U.S. patent number 3,911,776 [Application Number 05/411,764] was granted by the patent office on 1975-10-14 for sound effects generator.
This patent grant is currently assigned to Musitronics Corporation. Invention is credited to Michael L. Beigel.
United States Patent |
3,911,776 |
Beigel |
October 14, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Sound effects generator
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.
Inventors: |
Beigel; Michael L. (Warwick,
NY) |
Assignee: |
Musitronics Corporation
(Rosemont, NJ)
|
Family
ID: |
23630219 |
Appl.
No.: |
05/411,764 |
Filed: |
November 1, 1973 |
Current U.S.
Class: |
84/700; 84/DIG.9;
84/DIG.19; 84/702; 333/174; 984/328; 327/553; 327/47; 330/59;
330/144; 330/107; 330/149 |
Current CPC
Class: |
G10H
1/14 (20130101); Y10S 84/09 (20130101); Y10S
84/19 (20130101) |
Current International
Class: |
G10H
1/06 (20060101); G10H 1/14 (20060101); G10H
001/02 (); H03H 007/10 () |
Field of
Search: |
;84/1.01,1.09-1.12,1.19,1.21,1.22,1.24,1.27,DIG.9,DIG.19,1.25
;179/1D ;307/271 ;328/127,149,167,175,209 ;330/52,75,98-100,103,109
;333/7R,7CR |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
GE. Transistor Manual, General Electric Company, copyright 1964, p.
371..
|
Primary Examiner: Hix; L. T.
Assistant Examiner: Witkowski; Stanley J.
Attorney, Agent or Firm: Curtis, Morris & Safford
Claims
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 peak response
frequency of said voltage controlled filter means varies directly
with changes in the magnitude of the control voltage generated by
said voltage generating means.
3. Apparatus in accordance with claim 1 wherein the peak response
frequency of said voltage controlled filter means varies inversely
with changes in the magnitude of the control voltage generated by
said control voltage generating means.
4. Apparatus in accordance with claim 1 wherein said filter means
comprises a low-pass filter.
5. Apparatus in accordance with claim 1 wherein said voltage
controlled filter means comprises a band-pass filter.
6. Apparatus in accordance with claim 1 wherein said voltage
controlled filter means comprises a high-pass filter.
7. Apparatus in accordance with claim 1 wherein said voltage
controlled filter means selectively exhibits low-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.
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 said envelope
follower means further includes selecting means coupled to said
low-pass filter means for selectively producing said control signal
directly proportional to the envelope of the signals transmitted by
said operational amplifier rectifying circuit or inversely
proportional to 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 circuit coupled to said low-pass
filter means; a source of reference voltage; and a switch for
selectively coupling said source 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:
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 light-emitting
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 source of reference voltage to said difference
circuit; whereby the intensity of the radiant energy emitted by
said light-emitting diode increases as said sound generating signal
envelope increases and decreases as said sound generating signal
envelope decreases when said source of reference voltage is not
coupled to said difference circuit, and the intensity of the
radiant energy emitted by said light-emitting diode decreases as
said sound generating signal envelope increases and increases as
said sound generating signal envelope decreases 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.
Description
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 large-scale
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 selectively obtain the sound effects he particularly
desires, such operation requires significant concentration which
necessarily interferes with his musical performance.
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.
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 forthcoming detailed description of the present invention will
be readily understood in conjunction with the accompanying drawings
in which:
FIG. 1 is a block diagram, with typical wave forms, of a preferred
embodiment of the present invention;
FIGS. 2A-2C are graphical representations of the filter
characteristics of the voltage-controlled filter of the present
invention;
FIG. 3 is a graphical representation of one relationship between
the peak response frequency and the applied control voltage of the
voltage-controlled filter of 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 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.sub.C 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.sub.C 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.sub.C 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 of extracting 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 voltage-controlled 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.sub.o 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
frequency f.sub.o, 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.sub.o. 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.sub.audio, having the typical wave form 13, is applied to
the filter 14 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.sub.C
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 voltage-controlled
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.sub.o may vary throughout a given range of frequencies
in accordance with the amplitude of the control voltage V.sub.C.
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.sub.o to a higher
frequency f'.sub.o. Hence, as the control voltage V.sub.C increases
in amplitude, the peak response frequency of the filter 14
increases from its lower value f.sub.o to its higher value
f'.sub.o. 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.sub.C decreases, the peak response frequency of the
filter 14 correspondingly decreases from a maximum frequency value
f'.sub.o to its lower frequency value f.sub.o. It is appreciated
that, since the envelope follower 16 preferably extracts only the
positively poled envelope of the audio signal, the control voltage
V.sub.C 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.sub.C. 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.sub.o
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.sub.C 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.sub.C extracted by the
envelope follower 16 to thus vary the peak response frequency
f.sub.o in a similar manner. In a still further alternative
embodiment, the control voltage V.sub.C 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. 2B. 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 f.sub.o 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 frequency
f.sub.o is determined by the instantaneous amplitude of the control
voltage V.sub.C 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.sub.o to a higher frequency value
f'.sub.o . Conversely, as the control voltage amplitude decreases,
the peak response frequency likewise decreases from the higher
frequency value f'.sub.o to the lower frequency value of f.sub.o.
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.sub.C 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.sub.o and, additionally, a limited bandwidth or Q about
the peak response frequency. As in the previously described
embodiment, the peak response frequency may be varied between a
lower frequency value f.sub.o and a higher frequency f.sub.o ' in
accordance with variations in the amplitude of the control voltage
V.sub.C applied to the filter 14.
It should be appreciated that the selection of low-pass, band-pass
or high-pass characteristics of the filter 14 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.sub.o may be varied can be altered as desired. That is,
the absolute frequency range f'.sub.o -f.sub.o 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'.sub.o / f.sub.o 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
non-inverting 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 by-pass 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 non-inverting 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 non-inverting
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 of the 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
non-inverting 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 110 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
of change 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 vary. 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 (FET's) wherein the resistance value is dependent upon
the control voltage applied thereto might be employed. However such
FET's 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 FET's is significant.
Alternatively the resistors 116 and 132 may comprise conventional
resistance elements disposed for high frequency FET-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 photo-resistors 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 envelope 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 170 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 162
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
190 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.sub.C.
However, as is well known, conventional LED's 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 high-pass, band-pass 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 10 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 of each 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 170 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 a.c. 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 12 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 the relatively 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 amplifier-rectifier 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.sub.o to the right and a decrease
in the audio signal envelope correspondingly results in a sweep of
the peak response frequency f.sub.o 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 voltage-to-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 photo-resistors
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.sub.o 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.sub.o 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 170 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 187 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.sub.o 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 operator's 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, respectively,
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.sub.o 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.
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
desired. 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.
* * * * *