier, or may be placed atop a piano. Within the casing is an electronic system comprising a power supply, a generator of selective reference frequencies, a high-gain saturating amplifier for the string frequency being tuned, a comparator gate receiving the reference and string frequencies and a connection from the comparator gate to the glow lamp to visually indicate the frequency difference. The string frequency from the pickup of the instrument is converted to a square wave while the reference frequency is converted to pulses and the two are combined in the gate in such manner that the lamp is energized by the gate only when square wave input and pulse input are simultaneously present and in phase.
Connections to the device include a plug-in input cable from the instrument pickup, an input cord for A. C. power and a plug-in output cable from the device to deliver the reference frequency to the usual instrument amplifier. However, if the device is. built into the usual amplifier for the musical instrument, the cable and cord connections may be replaced by permanent wiring and a switch used to deliver the string signal either to the device or to the instrument amplifier.
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a system diagram indicating the major circuits involved.
FIG. 2 shows the device in a commercial package with appropriate input and output.
FIG. 3 indicates the waveforms present at several points in the circuit for a fundamental and an octave higher input.
FIG. 4 is a wiring diagram showing one embodiment of the pulse forming circuit, saturating amplifier, comparison gate and driver and indicator lamp.
FIG. 5 is a wiring diagram showing one embodiment of the selectable standard frequency oscillator.
FIG. 6 is a wiring diagram of a power supply suitable for powering the above mentioned circuits.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 a selectable standard frequency oscillator provides for guitar tuning purposes, one of six sinusoidal signals to the input of a pulse forming circuit. The sinusoidal signals must have a high degree of frequency stability and at least short term amplitude stability. For tuning a conventional electric guitar to American Standard pitch, the six frequencies should be: 82.4H, (E 110.0H (A 146.8H, (D 196.0H (G 246.9I-I (B and 329.6H, (E
With a sinusoidal signal input, the pulse forming circuit will produce a sample pulse for every cycle of the input signal. The time duration of the sample pulse will be a fixed percentage of the period (l/f) of the input signal. The duration of the sample pulse, determined by the circuit parameters, is made significantly less than 25 percent of the period as will hereinafter be explained.
The sample pulse thus generated is presented to one input of a two-input comparison gate.
The signal produced by a plucked instrument string will approximate a damped sinusoid. If on an electric guitar the string is plucked at the twelfth fret, a strong fundamental with a minimum of harmonics with be produced by the pick-up. In any case the string should be plucked at the mid-point of its effective length.
Since the signal from the string decays rapidly, if a usable output from the saturating amplifier is to be maintained for a protracted length of time, i.e., 5 to 10 seconds, the amplifier must possess high gain. The function of the saturating amplifier and its circuitry is to produce a lasting square wave output signal ofthe same frequency as the damped sinusoid string signal at the input of the amplifier.
This square wave signal is presented to the second input of the two-input comparison gate.
The comparison gate inputs are constrained to have signal swings limited by ground and plus 3.6 volts. The comparison gate is of the NOR" type, i.e., if either input is high (near +3.6 volts) the output of the gate will be low" (near-ground) and if both inputs are low" the output will be high.
The comparison gate output will switch between two voltage levels. The output voltage level corresponding to the condition of the inputs in which both inputs are at the level of the sample pulse will cause the driver to turn on the indicator lamp which is an instantly responsive neon type. When either one or both of the cornparison gate inputs are not at the voltage level of the sample pulse, the output of the comparison gate will be at the other voltage level and the driver will extinguish the lamp.
If the guitar signal causes the saturating amplifier to produce a square wave that has exactly the same frequency as the selectable standard frequency oscillator, the sample pulses and the square wave will have one of the following time relationships: the square wave voltage level is the same as the sample pulse when the sample pulse occurs, the square wave voltage level is not the same as the sample pulse when thesample pulse occurs, or the square wave is switching during the sample pulse duration.
In the first relationship the output of the comparison gate will be a train of pulses (the sample pulses propagating thru) each one of which will cause the driverto light the indicator lamp for its duration..The indicator lamp flashes with each sample pulse. Even the lowest musical frequency is higher than the highest visually perceivable flicker rate. Thus the eye perceives the indicator lamp, which is actually flashing at a rate determined by the selectable standard frequency oscillator, to be constantly on.
In the second relationship the square wave voltage level is not the same as the sample pulse and the output of the'comparison gate will be a steady voltage level which will, thru the driver, keep the lamp extinguished.
