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Description  |
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FIELD OF THE INVENTION
This invention relates to musical instruments, and, more particularly,
relates to an electronic sensing system for a stringed and fretted musical
instrument.
BACKGROUND OF THE INVENTION
Stringed and fretted musical instruments, such as guitars, have long been
well known, and, more recently, electronic stringed and fretted musical
instruments have also been suggested and are widely utilized. Still more
recently, microprocessor-based electronic musical instruments also have
been suggested and/or utilized and have, heretofore, been utilized in
conjunction with keyboards to provide a variety of pitch information (see,
for example, U.S. Pat. Nos. 4,409,877, 4,301,704 and 4,437,378).
Electronic guitar sensing tone reproduction systems have also heretofore
been suggested and/or utilized, and have included, for example, a system
employing fret-linked switch detectors, tone reproduction oscillators and
touch sensitive string activation circuitry (see U.S. Pat. No. 4,372,187).
SUMMARY OF THE INVENTION
This invention provides an electronic sensing system for use in conjunction
with a stringed and fretted musical instrument. Sensing of the musical
pitch of each string played is accomplished by a resistance-based
fret-string detecting circuit operating under control of a microprocessor
with the vibration state and bending of the strings to vary pitch being
separately detected under the control of the microprocessor so that the
resulting information conveyed to the microprocessor is utilized to
produce an output suitable for coupling to a synthesizer to produce the
music created by the performer utilizing the stringed and fretted musical
instrument.
It is therefore an object of this invention to provide an improved sensing
system for a stringed and fretted musical instrument.
It is another object of this invention to provide an improved sensing
system for a stringed and fretted musical instrument suitable for use with
a synthesizer.
It is still another object of this invention to provide an improved sensing
system for a stringed and fretted musical instrument capable of creating
electronic signals truly representive of the music created by the musical
instrument.
It is still another object of this invention to provide an improved sensing
system for a guitar.
It is still another object of this invention to provide an improved sensing
system for a stringed and fretted musical instrument that is
resistance-based with respect to detection of musical pitch.
It is yet another object of this invention to provide an improved sensing
system that is microprocessor controlled having accurate pitch selection
circuitry and synthesizer compatibility for greater musical versatility
while preserving the musical instrument's inherent tactile and expressive
qualities to the performer.
With these and other objects in view, which will become apparent to one
skilled in the art as the description proceeds, this invention resides in
the novel construction, combination, and arrangement of parts
substantially as hereinafter described, and more particularly defined by
the appended claims, it being understood that changes in the precise
embodiment of the herein disclosed invention are meant to be included as
come within the scope of the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings illustrate a complete embodiment of the invention
according to the best mode so far devised for the practical application of
the principles thereof, and in which:
FIG. 1 is a perspective view of a stringed instrument (illustrated as a
guitar) having the sensing system of this invention incorporated therein;
FIG. 2 is a side-sectional view of the guitar shown in FIG. 1;
FIG. 3 is a block diagram illustrating the invention;
FIG. 4 is a block diagram of the fret-string detection circuitry shown in
FIG. 3;
FIG. 5 is a simplified schematic circuit illustrating the fret-string
circuitry at the neck of the guitar;
FIG. 6 is a simplified equivalent circuit of the fret-string circuitry;
FIG. 7 is a schematic diagram illustrating one string selective unit for
string selection;
FIG. 8 is a simplified schematic diagram illustrating the fret comparison
circuitry;
FIG. 9 is a schematic diagram of the string vibration state detector as
shown in the block diagram of FIG. 3;
FIG. 10 is a partial end view illustrating string location relative to an
optical sensor;
FIG. 11 is a schematic diagram of the bending detector, shown in block
diagram of FIG. 3, using a strain gauge;
FIG. 12 is a partial perspective view illustrating location and mechanical
operation of the strain gauge; and
FIG. 13 is a block diagram illustrating in more detail the components of
the microcomputer shown in the block diagram of FIG. 3.
