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The present invention pertains to tuning aids. More particularly, it
relates to digitally-operated devices which enable the user readily to
bring a musical instrument into proper tune.
Using but a single tone from a reference source, such as a pitchpipe, a
person with sufficient talent and skill can achieve reasonably accurate
tuning of a musical instrument throughout its range of different possible
tones. In some cases, greater precision suggests the use of more accurate
reference sources such as tuning forks. With complex instruments, such as
a piano which has many strings, the task may be made simpler by
utilization of at least a small plurality of different tuning forks or the
like.
In a multiply-stringed instrument or the like, the person seeking to tune
the instrument in the past has made use of one or more such reference
sources and has relied upon a highly-developed sense of hearing and
appreciation of what is happening to detect the presence of harmonics,
subtones and overtones, as well as beats between different ones of such
tones, in order to be able to achieve tuning as to many different notes
distributed throughout a wide range of musical scale. While many
practitioners have developed an ability for achieving tuning of even the
most complex musical instrument by means of such an approach, they have
had to rely on substantial talent developed through careful training and
experience and have depended upon expert usage, and even the existence of,
their auditory human facility.
Recognizing the limitations inherent when reliance is placed upon the human
auditory response, and upon the necessity for the development of a talent
in the use of such response, the prior art has come forth with attempts to
enable tuning of a musical instrument by the use of electronic apparatus.
The general approach has been that of detecting and comparing a signal
produced by the instrument with a calibrated reference signal selected to
have a frequency corresponding with the ultimately desired tone. The
instrument is then adjusted until the signal it produces has a frequency
which is equal to the frequency of the selected reference signal.
Typically, an analog-type meter is employed for the purpose of indicating
the degree of correspondence, and the direction of any non-correspondence,
between the sensed and the reference signals. That is, the user reads a
meter which indicates degree of frequency deviation as between the two
signals.
With the development of logic systems in the field of electronics, it was
recognized that this form of approach could be beneficially used in
connection with the tuning of musical instruments. Thus, it has become
known to employ a crystal-controlled oscillator in order to provide an
accurate reference signal and to process that reference signal in a
digital frequency divider chain in order to achieve the production of a
selected reference tone. A signal received from the musical instrument is
processed, as by squaring and filtering, and compared with the reference
tone for permitting the detection of any difference between the signal
under measurement and the reference tone. Lack of coincidence is displayed
on an analog type meter. For obtaining the desired readout, it also has
been known to employ a bi-directional counter for determining which side
of an intermediate value is to be indicated.
Certainly, the implementation of digital electronic systems in an effort to
provide means for tuning a musical instrument has been commendable.
Reliance upon the acquisition of substantial talent or skill in detecting
the relationship between different notes has been minimized. Moreover, it
appears that at least theoretically more-accurate tuning can be achieved
through the use of such systems (provided, of course, that the selection
of the reference tone, to which the instrument is to be compared, is
faithfully made in correspondence with proper musical requirements and
knowledge).
While the above-described developments in the art have, at least seemingly,
enabled the production of apparatus capable of performing the assigned
tasks, such apparatus has tended to be rather costly. At least much of it
has been both cumberson and fragile. At the same time, the implementation
of digital logic circuitry has resulted in a degree of sophistry that may
result in improper measurement. That is, minute variations in phase as
between successive time-increments of the signal under measurement may be
detected by the prior apparatus as actual musical pitch variations. When
that happens, of course, an erratic output indication is given. Apart from
such details, it has also been noted that tuning aids of the type in
question have tended to be available only at rather substantial cost.
While such cost might easily be absorbed by a professional engaged in the
tuning field, it may become rather exorbitant if viewed by the ultimate
user who wishes to tune his own instrument.
At least usually, prior such apparatus includes a read-out indicator that
is always energized whenever the tuning aid is turned on. That wastes
power when no signal to be tested is present. It also may provide a false
indication when a signal under test is too weak to be properly processed.
In addition, typical prior apparatus has required use of often costly and
fragile analog type indicators in order to provide a representation of
degree of nearness to an exact "on-tune" condition.
