US3306969A - Percussion tone generating device - Google Patents

Description

SPEAKER R. O. BARBER 2 Sheets-$heet l AMPLIFIER TIMING nd SWITCHING TIMING and SWlTCHING Ronald 0. Barber BY wwzuwm Feb. 2 8, 1967 PERCUSSION-TONE GENERATING DEVICE Filed March 23, 1964 Q KEYER KEYER TONE GEN. l

TONE

PERCUSSION TONE GENERATING DEVICE Filed March 23, 1964 2 Sheets-Sheet 2 INVENTOR Ronald 0. Barber BY flammww ww Affws United States Patent ()fitice 3,306,969 PERCUSSIDN TGNE GENERATING DEVICE Ronald 0. Barber, Fort Wayne, Ind, assignor to The Magnavox Company, Fort Wayne, Ind, a corporation of Indiana Filed Mar. 2.3, 1964, Ser. No. 353,905 12 (Ilaims. (Cl. 84-124) This invention relates generally to electronic musical instruments, and more particularly to a circuit system whereby the tone producing means of a key or buttonoperated electronic musical instrument can produce an output simulating that produced by a piano or other percussion instrument.

Heretofore, various efforts have been made to cause an electrical musical instrument to simulate the sounds of a variety of musical instruments. These efforts have achieved various degrees of success but have not been entirely satisfactory when directed to the simulation of sounds produced by percussion types of musical instruments, particularly pianos.

It is a general object of the present invention to provide an improved circuit for use in electrical musical instruments to simulate the sounds produced by percussion types of musical instruments.

A further object is to provide a circuit operable by the usual keys of a key-operated electrical musical instrument and responsive to key actuation to control the tone producing means of the instrument and simulate the sound produced by a piano.

A further object is to provide a system which not only responds to key actuation itself but to the velocity of actuation of keys and the manner of their operation.

Described briefly, a typical embodiment of the present invention incorporates a system of four timing circuits for each playing key of a manual of an electronic organ. The circuits for each key are activated by switching functions effected by actuation of the key and by actuation of the sustain pedal of the organ.

The switching functions are accomplished in a predetermined sequence of switch events as the key is actuated, to provide a control voltage output whose amplitude-time characteristic is analogous to that of the sound which would be produced by striking the key of a piano in the same way. The output voltage is used to control the tone signal output of the tone generator producing the tone signal output for the key, to produce a sound output much like that of a piano.

The full nature of the invention will be understood from the accompanying drawings and the following description and claims.

FIG. 1 is a schematic diagram of the circuit of the present invention.

FIG. 2 is an example of a waveform produced by the circuit of the illustrated embodiment, when used to simulate a piano sound.

FIG. 3 is a mechanical schematic diagram illustrating a typical key switch mechanism for achieving a sequence of switching functions by the normal operation of a key of an electronic musical instrument, in order to control the circuit of the present invention to obtain the desired output.

Referring to the drawings in detail, and particularly FIG. 1, thereof, several note playing keys 6 of an electrical musical instrument are indicated, together with a tone generator 7 and keyer 8 for each, and the timing circuitry and switches 9 of the present invention for each. Switch contacts 11, 12, 13, and 14 are shown. A negative direct current potential is normally applied at all times to the contact 11 by a buss connected to the terminal 16. An example of the level of potential is volts below ground. A positive direct current potential is nor- 3,306,969 Patented Feb. 28, 1967 mally applied from terminal 18 to the movable contact 17 which is normally closed with the fixed contact 14, these being contacts of a pedal sustain switch. Fifteen volts positive potential is normally applied to terminal 18. Both of these sources of potential are of the low impedance type. Reference numeral 19 designates ground in the circuit.

A direct current voltage is provided on output conductor 21 of the circuit of this invention as a result of the operation of the circuit, and is of the form shown in FIG. 2. This voltage is applied to a music signal source such as provided by the tone generator 7 and keyer 8, for example. The music signal output of the tone generator is provided at terminal 24 and its amplitude follows the characteristic of the control voltage on the conductor 21. It may be coupled on conductor 23 to output amplifier 25 for amplification and then translation to sound at the speaker.