DESCRIPTION OF THE INVENTION
Referring now to the drawings, electronic sensor system 22 of this
invention is shown in FIG. 1 incorporated into a stringed and fretted
musical instrument (illustrated as a guitar) 23. Sensor system 22 detects
the fingering of strings 24 (more specifically referred to as strings
24(1) through 24(6) for the six strings of a guitar) on a neck portion 25
having thereon frets 26 (more specifically referred to as frets 26(1)
through 26(20) for the twenty frets commonly found on a guitar). The
specific objective is to sense, for each string, the position of each
selected fret contacting a string which determines the musical pitch of
that string which may then be processed by the processing circuitry 27
positioned in the body portion 28 of the guitar (as shown in FIG. 1) and
then coupled to a synthesizer 29.
In the ordinary fingering of a guitar neck by a performer, the performer's
finger (indicated for illustrative purposes in FIG. 2) presses one or more
strings downwardly toward the frets so that the string (or strings)
contact one or more frets (which could, for example, include pressing a
string down between two adjacent frets on the neck as indicated in FIG.
2). As also indicated in FIG. 2, the string, as is conventional, extends
between a tail piece 30 over bridge 31 and nut 32 to tuning peg 33. The
string is plucked or struck, commonly by a pick 34, between the frets 26
and bridge 31 so that string 24 vibrates and produces a musical note. The
pitch of this thus produced musical note is determined primarily by the
length of the free section of the string, between bridge 31 and the
fingered fret contacting the string that is closer to bridge 31 (fret
26(3) in the example illustrated in FIG. 2). It is therefore important
that the fret-sensing circuit determines which fret contacting the picked
string is closest to bridge 31 (the closest fret is referred to herein as
the "critical fret").
The sensory system of this invention includes three detecting sections, as
shown in FIG. 3, including a fret-string detecting section 35, a string
vibration detecting section 36 and a string bend detecting section 37. All
three of these detecting sections provide interfaces with TTL-standard
signals to a digital control system, which is specifically shown in FIG. 3
as a microcomputer 38.
As indicated in FIG. 3, microcomputer 38 includes a microprocessor 39, a
program ROM 40 connected with microprocessor 39, control switch 41
connected with microprocessor 39, and MIDI interface unit 42 connecting
microprocessor 39 with the synthesizer. As shown, microprocessor 39
provides an output to fret-string detecting section 35 to sequentially and
repeatedly energize each string, as is necessary to determine the
fret-string combination that is then being played (strings 24 and frets 26
are metallic and therefore electrically conductive). For a guitar having
six strings and twenty frets, fret-string detecting section 35 receives
six digital inputs from the microprocessor for string selection to
successively energize each string for sensing purposes, and the
microprocessor receives an output for each of the twenty frets on the
guitar to indicate when that fret is the critical fret for the then
selected (i.e., energized) string.
Fret-string detecting section 35 utilizes a resistance-based technique for
determining the fret-string combination then selected with a resistance
bridge network being established by the frets, strings and associated
resistors in fret-string detecting section 35. Since metallic guitar
strings have measureable electrical resistance, the string resistances
between the frets and main bridge are used as elements of the resistance
bridge network to discriminate the critical fret.
As indicated in FIGS. 3 and 4, strings 24 are connected with string
selection circuitry 43 which receives the digital input signals from
microcomputer 38, while frets 26 are connected with fret detector unit 44
which supplies digital output signals to microcomputer 38. Under control
of the control system (microcomputer) 38, string selector 43 successively
drives a DC current through each selected string, while leaving the other
strings inactive. This current flows through the then selected string and
fret combination of a neck network 45, causing voltage differences between
the frets (when one or more frets are in contact with the then selected
string). The neck network is designed to produce the most extreme voltage
on the critical fret containing the then selected string. Fret detector 44
compares the voltages on all the frets, discriminates the most extreme
one, and signals control system 38 through the corresponding output
signals.
String selection circuitry 43, under the control of control system 38,
steps through each of the six strings by successive energization of the
strings, and reads the fret detector in each case (after a short settling
delay). In this way, control system 38 uses this sensor to quickly, and
repeatedly, obtain a complete reading of each guitar neck fingering.
This sensing technique works for either choice of polarity of current flow
through the strings. In a working prototype (made in accordance with the
invention as particularly described herein), the polarity has been chosen
such that conventional circuit flow is from the neck network into the
string selector circuit. Accordingly, the string selector circuits apply
negative voltage to the strings, relative to the neck reference node
(NECK-GROUND), and so the critical fret has a more negative voltage than
the other frets.
