Apparatus for automatically tuning an electronic musical instrument

An apparatus for the automatic tuning of an electronic musical instrument comprising a phase comparator, a converter circuit connected to the comparator for converting an output signal of the phase comparator into a voltage of a magnitude corresponding thereto, a sample hold circuit connected in series with the converter circuit, a calculation circuit having a first input connected to the sample hold circuit and a voltage control type oscillator for generating a musical tone signal connected in series with the calculation circuit. The voltage control type oscillator is connected to an input of the phase comparator and another input of the phase comparator is connected to a circuit for producing a standard signal. A key signal generating circuit serves to generate a voltage corresponding to a depressed key and the key signal generating circuit is connected to an input of the calculation circuit, the key signal generating circuit being provided with a gate circuit which passes a key signal corresponding to a specific key. The gate circuit and the sample hold circuit have control electrodes connected to the output of a monostable multi-vibrator whose operation is controlled by a tuning switch.

FIELD OF THE INVENTION 
This invention relates to an apparatus for the automatic tuning of an 
electronic musical instrument using a voltage controlled type oscillator. 
BACKGROUND 
Hitherto, for tuning an electronic musical instrument, such a means has 
been employed in which an oscillation tone is generated and an operator 
effects a manual adjustment while listening to the tone or an oscillation 
frequency is measured and is manually adjusted to be in conincidence with 
a predetermined oscillation frequency. These means are, however, 
disadvantageous in that they require much trouble and are difficult for an 
amateur. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an apparatus in which for 
removing such disadvantages tuning can be carried out automatically by 
operation of a tuning switch. 
The apparatus in accordance with the invention comprises a phase 
comparator, a converter circuit for converting an output signal of the 
phase comparator into a voltage at a level corresponding thereto, a sample 
hold circuit, a calculation circuit and a voltage controlled type 
oscillator for generating a musical tone signal connected in series, and 
an output terminal of the voltage controlled type oscillator connected to 
an input terminal of the phase comparator and another input terminal of 
the phase comparator formed as an input terminal for a standard signal, 
and an output terminal of a key signal generating circuit for generating a 
voltage corresponding to a depressed key connected to a calculation input 
terminal of the foregoing calculation circuit, the key signal generating 
circuit being provided with a gate circuit which generates a key signal 
corresponding to a specific key, and control electrodes of the gate 
circuit and the sample hold circuit connected to an output terminal of a 
monostable multi-vibrator which is operated by operation of a tuning 
switch. 
Embodying examples of this invention will next be explained with reference 
to the accompanying drawings.

DETAILED DESCRIPTION 
Referring to FIG. 1, numeral 1 denotes a voltage controlled type oscillator 
for generating a musical tone signal (called hereinafter VCO 1) and an 
output terminal of VCO 1 is connected to a speaker through a voltage 
controlled type filter 2 (called hereinafter VCF 2) and a voltage 
controlled type amplifier 3 (called hereinafter VCA 3). 
Numeral 4 denotes a phase comparator, and an output terminal thereof is 
connected to a control electrode of VCO 1 through a converter circuit 5 
for converting an output signal of the phase comparator 4 into a voltage 
at a level corresponding thereto, e.g., a low-pass filter 5, a sample hold 
circuit 6, a calculation circuit 7 and an anti-log amplifier 8. An output 
terminal of VCO 1 is connected to an input terminal 4a of the phase 
comparator 4 and another input terminal 4b of the phase comparator 4 is 
connected to a standard signal oscillator 9. Numeral 10 denotes a key 
signal generating circuit for generating a voltage corresponding to a key 
which is depressed, the key signal generating circuit 10 comprising a 
plurality of resistances 13-1 . . . 13-n which are connected in series 
with one another and are connected through a constant current circuit 12 
to a power source 11, and a plurality of key switches 14-1 . . . 14-n 
connected at respective end terminals to respective connecting points of 
resistances 13-1 . . . 13-n, the other end terminals of the key switches 
14-1 . . . 14-n being connected together to form a common terminal 15, an 
output terminal 16 from circuit 10 being connected to a calculation input 
terminal of the calculation circuit 7. 
Numeral 17 denotes a gate circuit connected in parallel with a specific key 
switch 14-5, and a control electrode of the gate circuit 17 and a control 
electrode of the sample hold circuit 6 are connected to an output terminal 
of a monostable multi-vibrator 19 arranged to be operated by operation of 
a tuning switch 18. 