A resistor 26 and capacitor 27 in parallel are connected between supply terminal 18 and the junction 28 which is connected to the movable contactor 29 which is engageable alternatively with the switch contact terminals 11 and 12. Terminal 12 is connected through the resistance 31 and diode 32 and capacitor 33 in series to ground. The diode 32 is oriented such as would provide low resistance therein to the flow of positive current in a direction from ground through diode 32 to terminal 12.

Capacitor 34 and resistor 36 are connected in parallel with resistor 37, this parallel arrangement being connected between the output conductor 21 and the junction 38 of the capacitor 33 and diode 32.

A movable contactor 39 is normally closed with the fixed contact 13 and is connected through the diode 41 and resistor 42 in series to the junction 38. Diode 41 is oriented such as to provide low resistance therein to the flow of positive current in a direction from contactor 39 to resistance 42.

Referring now to FIG. 3, the various parts corresponding to contact terminals and contactors in FIG. 1 are assigned the same reference characters.

For each note playing key, a switch actuator arm 46 is coupled to the key, the actuator arm being pivotally mounted at 47 to a stationary frame 48. A contact wire support block 49 is afiixed to the frame 48 and supports three spring contact wires 12, 29, and 39 in cantilever fashion. Wire 12 normally extends straight out from the block and has a downturned free outer end 12a. The outer ends of wires 29 and 39 are received in apertures 52 and 53 in the nonconductive downturned leg 51 of the actuator arm.

When the key is not actuated, the return spring 55 holds the actuator arm down so that wire 29 normally engages a buss contact 11 and wire 39 normally engages buss contact 13. These wires are resilient and highly conductive.

When the key coupled to the actuator arm is depressed, and the leg 51 moves upwardly, the outer ends of wires 29 and 39 are raised by the arm to break contact thereof with the contacts 11 and 13. Then upon release of the key, the return spring will return the arm and re-engage wires 29 and 39 to engagement with the contacts 11 and 13, respectively.

It is desirable that upon depressing a note playing key coupled to the actuator arm 46 pivoting at 47, a certain sequence of switching events takes place. The first event is that the contact wire or movable contactor 39, which is normally closed with the buss contact 13, opens when the key is depressed approximately one-half the full key travel. Then when the key reaches approximately threequarters depression, the contact wire or movable contactor 29 which is normally closed with the buss contact 11 opens. The opening of contacts 29 and 11 after the opening of contacts 39 and 13 can be obtained by proper 3 location of the apertures 52 and 53, the wires 29 and 39, and the contacts 11 and 13.

Then when the key reaches full depression, the movable contactor 29 closes with the contact wire 12. If the key is held in this position, the condition of the switches will remain unchanged.

Then when the key is released, as the first event, the movable contactor 29 moves away and breaks from contact 12. Then at approximately one-quarter of the distance from full key depression, the movable contactor 29 closes with the fixed contact 11. Then when the key rises to approximately one-half full depression, the movable contactor 39 again closes With the fixed contact 13.

During all of this time, the movable contactor 17 remains closed with the fixed contact 14, this movable contactor 17 opens only when the sustain pedal 43 is depressed, and it closes immediately when the sustain pedal is released.

It has been stated that there are four basic timing circuits which shape the output of the circuit of the present invention. These are: the velocity timing circuit, the impulse timing circuit, the decay timingcircuit, and the release timing circuit. Each of these circuits will be best understood when related to the operation of the invention.