As shown, the neck network 45 includes strings 24 and frets 26 in their
conventional arrangement on a guitar. In addition, and as shown in FIGS. 5
and 6, resistors 46 (designated particularly as 46(1) through 46(20) in
the drawings) are connected with a different resistor 46 between each fret
and NECK-GROUND. Strings 24 (for simplicity only two strings are shown in
FIG. 5, but all of the strings are connected in the same manner as
illustrated for strings 1 and 2 in FIG. 5) are connected to string
selector 43 near bridge 31 for coupling input signals thereto. Frets 26
are connected to fret detector 44 (only four frets are illustrated, but a
like connection is separately made for each fret as illustrated for the
four frets as shown in FIG. 5).
The strings are electrically isolated from each other by insulated fittings
or mountings at several points on the guitar. The frets are insulated from
each other by their mounting on an ordinary non-conducting guitar neck.
When the fret board is fingered, electrical contacts are made between
strings and frets at the points of physical contact therebetween. During
such contact, electrical current flows from the string selector on the
NECK-GROUND signal, through resistor (or resistors) 46, through the frets
contacting an energized string, through the fret-string contacts, through
the selected string (i.e., the string then energized) and then back to the
string selector (which is connected with a power source).
Since each segment of string between two frets has electrical resistance
and the string-fret contact functions as a switch 47 (considered an ideal
switch herein), the neck circuit acts as a network of resistors and
switches as illustrated in the equivalent circuit shown in FIG. 6. This
network can be thought of as a multiple-node resistance bridge.
Current flowing through any fret must subsequently pass through that fret
which is closest to bridge 31, and contacting the selected string, since
the bridge 31 side of the selected string is the sole current return path
to the string selector. Since all frets are separated by string
resistance, this causes the circuit having the closest fret (i.e., the
critical fret) to have a lower voltage than all other circuitry having the
other frets therein.
A significant feature of this sensing technique is that it will properly
discriminate the critical fret contacting the selected string, even in
fingering patterns where frets even closer to the main bridge are
connected to the critical fret through fingering contacts on other
strings.
It is important to keep the contact resistance between strings and frets
substantially lower than the resistance of string segments between frets
(while conventional strings and frets made of, for example, stainless
steel and brass may be utilized, the contact resistance could be further
reduced, if needed, or desired, by gold-plating the strings and frets).
The resistance of the frets (which are massive relative to the strings and
are commonly made of brass) is very low, and therefore can be ignored.
String selector 43 includes six independently switchable constant-current
drivers 48, one for each string. It is the function of the external
control system 38 to supply logic signal inputs (STR1-STR6), to separately
select each of the six strings.
Each constant-current driver 48 provides enough current to cause a suitable
voltage difference between frets in worst-case fingering patterns, for
example, those contacts at frets 26(18) through 26(20) where several
strings 24 in that range are being fingered. The generated voltage
difference between frets must be large enough to exceed the maximum input
offset voltage error of the fret detector circuit. In an operating
prototype, it was determined that a string current of 1.0 amps produced
sufficient voltage difference between frets to enable satisfactory
detection by the fret detector.
A typical constant-current string driver circuit 48 is shown in FIG. 7.
This circuit switches the output current on when the TTL input is low (at
appoximately 0.2 volts). The output current is off when the input is high
(at approximately 2.0 to 5.0 volts). When the output is on (i.e.,
producing an output), a constant current of about 1.0 amps is produced.
This current is approximately constant across the operating range of the
STRING1-STRING6 output signals (from about -1.5 to 0.0 volts). When a
string is not contacting any fret, the output current must drop to zero.
In this case, transistor 50 saturates, and the STRING1-STRING6 output
signal drops to about -4 volts.
Transistors 50 and 51 operate as inverting amplifying switches. They are
both off when the circuit is "off", and both are in linear mode when the
circuit is "on". Transistor 51 provides sufficient gain so that the input
may be driven by an LS (Low-power Schottky) TTL device. Transistor 50 is a
Darlington high-gain power transistor with an operating base-emitter
voltage of about 1.4 volts.
Emitter resistor 53 is connected between the +5 volt power supply and the
emitter of transistor 51, emitter resistor 54 (connected between the
emitter of transmitter 50 and the -5 volt power supply) stabilizes the
output current at approximately 1 amp, and the collector of transistor 51
is connected with the -5 volt power supply through resistor 55. Since only
one output is normally on at a time, resistor 54 may be shared between all
six string drivers.
