US3569603A

US3569603A - Moving formant band-pass amplifier for an electronic musical instrument

Info

Publication number: US3569603A
Application number: US749334A
Authority: US
Inventors: Donald R Kern
Original assignee: Hammond Corp
Current assignee: Marmon Co
Priority date: 1968-08-01
Filing date: 1968-08-01
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09
Also published as: DE1939029A1; GB1210186A; BE736933A; DE1939029B2

Abstract

A circuit for achieving the moving or changing formant effect in an electronic musical instrument such as an electric organ. The circuit employs a band-pass amplifier with parallel T resistivecapacitive feedback circuits in which an FET (Field Effect Transistor) is employed as a variable resistance to cut in, by degrees, one of the T feedback circuits. The effective resistance of the FET and thus the band-pass of the amplifier are varied by a capacitive and resistive timing circuit that is controlled via a keying circuit from one or more keys or other player-operated controls, such as the manual keys of an electric organ.

Description

United States Patent Donald R. Kern Buffalo Grove, Ill. 749,334

Aug. 1, 1968 Mar. 9, 1971 Hammond Corporation Chicago, Ill.

[72] Inventor [21 Appl. No. [22] Filed [45] Patented [73] Assignee [54] MOVING FORMANT BAND-PASS AMPLIFIER FOR AN ELECTRONIC MUSICAL INSTRUMENT 11 Claims, 2 Drawing Figs.

[52] U.S.Cl 84/l.19, 84/1.25,84/1.11,330/86,330/l03 [51] lnt.Cl GlOh 1/02 [50] Field olSearch 84/l.01, 1.1 l, 1.19, 1.24, 1.25, (F), (FB), (B); 330/85, 103, 86

[5 6] References Cited UNITED STATES PATENTS 2,927,282 3/1960 Gardberg 331/142 2,971,161 2/1961 Clevenger 330/85 3,205,294 9/1965 Maynard 84/1.11 3,433,937 3/1969 McCarthy 330/103 2,245,365 6/1941 Riddle 84/(FB) 3,379,820 4/1968 Olson 84/1.24

Primary Examiner-Milton O. Hirshfield Assistant Examiner-Stanley J. Witkowski Attorney-Gradolph, Love, Rogers & Van Sciver ABSTRACT: A circuit for achieving the moving or changing formant effect in an electronic musical instrument such as an electric organ. The circuit employs a band-pass amplifier with parallel T resistive-capacitive feedback circuits in which an FET (Field Effect Transistor) is employed as a variable resistance to cut in, by degrees, one of the T feedback circuits. The effective resistance of the FET and thus the band-pass of the amplifier are varied by a capacitive and resistive timing circuit that is controlled via a keying circuit from one or more keys or other player-operated controls, such as the manual keys of an electric organ.

BRIG/l7 Bus PATENIED HAR 91911 SHEET 1 UF 2 PATENTED MAR 9 I87! SHEET E OF 2 MOVING FORMANT BAND-PASS AMPLIFIER FOR AN ELECTRONIC MUSICAL INSTRUMENT FIELD OF THE INVENTION Formanting circuits for electrical musical instruments.

DESCRIPTION OF THE PRIOR ART While some attempts of achieving moving formant effects, such as those disclosed in the US. Pat. to R. H. Peterson; No. 3,316,341, entitled Electrical Musical Instruments," which issued on Apr. 25, 1967, have been attempted before, these usually involve expensive constructions or accomplish limited effects which may be generally similar to those presently proposed.

SUMMARY OF THE INVENTION The present invention has as its general object the provision of a moving formant device of a new and improved construction. A more specific object of the invention is the provision of such a device that is both effective in use and economical to make and maintain. One object of the present invention is the provision of a moving formant device which may affect only a portion of the note voiced while leaving the other portion unaffected.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several FIGS. of which like reference numerals identify like elements, and in which:

FIG. 1. is an electric circuit diagram of a moving formant device constructed in accordance with the principles of the present invention; and

FIG. 2 is an electric circuit diagram of another moving formant device constructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 the device there depicted is generally designated by the numeral and forms part of an electrical musical instrument of the electric organ type. The organ includes a bright bus 11 from which an electrical musical tone may be taken. Playing key contacts or keyers, not shown since they may be conventional, will key the proper signals to the bus 11 as particular playing keys areactuated. A selector switch 12 couples the output of bus bar 11 to a band-pass amplifier generally designated 9.

