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Publication numberUS3911776 A
Publication typeGrant
Publication dateOct 14, 1975
Filing dateNov 1, 1973
Priority dateNov 1, 1973
Publication numberUS 3911776 A, US 3911776A, US-A-3911776, US3911776 A, US3911776A
InventorsMichael L Beigel
Original AssigneeMusitronics Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Sound effects generator
US 3911776 A
Abstract
A sound effects generator for modifying a sound generating signal is comprised of a voltage-controlled filter having a variable peak response frequency for transmitting at a maximum amplitude those frequency components included in the sound generating signal that correspond to the instantaneous filter peak response frequency. The peak response frequency is determinable by a control voltage applied to the voltage-controlled filter; such a control voltage being proportional to the envelope of the sound generating signal as extracted by a control voltage generator. The filter peak response frequency is thus varied in accordance with the varying envelope of the sound generating signal.
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United States Patent [19] Beige] [4 1 Oct. 14, 1975 SOUND EFFECTS GENERATOR [75] Inventor: Michael L. Beigel, Warwick, NY.

[73] Assignee: Musitronics Corporation, Rosemont,

[22] Filed: Nov. 1, 1973 [21] Appl. No.: 411,764

84/1.19, 1.21, 1.22, 1.24, 1.27, DIG. 9, DlG.l9, 1.25; 179/1 D; 307/271; 328/127, 149, 167, 175, 209; 330/52, 75, 98-100, 103, 109; 333/70 R, 70 CR [56] References Cited UNITED STATES PATENTS 2,589,723 3/1952 Miller, Jr. 333/70 R X 3,255,296 6/1966 Peterson 84/1 .24 3,316,341 4/1967 Peterson 84/1.24 3,493,669 2/1970 84/l.25 X 3,573,641 4/1971 328/167 X 3,644,847 2/1972 328/167 X 3,668,294 6/1972 Kameoka et al.. 84/1.01 3,696,252 l0/1972 Chapman 328/167 X 3,708,603 l/1973 Keagle et a1. 84/1.1 1 3,708,746 l/l973 Willett 328/167 X 3,749,809 7/1973 Niinomi 84/l.24

MUS/C41.

Vwom

3,762,265 10/1973 Adachi 84/l.24 3,767,833 l0/l973 Noble et a1. 84/l.01 3,767,939 10/1973 Chamran et a1. 328/167 X 3,787,602 1/1974 Okudaira 84/1.24 3,795,756 3/1974 Suzuki 84/l.22 3,828,110 8/1974 Colin 84/l.01 3,835,399 9/1974 Holmes 328/167 OTHER PUBLICATIONS G.E. Transistor Manual, General Electric Company, copyright 1964, p. 371.

Primary ExaminerL. T. Hix Assistant ExaminerStanley .1 Witkowski Attorney, Agent, or Firm-Curtis, Morris & Safford [57] ABSTRACT A sound effects generator for modifying a sound generating signal is comprised of a voltage-controlled filter having a variable peak response frequency for transmitting at a maximum amplitude those frequency components included in the sound generating signal that correspond to the instantaneous filter peak response frequency. The peak response frequency is determinable by a control voltage applied to the voltagecontrolled filter; such a control voltage being proportional to the envelope of the sound generating signal as extracted by a control voltage generator. The filter peak response frequency is thus varied in accordance with the varying envelope of the sound generating signal.

24 Claims, 6 Drawing Figures A74 75? 007- /6 Va ewra 0P5 7 mum/m v m fit; FE m 8\ i l m wm U.S. Patent Oct. 14, 1975 Sheet 1 of2 Sheet 2 of 2 US. Patent Oct. 14, 1975 SOUND EFFECTS GENERATOR BACKGROUND OF THE INVENTION This invention relates to apparatus for varying a sound generating signal and, in particular, to a sound effects generator for automatically modifying a sound generating signal produced by a musical'sound generating instrument in accordance with a preselected control signal.

In the field of sound generation, and in particular the field of musical tone generation and synthesis, it is frequently desirable to produce various types of electronic musical effects in accordance with the playing of a musical instrument. The contemporary practice of electronically generating musical tones by tone synthesizing generaters or other types of devices employed with suitable electronic transducers is frequently accompanied by modifying such electronic signals to produce a variety of audio responses or sound effects.

Numerous types of sound generation and effects are readily obtained by the use of conventional electronic music synthesizers. However, such synthesizers are generally expensive and usually require the unique skill of a trained operator. Consequently, the advantageous use of such synthesizers, although justifiable for largescale public preformances, has not been wide spread for private or rather limited entertainment. The prior art has,.therefore, sought an inexpensive device that is relatively simple to operate which can produce sound effects similar to those obtainable from electronic music synthesizers and that is operable with conventional musical instruments.

An exemplary tone signal modifier that has been employed to produce desirable sound effects comprises an electronic filter capable of generating a vowel-like effect upon a tone signal that is transmitted therethrough. This filter may be either a high-pass filter, a band-pass filter or a low-pass filter having a peak frequency response that is variable over a given frequency range. The peak response frequency of this filter is, of course, that frequency at which an input signal is transmitted therethrough with maximum gain or amplitude. That is, those frequency components included in the frequency spectrum of an input signal that correspond to the peak response frequency of the filter will be subjected to a minimum amount of attenuation by the operation of the filter. The aforenoted vowel-like effect is produced by varying the peak response frequency or the cut-off frequency of the filter during a filtering operation. Such variation in the frequency characteristic of the filter functions as a simple electronic analog of the complex acoustic filtering" of speech tones produced by the human vocal cords. Unfortunately, such variations in the filter characteristics have heretofore been obtainable only by a manual operation executed by, for example, a sound technician or the instrumentalist himself. In the latter respect, although the musician may selec tively obtain the sound effects he particularly desires, such operation, requires significant concentration which necessarily interferes with his musicalperformance.

Therefore, it is an object of-the present invention to.

provide an improved sound'effects generator that automatically produces sound effects without the requirement of manual control or operation.

It is another object of the present invention to provide a sound effects generator that operates in synchronism with a produced sound generating signal.

It is a further object of the present invention to provide a sound effects generator that may be advantageously employed with musical tone generators to modify the sound generating signals produced by such musical tone generators.

Yet another object of the present invention is to provide a voltage-controlled filter for-varying a sound generating signal to produce desirable sound effects.

A still further object of the present invention is to provide apparatus for varying a sound generating signal in accordance with the changes in the signal itself.