It is, therefore, a general object of the present invention to provide a
new and improved tuning aid which serves to overcome deficiencies and
objections adverted to hereinabove.
Another object of the present invention is to provide a new and improved
tuning aid which is capable of being packaged in compact form, utilizing
off-the-shelf electronic components, and resulting in a cost at least
reasonably attractive to the potential user.
A further object of the present invention is to provide a new and improved
tuning aid which simplifies its utilization for its intended purpose.
Still another object of the present invention is to provide a new and
improved tuning aid which avoids erroneous response to mere phase
variations in a signal being tested.
A still further object of the present invention is to provide a new and
improved tuning aid in which a read-out display is disabled whenever a
signal to be measured is either absent or too weak for proper response.
Yet another object of the present invention is to provide a new and
improved tuning aid in which a digital read-out display is enabled to
provide a representation of degree of nearness to an exact "on-tune"
condition.
A tuning aid constructed in accordance with the present invention includes
a stable source of reference signal having a predetermined frequency. A
divider coupled to that stable source divides the frequency of the
reference signal by a selected amount so as to produce a given tone
signal. Of course, the aid includes means for sensing a note developed by
an instrument and developing a corresponding sensed signal. In accordance
with one feature, variations in phase between successive time increments
of the sensed signal are averaged for the purpose of providing a
measurement signal. The aforesaid tone and measurement signals are
compared in order to produce an indication signal. That indication signal
is responded to for depicting the relationship between the tone and
measurement signals. Preferably, the indication signal is employed to
digitally display one of the conditions of a sharp, flat or on-tune of the
signal under measurement. As another principal feature, a signal presence
detector is included to disable the display in the absence of a meaningful
sensed signal. A further desirable inclusion is means for effecting
flicker of the display at a rate representative of the degree of nearness
to an exact match between the tone signal and the sensed or measurement
signal.
The features of the present invention which are believed to be novel are
set forth with particularity in the appended claims. The organization and
manner of operation of the invention, together with further objects and
advantages thereof, may best be understood by reference to the following
description taken in connection with the accompanying drawings, in the
several figures of which like reference numerals identify like elements,
and in which:
FIG. 1 is a block diagram of one embodiment of a musical instrument tuning
aid;
FIG. 2 is a schematic diagram of an implementation of a portion of the
system of FIG. 1;
FIG. 3a is a general state transistion diagram employed in the logic system
of a portion of the system of FIG. 1;
FIG. 3b is a simplified version of the state transistion diagram of FIG. 3a
as actually implemented;
FIG. 4 is a timing diagram of the operation of different ones of the
components of the system depicted in FIG. 1;
FIG. 5 is a block diagram of a second embodiment of a musical instrument
tuning aid;
FIG. 6 is a timing diagram illustrating the basic operational relationships
present in the system of FIG. 5;
FIG. 7 is a schematic diagram of one component shown in FIG. 5; and
FIG. 8 is a schematic diagram of another component shown in FIG. 5.
At least most always when one deals with electronic logic circuits, it is
rather readily recognized by the person skilled in the art that a variety
of possible logic-circuit approaches present themselves when attempting to
decide on how to proceed from a given set of input information so as to
achieve a desired set of output information. As a fundamental example, it
is well recognized that at least most circuits which might originally be
implemented with the use of AND-gate circuitry could as well have been
implemented by the use of NAND-gate circuitry. Thus the following detailed
explanation represents only those which are presently preferred
embodiments, without limitation upon the numerous obvious variations of
digital-logic approach.
In FIG. 1, a crystal-controlled oscillator 10 constitutes a source of
reference signal exhibiting substantial stability in frequency and having
a predetermined value of that frequency; as herein illustrated, that
reference signal frequency is 841,280 hertz. This particular value of the
reference frequency is selected for its capability of being divided down
in a manner to produce each of the different tones desired for the purpose
of tuning a wide-range instrument such as a piano.