The amplifier 9 may be consideredto be a two transistor parallel T feedback oscillator in which one of the feedbacks T has been slightly detuned, thus preventing oscillation but instead resulting in a resonant peak in the audio spectrum at approximately the oscillator frequency.

The switch 12 is directly connected to one side of a capacitor 13. A resistance 14 is connected from that side of capacitor 13 to a plane of reference potentialor ground. The other side of capacitor 13 is connected through a resistor 15 to the base of a first transistor 16 of the NPN type. Also connected to the base of transistor 16 is one end of a resistor 17, whose other end is grounded, and one end of a resistor 18 whose other end is connected to a source of direct current biasing potential B+.

The emitter of the first transistor 16 is connected to the base of a second NPN transistor 19. A resistor 20 is connected from the interconnected emitter of transistor 16 and base of transistor 19 to ground. The collectors of both

transistors

16 and 19 are connected together and through a resistor 21 to the voltage source 13+. The emitter of transistor 19 is connected through a resistor 22 and a parallel connected capacitor 23 and resistor 24 to ground.

The collectors of the

transistors

16 and 19 are connected together and through a direct current blocking capacitor 25 to an output terminal 26, connected in turn to conventional musical instrument amplifiers and mixing circuits which supply signals at an appropriate level to one or more speakers 71. The terminal 26 also is connected via a line 27 to a feedback network generally designated 28. The network 28 feeds back the output at terminal 26, via a line 29 and through a capacitor 30, to the base of the transistor 16. The circuit 28 includes a first T network of a pair of series connected

resistors

31 and 32 whose joined ends are connected through a capacitor 33 to ground and whose free ends are connected respectively to the lines 27 and 29. The circuit 28 also includes a second T network comprising a pair of series connected capacitors 34 and 35 whose common ends are connected through a variable resistance 36 to ground. The free end of capacitor 35 is connected to line 29 and also through another capacitor 37 to ground. The free end of capacitor 34 is connected through a resistor 38 to ground.

Connected between the junction of the free end of the capacitor 34 and the resistor 38 and the line 27 is a Field-Effect Transistor (FET) 40 of the p-channel type. The FET is a well known device now in commercial use. Those unfamiliar with its characteristics are referred to "Field Effect Transistors," by L. .I. Sevin, Jr., McGraw, Hill, 1965. Reference also may be made to Application Note AN-2l l of Motorola Semiconductor Products, Inc. entitled Field Effect Transistors in Theory and Practice" by J F. Kane et al., dated Sept. 1966. The drain of the FET 40 is connected to line 27 and the source of FET 40 is connected to the series junction of capacitor 34 and resistor 38. The gate of the FET 40 is connected to the movable tap of a variableresistor 41 which has one end grounded and the other end connected to one end of a fixed resistor 42. The junction between these resistors is connected to ground through a timing capacitor 43. The other end of the fixed resistor 42 is connected through the collector emitter circuit of a transistor 45 to ground and through a resistor 46 to 8+.

The transistor 45 is of NPN type, has its emitter grounded and has its base connected to one contact 44 of a single pole, single throw selector switch 47, the other side of which is grounded. When the switch 47 is open, the bias of base of transistor 45 is determined by a keying circuit generally designated by the numeral 50.

The keying circuit 50 may be anything suitable. but as shown consists of a circuit from the base of transistor 45 through a current limiting resistor 48 to a pair of

NPN transistors

51 and 52. The transistor 51 has its collector connected to the resistor 48 and also to the bias source 13+ through a resistor 53. Its emitter is grounded and the base is connected to the collector of the transistor 52 and through a resistor 54 to the bias source B+.

The base of the transistor 52 is connected to the series junction of a pair of resistors 55 and 56 which are connected in order between the bias source 8+ and ground. In parallel connection with the resistor 56 is a diode 57 oriented to pass current from the base of transistor 52 to ground.