Velocity timing circuit The velocity timing circuit consists of the resistance 26 and capacitor 27 parallel, these being normally connected in series between supply terminal 18 and the source of negative potential 16 through the movable and fixed contacts 29 and 11, respectively, of the normally closed switch in series therewith. As mentioned previously, this first normally closed switch opens when the key is depressed approximately three-quarters. While the switch is closed, capacitor 27 is charged to the potential difference between the supply at 16 and that at 18, this being normally 35 volts. As the key is depressed from three-quarters travel to its full extent of depression, capacitor 27 discharges through resistance 26. Accordingly, the magnitude of voltage remaining on capacitor 27 when the key reaches full depression and closes the movable contactor 29 with contactor 12, is proportional to the time required to move the key from three-quarters to full position. This means that the voltage remaining on capacitor 27 is proportional to the velocity of key movement.

Impulse timing circuit Initially, before the key was depressed, capacitor 33 had charged to the positive potential of terminal 18. The control voltage Ep at point 72 was of a positive value (point 55 on curve, FIG. 2) approximately equal to that of terminal 18. This voltage, being positive, back biased the keyer diode 56 so that no signal appeared at the tone signal output terminal 24.

As the key was depressed, it first broke the contact between damper switch contacts 13 and 39. Then, when the movable contactor 29 closes with the contactor 12 as the key reaches its full depression, the positive charge on capacitor 33 is discharged through the diode 32 and resistor 31, and capacitor 33 begins (time t=0, point 61 on the curve of FIG. 2) to be charged negatively to approximately the same voltage as remains on capacitor 27, the latter capacitor being large compared to capacitor 33. Resistor 31 determines the rapidity of the charging action.

As capacitor 33 charges negatively and reaches a potential more negative than the instantaneous potential existing at point 68, diode 56 becomes forward biased and the tone signal is coupled to point 24. At the same time, current flows either through resistance 66 and transistor 67 or through resistance 71, then through diode 56 and resistance 37 and becomes part of the discharge current for capacitance 27. As the potential on capacitance 33 becomes more negative, the signal coupled to point 24 increases in amplitude. Maximum signal is reached when the negative potential on capacitance 33 is more negative than the peak negative swing at point 63. For

example, if transistor 67 were operated as a switch by driving it rapidly from saturation to cut-oif, the peak negative voltage at point 68 would equal the negative potential at point 69 and maximum output would occur when the potential on capacitance 33 was more negative than the potential at point 69.

As the voltage on capacitor 33 approaches the voltage on capacitor 27, the diode 32 begins to turn off and the RC time constant of thevelocity timing circuit has no further elfect on the circuit action. Then (time i=1, point 62, FIG. 2) diode 32 has turned off, allowing capacitor 27 to discharge without disturbing the charge on capacitor 33. Capacitor 33 begins to discharge through resistor 37, and to charge capacitor 34 through resistor 36.

Initially, as capacitor 33 begins to discharge, it sees a low impedance path formed by resistor 36, capacitor 34, keyer diode 56, transistor 67, and resistor 66. Of course, resistance 37 in parallel with resistor 36 and capacitor 34 contributes somewhat to the low impedance. Consequently, capacitor 33 discharges quickly at first until it has charged up capacitor 34. The effect of this rapid discharge is that the control voltage output of. the circuit on conductor 21 falls rapidly from point 62 to points 63 on the curve of FIG. 2. Thus, it is seen that the impulse timing circuitry produces a control voltage impulse for the signal source, this impulse being of the character represented between points 61 and 63 on the curve in FIG. 2. The tone signal output amplitude is controlled accordingly.

Decay timing circuit After capacitor 34 charges up (time t:2, point 63, FIG. 2), the low impedance charging path through capacitor 34 in parallel with resistance 37 is effectively terminated. Therefore, the impedance to charging current for capacitor 33 then rises sharply. Because resistance to charging current in the keyer is low by comparison with resistance 37, the decay rate for the circuit output voltage on conductor 21 is determined primarily by resistance 37 and the voltage slowly drops. The effect is shown on the curve of FIG. 2 from point 63 to point 64. The keyer diode 56 will remain forward biased and continue to conduct tone signals to the output terminal 24. The amplitude thereof will decay according to the decay of the control voltage shown on FIG. 2.

