Gate Circuit For Electronic Musical Instruments

Abstract

A sustaining gate circuit for musical instruments comprising a differential amplifier stage including a pair of transistors the emitters of which are connected in common and a constant current transistor connected to said commonly connected emitters, a key switch for applying the gating signal to the base of said constant current transistor, and a time-delay circuit connected in the base circuit of said constant current transistor for obtaining sustaining effect in fade-out of the output.

Description

This invention relates to a gate circuit, and more particularly to a sustaining gate circuit for use in electronic musical instruments.

In electronic musical instruments, a desired signal or signals are selected from the electric signals of various frequencies generated from tone generators by pressing keys for playing the instrument. This selection is done through an electronic gate circuit driven by the mechanical action of pressing keys. Such a gate circuit is provided for each signal and thus the number thereof becomes vary large. Therefore, such a gate circuit should have a simple structure for minimizing the size and enhancing the reliability thereof. The outputs of respective gate circuits are mixed at the output to form an electric signal waveform corresponding to a desired sound and this mixed output is transmitted to a timbre circuit.

The total noise level at the output of the gate circuit system is the sum of noise level of the respective gate circuits. Thus, for minimizing the noise level it is necessary to minimize the leakage currents of the gate circuits when keys of the musical instrument are not hit. The signal to noise ratio of the total gate circuit system depends on the ratio of the output signal level of the respective gate circuits when the gates are open or in the on-state with respect to that when they are closed or in the off-state, i.e. on-off ratio. Thus, it is necessary for the respective circuits to have large on-off ratios. Further, it is preferable that musical sounds fade out naturally with the appropriate reverberation. For achieving this effect, the output of a gate circuit should not diminish immediately but decrease exponentially at a constant rate when a corresponding key switch is turned off. However, the on-off ratios of the conventional gate circuits are not much larger than 50 db and their damping characteristics after turning off the key switches take deformed exponential shapes so that the output sound diminishes unnaturally. Yet further, for forming a musical sound, the appropriate amount of higher harmonics should be mixed with the fundamental tone. Since a gate circuit is provided for each signal frequency and a time difference may occur in the gating action of the respective gate circuits, sometimes the desired electric signal waveform corresponding to the desired musical sound is not produced. Further, an output sound may be deformed since similar damping characteristics are hard to be obtained in the respective gate circuit.

An object of the invention is to provide a gate circuit having a large on-off ratio.

Another object of the invention is to provide a gate circuit having a sustaining effect in which the output of a gate decreases exponentially upon turning off a corresponding key switch and the length of time required for the output to decrease to a desired level can be arbitrarily and easily changed.

A further object of the invention is to provide a gate circuit which can simultaneously gate a plurality of input signals by one key switch and which can give a common damping characteristic to a plurality of output signals corresponding to said plurality of input signals.

Yet a further object of the invention is to provide a sustaining gate circuit operated by a small electric current and capable of damping the output exponentially without any distortion.

Another object of the invention is to provide a gate circuit for use in electronic musical instruments which is especially effective when applied to an integrated circuit.

To achieve the above objects, a differential amplifier can be used. In this case, a differential amplifier is formed by connecting the emitter electrodes of a pair of transistors in common and further connecting another transistor working as a constant current source to said common emitter electrode to vary the mutual conductance or the transmission conductance of the differential amplifier stage by the current flowing through said constant current transistor thereby to perform a gating action. Namely, when a control signal is cut off by opening the key switch connected to the constant current transistor, the current flowing through the constant current transistor decreases exponentially by the effect of a time delay circuit having a sustaining function and connected to the input electrode of the constant current transistor and accordingly the mutual conductance of the differential amplifier stage decreases exponentially. Thereby, a desired output characteristic is obtained at an output terminal of the differential amplifier stage.

According to an embodiment of this invention, the time constant of the time delay circuit used as an envelope forming circuit can be arbitrarily altered by adjusting the duty cycle of the control pulse signal to be applied to the switching element.

This invention will be described hereinafter with reference to the accompanying drawings in which:

FIG. 1 is a circuit diagram of an embodiment of a gate circuit for electronic instruments according to the invention;

FIG. 2 is a circuit diagram of another embodiment of a gate circuit for electronic instruments according to the invention; and

FIGS. 3a to 3c show signal waveforms detected at various points in the circuit shown in FIG. 1.