The emitter of transistor 52 is connected to ground through a diode 58 and resistor 59in parallel. The diode 58 is oriented to allow current flow only from the emitter of transistor 52 to ground. a

The emitter of transistor 52 is also connected to one side of a normally open control'switch 60 which typically will be closed by actuation of the organ playing keys. The other side of the switch 60 is connected, through a current limiting resistor 61 to a source of positive potential, designated B'+ at above 4v.

The biasing for the

transistors

16 and 19 as established by the voltage source 8+ and the resistors associated with the transistors, is such as to cause them to operate in class A amplification. The FET is similarly biased as to operate as a variable resistance between its drain and source. I

In overall operation the device 10 may deliver to the amplifying and m'iiting circuit 70 a musical tone signal which exhibits the moving formant effect in response to the closing of the key switch 60. With the selector switch 47 closed, so that the contact 44 is grounded, the circuit does not respond to the key contacts 60, but delivers a musical tone signal at its output terminal 26 which approximates a trumpet note in its character and does not exhibit a moving formant effect.

The signal from the bright bus 11 is transferred through the capacitor 13 and resistor 15 to the base of the transistor 16. The

transistors

16 and 19 amplify this signal and supply it to the output terminal 26 through the capacitor 25. The feedback circuit 28 has two feedback paths, one via line 27, the T circuit of

resistors

31 and 32 and line 29; and another through the drain-source circuit of the FET 40, the capacitors 34 and 35 and line 29. The effective resistance of the drain source circuit of the FET 40 controls or modulates the signal that passes through this second path. In effect, therefore, the feedback circuit is an RC twin T-frequency rejection network which has been split such that the output signal of transistor 19 is applied to half of the network (31-32-33) while a variable voltage, but otherwise similar signal, is applied to the other network half (34-35-36). The rejection frequency, therefore, varies depending upon conduction through the FET.

The amplifier 9 is, therefore, of the band-pass type and has its band-pass determined by the capacitive resistive feedback circuit 28. When the efiective resistance of drain source circuit of the FET 40 is high, the second feedback path is effectively cut out of the circuit and the band-pass of the amplifier 9 is centered about one high resonance peak. When the drain source resistance of the FET 40 is low, the alternate path is fully open and the band-pass of the amplifier 9 is centered about a low resonance peak.

The movement of the band-pass of the amplifier 9 from its low to high resonant peak is thus governed by the effective drain source resistance of the FET 40 which resistance is in turn governed by the direct current bias on its gate. The voltage on the gate is determined by the timing circuit of the resistances 41 and 42 and the capacitor 43.

Normally, when the transistor 45 is saturated, that is, when the resistance of the collector-emitter circuit of the transistor 45 is low, the junction of the resistors 42 and 46 is effectively at ground potential and thus the gate of the FET 40 is also at ground potential. This keeps the FET 40 source drain resistance low and the band-pass of the amplifier 9 low. However, when the transistor 45 is rendered nonconductive, current flows from the 13+ source through the resistors 46 and 42 and to the parallel capacitor 43 and resistor 41 circuit. Were the timing capacitor 43 not in this circuit the voltage at the top of the resistor 41 would jump almost immediately to a level determined by the voltage dividing resistors 41, 42 and 46. However, as the capacitor is in the circuit the current is diverted initially into charging that capacitor. As the charge on the capacitor 43 rises, the potential at the center tap of resistor 41 rises and the potential change at the FET gate is substantially as depicted by the curve 72 on the small graph of FIG. 1.

If after approximately reaching its ultimate value, at which time the capacitor 43 is fully charged, the transistor 45 is again rendered conductive, the capacitor 43 discharges through resistor 42 as well as through permanently connected resistor 41. If the value of the resistor 42 is chosen to be small in comparison to that of resistor 41 a fast decline of the voltage at the gate of the FET 40 will be achieved, approximating the curve 73 of the small graph of FIG. 1.