In the third time relationship the square wave switches during the occurrence of the sample pulse. The comparison gate output will be a train of narrow pulses since the gate will propagate thru only that portion of the sample pulse that occurs when the voltage level of the square wave is the same as the voltage level of the sample pulse.
If the guitar signal thru the saturating amplifier produces a square wave that is slightly lower or higher in frequency than the output of the selectable standard frequency oscillator, the neon indicator lamp will appear to blink on and off. As an example let there be a one fifth of a cycle per second difference between the guitar signal and the selectable standard frequency oscillator. This means that the time relationship (or phase) of the square wave will shift with respect to the sample pulse and will be shifting at the rate of one cycle every five seconds. While the square wave voltage level is the same as the sample pulses during the pulses occurrence there will be an apparent steady on condition of the indicator lamp. As the phase drifts the lamp will appear to dim as the sample pulse occurs while the square wave switches and finally the lamp will extinguish as the voltage level of the square wave is different than that of the sample pulse during the occurrence of the sample pulse. As long as there is a frequency difference this drift will continue and there will be a blinking of the indicator lamp.
The blink rate as perceived by the eye may be used to determine the frequency difference by the following formula:
Blinks/sec cycles/sec of frequency difference.
. string frequency and the standard frequency.
As previously noted the sample pulse width is made significantly less than 25 percent of the period of the fundamental. This is done in order to achieve the same tuning indication when the guitar input is one octave higher in frequency. On a standard guitar depressing any string on the twelfth fret will raise the pitch of the string by an octave if the bridge is properly adjusted. If after using the system to tune an open string the string is depressed at the twelfth fret and plucked, the lamp flicker will indicate the degree of bridge adjustment. By alternately adjusting the bridge and the string tension, the pitch of the open string and the pitch of the twelfth fret octave may both be tuned. When this is done the other fret positions will .be very closely tuned.
FIG. 2 illustrates one commercial form of the instrument. A small metal enclosure 1 has a power-on and string-frequency-selector switch 4. The switch is shown with the pitches of the six strings of a standard electric guitar: B A D G B and E Other pitches could be incorporated to accomodate the tuning of other instruments.
Indicator neon lamp 3 provides the visual means of indicating the degree of tuning.
Jack 2 accepts the signal from electric guitar 6 by way of a cable and phone plug 5 which is inserted.
Audio connector 8 may be inserted in a jack 93 which is mounted on the back of the instrument.
Jack 93 carries a'signal from the selectable standard frequency oscillator. If, via audio connector 8 and a cable, the signal is connected to the input of an instrument amplifier-speaker system it would allow a number of musicians to tune by ear" to the standard pitch selected by switch 4. This does not prevent the guitarist from tuning visually at the same time and he may thus compare his aural sense of tuning with the more accurate visual indication.
Plug 7 provides a power connection for the instrument to the AC. mains.
In FIG. 4 the jack 2 is shown as providing the input connection for the guitar signal. The guitar signal is coupled thru electrolytic capacitor 9 to the base of transistor 14. Collector load resistor 11, base bias resistor l0, emitter resistors 15 and 17, by-pass capacitor 16 together with transistor 14, form a linear amplifier. Base bias resistor 10 provides some negative feed-back to the base. This amplifier stage is designed to have a quiescent collector voltage of one half the supply voltage (+10 volts). Resistor 13 couples the linear amplifier output to the non-inverting input (pin 5) of operational amplifier l9. Resistor 12 together with resistor 20 form a negative feed-back loop from the output (pin 10) to the inverting input (pin 4) of the operational amplifier. This loop establishes the output quiescent point of amplifier 19 at approximately the same voltage as is present at the collector of transistor 14.
Resistor 13 is made approximately equal to the parallel combination of resistors 12 and 20, to minimize drift due to the amplifier input currents.
Under steady state conditions the quiescent voltage at the collector of transistor 14 is present at both the inverting and non-inverting inputs of amplifier 19. However, when a signal is present at the collector of transistor 14 it willbe decoupled from the inverting input by capacitor 18.
The operational amplifier 19 in this configuration has good quiescent output. voltage stability and yet exhibits an open loop gain for the signal present at its non-inverting input. A low amplitude signal from the linear amplifier will cause the output of amplifier 19 to switch between the limits of the power supply voltage thereby producing a square wave of the same frequency as the input.