FIG. 1 shows a circuit which gates or selects with respect to time an electrical signal from a plurality of electrical signals in the frequency range of 40 to 4000 Hz by a gate signal supplied through one key switch. The circuit comprises active elements Q.sub.1A, Q.sub.2A and Q.sub.3A having, respectively, input, output and common electrodes, each active element being a bipolar transistor having a base, a collector and an emitter electrode. A differential amplifier is formed by the transistors Q.sub.1A and Q.sub.2A with the base electrode of one transistor Q.sub.1A connected to an input terminal 1, the collector electrode of the other transistor Q.sub.2A connected to an output terminal 2 and the emitters of both transistors Q.sub.1A and Q.sub.2A connected in common. To this common connection point, the collector of another transistor Q.sub.3A is connected. The collector electrode of the transistor Q.sub.1A is connected to a first voltage supply terminal 3 applied with voltage V.sub.CC and the collector electrode of the transistor Q.sub.2A is also connected to the first terminal 3 through a resistance R.sub.3A. The base electrodes of the transistors Q.sub.1A and Q.sub.2A are connected to a second voltage supply terminal 4 applied with voltage V.sub.BB through resistances R.sub.1A and R.sub.2A, respectively, so as to apply a certain biasing voltage. The emitter of the transistor Q.sub.3A is connected to a reference voltage terminal 6 through an emitter resistance R.sub.4A. The base electrode of the transistor Q.sub.3A is on one hand connected through a resistance R.sub.5 to a key switch S which is interlocked with one of the keys of the instrument and connected to the second terminal 4 via terminal 8 through a forwardly biased diode D.sub.1, and on the other hand connected to an envelope forming circuit comprising a capacitance C.sub.2, a diode D.sub.2, resistances R.sub.6 and R.sub.7 and a transistor Q.sub.4 and working as a time delay circuit. The diode D.sub.2 is inserted between the base of the transistor Q.sub.3A and the reference voltage terminal 6 through the resistance R.sub.6 and the switching transistor Q.sub.4 in a direction so as to be biased in the same direction as that of the base-emitter junction of the transistor Q.sub.3A. The base electrode of the switching transistor Q.sub.4 is connected through a resistance R.sub.7 to a terminal 5 for introducing control pulse input. The capacitor C.sub.2 is connected between the reference voltage terminal 6, thereby also to the emitter electrode of the transistor Q.sub.4, and the base of the transistor Q.sub.3A. An input signal is introduced from an input terminal 1A to the input electrode 1 through a capacitor C.sub.1A and the output signal is derived from the output electrode 2 to an output terminal 2A through a capacitor C.sub.3A. The diode D.sub.1 is inserted as shown in the figure so as to decrease the emitter voltage of the transistors Q.sub.1A and Q.sub.2A, which is equal to the collector voltage of the transistor Q.sub.3A, to a value not more than the base voltage of the transistor Q.sub.3A. Without this diode D.sub.2, the base voltage of the transistor Q.sub.3A is maintained higher than the collector voltage of the transistor Q.sub.3A. Therefore, when the key switch S is opened or turned off, a saturation current flows through the transistor Q.sub.3A until the base voltage becomes lower than the collector voltage. The use of the diode D.sub.1 prevents this phenomenon and is effective to decrease the output current smoothly when the key switch S is opened. The diode D.sub.2 prevents such a phenomenon that when the key switch S is opened and the base voltage of the transistor Q.sub.3A decreases exponentially, the base voltage of the transistor Q.sub.3A rapidly becomes smaller than the threshold voltage V.sub.th of the transistor Q.sub.3A due to the fact that the transistor Q.sub.4 is in the on-state. Without this diode D.sub.2, though the output decreases exponentially until the base voltage of the transistor Q.sub.3A reaches the threshold voltage V.sub.th, since the base voltage decreases further on, the output is rapidly damped and the sound disappears unnaturally. By the insertion of diode D.sub.2 having a threshold voltage substantially equal to that of the transistor Q.sub.3A, the base voltage of the transistor Q.sub.3A is held substantially at the threshold voltage, therefore a damping characteristic with a close approximation to the exponential function is obtained. The resistance R.sub.5 is connected in series to the key switch S to prevent the transistor Q.sub.3 from an input signal being applied suddenly when the key switch S is turned on and to generate a sound smoothly becoming louder with a certain rising time length.