The transistor 45 is rendered nonconductive by grounding its base. This may be done by the trumpet selector switch 47 or by the keying circuit 50 in response to closure of the key contacts 60. When done by the switch 47, the band-pass of the amplifier is fixed at its high peak and the tone signal produced at output terminal 26 is like a trumpet and does not have the moving formant effect. However, when switch 47 is open, the band-pass amplifier 9 will normally be at its low band-pass peak and a muted trumpet sound electrical signal will be translated through the amplifier 9 to the output terminal 26. When the key contacts 60 are closed, the potential at the emitter of the transistor 52 is caused to rise to the threshold value of diode 58, cutting off that transistor and turning on the transistor 51. This causes the voltage at the collector of transistor 51 to fall from approximate B+ to ground which in turn effectively grounds the base of the transistor 45. When this occurs the voltage at the gate of the FET rises as the curve 72 and the band-pass of the amplifier 9 goes from its low peak (mute) to its high peak (trumpet) resulting in the moving formant effect simulating opening a muted trumpet.

For purposes of definiteness of this disclosure and not for limitation, the following component values are submitted. It will be understood by those skilled in this art that numerous variations in values, as well as reorientation of components and substitution of equivalents for elements or groups of elements are possible and that these are contemplated to be within the scope of the present invention.

Resistors: Ohm

15 680K 17, 53

K

18, 48

470K

20, 21 10K 22 a K 24 2.7K

6 0-100K 38, 42 6.8K 41 0-1000K 46 270K 54 t 220K 55 12K 56, 61 1K 59 Capacitors: Mf.

Transistor 16 is of the 2N339lA type,

transistors

19, 45, 51 and 52 are of the 2N3393 type. The FET 40 is of 2N3819 type.

The diodes 57 and 58 are of typical silicon junction type and are used to establish a potential at their threshold value of about 0.6V. They aid in temperature compensating the circuit.

The impressed signal from the bright bus bar 11 is normally at about 10.7v. peak-to-peak.

The components were chosen to provide a formant movement of about l /aoctaves, with the high resonant peak providing a formant similar to the normal 8' trumpet stop on a typical electrical organ.

Referring to FIG. 2, an alternative circuit for the moving formant device is there depicted and is similarly generally indicated by the numeral 10'. Elements of this embodiment which are similar to those of the previous embodiment will be identified with primes of the same numbers, it being understood that they are connected as in the previously described embodiment unless noted to the contrary. The values of the otherwise similar components may be different, however.

The device 10 includes an amplifier 9 generally similar to the amplifier 9 of the previous embodiment, except that it includes an NPN transistor 80 in place of the FET 40 and a parallel inductor 81 and capacitor 82 in place of the load resistor 2].

The resistance of the emitter-collector circuit of transistor 80 is controlled by either a simplified keying circuit 83 or by a low-frequency oscillator circuit 84, either of which may be connected to the base of transistor 80, depending upon the position of a selector switch 85.

More specifically, the amplifier 9 includes an input terminal 86 which is connected to one end of a resistor 15' whose other end is connected to the base of an NPN transistor 16'. This base also is connected through a resistor 18 to B+ and through a resistor 17' to ground. The emitter of the transistor 16' is connected to the base of an NPN transistor 19 and the collectors of both transistors 16' and 19' are connected together and through a parallel capacitor 82-inductor 81 circuit to B+, and also to a signal output terminal 26 and to a feedback circuit 28 via a line 27. The emitter of transistor 19 is connected to ground through a parallel connected resistor 24' and capacitor 23'. The circuit 28 comprises a pair of T circuits, one of resistors 31', 32 and capacitor 33 and the other of capacitors 34, 35' and variable resistor 36, the latter being connected to lead 37 by way of the collectoremitter circuit of a transistor 80. The other side of circuit 28' is connected to the base of transistor 16 through a line 29' and a capacitor 30.

The collector of transistor 80 is connected to lead 27 and a capacitor 87 is connected between the collector and its base. The base is also connected through a resistor 46 to B+ and through a resistor 41' to ground and through a resistor 88 to the blade ofthe previously mentioned switch 85. The switch blade is also connected to ground through a timin gcapacitor 43'.