Another object of the invention is to provide a sound effects generator that is automatically controlled by a sound generating signal upon which the generator operates. I

Various other objects and advantages of the present invention will become apparent from the detailed description set forth below and the novel features will be particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION In accordance with the present invention apparatus is provided for varying a sound generating signal, including a voltage-controlled filter that has a variable peak response frequency for passing, at a maximum amplitude, those frequencies included in the sound generating signal that correspond to the peak response frequency, the peak response frequency being variable over a given range of frequencies and being determinable by a control voltage applied to the filter; and a control voltage generator that is operable upon the sound generating signal to derive therefrom a sound generating signal envelope, such envelope being used as the control voltage to determine the peak response frequency of the filter.

BRIEF DESCRIPTION OF THE DRAWINGS the present invention; and

FIG. 4 is a schematic circuit diagram of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF ONE OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and in particular to FIG. 1, there is illustrated a block diagram of a preferred embodiment of the present invention comprising a sound effects generator 10. The sound effects generator is adapted to modify a sound generating signal applied to an input terminal 11 by, for example, a musical sound generating instrument 9, such that the resulting output signal is capable of deriving a synthesized or vowel-like tone. The sound effects generator 10 here I comprises a preamplifier 12, a voltage-controlled filter 14 and an envelope follower 16. The preamplifier 12 is, preferably, a variable gain amplifier adapted to amplify a produced sound generating signal to predetermined levels. The preamplifier is connected to input terminal 11 and includes an output that is coupled, in common relationship, to the voltage-controlled filter 14 and the envelope follower 16. In the preferred environment of the present invention, the preamplifier 12 is operable upon signals that are derived from musical instruments or are produced by conventional musical sound synthesizers. Hence, the audio signal coupled to the input terminal 11 may be produced by any conventional musical sound generator including electronic transducers or the like.

The voltage-controlled filter 14 is, preferably, a low sensitivity active filter of the type described in the paper synthesizing Active Filters by S. K. Mitra, IEEE Spectrum, January 1969, pages 47-63. In particular, the filter 14, which is shown and described in greater detail hereinbelow, exhibits a peak response frequency that is variable over a given frequency range in accordance with the magnitude of a control voltage applied thereto. As noted hereinabove, a peak response frequency is a predetermined frequency, or narrow band of frequencies, at which an input signal is transmitted with maximum amplitude (or, stated otherwise, with minimum attenuation). Hence, if an input signal applied to the voltage-controlled filter 14 admits of a frequency spectrum, those frequency components corresponding to the peak response frequency will be transmitted by the filter at a maximum amplitude with respect to the remaining frequency components. The filter 14 may comprise a low-pass filter, a band-pass filter or a high-pass filter. In accordance with a preferred embodiment of the present invention, the filter exhibits high-, band-and low-pass filtering characteristics, one of which may be individually selected by an operator of the sound effects generator.

The particular peak response frequency of the filter 14 is determined by the magnitude of a control voltage V applied thereto. Hence, if the control voltage is a time varying signal then the peak response frequency will correspondingly vary with respect to time. As will be apparent from the forthcoming description thereof, the peak response frequency of the filter 14 is capable of being rapidly varied so as to synchronously follow all expected variations in the control signal V applied to the filter. In one preferred embodiment thereof, the peak response frequency varies directly with changes in the control voltage. That is, as the control voltage increases, the peak response frequency will likewise increase; and as the control voltage decreases, the peak response frequency similarly decreases. In an alternative embodiment, the peak response frequency of the filter 14 varies inversely with changes in the control voltage.

The envelope follower 16 is adapted to produce the control voltage V determinative of the peak response frequency of the filter 14. This control voltage is capable of varying in synchronism with variations in the sound generating signal applied to the preamplifier 12 to thereby permit the corresponding synchronous variations in the peak response frequency of the filter. Since the actual wave form of the electronic audio signal is expected to include relatively high frequency variations having peak amplitudes that define an envelope therefor, it is appreciated that the envelope follower 16 comprises a conventional filtering circuit capable ofextracting the audio signal envelope. Preferably the envelope follower is comprised of a conventional rectifier and low-pass filter to thus extract either a positive or negative audio signal envelope. In the preferred embodiment of the present invention, a positive audio envelope is extracted and supplied as a control voltage to the filter 14.

In operation, let it be assumed that the voltagecontrolled filter 14 comprises a low pass filter having a frequency response characteristic graphically represented in FIG. 2A. The ordinate of this graphical representation depicts the amplitude of the output signal transmitted by the filter and the abscissa of the graphical representation depicts the frequency components of the transmitted signal. As may be observed the filter 14 admits of a peak response frequency f, whereat the output signal transmitted thereby exhibits maximum amplitude. That is, if the sound generating or audio signal applied to the filter 14 includes a component at the frequencyf that component will be transmitted by the filter at relatively maximum amplitude. As further depicted in FIG. 2A, the desirable or limited bandwidth, or Q is established about the peak response frequency f This Q serves to accentuate preselected frequency components included in the audio signal.

The sound generating audio signal applied to the input terminal 11 by the suitable musical instrument, transducer, sound synthesizer or the like, is amplified by the preamplifier. The amplified audio signal V having the typical wave form 13, is applied to the filter l4 and, concurrently therewith, to the envelope follower 16. As is apparent, the higher frequency components of the audio signal waveform vary in amplitude to define a time varying envelope that changes at a slower rate. The envelope follower 16 serves to extract the slower time varying envelope and to supply such extracted envelope to the filter 14 as a control voltage V therefor. The extracted envelope admits of a waveform 17. Although the audio signal 13 is seen to include a positive and negative envelope, the envelope follower 16 preferably extracts only a single polarity envelope. In the preferred embodiment described herein, the positively poled envelope is extracted by the envelope follower and applied as a control voltage to the voltagecontrolled filter 14. Hence, the control voltage applied to the filter varies in synchronism with the variations in the effective volume of the audio signal aupplied to the input terminal 11.

Again referring to FIG. 2A, it is apparent that the peaks response frequency f may vary throughout a given range of frequencies in accordance with the amplitude of the control voltage V That is, the instantaneous peak response frequency is determined by the instananeous amplitude of the control voltage. In the illustrated embodiment, the peak response frequency is variable throughout a range from a lower frequency f, to a higher frequency f,,. Hence, as the control voltage V increases in amplitude, the peak response frequency of the filter 14 increases from its lower value 1], to its higher value f,,. As the peak response frequency varies, it may be appreciated that the graphical representation of the filtering characteristics of the filter 14 is expanded, or stretched", to the right in FIG. 2A. Conversely, as the amplitude of the control voltage V decreases, the peak response frequency of the filter 14 correspondingly decreases from a maximum frequency value j, to its lower frequency value f,. .It is appreciated that, since the envelope follower 16 preferably extracts only the positively poled envelope of the audio signal, the control voltage V remains at a positive value.