Fret detector 44 compares the voltages appearing on all of the frets 26
through input signals (FRETl, FRET2, etc.), and discriminates, or detects,
the lowest one. A signal is then supplied to the external control system
through the corresponding logic signal output (F1, F2, etc.) to indicate
the fret sensed to have the lowest voltage.
The voltage difference between the critical fret and adjacent frets can be
very small, and can be, for example, as low as 5 millivolts with a string
selector current at 1 amp. Therefore, it is important that the fret
detector have a low input offset voltage error that is lower than the
minimum expected inter-fret discrimination voltage.
A typical fret detector circuit for fret detector 44 is shown in FIG. 8
(while only three detectors are shown in FIG. 8, it is to be realized that
a like detector is provided for each fret of the stringed and fretted
instrument). The overall unit is a multiple-input minimum-seeking
comparator consisting of one comparator section 57 for each fret input,
and a threshold circuit 58 to handle the special condition when no frets
are contacting the selected string.
The comparator sections communicate with each other by a V-COMPARE signal
to seek the minimum voltage. The circuit having the lowest fret input
voltage drives V-COMPARE down to approximately the same voltage as the
input. All other circuits exert no influence on V-COMPARE as long as their
inputs are higher than V-COMPARE. Resistor 60 tends to pull V-COMPARE
high, up to the point at which one of the comparator sections responds and
holds V-COMPARE at a fixed level. In this manner, the circuits
successfully discriminate the fret input with the lowest voltage.
Each comparator section consists primarily of an operational amplifier 61
(shown as 61(1) through 61(20) in FIG. 8), and a feedback signal diode 62
(shown as 62(1) through 62(20) in FIG. 8). When the fret input is lower
than the inputs to all other sections, the operational amplifier drives
its output low, typically to about -0.7 to -0.8 volts. At this point,
feedback diode 62 turns on and brings V-COMPARE low, to about -20 to -100
millivolts. The operational amplifier feedback loop thus stabilizes with
V-COMPARE approximately matching the fret input.
When the fret input is higher than one or more inputs to other sections,
V-COMPARE is forced by another section to a lower voltage than this input.
The operational amplifier output is then driven to positive saturation, at
about 4 volts. The feedback diode stays off, and this section has no
influence on V-COMPARE.
The threshold circuit 58, consisting of amplifier 63 and diode 64, operates
in the same manner as the rest of the comparator sections. Its input
voltage, however, is set by the threshold adjustment circuit consisting of
potentiometer 65 and resistor 66, rather than being connected to any fret,
with the input voltage being typically set at about -10 millivolts.
The function of the threshold circuit is to force all other comparator
sections off, i.e., inactive, whenever the selected string is not being
fingered, and thus is not contacting any of the frets. In this condition,
the fret inputs are all at 0 volts (the level of NECK-GROUND). Without the
threshold circuit, the operational amplifiers would select a random lowest
fret, based on the random variations of the input offset voltage errors of
the operational amplifiers. However, the threshold circuit acts as a
"lowest fret" in this situation, and thereby causes all frets to signal as
off. The ground reference in the threshold adjustment circuit is taken
directly from the NECK-GROUND signal, to minimize common ground-path
interference from other circuits in the system.
Diode 68 (shown as 68(1) through 68(20) in FIG. 8) and resistor 69 (shown
as 69(1) through 69(20) in FIG. 8) comprise an analog-to-digital level
converter for each comparator section. When the fret input is lowest, the
operational amplifier output is about -0.7 to -0.8 volts (see above),
diode 68 is off, and resistor 69 pulls the digital TTL output signals (F1,
F2, etc.) down to a logic low level of about 0.2 volts. When the fret
input is not lowest, the operational amplifier output is about 4 volts,
diode 68 is on, and the output signal is pulled up to a logic high level
of about 3.3 volts.
During the fingering and releasing of the strings, variations in string
pressure can cause momentary variations in string-fret contact resistance,
and thereby variations in the fret voltages. Because of this, two adjacent
frets can momentarily have similar voltages, and the comparator will then
momentarily indicate both frets on. It is seen from the technique of
fingering that the fret closest to bridge 31 is the critical fret in this
condition. Therefore, in order to minimize false interpretations from this
effect, the control system must give priority to frets closest to bridge
31.