The keying circuit 83, by way of a line 89, is connected to one of two contacts that the blade of switch 85 may engage. Line 89 is connected to the collector of a transistor 90 and also through a resistor 91 to B+. The emitter of the transistor 90 is coupled through a resistor 92 to ground, and the base of that transistor is connected through a resistor 93 to B'+ and through a resistor 94 to ground. The base of transistor 90 is also connected through a current limiting resistor 95 to B'+ by way of key actuated contacts 60.

The low-frequency frequency oscillator 84 which runs at vibrato rate, about 6.5 H includes a pair of NPN transistors 96 and 97 whose collectors are connected in common and,

through a resistor 98, to 3+ and through a series connected capacitor 99 and variable resistance 100 to the other terminal of the switch 85.

The emitter of transistor 96 is connected through a resistor 101 to ground while the emitter of transistor 97 is connected to the base of transistor 96. The base of transistor 97 is connected through a resistor 102 to its collector and through a resistor 103 to ground. The base is also connected to one side of a capacitor 104, the other side of which is connected, through a variable resistor .105, to ground and to one side of another capacitor 106. The other side of this capacitor 106 is connected through another variable resistor 107 to ground and to one side of yet another capacitor 108. The other side of this capacitor is connected to the collectors of the transistors 96 and 97.

The variable resistors are mechanically ganged and manually controlled to vary the oscillation rate. The variable resistance 100 varies the swing limits or the degree of movement per oscillation when the oscillator 84 drives the moving formant of the amplifier 9.

Basically, in operation, the device when the blade of the selector switch 85 is connected to the oscillator circuit 84, functions to move the formant signal transferred through the amplifier 9 cyclically at a rate determined by the oscillator 84, for example at 6.5 to 7 cycles per second. This yields a pleasing musical effect which is somewhat like a vibrato.

When the switch 85 has its blade connected to line 89, it functions much as the embodiment of FIG. 1 when the switch 47 of thatembodiment is open. That is, a formant movement is produced in response to the closure of the key actuated switch *60.

In the circuit of FIG. 2 the following values may be employed for the various components. These specific values are,

of course, suggested by way of example and for completeness of disclosure. It should be borne in mind, however, that nution and arrangements can be successfully used.

Resistors: Ohm

103 2000K 01000K 46

1000K

105, 107 0-20OK 18 K 15, 88, 101 100K 91 68K 93

56K

31, 32 27K 36 0-25K 17', 41' 22K 38, 94 10K 98 5. 6K 102 4. 7K 95 2. 7K 92 1. 2K 24' 1K Capacitors: Mf.

Inductor:

1 2300 T, Ferrite Cup Core.

The bias source B+ is 15 volts, while that of B'+ is 4 volts as previously stated.

This device 10 as shown and described effectively moves a formant slightly more than two octaves either under control of the playing keys or optionally under the influence of a repetitive voltage sources such as the vibrato oscillator. The trumpet stop of an organ played through this device under key control sounds like a mute trumpet being opened up as the playing key is held.

One of the effective uses of the device 10 or 10' is in keying it with a touch responsive system on the upper manual and with the bright wave voices fed through it. When this device is driven at the vibrato rate, it sounds very much like vibrato on the frequencies in the formant range. Since the formant range with the components as shown in FIG. 2 and values as detailed above, is from 400 cycles to l900 cycles, it is possible to play low-frequency bright wave tones in which the higher frequencies appear to have vibrato without the lower frequencies being affected. With proper selection of formant range, this device is effective to provide rnanual selection of vibrato within a limited frequency range on organs that normally have only unitary vibrato.

It is apparent that this invention provides a new and improved moving formant device that is versatile and capable of several applications in electrical musical instruments. Furthermore, it is relatively inexpensive as it employs low cost components, while effectively accomplishing its intended ends in an efficient manner.