Release timing circuit Normally, when a piano key is released, a damper is applied to the piano strings. The same effect is obtained in the circuit of the present invention.

To achieve this effect, it is desired to stop the signal before it has completed its normal decay through resistance 37. For this purpose, the switch incorporating movable contactor 39 and fixed contactor 13 is employed. This switch is normally closed when the key is in the undepressed condition. The movable contactor 39 moves away from the fixed contactor 13 when the key is depressed approximately one-half its full travel. Thus, the switch is open at that time. The switch remains open during the remainder of the key travel and will continue to remain open until the key again returns to approximately one-half its full travel. Thus, it is seen that when the key is released, the switch contactor 39 will close with the contactor 13 when the key rises to approximately one-half of travel. Closure of these contacts (time t=3, point 64, FIG. 2) provides a discharge path for capacitor 33 through the resistance 42, diode 41, switch contacts 39 and 13, and the normally closed sustain switch contacts 17 and 14 to the source of positive potential at terminal 18. This results in a quick discharge of capacitor 33 upon release of the key, which simulates the action that takes place when the damper in a piano is applied to the piano strings upon release of the piano key. The effect on the control signal is shown in FIG. 2 be' tween points 64 and 65. The effect on the tone signal output at terminal 24 is the same and keyer diode 56 again becomes back biased.

Sustain pedal The sustain pedal action of a piano is also simulated with the circuit of the present invention. As mentioned previously, the switch contacts 14 and 17 are used to provide the sustaining action. When these contacts are closed, the release timing circuit provides for rapid discharge of capacitor 33 upon release of the key, as previously described. .However, when the sustain pedal 43 is depressed, the sustain switch is opened. This opens the release circuit to disable it, and the capacitor 33 must continue to discharge through resistances 37 and 66 even though the key is released. Consequently, the release portion of the curve will continue as indicated by the dotted outline 76 and the tone output of the instrument is sustained. One sustain switch serves the circuits for all keys, according to this invention.

As indicated above, a keyswitch mechanism such as shown in FIG. 3, or one useful to obtain the same sequence of switching events is provided for each noteplaying key. However, the contacts 11 and 13 may each be a buss b-ar common to the keyswitch mechanism for all of the keys. Interaction between notes while the sustain switch is open is prevented by the diode 41. In the absence of diode 41 in the circuit for each key, a negative voltage appearing on capacitor 33 at junction 38 for one or more keys might cause current flow through the buss b-ar connected to contact 13 and therefore, through resistances 42 of the circuits for other keys, which could have the effect of keying action on another tone generator. This action is avoided by use of a diode 41 in each of the circuits according to this invention.

Examples of component values useful in circuitry according to the illustrated embodiment of this invention are as follows:

Potential at terminal 16, volts D.C. 20 Potential at terminal 1%, volts D.C. +15 Potential at terminal 69, volts 11C. l8

Potential at ground 19, volts D.C

Resistors '70, 71, ohms 10,000 Resistor 66, ohms 4'70 Resistor 37, megohms 2.2 Resistor 36, ohms 680,000 Resistor 42, ohms 390,000 Resistor 31, ohms 3,300 Resistor 25, ohms 15,000 Capacitor 34,, mid. 0.1 Capacitor 33, mfd. 0.47

Timing sequence The release phase is that beginning with release of the key from the stop condition until the key returns to its initial position. Its events are:

(l) Contact 29 breaks from 12. (2) Contact 29 contacts 11. (3) Contact 39 contacts 13.

While the invention has been disclosed and described in some detail in the drawings and foregoing description, they are to be considered as illustrative and not restrictive in character, as other modifications may readily suggest themselves to persons skilled in this art and within the broad scope of the invention, reference being had to the appended claims.