The quiescent output voltage of amplifier 19 is blocked by capacitor 21. The square wave output signal when present is coupled thru capacitor 21 across a resistive divider consisting of resistors 22 and 23. The resistive divider reduces the amplitude of the square wave and reduces the loading of the output of amplifier 19. The output of the resistive divider is, connected to the base of transistor 25 and the cathode of diode 24. The square wave signal from the divider will be symetrical with respect to ground. Since the base of transistor 25 will be conducting for only the positive half cycle, diode 24 must be'used to conduct during the negative half cycle. Otherwise a base rectification effect would occur.
With collector load resistor 26 returned to plus 3.6 volts the collector output of transistor 25 will be a square wave limited by ground and plus 3.6V.
The square wave output thus limited is connected to one input (pin 1) of a two-input NAND/NOR gate, one of two such gates contained in integrated circuit 32. If both inputs tothis gate are low (near zero volts) the output will be high (near +3.6 volts). For either or both inputs high the output will be low.
Circuit point 37 is connected to the output of the selectable standard frequency oscillator hereinafter to be described. Therefore a sinusoidal signal of several volts amplitude will-be present at circuit point 37.
The charging and discharging of coupling capacitor 27 is determined by the subsequent network. For any long term average the charging current must equal the magnitude of the discharging current for each cycle of the input signal. For a positive-going input signal the charge path would be resistor 28 in parallel with the series combination of resistor 29 and the emitter-base junction of transistor 30. A negative-going input signal would have a discharge path consisting of only resistor 28 (the emitter-base junction of transistor 30 would be reverse biased and consequently non conducting).
Assuming the base-emitter resistance and forward voltage drop of transistor 30 to be negligible, the transistor could be made to draw base current for only 60 of the input sinusoidal signal by setting the resistance ratio of resistor 29 to resistor 28 equal to l/27.
With a high current gain, transistor 30 will be saturated for 60 of the input sinusoidal signal and cut off for the remaining 300.
Resistor 31, the collector load for transistor 30, is connected to the plus 3.6 volt supply. The output signal at the collector of transistor 30 will be a series of pulses limited by plus 3.6 volts and ground.
Referring to FIG. 3, waveform A is a square wave signal such as would be present at the collector of transistor 25- when the fundamental guitar signal is above the frequency of the selectable-standardfrequency oscillator.
Waveform B is the signalpresent at the collector of transistor 30 when the proper frequency is being generated by the selectable standard frequency oscillator.
in H0. 4 pin 1 and 2 of integrated circuit 32 are inputs to a NAND/NOR gate the output of which is on pin 7 With the guitar-signal-derived square wave provided as an input to pin 1 and the sample pulses, derived from the selectable standard frequency oscillator, provided as an input to pin 2, the output signal of the NAND/NOR gate is shown as waveform C in FIG. 3. The first two sample pulses (B) are in phase with the square wave (A) and are propagated thru the gate to appear inverted at its output (C). The third sample pulse (B) is propagated thru only until the square wave switches. When the square wave switches it disables the gate. Thus only the first part of the third sample pulse. is
propagated thru the gate.
The fourth, fifth and sixth sample pulses (B) occur out of phase with the square wave (A), which disables the gate, and are thus'not propagated thru. Theresult from output (C) is a blinking of the indicator lamp at 'an observable frequency equal to the difference between the frequencies of (A) and (B).
Waveform D is a square wave signal such aswould be present at the collector of transistor 25 when the guitar signal is raised by one octave. Waveform E is the same as waveform B.
The waveform D is higher in frequency than twice the frequency of the selectable standard frequency.
Sample pulses 1, 3 and 6 (E) are not propagated thru the gate for their full duration due to the square wave (D) switching during their duration. Sample pulse 2 (E) is out of phase with the square wave (D) and thus is not'propagated to the output of the gate (F). Sample pulses 4 and 5 (E) occur in phase with the square wave (D) and are fully propagated thru the gate. They appear inverted at the output (.F). The result from output (F) is again a blinking of the indicator lamp but at a higher rate than for output (C).
When the guitar signal fundamental frequency matches the frequency of the selectable standard frequency oscillator, the output of the gate will either be a continuous stream of pulses or no pulses at all. Since the two frequencies match, there will be no relative shift of phase and the gate output will be determined by the random phase relationship established when the instrument string is plucked.