Responsive to the reference signal from oscillator 10 is a divider circuit
12 that divides the frequency of the reference signal by a selected amount
so as to produce a given tone signal. The amount of division selected at
any time is under the control of a note-select switch 14 that governs the
operation of divider circuit 12. Of course, the use of a
crystal-controlled oscillator in conjunction with an adjustably-divisable
divider circuit constitutes that which is well known to the person skilled
in the art. The sole purpose of such an arrangement is to produce an
output tone signal of the given frequency desired. This scheme, in itself,
has often been used in the communication arts for the purpose of selecting
a desired local-oscillator frequency in a communications receiver or a
radiating frequency in a communications transmitter. The general approach
is to start with an oscillator, such as oscillator 10, productive of a
frequency above the range intended for use and then selectively to divide
down to whatever specific frequency is desired at a given time.
Responsive to the output of divider circuit 12 is an optional audio output
circuit 16 that may feed a speaker 18. This enables the user to obtain, if
desired, a direct audible production of the reference signal as finally
selected with respect to a specific given tone. While the provision of
this feature may not be necessary in the ultimate usage of the device, it
can at least serve the purpose of enabling the person originally skilled
in more fundamental approaches at tuning to verify his own impressions of
correct tuning by means of comparing beats and half tones.
At the other side of the system of FIG. 1 is a microphone or audio pickup
20 which senses a note developed by the musical instrument under
measurement or test. Thus, microphone 20 develops a sensed signal.
Typically, the sensed signal from microphone 20 desirably is amplified so
as to be capable of driving subsequent circuitry. At the same time, the
implementation of digital logic circuits hereinafter to be described
suggests that the signal received from the musical instrument be shaped
into a squared waveform that still exhibits the basic cyclic change
representative of frequency. Accordingly, microphone 20 feeds an
amplifier-shaper 22. The output from amplifier-shaper 22 is an at least
substantially-squared wave the cyclic transistion of which represents the
frequency of the signal detected by microphone 20. Squaring-type shapers
of this kind also are well known to the person skilled in the art.
In producing tones from many musical instruments, successive
time-increments of the detected signal under measurement often exhibit
minute variations in phase. In a logic system, such phase variations are
capable of being interpreted as pitch variations that could cause an
erratic determination. To the end of minimizing the effect of such phase
variations, the system includes a phase averager 24 responsive to the
sensed signal detected by microphone 20 and developed by amplifier-shaper
22. As herein embodied, phase averager 24 is simply a frequency divider
that divides the frequency of the signal received from amplifier-shaper 22
by an amount of four. Since the phase variations in the received and
processed signal under measurement tend to be equally positive and
negative about a zero value, the frequency divider circuit of phase
averager 24 serves to reduce the value of such phase excursions at least
toward zero.
By operating upon a dividing-down principle, phase averager 24 serves to
reduce the logic count of signals in that channel of the apparatus.
Compensatorily, a normalizer 226 is included in series with the output of
divider circuit 12. Normalizer 226, in this embodiment, divides down by
the same amount as is effected by phase averager 24. In a given system,
the amount of dividing-down accomplished by normalizer 226 could be
incorporated within the function of divider circuit 12.
The processed reference tone and measurement signals are ultimately
compared in a digital frequency comparator 26 served by a clock 28 so as
to facilitate coherent timing of the comparison operation. Comparator 26
serves to produce an indication signal representing the relationship
between the tone and measurement signals ultimately produced at the
outputs of normalizer 226 and phase averager 24. The different possible
outputs from digital frequency comparator 26 are that of "sharp," "flat"
or "on-tune." These outputs preferably are displayed by use of a visual
readout such as a light-emitting diode. That is, there is some means
responsive to the indicating signal received from comparator 26 such as a
sharp display 30, a flat display 32 and an on-tune display 34.
FIG. 2 illustrates a preferred implementation of comparator 26. The
clock-frequency from clock 28 is fed to the C input of each of a
respective pair of input flip-flops 36 and 38. The clock frequency signal
is also fed to the C input of a subsequent pair of flip-flops 40 and 42.