As indicated in FIG. 3, string vibration detecting section 36 includes
level detectors 70 connected to receive the outputs of level optical
pickups 71 and provide outputs to microprocessor 39 of microcomputer 38.
String vibration detecting section 36 is shown in the schematic diagram of
FIG. 9. Only one portion is shown, but it is to be realized that identical
portions would be provided for each string so that six such portions would
be provided for a guitar with six strings, for example.
As shown in FIG. 9, a photo-optical pickup 71 is utilized to sense string
movement. This pickup may be conventional and may be, for example, a
slotted optical switch, type OPB 813, manufactured by Optron, Inc.,
Carlton, Tex.
As shown in FIG. 10, photo-optical pickup 71 is positioned adjacent to slot
72 on board 73 so that string 24 intercepts the light emitted by LED 74
(of pickup 71) that is directed toward phototransistor 75 (of pickup 71).
Phototransistor 75 is connected through capacitor 76 (having a resistor 77
at one side to the +5 volt power supply and a resistor 78 at the other
side to ground) to the positive input of amplifier 79 of level detection
circuitry 70. The negative input of amplifier 79 is connected through
series connected resistor 80 and capacitor 81 to ground, with the negative
input also having a feedback resistor 82 to the output of the amplifier.
The output from amplifier 79 provides an audio output signal and the output
is also coupled to detector and analog-to-digital converter section 83.
This section includes amplifiers 85 and 86 having resistors 87 and 88
connected to the negative inputs of the amplifiers, with the output of
amplifier 85 being connected with the negative input of amplifier 86
through diode 89 and resistor 90, with amplifier 85 having a feedback
diode 91 to the negative input, with resistor 92 being connected with the
junction of diode 89 and resistor 90 and to the negative input of
amplifier 85, and with amplifier 86 having parallel connected feedback
resistor 93 and capacitor 94 connected from the output to the negative
input.
With a string vibrating, an AC output is provided by the pickup reflecting
the frequency of vibrating string with the AC output signal having an
amplitude as well as a frequency suitable for use as a "traditional"
peripheral guitar signal. A DC signal reflecting amplitude over time is
converted by the A/D circuitry 83 and coupled to the microprocessor for
interpretation according to the following program: A (as illustrated in
the representative waveform in FIG. 9) equals an arbitrary (preprogrammed)
"note on" command threshold; C equals an arbitrary (preprogrammed) "note
off" command threshold; and B equals peak amplitude.
It is to be understood that interconnection of the pickup 71 to the
microprocessor is not necessary to the functioning of fret-string
detection section 36 and tone signal production thereof, but is, instead,
one useful program mode which preserves to the performer some of the
instrument's inherent tactile and expressive qualities.
Another optional circuit is illustrated by bend detecting section 37, shown
in FIG. 3 to include bend detectors 95 connected to receive the outputs
from the strain gauges 96 and to provide outputs to microprocessor 39 of
microcomputer 38. Bend detecting section 37 is shown in schematic form in
FIG. 11. Here again, only one section is shown, but it is to be realized
that separate like sections are provided for each string. As shown in FIG.
11, and more particularly by the perspective view of FIG. 12 showing the
mechanical arrangement, strain gauges 96 are utilized to sense bend for
each string. Strain gauges 96 may be conventional and may be, for example,
EASS 125 AC 350 strain gauges manufactured by Measurements Group, Inc.
As shown in FIG. 12, each strain gauge 96 is preferably mounted on the
underside of tail piece 97. L shaped levers 98 (one for each string) are
mounted for pivotal movement about pivot rods 99 at the end of the tail
piece with one end of each lever being attached to a different string 24
and the other end of each lever being attached to one end of a flat strip
of spring steel (separate strips are provided for each string), with the
other end of each strip 100 being attached to stop plate 101. Each strain
gauge 96 is mounted (as by gluing, for example) on spring steel strip 100
to sense the motion thereof.
As shown in FIG. 11, each strain gauge 96 is connected as one leg of a
Wheatstone bridge 103. The other legs of the Wheatstone bridge are made up
of resistors 104 and 105 in opposite legs and series connector resistor
106 and variable resistor 107 in the leg opposite the strain gauge.
Amplifier 109 is connected to bridge 103 (to detect bridge imba | | |