The calculation circuit 7 may be either an addition circuit or a 
subtraction circuit, but it will be described with reference to the case 
where the addition circuit is used. 
A circuit composed of the phase comparator 4, the low-pass filter 5, the 
sample hold circuit 6, the calculation circuit 7, the anti-log circuit 8 
and the VCO 1 forms a phase-locked loop (PLL), and the operation of the 
apparatus will be explained hereinafter with respect to the operational 
principle of the PLL. 
If the tuning switch 18 is pushed to closed position, a pulse A is 
generated from the monostable multi-vibrator 19 and this pulse A not only 
opens the gate circuit 17 but also releases the sample hold circuit 6 from 
its holding state. Thus, the VCO 1 oscillates and its output signal is 
applied to the phase comparator 4 along with the standard signal of the 
oscillator 9 applied thereto, so that the two signals may be compared to 
one another in phase or frequency. If a difference therebetween in phase 
or in frequency is detected, a difference signal is supplied to the 
low-pass filter 5 for being converted into a voltage signal of a magnitude 
corresponding thereto. This voltage signal is applied through the sample 
hold circuit 6 to the calculation circuit 7, and there is carried out in 
this calculation circuit 7 an addition thereof to an output voltage of the 
specific key-switch 14-5 through the gate circuit 17. The resultant signal 
thereof is applied through the anti-log amplifier 8 to the VCO 1, and, the 
the VCO 1 oscillates in such a condition that the oscillation frequency 
thereof has been made equal to the input frequency of the standard signal 
input terminal 4b of the phase comparator 4. 
If, thereafter, the output signal A of the monstable multi-vibrator 19 
vanishes, the gate circuit 17 is closed and the sample hold circuit 6 is 
brought into its hold condition. Thus, the voltage at that time held in 
the sample hold circuit 6 is thereafter taken out so as to be applied to 
the calculation circuit 7, so that the VCO 1 under goes an oscillation as 
already has been tuned to the standard signal. 
The standard signal input terminal 4b of the phase comparator 4 is 
connected to the standard signal oscillator 9 in the embodiment shown in 
FIG. 1, but such a modification can be considered that the same may be 
connected to an output terminal of an oscillator of a corresponding 
frequency in another electronic musical instrument, whereby to the other 
musical instrument can be effected. 
A modification can also be considered, as shown in FIG. 2, in which the 
standard oscillator 9 is replaced by a monostable multi-vibrator 9' , and 
an output terminal of the VCO 1 is diverged so as to be connected to an 
input terminal 9a of the monostable multi-vibrator 9' . If, in this case, 
the frequency which is to be tuned is high, a frequency divider 21 is 
interposed in a circuit connected between the output terminal of the VCO 1 
and the input terminals 4a, 4b. Thus, the loop is locked when the output 
pulse width of the VCO 1 and the output pulse width of the monostable 
multi-vibrator 9' become equal one to another. 
As shown in FIG. 3, the phase comparator 4 comprises two flip-flop circuits 
22,23 and a charge pump 24. The flip-flop circuits 22,23 comprise a D-type 
flip-flop in which when the data terminal D is "H," the output is "H" by 
change of the input terminal T from "H" to "L," and when the data terminal 
D is "L," the output is "L" by the input terminal T being "L." 
If, now, the oscillation frequency of the VCO 1 is higher than the standard 
frequency, the output of the frequency divider 21 is as shown in FIG. 4 
A-1, and the output of the monostable multi-vibrator 9' is as shown in 
FIG. 4 A-2. Accordingly, it will be clear that the output signals of the 
output terminals F22, F23 are as shown in FIGS. 4 A-3, A-4. Accordingly, 
the input terminal PD of the charge pump 24 is applied with "H," and the 
same is inverted by the invertor 24a. The input terminal PU is applied 
with "L." Thus, the output of the diode 24b is pulled in by "L" of the 
anode of the diode 24c, and the output of the output terminal 24d of the 
charge pump 24 rapidly becomes "L." This "L" is applied through the 
low-pass filter 5 to the sample hold circuit 6, and the sampling condenser 
is discharged and the oscillation frequency of the VCO is lowered. 
If the oscillation frequency of the VCO1 is lower than the standard 
frequency, the output of the frequency divider 21 becomes as shown in FIG. 