The reference tone signal from normalizer 226 is fed to the D input of
flip-flop 38 and also to one input of a NOR gate 44. Similarly, the input
frequency under measurement, and as derived from amplifier shaper 22 and
through phase averager 24, is fed to the D input of flip-flop 36 and also
to one input of a NOR gate 46.
The Q output of flip-flop 36 is fed to the other input of gate 46, while
the similar Q output of flip-flop 38 is fed to the other input of gate 44.
The Q output from flip-flop 36 is fed to the D input of flip-flop 40,
while the similar Q output of flip-flop 38 is fed to the D input of
flip-flop 42. From flip-flop 40, the Q output is fed as one input to a NOR
gate 48, while the Q output from flip-flop 36 is also fed to the other
input of gate 48. Similarly, the Q output of flip-flop 42 is fed to a NOR
gate 50 the other input of which is fed from the Q output of flip-flop 38.
The output from gate 48 is fed to the set input of a flip-flop 52. Again
similarly, the output of gate 50 is fed to the reset input of flip-flop
52. The output from gate 46 is fed as one input to a NAND gate 54, while
the similar output from gate 44 is fed to one input of a NAND gate 56. The
alternative outputs from flip-flop 52 are fed respectively to the other
inputs of gates 54 and 56. The output from gate 54 is fed through a
NOT-connective 58 to the set input of a flip-flop 60. Again similarly, the
output from gate 56 is fed through a NOT-connective 62 to the reset input
of flip-flop 60. The Q output from flip-flop 60 energizes a visual
indicator 66, such as a light-emitting diode, to represent the sharp
condition of the sensed signal. The Q output from flip-flop 60 similarly
energizes an indicator 64 such as a light-emitting diode so as to
represent a flat condition. The outputs from each of gates 54 and 56
likewise are fed as inputs to a NAND gate 68 the output of which is
connected through a resistor 72 to the base of a transistor 76. The
collector of transistor 76 is fed to the input of a threshold-setting
buffer-amplifier 82. The emitter of transistor 76 is returned to ground. A
resistor 78 is connected between a source of operating potential V+ and
the collector of transistor 76, while a capacitor 80 shunts that collector
to ground. The output of amplifier 82 is fed to another visual display
element 84.
The operation of the circuit of FIG. 2 follows the general state
transistion diagram depicted in FIG. 3a, as will be recognized by the
person skilled in the art. Moreover, the specific system illustrated
implements the simplified state transistion diagram of FIG. 3b. More
particularly, the timing diagram of FIG. 4 indicates the governing of the
system by the product of the clock and the particular relationship of its
operation with respect to a representative set of measured and reference
tone frequencies. The different arrows indicate, of course, the
transistions which occur.
It will be observed that the system of FIG. 2 includes what is known as an
up-down counter. FIGS. 3a and 3b reveal that comparator 26 is implemented
by the use of what is known as an excess false count algorithm. Basically,
transistions (either logical "0" to logical "1" or vice versa) of the
reference tone frequency and of the measurement frequency are entered into
the up-down counter until a net excess of either reference or measurement
pulses is detected. When such detection occurs, the sharp or the flat
signal is activated. The on-tune condition is met when the up-down counter
begins to alternate between a fully saturated state (i.e., all the way up
or all the way down) and an intermediate state. When the measured and
reference tone frequencies are exactly matched, except for a constant
phase angle, the intermediate state will be entered every cycle of the
reference tone frequency.
In more detail and with specific reference to FIG. 2, it will be observed
that flip-flop 52 serves to remember whether the reference tone or
measured frequency was the last to have a "1" to "0" transistion. Whenever
two transistions occur on either channel (reference or measured) before
the other channel undergoes a transistion, flip-flop 52 allows one of
gates 54 or 56 to generate a start or reset pulse for flip-flop 60. That
result establishes a sharp output for the case depicted in the state
transistions of the diagrams of FIGS. 3a and 3b. Since gates 54 and 56
generate pulses only when a frequency mismatch exists, the logical "OR" of
those pulses formed by gate 68 will approach zero frequency as the
frequencies approach a perfect match. The time constant established by
resistor 78 and capacitor 80 serves to set the limit to which the
frequencies must match in order that the on-tune condition be met. This is
represented by point T, in the diagram of FIG. 4, which indicates the
enablement of amplifier 82. For example, if the time constant is 0.1
second, then the frequencies must match to within approximately 10 hertz.