If the guitar signal is exactly an octave higher than the frequency of the selectable standard frequency oscillator the sample pulses will occur every other cycle of the square wave and the relative phase will not change. The output of the gate will again be determined by the random phase relationship established when the instrument string is plucked.
As can now be seen, the sample pulse is made less than 25 percent of the period of the fundamental frequency in order to check the tuning of the string ata frequency one octave higher than the fundamental. In theory the sample pulses may be made shorter to check higher octaves but other considerations make more than two octaves impractical. if the fundamental were the only frequency to be tuned the sample pulses could .be almost as wide as one half the period of the fundamental.
The output of the gate of FIG. 4, the operation of which was just described, (pin 7) is connected to an input (pin 3) of a second NAND/NOR gate in integrated circuit 32. The second input (pin 5) of the gate is grounded, therefore the gate secures only to invert the signal from the first gate. The output of the second gate (pin 6) willbe the complement of the output of the first gate (pin 7).
As was shown in FIG. 3 (waveforms C and F) the output of the first gate (pin 7) is low (near zero volts) in the absence of propagated sample pulses. The propagated sample pulses cause the output (pin 7) to go high (near +3.6 volts).
Since the second gate complements its input, the output (pin 6) will be high" in the absence-of propagated sample pulses and will go low with each propagated sample pulse. 7 v
The output of the second gate ,(pin 6), thru current limiting resistor 33, controls the base current of a high voltage switching transistor 35. When theoutput of the second gate (pin 6) is high, (near +3.6 volts) sufficient base current is provided to transistor 35 to saturate it. The collector voltage of transistor 35 will be near zero. All collector current thru collector load resistor 36 from the 100V supply will flow thru transistor 35 to ground and the glow-discharge indicator lamp 3 is extinguished.
A low output (near zero volts) from the second gate (pin 6) will be further reduced by the division of voltage by resistors 33 and 34, ensuring that the voltage at the base of transistor 35 will be below that required for base current flow.
Transistor 35 is thus in a non-conducting state and the collector voltage will rise, limited only by the ignition voltage of the glow-discharge indicator lamp 3 which is now lit.
Thus, when a sample pulse is propagated thru the first gate (pin 7) it will cause glow-discharge indicator selected between switch decks 4,,- and 4, and resistor 57 in series with the trimmer potentiometer selected by switch deck 4,. The second section consists of the capacitance selected between switch decks 4,, and 4 and resistor 76. The third section consists, of the capacitance selected between switch decks 4 and 4,, and the parallel combination of the resistance of resistors 77 and 79.
Switch 4 consists of 6 decks 4A, (shown in FIG. 6)4B,4C,4D,4E and 4F. All decks are shown in the full counter clockwise, or power off, position. Turning the switch one or more positions clockwise will turn power on as is shown in FIG. 6.
in the first clockwise position capacitors 75, 72, 69, 66, 63 and 60 are paralleled in the first section of the network, and the resistance is trimmed by potentiometer 51 for the proper frequency. In the second section of the network capacitors 74, 71, 68, 65, 62 'and 59 are paralleled. in the third section of the network, capacitors 73, 70, 67, 64, 61 and 58 are paralleled. This position of switch 4 corresponds to the lowest frequency, E (82.4 H,), of the oscillator. V
In the second clockwise position capacitors 75, 74 and 73 have been disconnected from the first, second and third network sections respectively. The total parallel capacitance of the sections is reduced and the frequency generated is higher, A (110.014,). Poten tiometer 52 provides means to adjust this frequency accurately.
in each successive clockwise position, the
, capacitance of each section of the network is reduced by the disconnecting of capacitors 70, 71 and 72 (third position), 67, 68 and 69 (fourth position), 64, 65 and 66 (fifth position) and 61, 62 and 63 (sixth position). in the sixth position, only capacitors 60, 59 and 58 are operative in the circuit. I
The frequencies may be adjusted by potentiometers 53, 54, 55 and 56 which corresponds to D; (146.8H,), G (l96.0H,), B (246.9H,) and E, (329.611,) respectively.