Although the two embodiments described in detail use a transistor or FET as a voltage responsive variable resistor to control the effectiveness of one of the two feedback paths, it

' will be appreciated that there are other voltage responsive claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A musical instrument moving formant devicecomprising an audio frequency amplifier having a music signal input and an amplified output, means providing a feedback :path connected forzfeeding back signal from said output to saidtinput,

said feedback path consisting of an RC, parallel T, frequency rejection circuit, one of the parallel branches of said circuit comprising a pair of resistors in series with a center connected capacitor and the other of the parallel branches comprising a pair of series connected capacitors with a center connected resistor. a voltage dependent variable impedance incorporated in one only of said branches and connected so that conduction through the last said branch from said output to said input depends upon the voltage applied to said voltage de- V pendent variable impedance, and means connected for applying a variable control voltage to said variable impedance to vary the conduction through one only of the parallel branches as a function of said control voltage.

2. The moving formant device as called for in claim 1 in which the means for applying the variable control voltage comprises a timing circuit of the type for providing a-gradual change of voltage for a few seconds followed by a steady voltage when activated and a rapid restoration of voltage to the original level when subsequently deactivated, and a manual control connected for activating and deactivating said timing circuit.

3. The moving formant device as called for in claim 1 in which the voltage dependent variable impedance device is a field effect transistor.

4. The moving formant device as called for in claim 1 in which the variable impedance device is incorporated in the branch which has the two capacitors in series and a center connected resistor.

5. The moving formant device as called for in claim 1 in which the rejection frequency of the parallel T circuit is lower when the impedance of said variable impedance is low than it is when the impedance of said variable impedance is high.

6. The moving formant device as called for in claim 5 in which the control voltage is applied to reduce the impedance of the variable impedance device for the duration of the voltage.

7. The moving formant device as called for in claim 1 in which the means for applying a variable control voltage is an oscillator operating at a vibrato frequency.

8. The moving formant device as called for in claim 2 in which the manual control is any of a group of playing keys of an electric organ.

9. The moving formant device as called for in claim 6 in which the means for applying the variable control voltage comprises a timing circuit of the type which provides a gradual voltage change as a function of time for a few seconds followed by a steady state voltage when actuated and a rapid restoration to the original state when subsequently deactivated, and manual means connected for activating and deactivating said timing circuit.

10. The moving formant device of claim 9 in which said manual means is any of a group of playing keys of an electric organ.

11. The moving formant device as called for in claim 9 in which the voltage dependent variable impedance is a field effect transistor.

{22 55 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 569,603 Dated March 9 1971 Inventor(s) Donald R. Kern It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Second paragraph of Abstract omitted, and should be inserted on top page of patent after first paragraph:

-A second embodiment of the moving formant circuit employs a transistor in place of the FET, an: inductive-capacitive load for the amplifier that com sates for the frequency shift of the amplifier s out to produce a generally constant subjective tone ampl: tude therefrom, and an alternate circuit for activat: the formant change comprising a variable low frequem oscillator. Driving the formant changing transistor the oscillator operating at vibrato speeds results i1 a musical effect similar to vibrato.--

Col. 2, line 67, "above" should read -abo1 Col. 5, line 14, "37" should be --27--.

Claim 11, "as called for in claim 9" shoul read --as called for in claim 10 Signed and sealed this 19th day of October 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOI'TSGHALK Attesting Officer Acting Commissioner of P

Claims

1. A musical instrument moving formant device comprising an audio frequency amplifier having a music signal input and an amplified output, means providing a feedback path connected for feeding back signal from said output to said input, said feedback path consisting of an RC, parallel T, frequency rejection circuit, one of the parallel branches of said circuit comprising a pair of resistors in series with a center connected capacitor and the other of the parallel branches comprising a pair of series connected capacitors with a center connected resistor, a voltage dependent variable impedance incorporated in one only of said branches and connected so that conduction throuGh the last said branch from said output to said input depends upon the voltage applied to said voltage dependent variable impedance, and means connected for applying a variable control voltage to said variable impedance to vary the conduction through one only of the parallel branches as a function of said control voltage.

2. The moving formant device as called for in claim 1 in which the means for applying the variable control voltage comprises a timing circuit of the type for providing a gradual change of voltage for a few seconds followed by a steady voltage when activated and a rapid restoration of voltage to the original level when subsequently deactivated, and a manual control connected for activating and deactivating said timing circuit.