The only qualification is that the actual match may be affected by the
exact switching thresholds of the different logic circuit components.
As so far described, the system depends on clock 28 for coherent governing
of the operation. While this is a desirable feature, it is not necessary
to satisfactory performance. Accordingly, clock 28 may be eliminated and
the system of FIG. 2 may be simplified in an obvious manner. That manner
of simplification is incorporated into the embodiment depicted in FIGS. 5
and 7.
With further reference to FIG. 2, it will be observed that different groups
of components have been enclosed by dashed lines to indicate group
function. Thus, components 36-52 all constitute that which functions as a
digital phase difference detector 90. Gates 54 and 56 combine, with their
connections, to serve as a phase shift detector 92. In turn, components
72-80 serve as a time-charge circuit 94.
Turning now to the embodiment of FIG. 5, microphone 20 again feeds
amplifier-shaper 22. A reference-frequency source 96 is, as herein
exemplified, composed of oscillator 10, divider 12 and switch 14. In the
particular arrangement depicted in FIG. 5, phase averager 24 and
normalizer 226 have not been shown. When included, phase averager 24 is
inserted immediately beyond amplifier-shaper 22, and normalizer 226 is
inserted just beyond or included in source 96.
In any case, the measurement signal from amplifier-shaper 22 is fed to an
input of a digital phase difference detector 90' as well as to inputs of
phase shift detector 92 and a signal presence detector 98. The tone signal
from source 96 is fed as an input to both detectors 90' and 92. Detector
90' is the same as detector 90 modified to eliminate provision for the
clock signal; a suitable embodiment is shown in FIG. 7. Detector 92 is
composed of gates 54 and 56 as shown in FIG. 2. It also receives the tone
and measurement signals directly.
The two outputs of detector 92 feed the respective inputs of flip-flop 60
and also the respective inputs of an OR gate 68'. The Q output of
flip-flop 60 feeds sharp indicator 66 as before. Output Q of flip-flop 60
feeds flat indicator 64 through an AND gate 100. The output of gate 68' is
fed as the reset input to circuit 94'. From the output of the latter, a
signal is fed through a lower-threshold-setting buffer-amplifier 82' to
indicator 84. The output of detector 98 is fed to the other input of gate
100.
Overall operation of the system of FIG. 5, as well as that of the system of
FIG. 1, may be better understood as a result of reading the following
discussion. FIG. 6 depicts a typical timing sequence. A cyclically varying
signal may be expressed:
.omega. = d.phi./dt, (1)
where .omega. is the cyclic frequency, .phi. is the phase and t is time. At
the output of detector 90', the frequency difference is
.DELTA..omega. = .omega..sub.t - .omega..sub.m, (2)
where .omega..sub.t is the frequency of the tone signal from source 96 and
.omega..sub.m is the frequency of the measurement signal from amplifier
22. When the output from detector 90' is constant,
.DELTA..omega. = d.phi./dt = d/dt [k] = 0, (3)
where k is a constant.
When, however, the frequency difference is other than zero, the output from
detector 90' is not a constant. Determination of d.phi./dt is then made by
choosing a convenient integral shift angle. To that end, detector 92 is
utilized. Its 360.degree. phase shift detection enables it to produce a
pulse every time the phase difference between the tone and measurement
signals exceeds the value of 360.degree.. Moreover, it provides separate
outputs respectively for a +360.degree. and a -360.degree. shift. Those
separate outputs respectively are fed to the set and reset inputs of
flip-flop 60. Through action of the latter, they cause energization of the
corresponding ones of indicators 64 or 66.