Resistors 77 and 79 (the parallel combination of which form the resistance of the last network section) form a voltage divider which biases the gate of an N- channel field effect transistor 78. Source load resistor 80 completes the source-follower circuit configuration. The source-follower circuit provides a high impedance input for minimum loading of the phase-shift network and'a low impedance non-inverting output with less than unity gain. Capacitor 81 couples the signal from The amplitude of the oscillation, normally limited by.
the supply voltage, is further limited by the loading of the resistive divider, consisting of resistors 86 and 87, thru diode 88. Quiescently, the collector voltage of transistor 84 is lower than the voltage at the cathode of diode 88. Thus, until the signal at the collector exceeds the voltage at the cathode of diode 88, the resistive divider does not load the collector of transistor 84. For
the period of time when the signal does exceed the voltage at the cathode of diode 88, the effective collector load resistance of transistor 84 will consist of the parallel resistance of resistors 83, 86 and 87. This parallel resistance is below the resistance required as a collector load to sustain oscillations. The signal is thus limited with negligible distortion.
Transistor 89 together with emitter load resistors 90 and 91 form an emitter-follower circuit. The collector output of transistor 84 is directly coupled to the base of transistor 89. The emitter-follower provides a high inputimpedance, negligible loading of transistor 84, and a low output impedance sufficient to drive the phase shift network. Circuit point 37, the output of the emitter-follower, is connected to the input of the pulse forming circuit shown in FIG. 4. A small fraction of the emitter-follower output signal is coupled to jack 93 by capacitor 92. Jack 93 is located on the rear of the metal enclosure 1 shown in FIG. 2. The usual instrument amplifier-speaker unit, with the signal from jack 93 as its input, may be used to provide selectable pitches for aural tuning. Several members of a band could thus tune aurally at the same time the guitarist is tuning visually.
In FIG. 6 circuit points7A and 7B are connected to the plug 7 shown in FIG. 2. A.C. power is thus supplied to transformer primary 37A of transformer 37 thru power switch 4A, operated with selector switch 4 of FIG. 2.
The output voltage of secondary winding 37 B is halfwave rectified by diode 38 and filtered by capacitor 39. The unregulated 100 V.D.C. supply thus formed is used to power the glow-discharge indicator lamp circuit.
Diodes 40 and 41 full-wave rectify the output voltage of secondary winding 37C. The rectified output voltage, filtered by capacitor 42, supplies two voltage regulators.
Current limiting resistor 44 together with zener diode 43 form a simple shunt voltage regulator. The regulated voltage (+3.6V) supplies integrated circuit 32 and the collector circuits of transistors and of FIG. 4.
Transistor 46 of FIG. 6 is the series regulator for the second regulated supply.
The base voltage of transistor 46, and consequently the output voltage, is controlled by the voltage reference and error amplifier 47. Base supply resistor 45 forms the load resistance for voltage reference and error amplifier 47. The output voltage is determined by the ratio of the resistance of resistors 48 and 49 and the internalvoltage reference in amplifier 47. Capacitor 50 serves to filter any higher frequency signals that would be generated by the driven circuits to which the regulator may not respond. This regulated supply voltage (+20V) powers the selectable standard frequency oscillator (FIG. 5) and the saturating amplifier (collector circuit of transistor 14 and integrated circuit 19) of FIG. 4.
While the foregoing are examples of specific circuitry and components, it will be understood by those skilled in the art that the invention is not limited thereto and may be practiced with other circuitry for the tuning of any stringed musical instrument.
1. An electronic tuning device for stringed instruments comprising: I
a. a high-gain saturating amplifier having an input for the picked-up signal of the string to be tuned; said amplifier including circuit means for converting the decaying sinusoidal signal of the string into a square wave output signal of the same frequency and lasting from 5 to 10 seconds; a generator of a reference frequency; circuit means connected to said generator for converting the reference frequency into output pulses of the same frequency; an electronic gate having a first input connected to receive said square wave output si nal and a second input connected to receive sai output pulses;
f. circuit means connected to said gate and operative to deliver an output from said gate only when the first and second inputs are simultaneously present and of the same phase;
. a glow lamp connected to receive the output from said gate to thereby indicate visually the frequency difference between the square wave and the pulses;
said circuit means converting the reference frequency into output pulses being constructed to produce a pulse width less than 25 percent of said square wave wavelength, whereby the lamp responds to tune a next higher octave.
2. The device of claim 1 having a plurality of generatory of reference frequencies and switching means to selectively render operable each of said generators.
3. The device of claim 2 wherein each of said generators is formed by a resistor-capacitor network and each generator includes a trimmer resistor to standardize the frequency thereof.
4. The device of claim I having connecting means leading from said generator to deliver a reference frequency to an amplifier-speaker for aural tuning of a band.