Description will now be made on how the circuit of FIG. 1 operates with reference to FIGS. 3a to 3c which show the waveforms of the input signal voltage V.sub.in, the base voltage V.sub.B3 of the transistor Q.sub.3A and the output signal voltage V.sub.out, respectively, with respect to time. Letter T indicates the sustaining length of time. In FIG. 1, the supply voltages V.sub.CC and V.sub.BB are applied to the first and second terminal 3 and 4 respectively and an input signal is introduced to the input electrode 1 from the input terminal 1.sub.A through the capacitor C.sub.1A. Supposing that the key switch S is turned on at the time t = t.sub.0, the base voltage of the transistor Q.sub.3A reaches the biasing voltage V.sub.B after a period of the rising time shown as a period between t.sub.0 and t.sub.1 in FIGS. 3a to 3c and then the collector Q.sub.3A works as a constant current source allowing the collector current I.sub.C3 to flow which is the sum of the emitter currents I.sub.e1 and I.sub.e2 flowing through the transistors Q.sub.1A and Q.sub.2A. If the current I.sub.e1 increases, the other current I.sub.e2 decreases since the sum is maintained constant. The differential amplifier stage has its operational voltage point determined by the second source voltage V.sub.BB transmits the input signal to the output terminal 2, the signal being amplifier according to the mutual conductance g.sub.m determined by the collector current I.sub.C3. The output signal voltage appearing at the output electrode 2 is applied to the output terminal 2A through the capacitor C.sub.3A. When the key switch S is turned off at the time t = t.sub.2, the base voltage V.sub.B3 of the transistor Q.sub.3A decreases according an exponential curve featured with the time constant of the time delay circuit connected to the base electrode of the transistor Q.sub.3A as is shown in FIG. 3b and thus the collector current of the transistor Q.sub.3A also decreases exponentially. Thereby, the output signal voltage decreases exponentially. When the base voltage V.sub.B3 of the transistor Q.sub.3A becomes lower than the threshold voltage V.sub.th of the transistor Q.sub.3A, the transistor Q.sub.3A is turned off thereby to cut off the collector current and then the differential amplifier stage. The output signal voltage therefore disappears from the output terminal. Namely, in the gate circuit according to the invention, the differential amplifier stage performs a switching action by controlling the total emitter current of the stage by changing the base voltage level of the transistor Q.sub.3A. In this embodiment, an on-off ratio above 100 db was obtained.

Further, the switching element Q.sub.4 which is a switching transistor in this embodiment is used to adjust the sustaining time length T arbitrarily. When the key switch S is turned off, the base voltage of the transistor Q.sub.3A decreases according to the discharge characteristic of the capacitor storing a charge therein featured by the time constant of the circuit including the capacitor C.sub.2. The capacitor C.sub.2 can be sufficiently discharged even by a small current. Further, the effective time constant .tau. can be arbitrarily varied by arbitrarily selecting the duty cycle of the control pulse signal, d (expressed by pulse width (sec.)/period (sec.) for a square pulse). That is, the effective time constant .tau. is determined as the product of the capacitance C.sub.2 and the resistance R.sub.6 of the envelope forming circuit, and the duty cycle d, that is, .tau.= C.sub.2.sup.. R.sub.6.sup.. d. Therefore, a desired time constant and a desired sustaining time length T can be obtained by appropriately varying the duty cycle d. For example, a square wave pulse train having a duty cycle of 1 to 50 percent, a frequency of 40 KHz and a peak voltage of 10V is applied to the terminal 5 in this embodiment.