In the foregoing description, the peak response frequency of the filter 14 is assumed to vary directly as the changes in the control voltage V Hence, the relationship between the control voltage and the peak response frequency of the filter is graphically represented in FIG. 3. However, it is contemplated that an inverse relationship may be established between the control voltage and the peak response frequency. That is, as the control voltage increases in amplitude, the peak response frequency f may decrease and, conversely, as the amplitude of the control voltage decreases, the peak response frequency increases. This relationship may be readily obtained merely by inverting the control voltage produced by the envelope follower 16, as by applying the control voltage to a conventional inverting operational amplifier, and then adding the inverted control voltage V to a predetermined reference voltage; the summed voltages being applied to the filter 14. In this modification, it may be appreciated that the voltage thus applied to the filter varies inversely with the control voltage V extracted by the envelope follower 16 to thus vary the peak response frequency f, in a similar manner. In a still further alternative embodiment, the control voltage V extracted by the envelope follower 16 may be subtracted from a predetermined reference voltage in a conventional subtracting, or differencing network. The resultant output signal produced by such subtracting or differencing network may then be applied to the filter 14 to thus determine the frequency value of the peak response frequency thereof. In yet another alternative embodiment, the envelope follower 16 may be adapted to extract the negatively poled envelope of the audio signal and to then sum this extracted negative envelope with a predetermined reference voltage. The resulting sum, if used as a control voltage for the filter 14, would thus vary the peak response frequency thereof in the desired inverse manner.

If the voltage-controlled filter 14 is assumed to comprise a band-pass filter, it is appreciated that the filtering characteristics thereof may appear as graphically represented in FIG. 28. Of course, as a band-pass filter, the voltage-controlled filter 14 is capable of passing only a predetermined band of frequencies included in the audio signal. However, such frequency pass band may be appropriately selected to encompass the desired frequencies expected in the audio signal. Nevertheless, such band-pass filter admits of a peak response frequency j", and additionally exhibits a predetermined bandwidth or Q about such peak response frequency to thus accentuate the filtered audio signal at certain selected frequencies while attenuating such filtered signal at other frequencies. As in the aforedescribed embodiment, the instantaneous value of the peak response frequencyfl, is determined by the instantaneous amplitude of the control voltage V applied to the filter 14 by the envelope follower 16. If it is desired to vary the peak response frequency directly as the control voltage changes, then it is appreciated that as the control voltage increases the peak response frequency increases from a lower frequency value f, to a higher frequency value f, onversely, as the control voltage amplitude decreases, the peak response frequency likewise decreases from the higher frequency value f to the lower frequency value off,. Of course, such variation in the peak response frequency is effected in synchronism with the envelope extracted from the audio signal by the envelope follower 16. As in the example described above, the peak response frequency of the band-pass filter may be varied inversely with the changes in the control voltage V if desired.

If it is now assumed that the filter 14 comprises a high-pass voltage-controlled filter, it is appreciated that the filtering characteristics thereof may be represented as depicted graphically in FIG. 2C. As there depicted, the filter exhibits a peak response frequency f,, and, additionally, a limited bandwidth or 0 about the peak response frequency. As in the previously described embodiment, the peak response frequency may be varied between a lower frequency value )1, and a higher frequency f,,' in accordance with variations in the amplitude of the control voltage V applied to the filter 14.

It should be appreciated that the selection of lowpass, band-pass or high-pass characteristics of the filter 1.4 is dependent upon the sound effects that are desired. That is, if the lower frequency tones are to be accentuated to produce a desired vowel-like effect, the low-pass filtering characteristics would be selected. Similarly, if the higher frequency tones of the audio signal are to be accentuated, the high-pass filtering characteristics would be selected. And if the intermediate frequency tones of the audio signal are to be accentuated the band-pass filtering characteristics would be selected. Nevertheless, in each of the pass-bands that are provided, and as will be described in greater detail hereinbelow, the absolute range over which the peak response frequency f, may be varied can be altered as desired. That is, the absolute frequency range f f, may admit of a low range or a high range to produce the vowel-like effect on a corresponding range of frequencies included in the audio signal. Preferably, however, the ratio f' f is constant for all frequency sweep ranges and for all filtering characteristics.

Referring now to FIG. 4, there is illustrated a schematic diagram of one preferred embodiment corresponding to the block diagram depicted in FIG. 1. The preamplifier 12 is seen to comprise a conventional operational amplifier 102 adapted for variable gain amplification. Accordingly, the input terminal 11 is coupled to the operational amplifier by a conventional a.c. coupling circuit 105 connected in series with a variable input resistor 104. The resistor 104 may, typically, comprise any conventional variable resistance element, such as a potentiometer, a rheostat, or the like. A feedback resistor 106 is provided between the output terminal of the operational amplifier 102 and the input terminal thereof. As is appreciated, the gain of the preamplifier 12 is a function of the ratio of the resistors 106 and 104. As resistor 104 is variable, the gain of the preamplifier is correspondingly variable. If desired, a conventional high frequency stop may be provided as, for example, the capacitive element 107 illustrated in parallel relationship with the feedback resistor 106. The operational amplifier 102 is conventional, as is well known to those of ordinary skill in the art, and may include an inverting input terminal supplied with an audio signal and a non-inverting input terminal coupled to a suitable reference potential, such as ground, by a conventional biasing network. Alternatively, the operational amplifier may include a single inverting or noninverting input terminal as desired.

The output terminal 158 of the preamplifier 12 is coupled to the voltage-controlled filter 14 and to the envelope follower 16. Additionally, the output terminal 158, which is thus seen to comprise the input terminal of the voltage-controlled filter 14, is coupled to a bypass switch 213 for a purpose soon to be described.

The voltage-controlled filter 14, which is a low sensitivity active filter, is adapted to provide a second order transfer function and, in this regard, is conventional as noted hereinabove and as described in the IEEE Spectrum publication. Accordingly, an input operational amplifier 110 is provided with a variable gain function and is connected in series relationship with first and second integrator circuits. Each integrator circuit is comprised of an operational amplifier disposed in conventional integrator circuit configuration and including input resistance and feedback capacitance.