The invention claimed is:

1. In an electrical musical instrument, the combination comprising:

a tone signal source having a tone signal output terminal;

a plurality of keys;

a plurality of switches;

means coupling said plurality of switches to one of said keys whereby switches of said plurality are operable in sequence by actuation of one of said keys;

first capacitance means coupled through a first switch of said plurality to a first terminal at substantially constant negative potential to charge said first capacitance means;

second capacitance means coupled through a second switch of said plurality of switches to a source of positive potential to establish an initial positive charge on said second capacitance means;

first resistance means coupling said tone signal source to said second capacitance means;

a third switch, said third switch being clos able after said second switch is opened, and said third switch being operable, when closed, to couple said second capacitance means and said first resistance means to said first capacitance means to thereupon initiate reversal of the charge of said second capacitance means by said first capacitance and initiate the negative rise portion of an impulse voltage applied to said tone signal source;

third capacitance means and second resistance means coupled to said second capacitance means to partially discharge said second capacitance means and provide a decrease in the impulse voltage applied to said tone signal source, said third capacitance means having less capacitance than said second capacitance means;

said third and second switches then being operable when again opened and closed, respectively, to provide a discharge path through said second switch to said source of positive potential to rapidly discharge the remaining negative potential from said second capacitance means and reestablish a positive charge thereon.

2. The combination of claim 1 and further comprising:

a fourth switch coupled between to said second switch and said source of positive potential and operable, when actuated, to prevent discharge of said second capacitance through said source of positive potential and thereby sustain the application of negative voltage to said tone signal source.

3. The combination of claim 1 and further comprising:

switch operator means in said coupling means and opening said first switch upon the depression of said key to a first extent and closing said third switch upon further depression of said key;

and third resistance means coupled in parallel with said first capacitance means and providing a discharge path for said first capacitance means, to discharge said first capacitance means during the interval between opening of said first switch and closing of said third switch as said key is depressed.

4. In an electrical musical instrument, the combination comprising:

a plurality of tone signal sources, each having a tone signal output terminal coupled thereto;

a plurality of note playing keys, each of said keys being coupled to a corresponding one of said output terminals to produce tone signals selectively at various ones of said output terminals;

a plurality of percussion signal amplitude simulating circuit systems, each of said systems being coupled to one of said keys and to the corresponding one of said output terminals, and each of said systems comprising;

a plurality of switches;

means coupling said switches to a key of said plurality of keys whereby the switches of said plurality are operable in sequence by actuation of the y;

first capacitance means coupled through a first switch of said plurality to a first terminal at substantially constant potential, to charge said first capacitance means;

second capacitance means and first resistance means coupled to one of said tone signal sources, said second capacitance means being coupled through a second switch of said plurality of switches to said first capacitance means when said second switch is closed to thereupon initiate charge of said second capacitance means by said first capacitance means and initiate the rise portion of an impulse voltage applied to said one tone signal source;

third capacitance means and second resistance means coupled to said second capacitance means to discharge said second capacitance means and cause a drop of said impulse voltage;

" a third switch coupled between said second capacitance means and a source of potential and operable when closed to rap-idly discharge said second capacitance means through said source of potential.

5. The combination of claim 4 and further comprising:

a fourth switch coupled to said third switch and said source of potential and operable, when actuate-d, to prevent discharge of said second capacitance through said source of potential.

6. The combination of claim 4 and further comprising in each of said systems:

switch operator means in said coupling means and opening said first switch upon the depression of said key to a first extent and closing said second switch upon further depression of said key, and third resistance means coupled in parallel with said first capacitance means and providing a discharge path for said first capacitance means, to discharge said first capacitance means during the interval between opening of said first switch and closing of said second switch as said key is depressed.

7. The combination of claim 4 and further comprising:

a sustain actuator member;

and a sustain switch coupled to said actuator member, said sustain switch being in the said discharge paths of said systems, and said sustain switch being operable upon actuation of said actuator member to break said discharge paths.