FIG. 2 shows another embodiment of the invention in which transistors Q.sub.1B, Q.sub.2B and Q.sub.3B forming a similar circuitry as that of the transistors Q.sub.1A, Q.sub.2A and Q.sub.3A of the former embodiment are added to the circuit. These two circuits are connected in parallel to the voltage source V.sub.CC. An envelope forming circuit comprising a capacitance C.sub.2, a diode D.sub.2, resistances R.sub.6 and R.sub.7, and a transistor Q.sub.4 is connected to the base electrodes of the transistors Q.sub.3A and Q.sub.3B in common. A series connection of a key switch S, a diode D.sub.1 and a resistance R.sub.5 is also connected to the base electrodes of the transistors Q.sub.3A and Q.sub.3B. In this arrangement, two kinds of electrical signal are subjected to gating operation by one key switch S. The operation of the circuit of FIG. 2 will be apparent since the structure is almost the same as that of FIG. 1. Although FIG. 2 shows a gating circuit for two input signals to be applied to the terminals 1A and 1B respectively, the number of the input signals can be arbitrarily increased, according to necessity, by connecting the desired number of circuits constituted by the transistors Q.sub.1A, Q.sub.2A and Q.sub.3A shown in FIG. 1 in parallel similarly as is shown in FIG. 2. Further, the gate circuit of the invention is especially effective when applied in an integrated circuit. In such a case, transistors to be used for a differential amplifier stage are formed in a unitary semiconductor body to be mutually adjacent thereto so that various characteristics of the transistors, such as output characteristic or current amplification coefficient can be easily made uniform. Since, the gate circuit of this invention does not need a large resistance, the integration thereof is easy in this respect, too. The diodes D.sub.1 and D.sub.2 preferably have characteristics which are compatible with those of the base-emitter junction of the transistors Q.sub.1A and Q.sub.3A, respectively, from the standpoint of their function. In this respect also, the transistors and the diodes are preferably formed simultaneously in a unitary semiconductor body. In integrated circuits, those portions as enclosed by the dotted line in FIGS. 1 and 2 may be formed in a single semiconductor.

Since this invention employs a differential amplifier stage, no direct coupling exists between the input and output side so that the leak signal between the input and output side is extremely small and that a large on-off ratio can be obtained. Further, since the base voltage of the transistor Q.sub.3A is subjected to a controlling operation, charge and discharge current flowing into and from the capacitor C.sub.2 can be small. This is important for integrating the circuit. Yet further, the addition of the diode D.sub.1 and D.sub.2 makes the shape of the damping characteristics of the output to resemble closely an ideal exponential curve. Thus, the sustaining gate circuit of the invention is very effective for use in electronic musical instruments.

Claims

What is claimed is:

1. A gate circuit comprising:

a pair of active elements each having a first, a second and a third terminal, the first terminals of the elements being connected in common, a signal being applied to the second terminal of one of said pair of active elements, and means for supplying desired bias voltages to the second and third terminals of the pair of active elements;

means connected to the third terminal of the other of said pair of active elements for deriving the signal from the other of said pair of active elements;

a third active element having a first, a second and a third terminal, the current path between the first terminal and the third terminal through the third active element being connected between the first terminals of the pair of active elements and a common reference potential;

means for applying a direct current gating signal to said second terminal of said third active element, said means including a mechanical switch and a direct current voltage source connected to the second terminal of said third active element; and

envelope forming circuit means connected to the second terminal of said third active element for receiving said gating signal.

2. A gate circuit according to claim 1, in which said switch is coupled to a biasing voltage supplying means providing one of said desired bias voltages and serving as said direct current voltage source and said gating signal is supplied from said biasing voltage supplying means through said switch.

3. A gate circuit according to claim 1, in which said envelope forming circuit means comprises a diode and a capacitor both connected to the second terminal of said third active element, said gate circuit further comprising a fourth active element connected to said diode in series.

4. A gate circuit according to claim 3, in which said first, second, third and fourth active elements are bipolar transistors.

5. A gate circuit comprising:

a first voltage source;

a plurality of differential amplifier means, each comprising a pair of transistors the emitter electrodes of which are connected in common and a third transistor having a collector connected to said emitter electrodes, said plurality of differential amplifier means being connected to said first voltage source in parallel;

switching means commonly connected to the base electrodes of the third transistors of said respective amplifier means;

a second voltage source for supplying a biasing voltage to the respective base electrodes of said third transistors through said switching means; and a time delay circuit means commonly connected to the base electrodes of said third transistors and through said switching means to said second voltage source, said time delay circuit means comprising a capacitor commonly connected to the base electrodes of said third transistors, a first diode commonly connected to the base electrodes of said third transistors so as to be forwardly biased by said second voltage source, a resistance connected to said first diode in series, and a fourth transistor connected to said resistance in series.

6. A gate circuit according to claim 5, in which the base electrodes of said pair of transistors of said respective amplifier means are biased by said second voltage source, said gate circuit further comprising a second diode connected between the base electrodes of each of said pair of transistors and the base electrodes of said third transistors so as to be forwardly biased by said second voltage source.