The input operational amplifier 110 includes a noninverting input terminal coupled to the preamplifier 12 via an input resistor 108. The inverting input terminal of the operational amplifier 110 is coupled, by resistor 114, to the output terminal 146 of the filter 14. Additionally, a feedback resistor 112 is provided between the output of the operational amplifier 110 and the inverting input terminal thereof. A variable feedback resistor 130 is provided between the output terminal 150 of the first integrator circuit 125 and the non-inverting input terminal of the operational amplifier 110. The variable resistor 130 may be similar to the aforedescribed variable resistor 104 and, therefore, may comprise a conventional potentiometer, a rheostat, or the like. It may be appreciated that the bandwidth or Q of the filter 14 is a function of the ratio between resistors 130 and 108. Additionally, since the gains of the low sensitivity active filter is proportional to the Q thereof, it may be appreciated that such gain is variable in accordance with the value of the resistor 130.

The output of the operational amplifier 110 is coupled to the input terminal 154 of the first series connected integrator circuit 125. Additionally, the input terminal 154 is coupled to a switch 204 for a purpose soon to be described. The first integrator circuit is comprised of an operational amplifier 122 having a noninverting input terminal coupled to ground and a feedback capacitor 124 intercoupled between the output terminal 150 thereof and the inverting input terminal thereof. The input resistance coupled to the inverting input terminal of the operational amplifier 122 is comprised of resistor 120 connected in parallel relationship with the series combination of resistors 116 and 118. The resistor 116 is a variable resistor that, preferably, admits of a resistance value determined by a control voltage, soon to be described. Thus, the resistor 116 may be considered to be a voltage dependent resistor, various embodiments of which are well known to those of ordinary skill in the art. As is appreciated, the time constant of the first integrator circuit is a function of the feedback capacitor and the input resistance. Furthermore, since the peak response frequency of the filter 14 is related to the charging time constant of the first integrator circuit, it is appreciated that a variation in the resistance value of resistor 116 effects a corresponding variation in the frequency value of the peak response frequency. Hence, the instantaneous value of the resistor 116 is at least determinative ofthe instantaneous peak response frequency of the filter 14.

A supplemental capacitor 126 is selectively connected in parallel with capacitor 124 by the series connected switch 128. It is recognized that the total feedback capacitance is increased when the switch 128 is closed. As the range over which the peak response frequency of the filter 14 may be varied is dependent upon the total feedback capacitance of each integrator circuit, it is appreciated that switch 128, together with switch 144 to be described, is adapted to establish the appropriate frequency varying range.

The output terminal 150 of the first integrator circuit is connected to a second integrator circuit 145 and, additionally, to the switch 204 for a purpose soon to be described. The second integrator circuit includes an operational amplifier 138 disposed in substantially identical configuration as the operational amplifier 122 of the first integrator circuit. Accordingly, a noninverting input terminal of the operational amplifier 138 is connected to ground and the output terminal thereof is interconnected with the inverting input terminal by a capacitor 140. The input resistance to the inverting input terminal of the operational amplifier 138 is comprised of the resistor 136 connected in parallel relationship with the series combination of resistors 132 and 134. Resistor 132 is substantially identical to the aforedescribed resistor 116 and, as may be appreciated, a variation in the resistance value thereof is effective to correspondingly alter the charging time constant of the second integrator circuit. Thus, a change in the resistance value of the resistor 132 is capable of resulting in a corresponding change in the frequency value of the peak response frequency of the filter 14. As will soon be described, the variable resistors 116 and 132 cooperate to substantially simultaneously vary the charging time constants of the respective integrator circuits to thereby effect a corresponding variation in the peak response frequency of the low sensitivity active filter 14.

A supplemental capacitor 142 is adapted to be connected in parallel relationship with the feedback capacitor 140 by the selective operation of switch 144. As indicated by the broken line depicted in FIG. 4, the switches 128 and 144 are adapted for ganged operation to thereby simultaneously increase or decrease, in accordance with the selective operation thereof, the total feedback capacitance of their respective integrator circuits.

The output terminal 146 of the second integrator circuit is interconnected, by resistor 114, to the input of the operational amplifier and, additionally, is coupled to the switch 204 for a purpose soon to be described.

The aforedescribed resistors 116 and 132 are adapted to function as voltage dependent resistors. That is, the resistance value exhibited by each resistor 116, 132 is seen to be a function of a control voltage. Hence, these resistors are electronically variable. it is here noted that the particular construction of the resistors 116 and 132 is dependent upon various considerations. The resistors should not detract from the preferably high signal-to-noise ratio to be exhibited by the filter 14. Additionally, the resistance values of the resistors should be variable at a sufficiently high rate such that the peak response frequency of the filter may vary in synchronism with the extracted envelope of the supplied audio signals. That is, the expected rate ofchange of the audio signal envelope should not exceed the peak response frequency variation capability. Furthermore, each resistor 116 and 132 should vary in a substantially identical manner so that the filtering characteristics of the filter 14 are substantially preserved over the frequency range within which the peak response frequency thereof is to varyv Also, the resistor 116 and 132 should preferably be inexpensive to thus facilitate a commercially feasible and marketable sound effects generator. In accordance with these considerations, various types of voltage dependent resistances are available. For example, dual matched field effect transistors (FETs) wherein the resistance value is dependent upon the control voltage applied thereto might be employed. However such FETs might be accompanied by leakage of control voltage transients into the audio signal path resulting in distortion of the filtered signals. Additionally, audio signals that exceed a modest level could be distorted, thus necessitating low level signal processing and an attendant poor signal-to-noise ratio. Furthermore, the cost of such dual matched FETs is significant.

Alternatively the resistors 116 and 132 may comprise conventional resistance elements disposed for high frequency PET-chopping. Such technique is known to vary the effective resistance value but has heretofore required considerable expense in implementation.

Therefore, in accordance with the preferred embodiment of the present invention, the resistors 116 and 132 desirably comprise photo-resistance elements such as conventional photocells, photo-resistors or other photo-electric devices having a resistance value that is dependent upon the intensity of radiant energy incident thereon. As a typical example, the resistors 116 and 132 may each comprise conventional photo-resistors in communication with a source of radiant energy such that the resistance value of each photo-resistor is determined by the intensity of the radiant energy transmitted thereto. Such photo-resistors are commercially available and satisfy the considerations noted hereinabove. It may be appreciated that the intensity of radiant energy emitted by a suitable source thereof may conventionally be a'function of the magnitude of a control voltage applied thereto. Although such source of radiant energy may comprise an incandescent lamp, it is known that the intensity of the radiant energy transmitted by an incandescent lamp cannot be rapidly altered. Thus, if an incandescent lamp is employed, it is possible that the resistance values of the respective photoresistors might not be varied at the rate of change of the audio signal envelope. Therefore, it is preferred to employ a conventional light emitting diode (LED) as the source of radiant energy. A single LED 198 may be provided to transmit radiant energy of substantially equal intensity to each of the photo-resistors 116 and 132. Preferably, these elements are provided in a suitable housing to prevent interference from ambient light.