8. A circuit for producing an output simulating the amplitude-time characteristic of the sound output of a percussion instrument, said circuit comprising:

a first normally-closed switch and a velocity timing circuit coupled in series across a first source of direct current potential, said velocity timing circuit including a first resistance and first capacitance;

a second capacitance, a first unidirectional device, a second resistance, and a second switch connected in series circuit relationship with said first capacitance to provide an impulse rise timing circuit, said second switch being normally open;

a third resistance connected in parallel with the series combination of a fourth resistance and third capacitance, said third resistance beingconnected in series circuit relationship with said second capacitance and a fifth resistance, to provide impulse fall timing circuit means; i

a normally-closed sustain switch, a normally-closed damper switch, a second diode, a sixth resistance, and said second capacitance, connected in series across a second source of direct current potential to provide arelease timing circuit;

and a manually operable actuator connected to said switches to first open said damper switch and then open said first switch and then close said second switch, when said actuator is moved in one direction, and first open said second switch and then close said first switch and then close said damper switch when said actuator is moved in the opposite direction, to thereby produce a sharp rise in negative potential applied to said fifth resistance when said actuator is moved in said first direction, followed by a sharp fall in negative potential applied to said fifth resistance and then followed by gradual fall of potential applied to said fifth resistance, and then followed by a sharp fall of potential applied to said fifth resistance when said damper switch is closed as said actuator is moved in said second direction;

and a sustaining actuator connected to said sustain switch and operable, when actuated, to open said sustain switch to disable said release timing circuit and prevent, when said sustain actuator is actuated, the last mentioned sharp fall of potential applied to said fifth resistance.

9. A percussion tone generating device comprising:

an electronic tone generator means including tone signal output means for an electronic musical instrument, said tone generator means having input means for a control signal to control amplitude of the output tone;

and control signal producing circuit means having control voltage output means coupled to said control input means for said tone generator means and having,

a manual actuator,

switch means coupled to said manual actuator and responsive to operation of said manual actuator,

a velocity timing circuit controlled by said switch means and including a first charge storage device to establish a potential dependent upon speed of actuation of said manual actuator,

an impulse forming circuit including a second charge storage device and controlled by said switch means to charge said second charge storage device according to the charge stored in said velocity timing circuit, said second charge storage device being coupled to said control voltage output means,

and a decay timing circuit coupled to said second storage device to accommodate discharge of said second storage device at a predetermined rate,

a release timing circuit controlled by said switch means to provide, when desired, a more rapid discharge of said second charge storage device than is possible through said decay timing circuit,

and sustain switching means in said release timing circuit to selectively disable said release timing circuit.

10. A percussion tone generating device comprising:

an electronic tone generator means including tone signal output means for an electronic musical instrument, said tone generator means having input means for a control signal to control amplitude of the output tone;

and control signal producing circuit means having control voltage output means coupled to said control input means for said tone generator means and having, a manual actuator.

switch means coupled to said manual actuator and re sponsive to operation of said manual actuator,

a velocity timing circuit controlled by said switch means and including a first charge storage device to establish a potential dependent upon speed of actuation of said manual actuator,

an impulse forming circuit including a second charge storage device and controlled by said switch means to charge said second charge storage device according to the charge stored in said velocity timing circuit, said second charge storage device being coupled to said control voltage output means,

and a decay timing circuit coupled to said second storage device to accommodate discharge of said second storage device at a predetermined rate,

and a release timing circuit controlled by said switch means to provide, when desired, a more rapid discharge of said second charge storage device than is possible through said decay timing circuit.

11. A circuit responsive to operation of a manual actuator to provide an output corresponding to the output of a percussion type of musical instrument and comprising:

switching means operable by said actuator;

a first circuit coupled to said switching means and to a source of potential and adapted to develop a charge proportional to the speed of movement of said actuator;

an impulse initiating circuit coupled through said switching means to said first circuit and including a charge storage element, said impulse initiating circuit being thereby adapted to rapidly charge the said storage element therein to a potential determined by the charge attained by said first timing circuit;

impulse shaping circuit means coupled to said charge storage element of said impulse intiating circuit and comprising means for discharging said charge storage element first at a rapid rate and then at a less rapid rate;

and a damper circuit coupled to said charge storage element and said switching means to accomplish rapid discharge of said charge storage element in response to a selected operation of said switching means.