Detector 98, in a well known manner as such and a suitable form of which is
shown in FIG. 8, latches on to the rising edge of the squared measurement
signal. Acting through gate 100, it disables energization of "flat"
indicator 64 in the absence of any measurement signal at all or whenever
the level of that signal is too weak to insure accurate processing of such
signal by the system. In the absence of a measurement signal, detectors
90' and 92 normally produce an indicating signal representing the flat
condition. This arrangement allows the person using the device to
determine if he is actuating the instrument under test sufficiently strong
to achieve proper measurement. It also saves power when the device is idle
but turned on.
When .DELTA..omega. is other than exactly zero, a reset pulse is
periodically generated and fed to circuit 94 from detector 92 through gate
68'. That pulse causes the output of circuit 94 to have a voltage value
below that of the threshold established by buffer 82'. Thus, "on-tune"
indicator 84 is turned off until such time as circuit 94 can bring the
indicator-driving voltage back above the threshold valve. As a result,
indicator 84 flickers at a rate proportional to the difference in the
frequency between the tone and measurement signals.
During any time that the frequency difference is such that its period is
less than the time required for circuit 94 to charge to the threshold
valve, indicator 84 is maintained in the "off" condition. On the other
hand, an exact match of the input frequencies results in indicator 84
being fully "on" without any flicker. Thus, the system provides a simple
and easily interpreted visual indication of whether the instrument being
tuned is either "sharp" or "flat" and, if one of those, providing a
further, and gradually-changing, indication as the instrument is tuned of
the degree of nearness to a perfect match between the reference tone and
sensed measurement signals.
Non-clocked digital phase detectors are now well known in the art. An
example of circuitry suitable for detector 90' is shown in FIG. 7. The
tone and measurement signals are fed to respective inputs of NAND gates
110 and 112 that are combined with corresponding cross-coupled NAND gates
114, 116 and 118, 120 all interconnected with a supervisory NAND gate 122
and corresponding control NAND gates 124 and 126 so as to provide
phase-differential outputs as indicated.
Analogously, signal presence detector 98 may take various forms. A
preferred detailed embodiment is shown in FIG. 8. Thus, the squared input
wave of the sensed or measurement signal are fed through a capacitor 130
and over a resistor 132 to the base of a transistor 134 the emitter of
which is returned to common. Its collector is connected to the junction
between a resistor 136 extending from a source of potential V+ and a
capacitor 138 returned to common. From that junction, the signal is fed
through a NOT-connective 140 that serves as a threshold gate.
When the square-wave signal applied to the input of detector 98 is of
sufficiently high frequency, transistor 134 discharges capacitor 138 at a
rate such that current through resistor 136 cannot charge that capacitor
to the threshold value of gate 140. Thus, gate 140 provides at its output
a logical "1" when the input frequency is greater than the time constant
of resistor 136 and capacitor 138. Otherwise, the output is a fluctuating
value of decreasing duty cycle as the input frequency decreases.
Broadly speaking, it will be seen that the approach hereinabove described
permits the achievement of the availability of tuning apparatus that may
provide a dependable output indication regardless of phase variations in
the signal being measured. Useful other improvements include the display
disablement in the absence of a signal and the flicker indication of
nearness to an "on-tune" condition. It also is attractive in providing a
direct-type readout. At the same time, the specific implementation of the
system involves the use of what amount to off-the-shelf components in
those portions of the implementation that utilize a logic approach.
Moreover, those components are capable of being entirely mounted upon a
single substrate or even integrated into a single chip. The end result is
an apparatus that may be exceedingly compact and very conveniently
transported and used. It is a simple apparatus capable of being hand-held
and yet fully utilizable by those who do not have an ear trained in note
production.
While particular embodiments of the invention have been shown and
described, and other modifications have been mentioned, it will be obvious
to those skilled in the art that changes and modifications may be made
without departing from the invention in its broader aspects, and,
therefore, the aim in the appended claims is to cover all such changes and
modifications as fall within the true spirit and scope of the invention.
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