The intensity of the radiant energy emitted by the LED 198 is determined by the control voltage applied thereto by the envelope follower 16. Preferably, the en'- velope follower comprises an operational amplifier rectifying circuit and a low-pass filter. As illustrated, the operational amplifier-rectifier is comprised of a conventional operational amplifier 170 disposed in precise rectifier configuration and admitting of an amplification factor. A non-inverting input terminal of the operational amplifier is coupled to ground by resistor 178. The inverting input terminal of the operational amplifier is coupled to the output terminal 158 of the preamplifier 12 by a conventional a.c. coupling capacitor I62 and a series connected resistor 164. A supplemental resistor 168 is adapted to be connected in parallel with the resistor 164 by switch 166 for a purpose soon to be described. Additionally, a rectifying diode 172 is provided to interconnect the output of the operational amplifier with the inverting input terminal thereof and is poled so as to be reverse biased in response to negative input signals. A further rectifying diode 174 is provided at the output of the operational amplifier 170 and is poled so as to be positively biased when positive signals are produced by the operational amplifier-rectifier. A feedback resistor 176 interconnects the rectifying diode 174 and the inverting input terminal of the operational amplifier 170. A series circuit comprised of resistor 180 and capacitor 182 is coupled between the output of the rectifying diode 174 and ground. The junction defined by the resistor 180 and the capacitor 182 is coupled to a suitable source of energizing potential V by a resistor 184. As will soon be described, the operational amplifier 170, disposed in the illustrated rectifying configuration, is adapted to provide the capacitor 182 with positive rectified signals. Hence, the capacitor 182 is capable of rapidly charging to the maximum value of the rectified signals. However, when the output of the rectifying circuit is a minimum, as when a positive component of the audio signal is applied to the operational amplifier 170, the charge potential provided at the capacitor 182 slowly discharges through the resistor 184. Accordingly, the resistor 184 is selected so as to provide a relatively high time constant for the filtering capacitor 182. A positive envelope of the audio signal is thus provided at the capacitor 182.

As noted hereinabove with respect to FIG. 1, it might be desirable in some instances to vary the peak response frequency of the low sensitivity active filter 14 directly as the envelope of the audio signal changes. In other instances it might be preferable to establish an inverse relationship between the variations in the peak response frequency and changes in the audio signal envelope. To this effect, a selective circuit including operational amplifier is provided. The operational amplifier is provided with an inverting input terminal coupled to the capacitor 182 by a resistor 186. Additionally, a non-inverting input terminal is selectively coupled to the capacitor 182 by a switch 187. The switch includes a movable contact 188 adapted to engage a first stationary contact designated UP and a secondary stationary contact designated DN. The DN contact is coupled to a source of reference potential. In the illustrated embodiment, the source of reference potential is derived from a voltage divider circuit comprised of series connected resistors 192 and 194 that are disposed between ground potential and a suitable source of energizing potential +V. Of course, any other conventional source of reference potential may be provided to supply the DN contact accordingly. It may be appreciated that, in the illustrated configuration, the operational amplifier 190 is adapted to selectively operate as a subtracting or a difference circuit. Hence, the operational amplifier may, alternatively, be replaced by a conventional subtracting circuit such as a resistance network, a differential amplifier, or the like.

The output signal provided by the operational amplifier 190 is adapted to operate as the aforedescribed control voltage V However, as is well known, conventional LEDs are current sensitive devices. Accordingly, the operational amplifier 190 is further adapted to function as a voltage-to-current converter and is provided with an appropriate feedback circuit comprised of the LED 198 and resistors 200 and 202. Additionally, current gain is obtained by the emitter-follower transistor 196 disposed in the feedback circuit of the operational amplifier. The junction defined by the LED 198 and the resistor 202 is interconnected with the inverting input terminal of the operational amplifier 190 by the resistor 200.

It may be appreciated that, depending upon the particular terminal included within the low sensitivity active filter 14 that is selected to provide an output filtered signal, the filter may accentuate highpass, bandpass or low-pass frequency components in the manner previously described with respect to FIG. 1. Accordingly, switch 204 is provided to select those particular filtering characteristics that are desired to produce the preferred sound effects. The switch 204 is comprised of a plurality of stationary contacts 148, 152 and 156 coupled, respectively, to the output terminal 146 of the second integrator circuit 145 the output terminal 150 of the first integrator circuit 125 and the input terminal 154 of the first integrator circuit. Additionally, a movable contact 206 is provided to selectively engage one of the stationary contacts of the switch. The movable contact 206 is coupled to the stationary contact 212 of switch 213 and hence via movable contact 214 to an output terminal by a conventional a.c. coupling circuit comprised of the series combination of capacitor 208 and resistor 210. Although not shown herein, it should be recognized that the output terminal of the illustrated sound effects generator may be connected to a suitable output jack to which further operating apparatus such as amplifier systems, may be connected.

The operation of the schematically illustrated sound effects generator is substantially similar to that described hereinabove with respect to FIG. 1 and, therefore, need only be briefly described. The audio signal supplied to the input terminal 11 of the preamplifier 12, as by any conventional audio signal generating device, such as a musical synthesizer, an electronic transducer, a musical instrument, or the like, is amplified to a desired level in accordance with the preselected gain of the preamplifier 12 as determined by the resistance value of variable resistor 104. Although the presence of the preamplifier 12 may be considered to be optional, the function thereof is preferable to permit the filter 14 and the envelope follower 16 to operate upon audio signals admitting of sufficient average amplitude. The preamplifier 12 thus amplifies the audio signal to the appropriate signal level suitable to the dynamic range of the filter 14 and the envelope follower 16.

The amplified audio signal is simultaneously applied to the filter 14 and to the envelope follower 16. By suitably selecting the appropriate resistance value of the resistor 130, the operator thereof establishes a desirable Q and gain of the active filter. These parameters may, of course, be varied in accordance with the preference of the operator and the desired sound effects obtained. As is appreciated, an increase in the Q of the low sensitivity active filter results in more pronounced filtering action such that at higher values of Q, individual frequency components of the audio signal may be emphasized.