4 B-1. The output of the monostable multi-vibrator 9' becomes as shown in 
FIG. 4 B-2. Accordingly, it will be clear that the output signals of the 
output terminals F22, F23 are as shown in FIGS. 4 B-3, B-4. 
Accordingly, the input terminal PD of the charge pump 24 is applied with 
"L," and the same is inverted by the invertor 24a and the anode of the 
diode 21b is applied with "H." Additionally, the input terminal PU is 
applied with "H." Thus, the "H" of the anode of the diode 21b is pulled in 
by the diode 21c and the output terminal 24d rapidly becomes "H." This 
output is applied through the low-pass filter 5 to the sampling condenser 
of the sample hold circuit 6 for the charging the same, whereby the 
oscillation frequency of the VCO is increased. 
Thus, it is repeated that the oscillation frequency of the VCO is lowered 
and if lowered in excess the same is increased, and the oscillation 
frequency closes to the standard value as near as possible. 
In this case, the low-pass filter 5 has direct current in its input, so 
that the same does not especially act on the loop. The filter acts only 
for removing unnecessary signals at the time of excess degree response 
operation and the like. 
FIG. 5 shows another embodiment in which the two input terminals 4a, 4b of 
the phase comparator 4 are connected through a logical circuit 26 to a 
luminous element 27. The luminous element 27 comprises a luminous diode 
and the logical circuit 26 comprises an Ex-NOR circuit. The cathode of the 
luminous diode 27 is connected to the output terminal of the Ex-NOR 
circuit 26 and the anode thereof is connected through a resistance 28 to a 
power source 29. Thus, the output signal of the Ex-NOR circuit 26 is "1" 
when an output pulse of the frequency divider 21 and an output pulse of 
the monostable multi-vibrator 9' coincide one with another, and the output 
signal thereof is "0" when the two pulses are not coincident so that for 
the output "0" at the time of non-coincidence an electric current flows 
from the power source 29 through the luminous diode 27, so that the 
luminous diode 27 goes on. With the output "1" at the time of the 
coincidence, the luminous diode 27 is de-energized, so that it goes out. 
Thus, if tuning is being effected in almost the same manner as in the 
foregoing case of FIG. 1 by pushing of the tuning switch 18, as long as 
the two pulse signals applied to the two input terminals 4a, 4b of the 
phase comparator 4 are partly different in phase, the luminous diode 27 
goes on and off, and when the two pulses are completely coincident in 
phase, the diode goes completely out, and thereby it is indicated that the 
oscillation frequency of the VCO 1 has reached a predetermined value. 
In the embodiment of FIG. 5, the tuning switch 18 is of a changeover type 
and a bistable multi-vibrator 30 is provided instead of the monostable 
multi-vibrator 19 so that an output signal obtained while the tuning 
switch 18 is pushed (as shown by dotted lines) may serve to open the gate 
circuit 17 and release holding of the sample hold circuit 6 whereas when 
the tuning switch 18 is released the gate circuit 17 may be opened and the 
sample hold circuit 6 may be brought into its hold condition. Accordingly, 
the tuning can be achieved by releasing the tuning switch 18 when the 
luminous diode 27 goes out completely. 
The sample hold circuit 6 is composed of a circuit including a holding 
condenser, so that when the hold of the circuit 6 is released, the output 
voltage corresponding thereto is sent out therefrom and when the circuit 6 
is brought into its hold condition the output voltage of the low-pass 
filter 5, at the moment when the loop is locked, is charged therein, so 
that voltage corresponding thereto is sent out and this output is 
continued. However, the holding condenser often discharges little by 
little over a long lapse of time and the hold voltage is varied, and, 
thus, it comes to be out of tune. Accordingly, it becomes necessary for 
the tuning switch to be pushed frequently. 
Any of such modifications can be considered also in this embodiment in 
which the standard signal is applied from another electronic musical 
instrument, instead of application from the standard signal oscillator 9, 
the standard signal oscillator 9 is replaced by a monostable 
multi-vibrator 9' as shown in FIG. 3, such an indicating device as shown 
in FIG. 5 is provided, or the tuning switch 18 and the monostable 
multi-vibrator 19 are replaced by the tuning switch 18' and bistable 
multi-vibrator 30 as shown in FIG. 5. 
Thus, according to this invention, tuning can be automatically effected in 
a short time merely by operating a tuning switch and can be carried out 
simply even by unskilled personnel.