12. In a sound control circuit for an electrical musical instrument, the combination comprising:

a source of positive direct current potential;

a source of ground potential;

first and second normally closed switches connected in series with a first diode, a first resistance, and a charge storage element, said series connection being connected from said source of positive potential to said ground, and said second switch being operable by a playing key of said instrument and said first switch being operable by a sustain pedal of said instrument;

and an output coupled through a resistance to a junction between said first charge storage element and said first resistance.

No references cited.

ARTHUR GAUSS, Primary Examiner. J. BUSCH, Assistant Examiner.

Claims (1)

1. IN AN ELECTRICAL MUSICAL INSTRUMENT, THE COMBINATION COMPRISING: A TONE SIGNAL SOURCE HAVING A TONE SIGNAL OUTPUT TERMINAL; A PLURALITY OF KEYS; A PLURALITY OF SWITCHES; MEANS COUPLING SAID PLURALITY OF SWITCHES TO ONE OF SAID KEYS WHEREBY SWITCHES OF SAID PLURALITY ARE OPERABLE IN SEQUENCE BY ACTUATION OF ONE OF SAID KEYS; FIRST CAPACITANCE MEANS COUPLED THROUGH A FIRST SWITCH OF SAID PLURALITY TO A FIRST TERMINAL AT SUBSTANTIALLY CONSTANT NEGATIVE POTENTIAL TO CHARGE SAID FIRST CAPACITANCE MEANS; SECOND CAPACITANCE MEANS COUPLED THROUGH A SECOND SWITCH OF SAID PLURALITY OF SWITCHES TO A SOURCE OF POSITIVE POTENTIAL TO ESTABLISH AN INITIAL POSITIVE CHARGE ON SAID SECOND CAPACITANCE MEANS; FIRST RESISTANCE MEANS COUPLING SAID TONE SIGNAL SOURCE TO SAID SECOND CAPACITANCE MEANS; A THIRD SWITCH, SAID THIRD SWITCH BEING CLOSABLE AFTER SAID SECOND SWITCH IS OPENED, AND SAID THIRD SWITCH BEING OPERABLE, WHEN CLOSED, TO COUPLE SAID SECOND CAPACITANCE MEANS AND SAID FIRST RESISTANCE MEANS TO SAID FIRST CAPACITANCE MEANS TO THEREUPON INITIATE REVERSAL OF THE CHARGE OF SAID SECOND CAPACITANCE MEANS BY SAID FIRST CAPACITANCE AND INITIATE THE NEGATIVE RISE PORTION OF AN IMPULSE VOLTAGE APPLIED TO SAID TONE SIGNAL SOURCE; THIRD CAPACITANCE MEANS AND SECOND RESISTANCE MEANS COUPLED TO SAID SECOND CAPACITANCE MEANS TO PARTIALLY DISCHARGE SAID SECOND CAPACITANCE MEANS AND PROVIDE A DECREASE IN THE IMPULSE VOLTAGE APPLIED TO SAID TONE SIGNAL SOURCE, SAID THIRD CAPACITANCE MEANS HAVING LESS CAPACITANCE THAN SAID SECOND CAPACITANCE MEANS; SAID THIRD AND SECOND SWITCHES THEN BEING OPERABLE WHEN AGAIN OPENED AND CLOSED, RESPECTIVELY, TO PROVIDE A DISCHARGE PATH THROUGH SAID SECOND SWITCH TO SAID SOURCE OF POSITIVE POTENTIAL TO RAPIDLY DISCHARGE THE REMAINING NEGATIVE POTENTIAL FROM SAID SECOND CAPACITANCE MEANS AND RE-ESTABLISH A POSITIVE CHARGE THEREON.

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