As the amplified audio signal is transmitted through the filter 14, the envelope thereof is extracted by the envelope follower 16 to thus vary the intensity of the radiant energy transmitted to the photo-resistors 116 and 132 by the LED 198. Accordingly, the peak response frequency of the filter 14 is correspondingly varied, the frequency range of variation being determined by the feedback capacitors of the first and second integrator circuits. In this latter regard, it is apparent that switches 128 and 144, which are adapted for simultaneous operation, may be closed to thus increase the feedback capacitance ofeach integrator circuit and to thus decrease the range of frequencies over which the peak response frequency of the filter 14 may be varied. For example, with switches 128 and 144 closed, the peak response frequency of the filter may be swept over a relatively low range. Alternatively, when the switches 128 and 144 are opened the range of frequencies over which the peak response frequency may be swept will be increased. It is recalled that, notwithstanding the frequency sweep range, the ratio of the lower frequency limit to the higher frequency limit will remain substantially constant. Hence, the octaves over which the peak response frequency is swept is constant.

Let it be assumed that the peak response frequency of the filter 14 is to be varied directly as the audio signal envelope changes. Accordingly, switches 166 and 188, which are adapted for simultaneous operation, are disposed at their respective UP contacts. Thus, the effective input resistance to the operational amplifier is comprised of resistor 164. It is appreciated that resistor 168 is here electrically isolated. The audio signal, having a typical wave form as depicted by wave form 13 in FIG. 1, is supplied to the operational amplifier 170 by the ac. coupling capacitor 162. The capacitor 162 isolates the operational amplifier 170 from any initial d.c. offset voltage that might otherwise be applied thereto. If the instantaneous amplitude of the audio signal coupled to the operational amplifier 170 admits of negative polarity, the inverting input terminal of the operational amplifier provides a positive amplified output signal. Such positive output signal reverse biases the diode 172 and forward biases the diode 174. Hence, the operational amplifier 170 operates as a conventional inverting amplifier for audio signals admitting of negative polarity. As is recognized, the gain of such inverting amplifier is determined by the ratio between the feedback resistance 176 and the input resistance 164.

If, now, the audio signal supplied to the operational amplifier 170 by the preamplifier l2 admits of positive polarity, the output signal produced by the operational amplifier in response thereto is negative. Such negative polarity serves to forward bias the diode 172. Consequently, the gain of the operational amplifier circuit is determined by the ratio between the forward bias resistance of diode 172 and the resistance 164. Since the forward bias resistance of the diode may, for practical purposes, be so negligible as to be substantially equal to zero, it is appreciated that a substantially zero output signal is produced by the operational amplifier 170 when positive audio signals are applied thereto. Thus, it is seen that the operational amplifier-rectifier functions as a half-wave rectifier having an amplification factor determined by the ratio between the resistances 176 and 164. Audio signals admitting of negative polarity are inverted and amplified by the operational amplifier-rectifier, whereas audio signals admitting of positive polarity are not transmitted therethrough. The audio signal is thus positively half-wave rectified.

The low-pass filter comprised of resistors 180 and 184 and filtering capacitor 182 transforms therelatively rapidly varying signal supplied thereto by the'operational amplifier 170 so as to produce an output signal that is proportional to the maximum amplitude obtained by each rectified peak voltage. More particularly, when the output signal produced by the operational amplifier 170 is positive, the capacitor 182 rapidly charges through the resistor 180 to the maximum or peak amplitude obtained by the rectified signal. It is recognized that the charging time constant of the capacitor 182 is a function of the resistance 180 and the capacitance of the capacitor. Hence, the resistance 180 may be selected to be relatively small to thus define a rapid charging time.

Now, when the output signal produced by the operational amplifier 170 is substantially equal to zero, as may be observed by the rectified waveform 179, the maximum voltage stored by capacitor 182 tends to discharge through the resistor 184. However, if resistor 184 is selected to have a relatively high resistance value to thereby define a relatively high discharge time, it is appreciated that the stored voltage slowly discharges from the capacitor 182. Consequently, when the next rectified peak, as indicated by the wave form 179, is coupled to the capacitor 182, the capacitor rapidly charges to the newly presented rectified amplitude. Thus, the rapid charge and slow discharge of the capacitor functions to extract the envelope wave form 183 from the rectified audio signal. Therefore, it is appreciated that the combination of the operational amplifierrectifier 170 and the low pass filter coupled thereto serves an an envelope follower to extract, for example, the positive envelope of the audio signal. Of course, should it be desired, the negative envelope of the audio signal may be readily extracted by modifying, in minor respects, the illustrated circuit.

In the presently described embodiment, it is desired to vary the peak response frequency of the filter 14 directly as the extracted audio signal envelope changes. That is, as the audio signal envelope increases in magnitude, it is preferred to increase the peak response frequency of the filter. With reference to FIGS. 2A-2C, an increase in the audio signal envelope results in a sweep of the peak response frequency f, to the right and a decrease in the audio signal envelope correspondingly results in a sweep of the peak response frequency f, to the left.

Since the variable resistors 116 and 132 are preferably photo-resistors that are responsive to radiant energy incident thereon, and since the source of radiant energy is preferably a light emitting diode, it is appreciated that the voltage representing the extracted audio signal envelope must be converted to a corresponding current to appropriately drive or vary the radiant energy intensity emitted by the LED. Accordingly, the

operational amplifier 190, in cooperation with the feedback resistors 200 and 202, functions as a voltageto-current converter. When the movable contact 188 of switch 187 is disposed at its UP contact, an increase in the amplitude of the audio signal envelope results in a corresponding increase in the current supplied to the LED 198 by the voltage-to-current converter. Conversely, a decrease in the amplitude of the extracted audio signal envelope results in a decrease in the current supplied to the LED. Thus, when the movable contact 188 is positioned at its UP contact, the illustrated voltage-to-current converter operates in the direct proportional relationship.

Therefore, as the audio signal amplified by the preamplifier 12 is filtered by the low sensitivity active filter 14, the resistance values of the photo-resistors 116 and 132 vary in response to the changes in the radiant energy emitted by the LED 198. That is, as the extracted audio signal envelope increases in magnitude, the intensity of the radiant energy transmitted to the photoresistors 116 and 132 likewise increases to thereby increase the charging time constant of the respective integrator circuits. The result, of course, is to vary, in an increasing manner, the peak response frequency f, of the filter 14. As the peak response frequency varies, the corresponding frequency components included in the audio signal are accentuated. In this instance successively higher frequency components of the audio signal will be accentuated to thus produce the vowel-like effect, aforedescribed.

Now, when the extracted audio signal envelope decreases in magnitude, the radiant energy transmitted to the photo-resistors 116 and 132 by the LED 198 likewise decreases. Hence, the resistance value of the corresponding photo-resistors decreases to thus decrease the charging time constant of the respective integrator circuits. Consequently, the peak response frequency f of the filter 14 is varied in a decreasing manner to now accentuate successively decreasing frequency components of the audio signal. As the filtering characteristics of the filter 14 are varied in syncronism with the changes in the audio signal envelope, the desired sound effects are obtained.

If it is desired to vary the peak response frequency of the filter 14 inversely as the extracted audio signal envelope changes, the switch 187 need merely be operated such that the movable contact 188 thereof is disposed at its DN contact. Simultaneous with the operation of the switch 187, the movable contact 166 is positioned at its DN contact. In this configuration, the effective input resistance of the operational amplifier is now comprised of parallel connected resistors 164 and 168 to correspondingly increase the gain of the operational amplifier-rectifier. Also, the operational amplifier 190 now operates as a difference or subtracting circuit. That is, the current produced by the illustrated voltage-to-current converter is now proportional to the difference between the voltages applied to the operational amplifier 190. Hence, the extracted audio signal envelope is subtracted from the reference voltage applied to the DC contact of the switch 187, resulting in an energizing current supplied to the LED 198 that now varies in a manner that is inversely proportional to the changes in the extracted audio signal envelope. Thus, as the audio signal envelope increases, the difference between the reference voltage supplied to the DN contact of the switch 187 and the audio signal envelope decreases to thus decrease the current supplied by the voltage-to-current converter. Conversely, as the audio signal envelope decreases, the difference between the reference voltage and the audio signal envelope now increases to correspondingly increase the energizing current supplied to the LED 198.

It is, therefore, seen that when the switch 18? is operated to position the movable contact 188 at its DN contact, the intensity of the radiant energy transmitted to the photo-resistors 116 and 132 varies inversely with the changes in the extracted audio signal envelope. Hence, the peak response frequency f of the filter 14 is caused to vary inversely with changes in the audio signal envelope to produce the corresponding sound effects.

As the filtering characteristics of the filter 14 are varied in accordance with the extracted audio signal envelope, an operator of the illustrated apparatus may select specific frequency components of the audio signal which he desires to accentuate to produce the corresponding sound effects. Thus, switch 204, which is coupled to the output terminal of the illustrated apparatus, may be selectively operated in accordance with the operators preferences. For example, if an operator prefers to derive the sound effects resulting from the emphasis of the higher frequency components of the audio signal, the movable contact 206 of switch 204 is positioned at the HP contact 156 and the movable contact of switch 213 is positioned at stationary contact 212. In this configuration, the input terminal 154 of the first integrator circuit is coupled by the switches 204 and 213 to the sound effects generator output terminal. Should it be desired to emphasize the intermediate frequencies of the audio signal, the movable contact 206 is positioned at the BP contact 152. As thus disposed, the output terminal 150 if the first integrator circuit is connected by switches 204 and 213 to the sound effects generator output terminal. And should it be desired to emphasize the low frequencies of the audio signal, the switch 206 is positioned at the LP contact 148 to thus connect the output terminal 146 of the second integrator circuit to the sound effects generator output terminal;

Thus, when the moveable contact 206 is positioned at a selected one of the respective contacts 148, 152, and 156, it may be appreciated that the emphasized audio signal supplied to the sound effects generator output terminal appears as if operated upon by an effective low-pass, band-pass, and high-pass filter, re-

spectively, exhibiting the frequency response characteristics graphically depicted in FIGS. 2A-2C, respectively. Additionally, an operator of the illustrated apparatus may selectively shift the absolute frequency range over which the peak response frequency f may vary merely by selectively operating the ganged switches 128 and 144.

If the aforedescribed sound effects are not desired, the movable contact 214 of switch 213 may be positioned at the by-pass contact 160 to thus interrupt the output channel from the filter 14 to the output terminal and avoid the result of the operation of the filter upon the audio signal. In this configuration, the audio signal amplified by the preamplifier 12 and provided at the output terminal 158 is directly coupled by the switch 213 through the coupling circuit to the sound effects generator output terminal. In an alternative embodiment switches 204 and 213 are combined in a single switching device.

6 Although the present invention has been described in detail with respect to a preferred embodiment of a sound effects generator, it should be readily apparent to one of ordinary skill in the art that the disclosed apparatus may be advantageously used for any application wherein a voltage-controlled filtering effect is de sired. Moreover, the foregoing has described a sound effect generator operable on audio signals produced by musical instruments, or the like. it is readily apparent, however, that the present invention may be utilized with virtually any sound or any tone generating device wherein the variable filtering effect is desired. Furthermore, the control voltage described herein to vary the peak response frequency of the voltage-controlled filter need not be limited solely to the described embodiment wherein an audio signal envelope is extracted to derive such control voltage. Alternatively. any suitable source of control voltage such as a time varying voltage wave form generator or the like, may be used. Additionally, the specific circuit components described hereinabove may be replaced by other fully equivalent devices. For example, a conventional highly precise rectifier may be substituted for the disclosed operational amplifier-rectifier, Also, various alternative amplifying devices adapted to perform substantially the same function as the operational amplifiers used in the present invention may be substituted therefor. Likewise, the low-pass filter connected to the operational amplifier-rectifier may be replaced by other equivalent filtering means capable of functioning as the described envelope follower circuit, and other conventional voltage-to-current converters or drive circuits may be used. Furthermore, various resistance configurations may be employed where desired to control the various gains of the exemplary circuits.

Therefore,'while the invention has been particularly shown and described with reference to a specific preferred embodiment thereof, it will be obvious to those skilled in the art that the foregoing and various other changes and modifications in form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims be interpreted as including all such changes and modifications.

What is claimed is:

1. Apparatus for varying a sound generating signal comprising:

voltage controlled filter means having a variable peak response frequency for transmitting at a maximum amplitude those frequencies included in said sound generating signal that correspond to said filter peak response frequency, said peak response frequency being determinable by a control voltage applied to said filter means;

envelope follower means comprised of rectifying means for transmitting sound generating signals of a single predetermined polarity and low pass filter means coupled to said rectifying means for generating a control voltage proportional to the amplitude of the envelope of said sound generating signal and for supplying said control voltage to said voltage controlled filter means;

and means for supplying said sound generating signal to said voltage controlled filter means and to said envelope follower means.

2. Apparatus in accordance with claim 1 wherein the 5 peak response frequency of said voltage controlled fil- 17 v 3. Apparatus in accordance with claim 1 wherein the peak response frequency ofsaid voltage controlled filter means varies inversely with changes in the magnitude of the control voltage generated by said control 1 I pass, band-pass and high-pass filtering characteristics and includes switch means for selecting one of said filtering characteristics.

8. Apparatus in accordance with claim 1 wherein said voltage controlled filter means comprises a low sensitivity active filter including variable resistance means having a resistance value that is a function of said control voltage, said resistance value being determinative of said peak response frequency. I

9. Apparatus in accordance with claim 8 wherein said variable resistance means comprises radiant energy sensitive means having a resistance value proportional to the intensity of radiant energy incident thereon, and further including a source of radiant energy supplied with said control voltage for emitting radiant energy having an intensity determined by said control voltage.

10. Apparatus in accordance with claim 9 wherein said low sensitivity active filter includes integrator circuit means having a charging time constant determined by the resistance value of said radiant energy sensitive means.

11. Apparatus in accordance with claim 10 wherein said integrator circuit means comprises first and second series connected integrator circuits, and wherein said radiant energy sensitive means comprises first and second photoresistors, said first and second photoresistors being included as input resistors of said first and second integrator circuits, respectively.

12. Apparatus in accordance with claim 11 wherein each of said integrator circuits includes switchable means operative to vary the range of frequencies over which the peak response frequency of said low sensitivity active filter is variable.

13. Apparatus in accordance with claim 12 wherein said switchable means comprises a capacitor and a switch for selectively switching said capacitor into the feedback circuit of said integrator circuit to thereby increase the value of the feedback capacitance of said integrator circuit.

14. Apparatus in accordance with claim 11 wherein said source of radiant energy comprises a light-emitting diode for transmitting radiant energy to said first and second phototesistors.

15. Apparatus in accordance with claim 1 wherein said rectifying means comprises an operational amplifier rectifying circuit.

16. Apparatus in accordance with claim 15 wherein portionalsto the envelope of the signals transmitted by said operational amplifier rectifying circuit.

17. Apparatus in accordance with claim 16 wherein said selecting means comprises a difference cir'cuit coupled to said low-pass filter means; a source of reference .voltage; and a switch forselectively coupling said said envelope follower means further includes selecting means coupled to said low-pass filter means for selecoperational amplifier rectifying circuit or inversely prosource of reference voltage to said difference circuit whereby said signal proportional to the envelope of the signals transmitted by said operational-amplifier rectifying circuit is subtracted from said reference voltage when said switch couples said source of reference voltage to said difference circuit to thereby produce said control voltage.

18. A sound effects generator for modifying a sound generating signal produced by a musical sound generating instrument, comprising: I

a preamplifier having variable gain for amplifying said produced sound generating signal to predetermined levels;

a low sensitivity-active filter coupled to the output of said preamplifier and exhibiting a peak response frequency that is variable over a given range of frequencies, said low sensitivity active filter including integrator circuit means comprising photoresistance means having a resistance value proportional to the intensity of radiant energy incident thereon, said resistance value determining the charging time constant of said integrator circuit means to thus be determinative of said peak response frequency;

means for selecting a predetermined band of frequencies in said sound generating signal to be modified;

a source of radiant energy for transmitting radiant energy to said photoresistance means, said source being responsive to a control signal applied thereto for emitting radiant energy having an intensity proportional to the magnitude of said control signal;

control signal generating means for supplying said control signal to said source of radiant energy;

and output means for receiving said modified sound generating signal.

19. A sound effects generator in accordance with claim 18 wherein said photoresistance means comprises first and second photoresistors and wherein said integrator circuit means comprises a first integrator circuit including said first photoresistor and a second integrator circuit including said second photoresistor, said first and second integrator circuits being connected in series relationship and each of said integrator circuits having a charging time constant determined by the resistance value of its associated photoresistor.

20. A sound effects generator in accordance with claim 19 wherein said means for selecting a predetermined band of frequencies to be modified comprises a switch having a movable contact adapted to selectively engage each of a plurality of stationary contacts including a first stationary contact coupled to the input of said first integrator circuit, a second stationary contact coupled to the output of said first integrator circuit and a third stationary contact coupled to the output of said second integrator circuit, whereby the high frequency components of said sound generating signal are modified and supplied to said output means when said movable contact engages said first stationary contact, the low frequency components of said sound generating signal are modified and supplied to said output means when said movable contact engages said third stationary contact and the intermediate frequency components of said generating signal are modified and supplied to said output means when said movable'contact engages said second stationary contact.

21. A sound effects generator is accordance with claim 20 wherein said source of radiant energy comprises a light-emitting diode.

22. A sound effects generator in accordance with claim 21 wherein said control signal generating means comprises envelope follower means coupled to the output of said preamplifier for extracting the envelope of said sound generating signal and for supplying a signal proportional to said extracted envelope to said lightemitting diode.

23. A sound effects generator in accordance with claim 22 further comprising a difference circuit coupled to said envelope follower means; a source of reference voltage; and a switch for selectively coupling said creases when said source of reference voltage is coupled to said difference circuit.

24. A sound effects generator in accordance with claim 23 wherein said low sensitivity active filter further includes means for selectively varying the range of frequencies over which said peak response frequency is variable.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 911, 776

DATED October 14, 1975 |NVENTOR(S) Michael L. Beigel It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column 17, line 7, before "filter means" insert voltage controlled Signed and Scaled this sixteenth Day Of March 1976 [SEAL] A ttes t:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner uj'Palents and Trademarks UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1,776

DATED October 14, 1975 |NVENTOR(S) 3 Michael L. Beigel It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 17, line 7, before "filter means" insert voltage controlled Signed and Scaled this sixteenth Day of March 1976 [SEAL] AUG-8'1.

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner oj'larents and Trademarks

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Classifications
U.S. Classification84/700, 330/59, 84/DIG.900, 330/107, 330/149, 984/328, 330/144, 84/DIG.190, 327/553, 84/702, 333/174, 327/47
International ClassificationG10H1/14
Cooperative ClassificationY10S84/19, Y10S84/09, G10H1/14
European ClassificationG10H1/14